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St. Croix River Crossing Preliminary Engineering

Concept Refinement Report

Prepared for:

Minnesota Department of Transportation and

Wisconsin Department of Transportation

Prepared by:

Parsons Brinckerhoff

June 2010





June 2010 i

Table of Contents

1 Executive Summary ....................................................................................................................... 1‐1

1.1 Pier Configuration ............................................................................................................ 1‐1

1.2 Box Girder Configuration ................................................................................................. 1‐2

1.3 Pedestrian Trail Location ................................................................................................. 1‐2

1.4

Cable Anchorage

Details

..................................................................................................

1‐2

1.5 Approach and Ramps Span Arrangement ........................................................................ 1‐2

1.6 Approach and Ramps Columns ........................................................................................ 1‐3

1.7 Bridge Lighting ................................................................................................................. 1‐3

2 Introduction .................................................................................................................................. 2‐1

2.1 Project Description .......................................................................................................... 2‐1

2.1.1 Proposed River Crossing Description ............................................................................... 2‐1

2.2 Report Purpose and Objective ......................................................................................... 2‐2

2.3 St. Croix River Crossing Development .............................................................................. 2‐3

2.4 General Definition of an Extradosed Bridge .................................................................... 2‐4

3 Acknowledgement of Commitments ............................................................................................ 3‐1

3.1

Reference Documents

.....................................................................................................

3‐1

3.2 Commitments .................................................................................................................. 3‐1

4 Risk Evaluation .............................................................................................................................. 4‐1

4.1 Risk Matrix and Summary ................................................................................................ 4‐1

4.2 Investigation and Evaluation of Risk Factors ................................................................... 4‐2

4.2.1 Structural Analysis ........................................................................................................... 4‐2

4.2.2 Bridge Lighting and Signing .............................................................................................. 4‐3

4.2.3 Visual Quality ................................................................................................................... 4‐4

4.2.4 Construction..................................................................................................................... 4‐5

4.2.5 Maintenance and Inspection ........................................................................................... 4‐6

5 Division I Structural Analysis ......................................................................................................... 5‐1

5.1

Introduction .....................................................................................................................

5‐1

5.2 Objectives ........................................................................................................................ 5‐1

5.3 Base Modeling ................................................................................................................. 5‐2

5.4 Longitudinal Analysis ....................................................................................................... 5‐2

5.5 Transverse Analysis .......................................................................................................... 5‐4

5.6 Superstructure Cross‐section Investigation ..................................................................... 5‐5

5.7 Inboard Pedestrian Trail .................................................................................................. 5‐6

5.8 Box Girder Depth ............................................................................................................. 5‐7

5.9 Single Box Girder Feasibility ............................................................................................. 5‐8

5.10 Transverse Diaphragm investigation ............................................................................... 5‐9

5.11 Pier Investigation ............................................................................................................. 5‐9

5.12

Fixity/Longitudinal Movement

Investigation

.................................................................

5‐12

5.13 Grade Induced Movement Investigation ....................................................................... 5‐13

5.14 Two Box Girder Load Distribution Investigation ............................................................ 5‐13

5.15 Wind Load—Vibration Analysis Investigation ................................................................ 5‐15

5.15.1 Back span Uplift/Maximum Back span Length Investigation......................................... 5‐17

5.16 Extradosed Stay Cable Analysis ...................................................................................... 5‐17

5.17 Points of Interest—Local Analysis .................................................................................. 5‐19





ii June 2010

6 Division II Structural Analysis ....................................................................................................... 6‐1

6.1 Introduction and Objectives ............................................................................................ 6‐1

6.1.1 Box Girder Width Transition Investigation ...................................................................... 6‐1

6.1.2 Box Girder Depth Economy Investigation ....................................................................... 6‐1

6.1.3 Fixity/Longitudinal Movement Investigation .................................................................. 6‐2

6.1.4 Grade Induced Movement Investigation ........................................................................ 6‐2

6.1.5 Box Girder Depth Transition Investigation ...................................................................... 6‐2

6.2 Investigation Methods: Longitudinal Analysis................................................................. 6‐2

6.3 Baseline Model Description ............................................................................................. 6‐4

6.3.1 Piers ................................................................................................................................. 6‐4

6.3.2 Box Girders ...................................................................................................................... 6‐6

6.3.3 Transition Span ................................................................................................................ 6‐7

6.3.4 Pedestrian Trail ................................................................................................................ 6‐7

6.4 Alternative R1 Investigation .......................................................................................... 6‐10

6.4.1 Grade‐Induced Movement Investigations ..................................................................... 6‐15

7 Bridge Lighting and Signing .......................................................................................................... 7‐1

7.1 Review of Requirements for Architectural and Roadway Lighting ................................. 7‐1

7.2

Lighting Criteria

...............................................................................................................

7‐1

7.3 Lighting Alternatives Analysis .......................................................................................... 7‐5

7.3.1 Source Selection .............................................................................................................. 7‐5

7.4 Architectural Lighting .................................................................................................... 7‐11

7.4.1 Option 1A ...................................................................................................................... 7‐12

7.4.2 Option 2A ...................................................................................................................... 7‐13

7.4.3 Option 2AA .................................................................................................................... 7‐14

7.4.4 Option 3 ......................................................................................................................... 7‐15

7.4.5 Option 4 ......................................................................................................................... 7‐16

7.4.6 Preferred Option ........................................................................................................... 7‐16

7.5 Roadway Lighting .......................................................................................................... 7‐17

7.6

Loop Trail

Lighting

.........................................................................................................

7‐18

7.7 Navigation and Obstruction Lighting ............................................................................. 7‐18

7.8 Light Trespass / Glare / Environmental Impacts ........................................................... 7‐18

7.9 System Maintenance ..................................................................................................... 7‐19

7.10 Box Section Inspection Lighting ..................................................................................... 7‐19

7.11 Signing ........................................................................................................................... 7‐20

7.11.1 Review of the Visual Quality Manual ............................................................................ 7‐20

7.11.2 Additional Concept Refinements .................................................................................. 7‐20

8 Visual Quality ................................................................................................................................ 8‐1

8.1 Introduction ..................................................................................................................... 8‐1

8.2 Objectives ........................................................................................................................ 8‐2

8.3

Refinement Process

.........................................................................................................

8‐2

9 Construction ................................................................................................................................. 9‐1

9.1 Introduction ..................................................................................................................... 9‐1

9.2 Construction Staging Areas ............................................................................................. 9‐1

9.3 Casting Yard ..................................................................................................................... 9‐1

9.4 Precast vs. Cast‐in‐place Construction ............................................................................ 9‐1

9.4.1 Precast Construction ....................................................................................................... 9‐2

9.4.2 Cast‐in‐ Place Construction ............................................................................................. 9‐3





June 2010 iii

9.5 Permitting Requirements ................................................................................................. 9‐3

9.6 Erosion Control and Environmental Compliance ............................................................. 9‐4

9.7 Foundation Construction Methods .................................................................................. 9‐4

9.8 Substructure Pier and Tower Construction ..................................................................... 9‐5

9.9 Reinforcing steel arrangements congestion and detailing .............................................. 9‐5

9.10 Post‐tensioning tendons and grouting ............................................................................ 9‐6

9.11 Stay Installation ............................................................................................................... 9‐6

9.12 Special architectural forming and finishing ..................................................................... 9‐6

9.13 Industry forum ................................................................................................................. 9‐7

10 Maintenance and Inspection ...................................................................................................... 10‐1

10.1 Introduction ................................................................................................................... 10‐1

10.2 Critical Elements ............................................................................................................ 10‐1

List of Figures

Figure 4‐1. Risk Assessment ................................................................................................................ 4‐1

Figure 5‐1.

Stick

(top)

and

Rendered

(bottom)

Isometric

View

of

Global

Model

...............................

5‐2

Figure 5‐2. Rendered Isometric View of Global Model Showing Construction Sequence .................. 5‐3

Figure 5‐3. Box Girder Stress along Length of Bridge .......................................................................... 5‐4

Figure 5‐4. Transverse Framing at Cable Connection .......................................................................... 5‐5

Figure 5‐5. Baseline Box Girder Superstructure .................................................................................. 5‐6

Figure 5‐6. Proposed Box Girder Showing the Reduced Depth and Integrated Pedestrian Trail ....... 5‐7

Figure 5‐7. Typical Sections ................................................................................................................. 5‐9

Figure 5‐8. VQM Baseline Pier Configuration .................................................................................... 5‐10

Figure 5‐9. Proposed Pier Configuration ........................................................................................... 5‐12

Figure

5‐

10.

Deflected

Pier

Shape

.....................................................................................................

5‐

13

Figure 5‐11. Superstructure between Cable Supports ...................................................................... 5‐14

Figure 5‐12. Transverse Analysis ....................................................................................................... 5‐14

Figure 5‐13. Closure Pour Detail ........................................................................................................ 5‐15

Figure 5‐14. Mode Shapes from the Eigenvalue Dynamic Analysis .................................................. 5‐16

Figure 5‐15. Extradosed Stay Cable Configuration ............................................................................ 5‐18

Figure 5‐16. Live Load Stress Range .................................................................................................. 5‐19

Figure 5‐17. Local Model of Integral Pier (Bottom View) .................................................................. 5‐20

Figure 5‐18. Local Model of Integral Pier (Top View) ........................................................................ 5‐20

Figure 5‐19. Pier Cross Girder Interface Model ................................................................................. 5‐21

Figure 5‐20. Pier 8 Elevation Showing Widened Superstructure and Full Super Elevation .............. 5‐22

Figure 5‐21. Span 8 Typical Section ................................................................................................... 5‐23

Figure 5‐22. Isometric View of Precast Truss .................................................................................... 5‐23

Figure 5‐23. Isometric View of 3D Analysis ....................................................................................... 5‐24

Figure 6‐2. Detail of Segments on Pier and Accurate Section Property Input .................................... 6‐3





iv June 2010

Figure 6‐1. Balanced Cantilever Construction Method (Modeling of Construction Stage 6,

Bridge 82048) ..................................................................................................................... 6‐3

Figure 6‐3. Stress Output Checked at 10 Locations in the Cross‐section ........................................... 6‐4

Figure 6‐4. Pier 1 on TH 36 (Bridge No. 82045) .................................................................................. 6‐5

Figure 6‐5. Typical Pier on TH 36 Approaches (Others Similar) .......................................................... 6‐6

Figure 6‐6.

Baseline

Approach

Box

Girders.........................................................................................

6‐7

Figure 6‐7. Baseline Model 82045E‐r0A, Bridge 82045 EB (EB TH 36) ............................................... 6‐8

Figure 6‐8. Plan and Elevation of Baseline Model, Bridge 82045 EB (EB TH 36) ................................ 6‐8

Figure 6‐10. Model 82045E‐r1A—Double‐stem Columns in Alternative R1 Span Arrangement ..... 6‐11

Figure 6‐9. Plan and Elevation of Alternative “R1” Models for EB and WB TH 36. .......................... 6‐11

Figure 6‐11. Model 82045E‐r1B—Single‐stem Columns in Alternative R1 Span Arrangement ....... 6‐12

Figure 6‐12. Plan and Elevation of Model 82045W‐r1B—WB TH 36 with Single‐stem Columns

in Alternative R1 Span Arrangement ................................................................................ 6‐12

Figure 6‐13. Bridge No. 82047 (NE Ramp) Stick Model and Rendered Model ................................. 6‐13

Figure 6‐14.

Bridge

No.

82047

Rendered

Plan

and

Elevation

from

Structural

Model

with

Details ............................................................................................................................... 6‐13

Figure 6‐15. Bridge No. 82048 (SE Ramp) Stick Model (Left) and Rendered Sectional Model

(Right) ............................................................................................................................... 6‐14

Figure 6‐16. Bridge No. 82048 (SE Ramp) Plan and Elevation View of Rendered Model ................. 6‐14

Figure 6‐17. Models Used in Investigating Grade‐induced Deflection ............................................. 6‐15

Figure 7‐1. Typical LED ........................................................................................................................ 7‐6

Figure 7‐2. Schematic of LED Operation ............................................................................................. 7‐6

Figure 7‐3. Typical Representative Spatial Radiation Pattern for White Lambertian ......................... 7‐8

Figure 7‐4. Typical Polar Radiation Pattern for White Lambertian ..................................................... 7‐8

Figure 7‐5.

Typical

Luminous

Flux

.......................................................................................................

7‐9

Figure 7‐6. Typical Light Output Characteristics Over Temperature (Cool‐White at Test

Current) ............................................................................................................................ 7‐10

Figure 7‐7. Typical Lumen Maintenance Values for Various Light Sources ...................................... 7‐11

Figure 7‐8. Base Architectural Lighting Option ................................................................................. 7‐11

Figure 7‐9. Lighting Option 1a ........................................................................................................... 7‐12

Figure 7‐10. Lighting Option 1b ........................................................................................................ 7‐12

Figure 7‐11. Lighting Option 2a ......................................................................................................... 7‐13


Figure

7‐

13.

Lighting

Option

2aa

.......................................................................................................

7‐

14

Figure 7‐14. Lighting Option 2bb ...................................................................................................... 7‐14

Figure 7‐15. Lighting Option 3 .......................................................................................................... 7‐15

Figure 7‐16. Lighting Option 4a ......................................................................................................... 7‐16


Figure 7‐18. Preferred Option—View from Sunnyside Marina ........................................................ 7‐16

Figure 7‐19. Preferred Option—View from Stillwater ...................................................................... 7‐16





June 2010 v

Figure 7‐20. Preferred Option—View Looking North ........................................................................ 7‐17

Figure 7‐21. Roadway Lighting .......................................................................................................... 7‐17

Figure 7‐22. Loop Trail Lighting Using Linear Luminaires .................................................................. 7‐18

Figure 7‐23. Navigation and Obstruction Lights ................................................................................ 7‐18

Figure 7‐24. LED Utility Light ............................................................................................................. 7‐19

Figure 7‐25. Typical Inspection Lighting ............................................................................................ 7‐20

Figure 8‐1. Section View Illustrates Curved Outside Edges of Cross Girder ........................................ 8‐3

Figure 8‐2. Tangent Segments Have Been Introduced Along Vertical Faces of Tower Forms

for Ease of Construction ...................................................................................................... 8‐4

Figure 8‐3. View along pedestrian trail located inside of tower line .................................................. 8‐5

Figure 8‐4. Section view of trail located on north edge along west bound travel lanes ..................... 8‐6

Figure 8‐5. Typical Pedestrian Overlook outside of tower on north elevation of bridge ................... 8‐7

Figure 8‐6. View of underside of Pedestrian overlook surrounding tower ......................................... 8‐7

Figure 8‐7. Higher curb height with 6” vertical picket spacing and integral LED light fixtures

incorporated into

post

on

pedestrian

hand

railing.

............................................................

8‐8

Figure 8‐8. North elevation view of structure with covered cable connections ................................. 8‐9

Figure 8‐10. View of Battered Abutment (Left) and Typical Single Stem Pier (Right) along

Minnesota Approach ........................................................................................................ 8‐10

Figure 8‐9. Enlarged view of covered cable connections .................................................................. 8‐10

List of Tables

Table 4‐1. Summary of Risk Assessment ............................................................................................. 4‐2

Table 5‐1. Box Girder Depth Cost Comparison .................................................................................... 5‐8

Table 6‐1.

Baseline

Span

Arrangement

...............................................................................................

6‐9

Table 6‐2. Span Arrangement “R1” ................................................................................................... 6‐10

Table 6‐3. Grade‐induced Movement Results ................................................................................... 6‐15

Table 7‐1. Illuminance Levels for Floodlighting Buildings and Monuments ........................................ 7‐2

Table 7‐2. Illuminance Method—Recommended Values ................................................................... 7‐2

Table 7‐3. Luminance Method—Recommended Values ..................................................................... 7‐3

Table 7‐4. Illuminance and Luminance Design Values (English) .......................................................... 7‐4

Table 7‐5. Recommended Illumination (Values in lux) ....................................................................... 7‐5

Table 7‐6. Flux Characteristics for LUXEON K2 with TFFC Junction and Case Temperature =

25C ..................................................................................................................................... 7‐9





June 2010 i

Acronyms and Abbreviations

2D two dimensional

3D three dimensional

AASHTO American Association of State Highway and Transportation Officials

ASBI American Segmental Bridge Institute

avg average

BMP best management practice(s)

cd/m2 candelas per square meter

CSL Crosshole sonic logging

cy cubic yards

DB design build

EB eastbound

EoR engineer of record

f’c Concrete 28‐day compressive strength

fc

Footcandles

FEIS Final Environmental Impact Statement

FGUTS Prestressing strand guaranteed ultimate tensile strength

FHWA Federal Highway Administration

f pu Prestressing strand specified ultimate tensile strength

ft feet

I severity of impact

IES Illuminating Engineering Society

IESNA Illuminating Engineering Society of North America

InGaN

indium

gallium

nitride

kips thousand pounds

ksi thousands of pounds per square inch

LED light emitting diode

Lpile A Program for the Analysis of Piles and Drilled Shafts Under Lateral Loads

LRFD Load and Resistance Factor Design

lux SI unit of illuminance and luminous emittance

mA milliamps

max maximum

min minimum

Mn/DOT Minnesota Department of Transportation

NDE Nondestructive evaluation

NE northeast

P probability of occurrence

PB Parsons Brinckerhoff (PB Americas, Inc.)

PGL Profile grade line





ii June 2010

PTI Post‐Tensioning Institute

RFP request for proposal

RGB Red green blue color model

SE southeast

sf square feet

SFEIS Supplemental Final Environmental Impact Statement

Tj junction temperature

UP R/R Union Pacific Railroad

USCG U.S. Coast Guard

UV ultraviolet

VE value engineering

VQAC Visual Quality Advisory Committee

VQM Visual Quality Manual VQRC Visual Quality Review Committee

WB

westbound

WisDOT Wisconsin Department of Transportation

Certification

I hereby certify that this report was prepared by me or under my direct supervision,

and that I am a duly Licensed Professional Engineer under the laws of the State of

Minnesota.

Paul J. Towell

PE No. 42965

June 30, 2010





June 2010 1‐1

1 Executive Summary

This Concept Refinement Report has been developed by Parsons Brinckerhoff (PB),

under contract with the Minnesota Department of Transportation (Mn/DOT). PB is

providing concept design and associated services for the proposed new St. Croix

River

Crossing

that

is

within

the

overall

St.

Croix

River

Crossing

Project.

These

services are part of the Preferred Alternative Package for the St. Croix River Crossing

Project as documented in the 2006 Supplemental Final Environmental Impact

Statement (SFEIS).

The purpose of the report is to document the results of investigations of the

following aspects of the bridge:

Structural Analysis

Bridge Lighting and Signing

Visual Quality

Construction

Maintenance and

Inspection

The report separates the structural analysis portion of the study into two divisions.

Division I is for the extradosed portion of Bridge 82045, and Division II is for the

approach spans of Bridge 82045 and the ramps designated Bridges 82047 and

82048. The other areas of study are reported in combined sections that cover both

divisions.

The starting point for the concept refinement is the concept design presented in the

Visual Quality Manual (VQM). This concept is based on the use of concrete

segmental bridges for the mainline and ramp structures. In addition, the river

crossing portion of the mainline structure is an extradosed bridge with short towers

above the

roadway

and

cable

support

of

the

girder.

The

objective

of

the

concept

refinement effort is to confirm the feasibility of the VQM concept and to develop

and refine the concept prior to advertising for final design and construction. A key

component of the concept refinement effort was coordination with the Visual

Quality Advisory Committee (VQAC). Meetings with the committee, consisting of a

subset of the stakeholder groups involved in the SFEIS process, were used to present

and discuss proposed concept refinements. Based on the results of the various

studies undertaken, and input from the committee, a number of refinements were

incorporated.

A summary of the results of the concept refinement effort are as follows:

1.1 Pier Configuration

The VQM concept three‐column pier with a center column under the box girder

girders was revised to a two‐column pier by eliminating the center column.

Structural analysis determined that the center column was not necessary, and with

post‐tensioning the cross girder between the two columns had sufficient strength to

support the box girders. From a visual quality aspect, the removal of the center





1‐2 June 2010

column had been the desire of the Visual Quality Review Committee during the

development of the VQM, but structural feasibility needed to be confirmed before

this goal could be attained. In addition the form of the pier columns was refined to

improve constructability.

1.2 Box Girder

Configuration

Two box girder configurations were studied for the roadway deck, a two‐box girder

configuration as shown in the VQM, and a one‐box girder configuration. Structural

analysis and constructability studies determined that both configurations were

viable.

1.3 Pedestrian Trail Location

The VQM concept with the pedestrian trail outboard of the box girders and passing

around the north pier column was revised to an inboard configuration with the

pedestrian trail located on the box girder and passing inside of the north pier

column. This

configuration

was

a recommendation

of

the

CRAVE

Study

completed

in

November 2008. Various studies confirmed that this arrangement was similar in cost

to the concept, but with the benefit of improved constructability and long‐term

durability. With the trail located inboard of the column, pedestrian overlooks were

at the piers to permit people to stop and view the scenic river while removed from

through bicycle traffic. After review by the VQAC, it was determined that three

overlooks located at alternating piers within the river would be appropriate.

1.4 Cable Anchorage Details

The VQM concept with the anchorages exposed along the side of the box girders

was revised to cover the side face of the anchorages providing a smooth line along

the roadway edge. This was a recommendation of Sumitomo Mitsui Construction

Co., Ltd., the technical advisor for the project. The saw tooth appearance of the

exposed anchorages did not seem to be in harmony with the smooth curving forms

of the bridge. Visually the covered anchorage gives the bridge a much cleaner

appearance and the added vertical face provides additional protection from the

elements for the cable anchorage.

1.5 Approach and Ramps Span Arrangement

The VQM concept assumed typical spans of approximately 300 feet for the

Minnesota approach spans and ramps to minimize the number of piers at the

approach spans

that

extend

from

the

bluff

toward

the

shoreline

in

Minnesota.

The

approach spans and two ramp structures cross four wetland areas, with the primary

impacts from construction in Basin Q and Basin Q Forested. The 300‐foot span

arrangement proposed in the concept design would have required extensive

falsework. To reduce the impacts from construction on falsework, an alternate span

arrangement was developed that permitted a greater amount of balanced cantilever

segmental construction and reduced the wetland impacts. The long spans proposed





June 2010 1‐3

in the concept design are not conducive to segmental construction, specifically at

spans adjacent to expansion joints and at bridge sections that vary in width. Both of

these conditions occur over the Basin Q wetlands.

1.6 Approach and Ramps Columns

The VQM

concept

has

twin

stem

piers

with

a 10

‐foot

space

between

the

stems.

Visually the piers had a heavy look and structurally the piers stiffness could not be

reduced without the stems becoming overly slender. To resolve these issues, an

alternate twin stem arrangement with a 5‐foot space between the stems and a

single stem pier were studied. The structural results proved that the single stem pier

was feasible and based on review by the VQAC, the single stem pier was selected. In

addition to structural and aesthetic benefits, the single stem column is more

economical and improves constructability.

1.7 Bridge Lighting

Bridge lighting

studies

were

performed

for

both

functional

lighting

and

architectural

lighting. Functional lighting consists of roadway lighting, pedestrian trail lighting,

aerial obstruction lighting, and navigation lighting. In addition a number of options

for architectural lighting were added to the functional lighting in photo visualizations

for review by the VQAC. Based on input from the VQAC it was determined that lower

lighting levels were preferred, and the proposed architectural lighting is limited to

lighting of the pier columns below the roadway. In combination with the roadway

and trail lighting, which provide low levels of light on the cables and columns above

the roadway, the desired effect is achieved.

This concept refinement report is not a final engineering report, and it is anticipated

that final

design

will

be

performed

by

others

under

a separate

contract.





June 2010 2‐1

2 Introduction

2.1 Project Description

This Concept Refinement Report has been developed by PB, under contract with the

MnDOT. PB will provide concept design and associated services for the proposed

new St.

Croix

River

Crossing

that

is

within

the

overall

St.

Croix

River

Crossing

Project:

Minnesota S.P. 8214‐114, S.P. 8214‐(82045), S.P. 8214‐(82047), S.P. 8214‐(82048)

and Wisconsin Project I.D. 1550‐00‐02. The overall St. Croix River Crossing Project

termini are along TH 36/STH 64 from TH 5 in Minnesota to 150th

Avenue in

Wisconsin. These services are part of the Preferred Alternative Package for the St.

Croix River Crossing Project as documented in the 2006 SFEIS.

2.1.1 Proposed River Crossing Description

The proposed St. Croix River Crossing is comprised of three separate bridges that are

identified as MnDOT Bridge Nos. 82045, 82047, and 82048.

The alignment

of

the

bridge

follows

a horizontally

curved

extension

of

the

TH

36

alignment that will pass just to the south of the Oak Park Heights water treatment

plant and north of the King Power Plant. The alignment continues on a tangent

across and approximately perpendicular to the river intersecting the bluffs in

Wisconsin, and continuing on to new alignment for STH 64. The bridge approach

spans, which extend from the Minnesota bluff to the river cross both high quality

wooded wetlands adjacent to the river and lower quality wetlands inland from the

river. An eastbound entrance ramp structure and westbound exit ramp structure

connect TH 36 to TH 95 in Minnesota.

Requirements for these bridges have been developed and are documented in two

reports, the

2006

SFEIS,

and

the

VQM

(January

2007).

Those requirements are summarized as follows:

The extradosed river spans will have no more than 6 piers in the water.

The extradosed towers vary in height from approximately 220 feet on the

Wisconsin side to approximately 170 feet on the Minnesota side (tower heights

are above normal pool elevation and include approximately 60 feet of tower

above the deck).

The extradosed span lengths are approximately 480 feet and the backspans

approximately 290 feet.

The extradosed typical section consists of two 12 foot lanes in each direction, 6

foot inside

shoulders,

10

foot

outside

shoulders,

and

a 12

foot

sidewalk

on

the

north side of the bridge.

Aesthetics, landscaping, and context sensitive designs are in the VQM.





2‐2 June 2010

Mn/DOT Bridge 82045 (WisDOT Bridge B-55-224) (Mainline TH 36 and

STH 64)

Bridge No. 82045 spans Minnesota TH 95, the Union Pacific Railroad(UP R/R),

wetlands, and the St. Croix River. Its west abutment is located immediately west of

TH 95. Its east abutment is located on the Wisconsin river bluff. The total bridge

length from

abutment

to

abutment

is

approximately

5,040

feet.

The

bridge

is

comprised of main river spans with a length of approximately 3,460 feet and

westerly approach spans with a length of approximately 1610 feet. The river spans

have an extradosed superstructure that combines concrete box girders with cable

stays. The approach spans have a concrete box girder superstructure. The transition

from approach spans to river spans occurs at a common pier located just inland from

the Minnesota shoreline. The Minnesota approach spans to the river crossing have

lengths ranging from 180 to 300 feet.

Mn/DOT Bridge 82047 (TH 36WB off -ramp to TH 95)

Bridge

No.

82047

spans

the

UP

R/R,

local

roadways,

and

wetlands.

This

bridge

has

a

concrete box girder superstructure that frames into an approach span of Bridge

82045. The structure depth is directly related to that of the Bridge 82045 river spans.

The typical section has a variable lane width ranging from 16 to 32 feet, with 4 foot

inside and outside shoulders.

Mn/DOT Bridge 82048 (TH 36EB on-ramp from TH 95)

Bridge No. 82048 spans the UP R/R, local roadways, and wetlands. This bridge has a

concrete box girder superstructure that frames into an approach span of Bridge

82045. The structure depth is directly related to that of the Bridge 82045 river spans.

The typical section has a variable lane width ranging from 16 to 32 feet, and 4 foot

inside and

outside

shoulders.

2.2 Report Purpose and Objective

The Concept Refinement Report that will look at a number of items for investigation

described in further detail within the report.

The content of the report is separated into two divisions:

Division I is for the extradosed portion of Bridge 82045 (B‐55‐224),

Division II is for the approach spans of Bridge 82045 (B‐55‐224) and the ramp

Bridges 82047 and 82048.

Items

for

study

and

evaluation

will

include:

Structural Analysis

Bridge Lighting and Signing

Visual Quality

Construction

Maintenance and Inspection





June 2010 2‐3

The report provides the results of the investigation and evaluation of risk for each of

the items listed above. The evaluation of risk for each item assesses and reports the

impacts to design feasibility, constructability, costs, schedule, environmental risks,

aesthetics, long term durability, maintenance, etc. In addition to detailed findings, a

risk analysis matrix is provided that summarizes the risk assessment for ease of

understanding.

The report is arranged in the following format and content:

Table of Contents

Executive Summary

Introduction

Acknowledgement of Commitments—The Report will cite commitments made in

project documents that affect the bridge. This includes such documents as the

SFEIS, Visual Quality Manual, etc.

Risk Evaluation

Content—Divisions I and II

Portions of the report may be incorporated into a future design build request for

proposal (RFP) for the project.

2.3 St. Croix River Crossing Development

A new river crossing near Stillwater to replace the aging lift bridge has been

discussed for many years. The first formal effort to replace the existing bridge began

in 1985 with a Draft Study and Scoping Document. The planning process moved

forward until 1995 when FHWA approved and Final Environmental Impact

Statement (FEIS). Final design began in 1995, but in 1996 the National Park Service

reacting to federal permit applications determined that the proposed bridge would

have an

adverse

effect

on

the

St.

Croix

River,

a part

of

the

National

Wild

and

Scenic

River System. With this determination, the federal permits could not be issued and

work on the new river crossing stopped.

Beginning in 1998 efforts to revive the river crossing began culminating in the

approval of an SFEIS in 2006. The visual appearance of the St. Croix River Crossing

and the context of the bridge within the wild and scenic riverway were critical

factors in the development of the new river crossing. As the SFEIS was developed

between 2004 and 2006, the extradosed bridge type was select for the main river

crossing through an extensive stakeholder process that involved local state and

federal government agencies, as well as, local and national citizen organizations. In

parallel with

the

SFEIS

process,

a VQM

was

developed

to

outline

the

aesthetic

values

for the project. A VQRC with member participation from all of the stakeholder

groups was a key part of the visual quality process. In addition the VQM process

included a public open house to gather public input for the aesthetic development

of the bridge.

For a complete history of the planning stages for the St. Croix River Crossing refer to

Chapter 1 of the 2006 Supplemental Final Environmental Impact Statement.





2‐4 June 2010

2.4 General Definition of an Extradosed Bridge

The extradosed bridge concept was first proposed by French engineer Jacques

Mathivat in 1988. In the past 20 years over 40 extradosed bridges have been built

around the world, with the majority of the bridges built in Japan. The extradosed

bridge combines a prestressed girder bridge with a cable stayed bridge. With a span

to depth ratio of 30 to 35 compared to a typical prestressed girder bridge with a

span to depth ratio of 20 to 25, the economy of a shallower girder is realized with

the extradosed bridge. The tower height: span ratio of 1:8 compared to 1:4 for a

cable stayed bridge gives a structure height that is much less imposing than a cable

stayed bridge. Finally because of the relatively stiff girder, the extradosed bridge

permits allowable cable stresses of 0.6 f pu compared to 0.45 f pu for a cable stayed

bridge. This leads to the economy of fewer strands in the cables and a reduction in

the number of post‐tensioning tendons needed to support the bridge.





June 2010 3‐1

3 Acknowledgement of Commitments

3.1 Reference Documents

2006 Supplemental Final Environmental Impact Statement

Visual Quality

Manual

3.2 Commitments

This Concept Refinement Report and associated Concept Drawings have been

developed by PB, under contract with Mn/DOT. PB has performed this work in

accordance with the concepts and commitments included in the 2006 Supplemental

Final Environmental Impact Statement and the Visual Quality Manual.





June 2010 4‐1

4 Risk Evaluation

4.1 Risk Matrix and Summary

Risk management is an integral part of all phases of project delivery, from planning

and preliminary engineering through final design and construction. Risk

management is

the

systematic

process

of

identifying,

analyzing,

and

responding

to

project risk. Risk management must be conducted throughout the life of a project. A

standard dictionary definition of risk is: “exposure to the chance of injury or loss; a

hazard or dangerous chance”. In the design and construction of large transportation

projects there are inherent risks that must be explored as the project develops. At

the beginning, these variables, such as, subsurface geology, material quantities and

costs, and material and labor availability, are uncertain. As a project develops, these

risks begin to be to become less variable as information is gathered and engineering

evaluation begins. Risk assessment is used to evaluate risks as part of risk

management. The first step is to identify the risk factors. For the St. Croix River

Crossing, eight

specific

risk

factors

have

been

identified.

Each

of

the

five

report

areas

are to be assessed for these eight risk factors, with an additional risks assessed as a

ninth risk factor.

Assessment of risk is based on a scale from 1 to 3 for the probability of occurrence

(P), and on a scale of 1 to 3 for the severity of impact (I). Scoring is qualitative, with

low defined as 1, medium defined as 2, and high defined as 3. The risk score is the

multiple of P x I, such that the lowest risk is 1 and the highest risk is 9. The process is

illustrated in Figure 4‐1.

Figure 4‐1. Risk Assessment

In some cases a category may not be applicable, in which case, a value of 0 will be

applied. Table 4‐1 shows the summary of risk assessment.

Low Med High

L o w

M e d

H i g h

Impact (I) P r o

b a b i l i t y o f O c c u r r e n c e ( P )

Significant

Risk Area





4‐2 June 2010

Table 4‐1. Summary of Risk Assessment

Risk Factor

Structural

Analysis

Bridge

Lighting &

Signing

Visual

Quality Construction

Maintenance

& Inspection

Design feasibility 3 1 1 1 2

Constructability

2

1

2

1

1

Cost 4 1 2 6 1

Schedule 1 1 1 6 0

Environmental risks 1 1 1 3 1

Aesthetics 1 1 1 1 2

Long‐term durability 2 1 1 2 3

Maintenance 2 1 1 2 3

Total 16 8 10 22 13

Average 2.0 1.0 1.25 2.75 1.63

4.2 Investigation and Evaluation of Risk Factors

4.2.1 Structural Analysis

Design Feasibility

Probability (P): 1 Implementation of a QA/QC procedure in the design process

will help confirm the design is feasible.

Impact (I): 3 When a major error is found during construction, the impact to

cost and schedule is severe.

Constructability

Probability (P): 1 Proper constructability reviews will significantly reduce the

chance of construction problems.

Impact (I): 2 Errors found during construction can be corrected.

Costs

Probability (P): 2 Overly conservative design and/or design errors will increase

construction cost.

Impact (I): 2 Conservative design will not significantly increase construction

cost, but design error may.

Schedule

Probability (P): 1 Design may impact schedule. The design process should be

clearly defined and the review process integrated into the

design process, to permit timely reviews and issuing of

construction drawings.

Impact (I): 1 Lack of approved construction drawings can slow or stop

construction.





June 2010 4‐3

Environmental Risks

Probability (P): 1 Low risk.

Impact (I): 1 Low impact.

Aesthetics

Probability (P):

1

Low risk.


Long-term Durability

Probability (P): 1 Proper analysis and design detailing will ensure the long‐term

durability of the structure.

Impact (I): 2 Serviceability issues can be corrected with remedial work, but

additional costs are incurred.

Maintenance

Probability (P):

1

Accessibility for

inspection

and

maintenance

must

be

considered throughout the design process.

Impact (I): 2 Accessibility for maintenance and inspection is of critical

importance.

4.2.2 Bridge Lighting and Signing

Design Feasibility

Probability (P): 1 Proposed bridge light and signing details are fairly standard.


Constructability


Impact (I): 1 Low Impact.

Costs



Schedule



Environmental Risks

Probability (P): 1 Spillover lighting can be assessed during lighting tests and

adjustments made as needed.






4‐4 June 2010

Aesthetics

Probability (P): 1 Planned aesthetic lighting levels are minimized to subtly

highlight the structure. Current modeling techniques allow

accurate design and visualization of the lighting system



Probability (P): 1 LED fixtures have long operating life and are low cost to

operate.


Maintenance

Probability (P): 1 LED fixtures have long operating life and are low cost to

operate.


4.2.3 Visual Quality

Design Feasibility

Probability (P): 1 Preliminary engineering has vetted the design feasibility of the

aesthetic design elements of the bridge.


Constructability

Probability (P): 1 The design features include many curved surfaces, but the

concept refinement has conformed the various surfaces to

standard geometric shapes.

Impact (I): 2 The many curved surfaces will provide more of a challenge than

construction with flat surfaces.

Costs

Probability (P): 2 The forming systems required for the various design elements

will cost more than standard forming systems.

Impact (I): 1 The repetitive use of these forming systems will minimize this

additional cost.

Schedule

Probability

(P):

1

Some

reduction

in

production

rates

may

occur

due

to

curved

form of the various design elements.

Impact (I): 1 As with all construction operations, there is a learning curve,

but as operations are repeated productivity increases.





June 2010 4‐5

Environmental Risks

Probability (P): 1 Visual impacts are addressed in the visual quality process,

which provides guidance to ensure the bridge fits into the

environment.


Aesthetics

Probability (P): 1 The visual quality process has developed aesthetic standards

for the bridge.



Probability (P): 1 The aesthetic features do no impact long‐term durability.

Impact (I): 1 Low Impact

Maintenance

Probability (P): 1 The aesthetic features do not impact maintenance.

Impact (I): 1 Low Impact

4.2.4 Construction

Design Feasibility

Probability (P): 1 The planned construction methods are included in the design

process using construction stage analyses to ensure that the

construction methods are feasible. The planned construction

methods are standard methods.

Impact

(I):

1

Coordinated

construction

planning

during

the

design

phase

will

limit design impacts.

Constructability

Probability (P): 1 The planned construction methods using concrete segmental

construction with either cast‐in‐place or precast segments are

standard methods used throughout the world.

Impact (I): 1 Coordinated constructability reviews during the design phase

will limit constructability impacts.

Costs

Probability

(P):

2

Foundation

construction

costs

are

less

defined

than

other

costs,

because preliminary foundation capacity is relatively low.

Impact (I): 3 Foundation costs are significant, and cost reduction or increase

can be significant.

Schedule

Probability (P): 2 Construction is the largest project activity and controls the

contract duration. Proper scheduling is critical.





4‐6 June 2010

Impact (I): 3 The construction schedule must be closely monitored to ensure

that progress toward schedule completion is maintained. If the

schedule begins to slip, work around schedules, or

supplemental schedules must be developed to get the project

back on track.

Environmental Risks

Probability (P): 1 Construction operations by their very nature impact the

environment. Proper environmental standards and controls are

necessary to ensure environmental compliance.

Impact (I): 3 The Minnesota approach spans cross environmentally sensitive

wetlands and the St. Croix River is a national scenic waterway

with mussel beds near the Wisconsin shore. Uncontrolled and

unmonitored construction operations can had serious

environmental impacts. Construction debris, excavated soils,

and petroleum products are among the numerous items that

can cause

environmental

damage.

Aesthetics

Probability (P): 1 Proper workmanship is utmost importance to produce finished

work of the specified quality.

Impact (I): 1 Typical finishing methods will remedy minor workmanship

errors.


Probability (P): 1 Material controls and construction QA/QC programs must be

implemented and maintained to ensure that the requirements

of the

plans

and

specifications

are

incorporated

into

the

work.

Impact (I): 2 Incorporating specified materials using proper construction

methods and workmanship are critical to long‐term durability.

Out of specification metal products and/or concrete will have a

detrimental effect on long‐term structure performance.

Maintenance

Probability (P): 1 See Long‐term Durability.

Impact (I): 2 See Long‐term Durability.

4.2.5 Maintenance and Inspection

Design Feasibility

Probability (P): 1 Design details will affect maintenance and inspection.

Maintenance and inspection should be considered throughout

the design process.

Impact (I): 2 Design that does not include consideration of maintenance and

inspection access will impact operations.





June 2010 4‐7

Constructability

Probability (P): 1 Maintenance and Inspection have little impact on

constructability.

Impact (I): 1

Costs

Probability (P): 1 Inspection and maintenance are directly related to cost.

Impact (I): 1 The cost of future maintenance and inspection is directly linked

to accessibility.

Schedule

Probability (P): 0 None.

Impact (I): 0 None.

Environmental Risks

Probability (P):

1

The maintenance

of

the

bridge

drainage

system

is

critical.

Impact (I): 1 Leakage of bridge drainage must be repaired quickly. The

overflow trough will limit leakage until repairs are made.

Aesthetics

Probability (P): 1 The architectural surface treatment is long lasting are requires

little maintenance.

Impact (I): 2 Lack of maintenance will have a negative impact on the

structures appearance.


Probability (P):

1

The long

‐term

durability

of

the

bridge

depends

on

the

inspection and maintenance programs, but concrete segmental

bridges are among the most durable bridges.

Impact (I): 3 Poor inspection and maintenance can greatly reduce the life

span of the bridge. For a concrete segmental bridge the parts

that are most vulnerable to wear and bearings and expansion

joints. In addition the stay cables of an extradosed bridge are

key structural components. The anchorages and cable

sheathing need to be inspected on a systematic basis.

Maintenance

Probability (P):

1

See Long

‐term

Durability.

Impact (I): 3 See Long‐term Durability.





June 2010 5‐1

5 Division I Structural Analysis

5.1 Introduction

The extradosed river spans for the St. Croix River Crossing are designated Mn/DOT

Br. No. 82045 (WisDOT Bridge B‐55‐224)(Mainline TH 36 and STH 64). For the

purpose of

this

report,

the

baseline

structure

is

defined

as

a structure

that

conforms

to the structural configuration presented in the VQM. The baseline structure is an

eight‐span, extradosed structure with a length of 3,460 feet, and a span

arrangement of 290 feet—480 feet—480 feet—480 feet—480 feet—480 feet—

480 feet—290 feet. Of the seven piers, six are located in the river with one pier

located on the Wisconsin bluff. The baseline structure’s configuration is such that

the seven piers consist of three columns below roadway level, with the two exterior

columns extending above the roadway as towers for anchoring of the extradosed

cables. The baseline superstructure is comprised of two concrete segmental box

girders connected by full‐depth diaphragms at the cable support locations. The

cables are

arranged

in

two

planes

and

anchor

in

an

anchor

pod

at

the

exterior

edge

of each box girder, with typical spacing of 20 feet between cables. There are a total

of 252 cables, with 36 cables anchored at each pier. The exterior piers extend

60 feet above the roadway, thus giving a tower vs. span ratio of 1:8, which is ideal

for an extradosed bridge. Various modifications to the baseline structure have been

made in order to satisfy the goals of the objectives. These modifications to the

baseline structure are described in the following sections of this report.

5.2 Objectives

The specific concept refinement program consists of developing appropriate

structural

models

and

performing

structural

analyses

to

determine

the

adequacy

and feasibility of the proposed concept design. As previously described, the baseline

model conforms to the structural configuration presented in the VQM. The

structural analysis utilizes a global model for longitudinal and transverse effects, and

localized modeling performed at two “points of interest” identified during the global

modeling. In addition to the general analysis task, specific investigations and

evaluations will include the following:

Base Modeling

Superstructure Cross‐section Investigation

Pier Investigation

Fixity/Longitudinal Movement Investigation

Grade Induced

Movement

Investigation

Two Box Girder Load Distribution Investigation

Wind Load—Vibration Analysis Evaluation

Back span Uplift Investigation

Extradosed Stay Cable Analysis

Points of Interest—Local Analysis





5‐2 June 2010

5.3 Base Modeling

The base model is configured according to the structure previously defined in the

VQM. The analysis of the bridge was carried out using a three‐dimensional (3‐D)

structural model of the bridge, including explicitly modeled foundation elements

with non‐linear springs to represent the soil properties. The superstructure and

substructure components are modeled using beam elements with prismatic section

properties. The stay cables are modeled as truss elements. The superstructure’s two

box girders are modeled independently and are connected by transverse members

at each cable connection location. The double stem portion of the pier columns are

also modeled independently to more realistically simulate the longitudinal stiffness.

The model simulates the anticipated construction sequence and duration to capture

the locked‐in dead load forces from construction and to simulate the long‐term

effects of creep and shrinkage on the structure. This model, as shown in Figure 5‐1,

is used for all global longitudinal analyses, with modifications incorporated as

preliminary results determined that proposed refinements were structurally

feasible.

Figure 5‐1. Stick (top) and Rendered (bottom) Isometric View of Global Model

5.4 Longitudinal

Analysis

As with all segmental bridges, the design of a segmental extradosed bridge is

dependent on the construction sequence used to build the bridge. For the St. Croix

River Crossing, the structural arrangement conforms to a balanced cantilever

construction sequence, as shown in Figure 5‐2. As with any segmental bridge built in

balanced cantilever fashion, the substructure consisting of the foundation and pier is

built first followed by construction of the superstructure girder. Segments of the





June 2010 5‐3

girder are either cast on a form traveler for cast‐in‐place construction or erected

using a deck mounted lifting system or land/water‐based crane for precast

construction. As each segment is cast, or in the case of precast, as each pair of

segments is erected, cantilever post‐tensioning tendons in the top flange that

anchor at the ends of the two cantilevers are installed and stressed. In addition, for

the extradosed

structure,

the

cables

are

installed

and

stressed

and

cantilever

construction progresses.

Figure 5‐2. Rendered Isometric View of Global Model Showing Construction Sequence

The construction at the various piers can proceed in a number of sequences, but

typically construction either proceeds from one end of the structure to the other

end, or from both ends toward the middle. As balanced cantilever construction is

completed at a pier nearest to an abutment, a number of segments adjacent to the

abutment are supported on temporary falsework. A closure segment is cast between

the two sections of girder, and continuity post‐tensioning tendons in the bottom

flange are installed and stressed to make the first continuous span. As cantilever

construction is completed at subsequent piers, a closure segment is cast between

adjacent sections

of

girder,

and

continuity

post

‐tensioning

tendons

in

the

bottom

flange are installed and stressed to make the span continuous.

In addition to the changing structural system as the cantilevers are constructed and

the spans are made continuous, time dependent effects due to increasing concrete

strength, concrete creep and shrinkage, and post‐tensioning steel relaxation must be

considered in the design of segmental structures. Once the structure is completed,

the time dependent effects continue to occur, and the analysis evaluates these





5‐4 June 2010

effects for a predetermined time interval, typically to 10,000 days, which is defined

as time infinity, when it is assumed all time dependent effects have occurred.

Following the 10,000‐day analyses, external loads such as truck and wind forces are

applied to the completed structure. Stresses are checked for conformance within

AASHTO LRFD limits. These stress checks include limit states during and after

construction. A

girder

stress

profile

displaying

service

load

stresses

is

shown

in

Figure 5‐3.

Figure 5‐3. Box Girder Stress along Length of Bridge

5.5 Transverse Analysis

The extradosed bridge configuration in the VQM consists of two cable planes, which

are connected to the superstructure along the outside edge of each box girder.

Transverse members between the box girders are required to transfer and balance

the forces imposed by the stay cables in addition to transferring unbalanced live

loads between

the

individual

box

girders,

as

shown

in

Figure

5‐4.

The

lower

member

primarily resists tension forces and the upper closure pour member resists

compression forces as well as vertical shear forces caused by unbalanced live loads.





June 2010 5‐5

Figure 5‐4. Transverse Framing at Cable Connection

To determine the demands on the transverse members, a 3D analytical model of a

representative span was created. The box girders were modeled using plate

elements and the transverse members were modeled using beam elements. Various

live load conditions were studied as well as the forces induced in the members

during the stressing of the stay cables during construction. As a result of the

analysis, it has been determined that a post‐tensioning tendon of 24 strands is

required and

the

4 foot

deep

closure

pour

of

the

top

flange

is

sufficient

to

resist

compression and shear.

5.6 Superstructure Cross-section Investigation

The superstructure of the baseline structure described in the VQM consists of two

concrete segmental box girders connected by full depth diaphragms at the cable

support locations, as shown in Figure 5‐5. The box girders are comprised of a 3‐cell

cross‐section, with two vertical interior webs and two inclined exterior webs. The

box girders are 20 feet deep and 42.25 feet wide at the top flange or roadway level,

with a 16.0‐foot width at the girder soffit. The exterior webs and soffit have circular

curved faces

with

a radius

of

66.0

feet.

The

individual

curves

meet

at

the

web

‐soffit

intersection, forming an angular break point. The exterior webs curve to vertical at

roadway level by the addition of a smaller radius curve at the top 3 feet of the web.

The top flange of the box girder is haunched adjacent to each web to provide room

for post‐tensioning anchorages on either side of the interior webs, and on the

interior side of the exterior webs. The bottom flange is similarly haunched adjacent

to the interior webs to permit the top surface of the flange to follow the curved

soffit face and reduce the flange thickness at the midspan between the webs.





5‐6 June 2010

Figure 5‐5. Baseline Box Girder Superstructure

The baseline

pedestrian

trail

cantilevers

from

the

edge

of

the

northern

box

girder,

outside the north cable plane. The trail consists of a sidewalk slab supported by

concrete brackets attached to the box girder at the same interval as the cable

diaphragms. This trail curves outward and around the north column at each pier.

Several aspects of the superstructure cross section were evaluated during the

project, which include girder depth, feasibility of a single box girder superstructure

and transverse diaphragm requirements. Another item investigated was the

feasibility and possible benefits of integrating the cantilevered pedestrian trail with

the box girder, as recommended in the Cost Reduction and Value Engineering

(CRAVE) Study.

5.7 Inboard Pedestrian Trail

After discussions with Mn/DOT, WisDOT and the VQAC, the cantilevered pedestrian

trail has been integrated with the box section. The trail is now located inboard of the

north cable plane. The recommendation is based on improved constructability,

improved operations, ease of maintenance and inspection, and potential cost

savings. More information regarding the benefits of integrating the pedestrian trail

with the box girder can be found in the Visual Quality Section of this report. The

overall shape and symmetry among the two box girders is maintained, however, the

median barrier is no longer located at the centerline of the bridge. Each box girder is

widened to a width of 49’‐3” and the median barrier is shifted 6’‐7” south of the

centerline of the bridge. The proposed section showing the revised configuration is shown in Figure 5‐6.





June 2010 5‐7

Figure 5‐6. Proposed Box Girder Showing the Reduced Depth and Integrated Pedestrian Trail

5.8 Box Girder Depth

As previously

mentioned,

the

box

girder

illustrated

in

the

VQM

has

a constant

depth

of 20 feet. After evaluating the longitudinal demands on the girder, it was concluded

that from a structural perspective, a 20 foot deep girder is not required. It has been

determined that a box girder depth of 16 feet is more advantageous. The 16 foot

deep girder was found to be more economical and more compatible with the

approach spans, while still maintaining the architectural proportions described in

the VQM. The shallower box girder is lighter and more efficient, especially since the

girder has a constant depth along the length of the spans. Our studies also indicate

that the 16‐foot deep girder alternative is approximately $12.50 per square foot of

bridge area less expensive than the 20‐foot deep girder alternative, as shown in

Table

5‐

1.

The

overall

shape

and

general

dimensions

of

the

shallower

girder

remain

the same.





5‐8 June 2010

Table 5‐1. Box Girder Depth Cost Comparison

Bridge length ................................................ 3,460 ft

Number of piers ............................................ 7

Pier cross girder width .................................. 20 ft

Girder length ................................................ 3,320 ft

Bridge area ................................................... 354,000 sf

Concrete unit cost ........................................ $700.00/cy

Reinforcing, epoxy coated unit cost ............. $1.25/lb

Reinforcing weight ........................................ 250 lb/cy

Box Girder

Depth (ft)

Cross‐

section

Area (sf)

Web Width

(ft)

Concrete

Volume (cy)

Concrete

Cost ($)

Reinforcing

Cost ($)

Total Cost

Reduction

20 175.140 1.25 43,071 30,150,027 13,459,833

16 157.313 1.50 38,687 27,081,142 12,089,795

Difference ‐

4,384

(3,068,885)

(1,370,038)

(4,438,923.00)

Difference per sf (12.54)

5.9 Single Box Girder Feasibility

According to the VQM, both a single box girder and a double box girder

superstructure are acceptable alternatives. The basic configuration of both

superstructure alternatives is shown in Figure 5‐7.

Both alternatives have been evaluated from structural, constructability, and

architectural perspectives. Based on our analysis and engineering judgment, it has

been concluded

that

the

single

box

girder

alternative

is

feasible.

It

has

also

been

determined that the optimal superstructure depth for the single box girder

alternative is 16 feet. From a constructability perspective, the double box girder

alternative would most likely be constructed using precast segments and the single

box alternative would most likely constructed by casting the segments in place using

a form traveler, due to the size and weight of the segment as the weight of a single

box girder is too large for typical erection equipment. In terms of construction

duration, it is believed that the double box girder alternative would require less time

to construct since the precast segments can be cast concurrently with the

construction of the bridge substructure.





June 2010 5‐9

Figure 5‐7. Typical Sections

5.10 Transverse Diaphragm investigation

Although it is not explicitly stated, it appears that the structural systems shown in

the VQM utilize full depth diaphragms located at each cable. After conducting a

transverse analysis on the structure, it has been determined that full depth

diaphragms are not required. Instead, a much lighter transverse tie member and

partial depth diaphragms inside the girder are proposed. The post‐tensioned tie

member will most likely be constructed as a precast element. For additional

information on the transverse diaphragms, please see the Transverse Analysis

section of this report.

5.11 Pier Investigation

As shown in the VQM (baseline structure) and in Figure 5‐8, the deck is supported at

each pier by the two outside columns, which extend above the deck for anchoring of

the extradosed cables and one center column that terminates at the cross girder at

deck level. The pier configurations shown in the VQM have a constant height of

60 feet

above

the

girder,

and

so

therefore

vary

in

height

above

the

water

line

from

170 feet at the pier closest to the Minnesota shore to 212 feet at the pier on the

Wisconsin bluff. The pier columns are generally conical in shape with varying

dimensions from top to bottom, with the smaller dimension at the top of the pier.

The interior column is split into two stems below the girder, and combines into a

single column 104 feet below the roadway level. The exterior columns are split into

two stems from the top of the pier to 104 feet below the roadway level, with the





5‐10 June 2010

exception of the tower anchor housing, which extends from just below the top of

column to 17 feet above the girder. The exterior column stem connection occurs

only at the upstream and downstream face of the column, creating a C‐shaped

column in cross‐section. The three columns in the baseline structure are connected

by a 20‐foot wide by 25‐foot deep rectangular shaped cross girder at the roadway

level. The

cross

girder

is

a hollow

section

between

columns

with

5‐foot

thickness

for

the top and bottom flanges, and the two webs. At the columns, the cross girder has

a solid cross‐section comprised of a diaphragm infill in addition to the flange and

web section. Since it has been determined to reduce the depth of the superstructure

box girder from 20 feet to 16 feet, the cross girder depth has also been reduced the

same amount to maintain similar proportions shown in the VQM. The proposed

cross girder depth is 21 feet.

Figure 5‐8. VQM Baseline Pier Configuration





June 2010 5‐11

The Visual Quality Advisory Committee’s preference is to have only the two stay

tower columns at each pier (eliminate the center column) to maintain the “light and

elegant” appearance of the chosen “Organic” design concept. The pier investigation

evaluated the feasibility of a two column design in place of the three column design

presented in the VQM. The evaluation considers the advantages and disadvantages

of cost,

constructability,

and

aesthetics

including

impact

to

the

overall

appearance

of the two column option compared to the three column option.

There are three critical components to be considered when determining the

feasibility of the two column alternative. The first item is the location and number of

the expansion joints on the bridge. It is preferred to have all eight extradosed spans

be continuous with expansion joints only at Pier 7 and the East Abutment. Having

the expansion joints only at the ends of the extradosed bridge create significant

demands on Piers 8 and 14 due to the longitudinal displacements from the thermal

loads and long term creep and shrinkage. The second item is the stiffness of the

individual pier columns. The pier cross girder strength is the third critical component

in determining

the

feasibility

of

tower

bents

consisting

of

two,

rather

than

three,

column legs. Since the superstructure is integrally connected to the substructure at

all piers, the pier cross girder is subjected to sizeable shear, vertical bending and

torsion.

Based on our analysis and our engineering judgment, it has been determined that

the two pier columns alternative is feasible. The two columns have adequate

capacity to resist the imposed loads and deformations. The relatively tall and slender

double stem section of the pier column provide adequate flexibility to accommodate

the longitudinal displacement demands from thermal effects and long term creep

and shrinkage. Although it is relatively minimal, additional flexibility of the

substructure is

provided

by

the

drilled

shaft

foundation.

Even

though

it

is

not

the

shortest pier, Pier 8 controls the design. This is due to shallow rock at the footing

location, resulting relatively short, stiff drilled shafts. The double stem sections of

the columns are the controlling component of the pier design. Utilizing the cracked

section properties, the double stem section will require approximately 2%

reinforcement, which is a feasible amount. It has also been determined that the

proposed cross girder has adequate strength to resist the imposed loads. The

proposed pier configuration is shown in Figure 5‐9. For additional information,

please see the Fixity / Longitudinal Movement Investigation section of this report.





5‐12 June 2010

Figure 5‐9. Proposed Pier Configuration

5.12 Fixity/Longitudinal Movement Investigation

The fixity/longitudinal movement investigation evaluated the pier and bearing fixity

requirements, pier stiffness effects due to thermal movements, and other issues

unique to long, continuous bridges. As the superstructure shortens due to thermal

effects and long‐term creep and shrinkage, the pier columns are deflected and the

pier cross girders are subjected to torsional effects. Based on our analysis and our

engineering judgment, it has been determined that the eight span extradosed

portion

of

Bridge

82045

(B‐

55‐

224)

can

be

made

continuous

with

expansion

joints

only at Pier 7 and the East Abutment. The relatively tall and slender double stem

section of the pier columns provide adequate flexibility to accommodate the

longitudinal displacement demands from thermal effects and long term creep and

shrinkage, as shown in Figure 5‐10. Although it is relatively minimal, additional

flexibility of the substructure is provided by the drilled shaft foundation. Even

though it is not the shortest (therefore stiffest) pier, Pier 8 controls the design. This





June 2010 5‐13

is due to shallow rock at the footing location, resulting relatively short, stiff drilled

shafts.

The expected movement of the extradosed bridge due to thermal effects is

approximately +/‐ 8 inches and the expected shortening of the superstructure due to

creep and shrinkage is approximately 6 inches. This results in the expansion joint at

the east

abutment

having

a movement

rating

of

26

inches

and

the

expansion

joint

at

Pier 7 having a movement rating of 38 inches. The increased expansion joint size at

Pier 7 accounts for the movement of the approach structure. Please see the Pier

Investigation section of this document for more information.

Figure 5‐10. Deflected Pier Shape

5.13 Grade Induced Movement Investigation

The bridge is on a constant +1.74% grade traveling toward Wisconsin. An

investigation to evaluate the tendency of the bridge to move permanently in the

downhill direction was conducted. It has been concluded that the bridge is not

susceptible to grade induced movements. The frame action of the integral pier

configuration alleviates this movement.

5.14 Two Box Girder Load Distribution Investigation

The two box girder load distribution investigation evaluated the requirements for

transfer of transverse loads in the regions along the bridge where cable stays and

diaphragms do not exist, as shown in Figure 5‐11.





5‐14 June 2010

Figure 5‐11. Superstructure between Cable Supports

Once the closure between the individual girders is complete, the two girders no

longer act independently, but instead behave as if they are one. As shown in

Figure 5‐12, the deflection of both box girders is approximately the same when only

one box girder is subjected to full live load.

Figure 5‐12. Transverse Analysis

It has been determined that the closure pour between the two box girders is

sufficient to transfer unbalanced live loads and to prevent differential deflection

between the individual box girders. To improve the long‐term service

performance of the closure, transverse post‐tensioning will be integrated into

the design.

A

detail

of

the

proposed

closure

pour

is

shown

in

Figure

5‐13.





June 2010 5‐15

Figure 5‐13. Closure Pour Detail

5.15 Wind Load—Vibration Analysis Investigation

Long span bridges tend to be excited by dynamic wind load. The St. Croix River

Bridge River Crossing is a long span bridge with six 480’ spans. While the bridge is

extradosed type, it is actually a box girder bridge with partially exposed post‐

tensioning tendons

and

therefore

the

bridge

behavior

is

very

close

to

traditional

box

girder bridges. There are long span box girder bridges in Japan and other countries

that have experienced large vibrations under wind load and there is a possibility that

this bridge might also experience wind induced vibration. A study for the

vulnerability to excessive vibration has been performed and is discussed below.

An Eigenvalue dynamic analysis was performed on the St Croix River Crossing and

some modes of vibration are shown in Figure 5‐14. The period of the first vertical

mode is approximately 1.0 second. The period of the first torsional mode is found to

be approximately 0.5 second. The ratio of the first vertical mode to the first torsional

mode is approximately 2. Based on previous experience, when this ratio is near the

range of

2 to

3,

the

bridge

has

higher

possibility

of

wind

induced

vibration.

Therefore, a detailed wind study is deemed necessary during final design of this

bridge.





5‐16 June 2010

Figure 5‐14. Mode Shapes from the Eigenvalue Dynamic Analysis

Bridge structural dynamic response to wind is commonly studied by performing

wind tunnel tests. An aero elastic model wind tunnel test can cover all modes of the

bridge and provide a realistic prediction of the bridge dynamic response to wind. A

partially completed structure is more vulnerable than a complete structure. The

most vulnerable condition of this bridge is before the spans are closed and while the

cantilever arms are the longest. Therefore, several critical construction conditions in

addition to the complete bridge are recommended for detailed study.

In accordance with these findings, it is recommended that the design criteria include

aerodynamic evaluation of the bridge with the testing criteria listed as follows:

13.3.3.1.10 Aerodynamic Evaluation

A wind expert shall perform wind data collection and analysis to

determine the design wind speed.

The aerodynamic stability of the structure shall be determined by

performing wind

tunnel

tests

on

aero

‐elastic

models

of

the

structure.

A buffeting analysis shall be performed based on the data measured

from the wind tunnel tests.

The buffeting analysis will generate equivalent static wind loads to

the structure. Aero‐elastic model wind tunnel tests shall include

critical construction conditions as well as the completed structure.





June 2010 5‐17

Large amplitude vibration of some stay cables in light rain and low wind has been

observed in of some cable stayed bridges around the world. This phenomenon is

called rain‐wind induced vibration. The stay cable industry has developed several

means to counteract this vibration. The most commonly used method is a spiral bar

placed around the PE pipe. This has been proven a very effective Measure and it is

recommended that

this

type

of

PE

pipe

be

required

for

the

St

Croix

River

Crossing.

The allowable stress in the cables of the extradosed bridge is 0.6 FGUTS or 162 ksi.

The allowable stress in the cables of cable‐stayed bridge is 0.45 FGUTS or 122 ksi.

The higher allowable tensile stress in the extradosed bridge has the effect of

lowering the possibility of excessive vibration.

The Contractor will be required to design the cables in accordance with PTI

Guide Specifications Recommendations for Stay Cable Design, Testing and

Installation, 3rd and 5th Editions. A properly designed cable is not expected to

experience wind induced vibration.

5.15.1 Back span Uplift/Maximum Back span Length Investigation

The back

span

uplift/maximum

back

span

length

investigation

evaluated

the

potential for uplift at the back span piers. It has been determined that the current

configuration of the spans does not result in uplift. One of the main reasons there is

no uplift is that the live load to dead load ratio on the extradosed bridge is quite low.

Another reason is that the proposed back span length is 290 feet, which is 50 feet

longer than half of the main span. The additional 50 feet of span length on the back

span effectively acts a counterweight to resist the uplifting forces when the main

span is fully loaded. The integral framing of the superstructure to the substructure at

the main piers is effective at resisting the longitudinal rotation of the superstructure

caused by the fully loaded main span, which also limits the ability of the back span to

uplift. It

had

been

determined

that

the

minimum

service

load

bearing

reaction

at

the

back span support is approximately 800 kips, based on a two bearing per girder

configuration.

5.16 Extradosed Stay Cable Analysis

The cables of the baseline structure are arranged in two planes and anchor in an

anchor pod at the exterior edge of each box girder, with typical spacing of 20 feet

between cables, as shown in Figure 5‐15. There are a total of 252 cables, with 36

cables anchored at each pier.





5‐18 June 2010

Figure 5‐15. Extradosed Stay Cable Configuration

The cables are connected to the tower starting 22 feet above the roadway level,

with typical

spacing

of

4 feet

between

cables

at

centerline

of

pier.

The

cables

are

either anchored to the tower within a steel anchorage housing. The cables are

connected to the box girder at anchorage pods spaced at 20 feet along the exterior

edges of the two box girders. There are nine anchorage pods along each edge up

station and down station from the pier. The first cable pair is located 55 feet from

the centerline of pier, with the ninth cable pair located 215 feet from centerline of

pier. This leaves a space of 50 feet in the midspan between the ninth cable pair of

adjacent piers. The angle of the cables from the horizontal plane varies from 14

degrees to 22 degrees. At each anchorage pod there is a diaphragm connecting the

box girders together.

The extradosed

cables

were

initially

sized

assuming

that

they

would

resist

approximately 60% of the dead load moment of the cantilever prior to closure.

Following this initial sizing, the average cable tension (1250 kips) was selected to be

applied to each extradosed cable upon installation.

According to the Cable Stays Recommendations of the French Interministerial

Commission of Pre‐stressing, stay cables with a change in stress caused by live load

of less than 7.2 ksi are considered extradosed. The standard limits the cable tension

to 60% of the guaranteed ultimate tensile strength, that is, 0.6 GUTS, under the

effects of maximum service loading. It has been determined that the stress variation

caused by live load (HL‐93 with no pedestrian load) on the St. Croix River Bridge is

approximately 4 ksi

(as

shown

in

Figure

5‐16),

which

defines

the

cables

as

extradosed. Utilizing the allowable tension limit of 0.6 FGUTS, all cables are comprised

of 37 0.6‐inch diameter strands.





June 2010 5‐19

Figure 5‐16. Live Load Stress Range

5.17 Points of Interest—Local Analysis Two “points of interest” were identified during the global modeling. The first

location is the integral pier cross girder to column connection, which is a complex

area and critical component to the feasibility of the structure. A localized finite

element model was generated to study the force flow, stress concentrations and

deformations within the integral connection. The forces input into the localized

models have been derived from the global analytical model. The results of the

localized analysis indicate that proposed integral connection is still feasible.

However, there will be areas of stress concentration that will need to be addressed

during final design. Various stress diagrams can be seen in Figure 5‐17 through

Figure 5‐19.





5‐20 June 2010

Figure 5‐17. Local Model of Integral Pier (Bottom View)

Figure 5‐18. Local Model of Integral Pier (Top View)





June 2010 5‐21

Figure 5‐19. Pier Cross Girder Interface Model

The second area of interest is the configuration of the first two spans (spans 8 and 9)

of the extradosed bridge, which have a variable width superstructure caused by the

merging of Ramps SE and NE into the mainline. The majority of the width variation is

within span 8, in which the width varies from 135 feet at Pier 7 to 110 feet at Pier 8.

Special consideration of the framing of span 8 is required due to its increased width.

It should also be noted that in addition to the increased width, the super elevation is

also varying along the length of span 8. Although it doesn’t have a major impact on

the framing, the varying super elevation must be considered in the cross girder and

extradosed stay cable design. There are several viable alternatives, but one solution

is shown in Figure 5‐20 through Figure 5‐22. The proposed configuration utilizes a

precast truss member between the two precast box girders. This maintains a similar

“open” appearance to the rest of the extradosed structure as well as maintaining

the same geometry for the box girders.





5‐22 June 2010

Figure 5‐20. Pier 8 Elevation Showing Widened Superstructure and Full Super Elevation





June 2010 5‐23

Figure 5‐21. Span 8 Typical Section

Figure 5‐22. Isometric View of Precast Truss

To

determine

the

demands

on

the

transverse

members,

a

3D

analytical

model

of

a

representative span was created as shown in Figure 5‐23. The box girders were

modeled using plate elements and the transverse members were modeled using

beam elements. Various live load conditions were studied as well as the forces

induced in the members during the initial stressing of the stay cables during

construction. Based on the results from the localized and global analysis, it can be

concluded that the proposed precast truss alternative is feasible. The light weight

strut design will also keep the extradosed stay cable force increase to a minimum.





5‐24 June 2010

Figure 5‐23. Isometric View of 3D Analysis





June 2010 6‐1

6 Division II Structural Analysis

6.1 Introduction and Objectives

There are three bridges in this portion of the work: Bridge No. 82045 (Mainline TH

36 approach units), Bridge No. 82047 (TH 36WB off ‐ramp to TH 95), and Bridge No.

82048 (TH

36EB

on

‐ramp

from

TH

95).

The

typical

mainline

superstructure

is

comprised of two parallel concrete segmental box girders connected by a closure

pour at the top flange, while the ramp superstructures are a single concrete

segmental box girder. The baseline model piers are twin stem wall type that is

connected monolithically to the superstructure, with the exception of the mainline

pier nearest to the abutment that is a single column with bearings. The approach

span adjacent to the extradosed river spans is a transition span that has a wider

cross‐section to accommodate the on‐ramp and off ‐ramp lanes.

The concept refinement program consisted of developing appropriate structural

models and performing structural analyses to evaluate the adequacy of the

proposed design

concept.

The

baseline

model

conforms

to

the

structural

configuration presented in the VQM and the Mn/DOT‐provided roadway design files.

Variations on the baseline model were developed as the layout and construction

impacts were examined for reduction of impact and overall technical enhancement.

These variations were designated R1, R2, etc., to indicate the layout version being

modeled. Subsequent sections describe the baseline model and interpretation of the

VQM.

The modeling approach utilized global modeling for longitudinal and transverse

effects, and localized modeling was performed at “points of interest” identified

during the global modeling. In addition to the general analysis task, specific

investigations and

evaluations

included

the

following:

6.1.1 Box Girder Width Transition Investigation

The box girder width transition investigation evaluated the transition in box girder

width at the ramp termini, on the Minnesota side, and investigated options for

transitioning the box girders at the ramp termini. The transition was kept off of the

back span of the extradosed portion of the river crossing to the greatest extent

possible.

6.1.2 Box Girder Depth Economy Investigation

The box girder depth economy investigation evaluated ways to economize the depth

of the box girder section from the west abutment to pier 6. As referenced in the

VQM, the depth of section of the first 6 spans varies from 10 feet to 20 feet. The 10‐

foot depth is required to provide vertical clearance over TH 95 in Span 1 and the 20‐

foot depth is required to match the depth of the extradosed spans.





6‐2 June 2010

6.1.3 Fixity/Longitudinal Movement Investigation

The fixity/longitudinal movement investigation evaluated pier and bearing fixity

requirements, pier stiffness effects due to thermal movements, and other issues

unique to long, continuous bridges. Continuous structures are desired with

expansion joints only expected to be at the ends of the extradosed spans and the

west abutment.

6.1.4 Grade Induced Movement Investigation

The grade induced movement investigation evaluated potential of the bridge to

move permanently in the downhill direction and provide performance requirements

for a Design‐Build RFP. The Minnesota approach spans of Bridge 82045 (B‐55‐224)

are on a ‐1.25% grade traveling toward Wisconsin.

6.1.5 Box Girder Depth Transition Investigation

The box girder depth transition investigation evaluated recommendations regarding

the location of the box girder depth transition. There are vertical clearance

restrictions over TH 95 at the first span of the river bridge approach spans that necessitate a 10‐foot maximum structure depth. The structure depth must increase

at some location after spanning over TH 95 to accommodate longer span lengths

and to match the depth of the extradosed spans. In the concept plan shown in the

VQM, the depth transition begins in Span 1 just east of TH 95 and terminates at

Pier 2.

6.2 Investigation Methods: Longitudinal Analysis

As with all segmental bridges, the design is dependent on the construction sequence

used to build the bridge. For the St. Croix River Crossing, the structural arrangement

conforms to

a balanced

cantilever

construction

sequence.

The

substructure

consisting of the foundation and pier is built first followed by construction of the

superstructure girder. Segments of the girder are either cast on a form traveler for

cast‐in‐place construction or erected using a deck mounted lifting system or

land/water‐based crane for precast construction. As each segment is cast, or in the

case of precast, as each pair of segments is erected, cantilever post‐tensioning

tendons in the top flange that anchor at the ends of the two cantilevers are installed

and stressed.

The construction at the various piers can proceed in a number of sequences, but

typically construction proceeds from one end of the structure to the other end. As

balanced cantilever

construction

is

completed

at

a pier

nearest

to

an

abutment,

a

number of segments adjacent to the abutment are supported on temporary

falsework. A closure segment is cast between the two sections of girder, and

continuity post‐tensioning tendons in the bottom flange are installed and stressed to

make the fist continuous span. As cantilever construction is completed at

subsequent piers, a closure segment is cast between adjacent sections of girder, and





June 2010 6‐3

continuity post‐tensioning tendons in the bottom flange are installed and stressed to

make the span continuous.

Cantilever construction and

the span closures change the

structural system. Coinciding

with these

changes

are

time

dependent effects due to

increasing concrete strength,

concrete creep and

shrinkage, and post‐

tensioning steel relaxation

which must be considered in

the design of segmental

structures. Once the

structure is completed, the

time dependent

effects

continue to occur, and the

analysis evaluates these

effects for a predetermined

time interval, typically to

10,000 days, which is defined as time infinity, when it is assumed all time dependent

effects have occurred.

Figure 6‐2. Detail of Segments on Pier and Accurate Section Property Input

Following the 10,000‐day analyses, external loads such as truck and wind forces

were applied to the completed structure. Stresses were checked for conformance

within AASHTO LRFD limits. These stress checks include limit states before and after

construction.

Figure 6‐1. Balanced Cantilever Construction Method

(Modeling of Construction Stage 6, Bridge 82048)





6‐4 June 2010

Figure 6‐3. Stress Output Checked at 10 Locations in the Cross‐section

6.3 Baseline Model Description

6.3.1 Piers

The VQM illustrates twin stem wall piers having a curved surface on the outward

faces looking

up

station

and

down

station.

These

double

‐stem

piers

vary

in

width

from top to bottom with the smaller dimension at the top of the stem walls. A

diagonal cut on the side faces provides the taper. From the side, the outer and inner

stem faces are plumb, but the diagonal cut provides a side face that decreases in

dimension from top to bottom. The top width of the stem walls matches the bottom

flange width of the box girder.

The double stem piers were examined through initial visual renderings, bridge

elevation views of each structure, approximate material quantities and an estimated

construction impacts. Through discussions with Mn/DOT and the VQAC, there was

general consensus that a single stem pier would be preferred to obtain

constructability, economic

and

visual

benefits.

Since

substructure

geometry

significantly affects the structural analysis, all models were suffixed with either an

“A” or “B” to indicate whether double‐stem piers or single‐stem piers were

modeled, respectively. In other words, structural model 82045_R1A was variation 1

on the baseline model and utilized double‐stem columns, whereas model

82045_R1B was baseline variation one with single‐stem columns.





June 2010 6‐5

From our analyses it is apparent that a single stem column is a viable option with the

caveat that expansion joint locations will require a wider platform for bearings and

jack area for bearing replacement. This wider platform would equate to a single

column with an approximately 14’‐6” longitudinal dimension at Ramp Pier 5R and

the three columns of Pier 7. Alternatively, double‐stem columns connected with a

pier cap

may

be

utilized.

Figure 6‐4. Pier 1 on TH 36 (Bridge No. 82045)





6‐6 June 2010

Figure 6‐5. Typical Pier on TH 36 Approaches (Others Similar)

6.3.2 Box Girders

The baseline box girders along the TH 36 mainline are comprised of a 3‐cell cross‐

section, with two vertical interior webs and two inclined exterior webs. The bridge

begins at the west abutment with 10‐foot deep box girders that transition to a 20‐

foot deep box girder from the end of span 1 throughout span 2. These girders are

typically 42’‐8” wide at the top flange or roadway level, with a 16.0‐foot width at the

girder soffit. The exterior webs and soffit have circular curved faces with a radius of

66.0 feet. The individual curves meet at the web‐soffit intersection, forming an

angular break point. The exterior webs curve to vertical at roadway level by the

addition of a smaller radius curve within the top 3 feet of the web.

The baseline N.E. and S.E. ramp box girders emulate the shape of the mainline

girders. The depth of these ramp girders was undefined prior to preliminary design,

although there was some indication in the proposed railroad and Plant Access Road





June 2010 6‐7

profiles that these girders would be 10 feet deep at these crossings. Considering this

indication, preliminary design proceeded with the assumption that the approach

ramp structures should also include a box girder depth transition from 10 feet to

20 feet. This assumption fit the ramp structures well because a constant 20’‐foot

depth appeared visually too deep for the ramps, especially as these structures

descended toward

the

respective

exit

or

entrance

intersection.

The baseline ramp box girders are typically 27’‐4” wide at the top flange or roadway

level, with a 12.0‐foot width at the girder soffit. The exterior webs and soffit have

circular curved faces with a radius of 50.0 feet for the exterior webs and 36.0 feet

for the soffit. The individual curves meet at the web‐soffit intersection, forming an

angular break point. The exterior webs curve to vertical at roadway level by the

addition of a smaller radius curve at the top 3 feet of the web.

Figure 6‐6. Baseline Approach Box Girders

The top flange of the box girder is haunched adjacent to each web to provide room

for post‐tensioning anchorages on either side of the interior webs, and on the

interior side of the exterior webs. The bottom flange is similarly haunched adjacent

to the interior webs to permit the top surface of the flange to follow the curved

soffit face and reduce the flange thickness at the midspan between the webs.

6.3.3 Transition Span

The approach span adjacent to the extradosed river spans is a transition span that

has a wider cross‐section to accommodate the on‐ramp and off ‐ramp lanes. The

cross‐section consists of two box girder similar to the mainline, but the box girder

dimensions vary to accommodate the additional roadway width.

6.3.4 Pedestrian Trail

A pedestrian trial cantilevers from the edge of the transition span and the northern

ramp box girder. The trail consists of a sidewalk slab supported by concrete brackets

attached to the box girder at approximately 20‐foot interval.

The six bridges are divided into four baseline models as follows:

The initial focus was on determining constructible and feasible box sections for use

in modeling. These design sections made use of repeatable forms and were





6‐8 June 2010

conceptually compatible with the VQM intent. Areas of wide variation were

relegated to cast‐in‐place construction on falsework. Such sections were kept

minimal to limit the environmental impacts and cost impacts, but nonetheless these

areas were accurately modeled in the analyses for construction staging and tendon

geometry. Bridge 82045 was modeled in a baseline model consisting of double‐stem

piers in

and

five

‐span

configuration

as

illustrated

in

Figure

6‐7 and

Figure

6‐8 and

Table 6‐1. Consistent with the model naming convention, the analysis is termed

82045E‐r0A to indicate Bridge 82045, Eastbound in the baseline configuration with

double‐stem columns.

Figure 6‐7. Baseline Model 82045E‐r0A, Bridge 82045 EB (EB TH 36)

Figure 6‐8. Plan and Elevation of Baseline Model, Bridge 82045 EB (EB TH 36)





June 2010 6‐9

Table 6‐1. Baseline Span Arrangement

Bridge No.

Length

(feet)

Span Arrangement from

West to East (feet) Comment

82045 (EB) 1609 180‐229‐300‐300‐300 Mainline eastbound

82045 (WB) 1609 180‐229‐300‐300‐300 Mainline westbound, similar to eastbound and

not uniquely

modeled

82045 (EB) 300 300 Transition eastbound

82045 (WB) 300 300 Transition westbound, similar to eastbound and

not uniquely modeled

82047 1031 125‐300‐300‐301 Off ‐ramp TH 36WB/TH 95

82048 1296 91‐300‐300‐300‐300 On‐ramp TH 36EB/TH 95

Pursuant to the study’s objectives, the baseline box girder depth of 20 feet was

investigated in both the extradosed and approach unit models and span

arrangements. In

the

extradosed

span,

an

optimal

depth

of

16

feet

was

determined

by balancing girder and cable requirements. In the approach units, however, the 16‐

foot depth was not structurally viable in the transition span with the baseline span

configuration. One reason for this is that the N.E. and S.E. ramp expansion joints

force the transition span to be a single‐span, separate structure. Precast, balanced

cantilever construction envisioned for much of the bridge construction is not

possible in a single‐span configuration. Therefore, the baseline configuration would

require a 20‐foot deep box girder in the 300‐foot transition span that would be

entirely cast on falsework.

This finding initiated a focus on the transition span and span alternatives. Four goals

were set

for

the

transition

span

investigation:

To enable construction via the precast balanced cantilever method

To minimize the falsework and associated permanent impacts in the wetlands

below

To move the expansion joints over the support locations and eliminate hinges for

maintenance reasons

To enable use of a 16‐foot deep structure that was compatible with the optimal

structure depth found for the extradosed spans

Decreasing the box girder depth to 16’ would also introduce savings in material and

support structure costs due to the increased weight of the superstructure.

Parallel to the girder depth investigation, the baseline spans were examined for

feasibility. Results from these analyses indicated Bridge 82045 appeared to have a

viable arrangement given the 20’ box girder depth, but the N.E. and S.E. ramps had

baseline span arrangements that were not only inefficient but appeared impractical

for the 10‐foot box girder depth near the end spans as suggested by the profiles. For

this reason, no baseline model was developed for the N.E. and S.E. Ramps according





6‐10 June 2010

to the VQM, or baseline, span arrangement. Instead, variations of span

arrangements were investigated using engineering judgment which suited:

The VQM box girder section

Site constraints such as wetland avoidance and railroad clearances

Balanced cantilever construction method to minimize heavy duty falsework in

sensitive areas

The resulting span arrangements were drafted and presented at a wetland

coordination meeting on September 2nd, 2009. The proposed changes to span

configuration as suggested by PB were agreeable to the stakeholders and it was

agreed to further advance these configurations as the preferred arrangement. These

preferred arrangements are described in detail in the next sections.

6.4 Alternative R1 Investigation

Alternative R1 is the result of girder depth investigations, span efficiency and site

constraints. The R1 span arrangements are indicated in Table 6‐2.

Table 6‐2. Span Arrangement “R1”

Bridge No.

Length

(feet)

Span Arrangement from

West to East (feet) Comment

82045 (EB) 1909 184‐224‐300‐300‐183‐161 Mainline eastbound

82045 (WB) 1909* 184‐235‐300‐300‐183‐161* Mainline westbound

82047 1031 104‐195‐243‐233‐202 Off ‐ramp TH 36WB/TH 95

82048 1296 126‐192‐199‐199‐199‐199‐168 On‐ramp TH 36EB/TH 95

*Indicates span measurement with respect to EB TH 36 alignment

Within the

R1

span

arrangement,

various

column

configurations

were

investigated.

These models were suffixed with a letter designation to indicate the column type

used in the investigation. The suffixes to date are as follows:

A = baseline model with double stems typical except at Pier 1, Bridge 82045

B= single stem columns with an 8‐foot longitudinal dimension and f’c=4 ksi

C = single stem columns with an 6‐foot longitudinal dimension and f’c=5 ksi

In all of these instances bridge modeling using true coordinates along the respective

alignment. However, a horizontal datum of 70’ was established for a constant girder

elevation while the column heights were correctly calibrated to the height from PGL

to top of footing. At the top of footing, 3 rotational and 3 translational springs serve

to emulate the group pile behavior. These springs were determined elastically from

a refined piling arrangement and checked through the use of industry‐standard Lpile

Group software.





June 2010 6‐11

For all bridges it is assumed that a

pair of bearings supports the

superstructure at the West

Abutments, Pier 1 and the end piers.

The introduction of bearings at the

West Abutment

and

Pier

1 are

due

to

the short height of these support

structures, which creates a high

stiffness and resistance against

thermal movements. The end pier

bearings are necessary because they

are expansion joint locations.

Bridge 82045 EB was modeled with

both a double‐stem column and a

single‐stem column. These two

models served

as

a basis

for

evaluating the viability of the single‐stem piers. Once verified, column option A

(Double‐stem columns) were eliminated due to Mn/DOT preference and column

option B was continued through analyses that included WB TH 36 and the N.E. and

S.E. ramps. Figure 6‐10 through Figure 6‐12 illustrate the various analyses that were

developed in this program.

Figure 6‐10. Model 82045E‐r1A—Double‐stem Columns in Alternative R1 Span Arrangement

Figure 6‐9. Plan and Elevation of Alternative “R1”

Models for EB and WB TH 36.





6‐12 June 2010

Figure 6‐11. Model 82045E‐r1B—Single‐stem Columns in Alternative R1 Span Arrangement

Figure 6‐12. Plan and Elevation of Model 82045W‐r1B—WB TH 36 with Single‐stem

Columns in Alternative R1 Span Arrangement





June 2010 6‐13

Bridge 82047, or the N.E. Ramp, includes a cantilevered sidewalk in the VQM which

was integrated with the box section after discussions with Mn/DOT. With the

inclusion of the sidewalk, this ramp has a width very comparable with the typical TH

36 approach unit segments and a consistent shape was utilized. At the West end,

two turn lanes are added on the structure necessitating variable width sections up

to 60’

‐6”

in

width.

A

depth

increase

of

6‐foot

is

introduced

in

span

four

to

match

the

depth of TH 36 box girder at Pier 5R. This depth increase suits the higher end span

moments and provides a visually appropriate transition.

Figure 6‐13. Bridge No. 82047 (NE Ramp) Stick Model and Rendered Model

Figure 6‐14. Bridge No. 82047 Rendered Plan and Elevation from Structural Model with Details





6‐14 June 2010

Bridge 82048 (SE Ramp) is the narrowest and the longer of the two ramps. The ramp

width is largest at the beginning where the roadway would permit three 12‐foot

lanes and narrows to a 24’‐0 roadway for two potential loading lanes. Although the

roadway width is amenable to a single‐cell box girder, this bridge has been

envisioned as a two‐cell box girder due to the lack of counter‐balancing overhangs.

Without cantilever

overhangs

to

decrease

the

top

‐slab

span,

high

restraint

forces

will develop directly under the barriers and necessitate more transverse post‐

tensioning and a thicker, heavier slab section. For these reasons, a two‐cell structure

presents more efficiency than a single cell girder. The box girder depth transition is

placed in the second span from the end of the ramp to enable the deeper girder’s

effectiveness in the end span. Model R1B is illustrated in Figure 6‐15 through

Figure 6‐16.

Figure 6‐15. Bridge No. 82048 (SE Ramp) Stick Model (Left)

and Rendered Sectional Model (Right)

Figure 6‐16. Bridge No. 82048 (SE Ramp) Plan and Elevation View of Rendered Model





June 2010 6‐15

6.4.1 Grade-Induced Movement Investigations

Grade induced movement was examined by using two models that were identical in

all respects except the grade. For this investigation the EB Bridge 82045 Alternative

R1 was used. To exaggerate the effects, a grade of 5% was modeled in lieu of the

actual 1.25% slope. Both models were run and the displacements at Pier 7, the

downhill end

support,

were

examined.

As can be seen in Table 6‐3, no appreciable difference is observed. The results are

taken at a time equal to 10,000 days after construction, which is an industry‐

standard time for which all construction‐induced load effects should be realized

(Self ‐weight, elastic shortening, creep, shrinkage, etc.).

Table 6‐3. Grade‐induced Movement Results

Model

Description Time

DX

(Feet)

DY

(Feet)

DZ

(Feet)

Vector Sum, Net

movement

(Feet)

Horizontal

T_INFINITY ‐

0.194 ‐

0.236 ‐

0.042

0.305

5% grade T_INFINITY ‐0.194 ‐0.236 ‐0.037 0.306

Figure 6‐17. Models Used in Investigating Grade‐induced Deflection





June 2010 7‐1

7 Bridge Lighting and Signing

7.1 Review of Requirements for Architectural and Roadway Lighting

The Visual Quality Manual discusses many aspects of the different types of lighting

for the bridge, roadway, and trail. These include specific concerns about the impact

of lighting

as

well

as

the

lighting

approach

to

be

used

for

different

areas.

Specific concerns included:

Design should minimize the negative impact that bridge lighting could have on

the scenic river valley.

Lighting should meet the needs for required safety levels but minimize

“spillover” into the riverway.

Architectural lighting must be thoughtfully designed to enhance the structure

without intruding into the sensitive natural environment.

Preliminary investigations were conducted during the preparation of the VQM into

various types

of

lighting

approaches.

The

results

of

these

investigations

arrived

at

the following recommendations.

Roadway lighting using davit poles with twin arms and contemporary style

luminaires mounted in the median.

A “white” light source such as metal halide is preferred due to aesthetic and

quality reasons

Trail lighting could use a low level lighting system to provide adequate

pedestrian visibility with minimized light trespass.

Navigation channel lighting is not required but lighting identifying the piers

locations for boaters is required for safety reasons.

FAA

obstruction

lights

are

required

at

the

tops

of

the

pier

towers.

These concerns and recommendation were carried into this phase of design and are

addressed and discussed in the various lighting approaches in this report.

7.2 Lighting Criteria

Aesthetic lighting for the St. Croix River Crossing will accent its unusual forms. These

derive from the extradosed structural design and the biomorphic aesthetic concept.

The effects under consideration include illuminating the cables from below with

narrow beam floodlights, grazing the tower surfaces either inside or outside, and

outlining selected features with direct view fixtures. For floodlighting options the

criteria of

the

Illuminating

Engineering

Society

(IES)

will

apply.

The

criteria

recommended in IES RP‐33‐99 Lighting for Exterior Environments is shown in

Table 7‐1 (excerpted from Table 2 of that document).





7‐2 June 2010

Table 7‐1. Illuminance Levels for Floodlighting Buildings and Monuments

Area Description

Average Target Illuminance (Vertical)

(lux/footcandles)

Bright surroundings and light surfaces 50/5

Bright surroundings and medium light surfaces 70/7

Bright surroundings and dark surfaces 100/10

Dark surroundings and light surfaces 20/2

Dark surroundings and medium light surfaces 30/3

Dark surroundings and medium dark surfaces 40/4

Dark surroundings and dark surfaces 50/5

The vertical value on the structural elements will be designed to 20 to 30 lux (2 to 3

footcandles) associated with the categories of Dark Surroundings and Light to

Medium Light Surfaces (note : RP‐33 has an error in the table description).

For roadway lighting criteria the recommendations of IESNA Recommended Practice

8‐00 and AASHTO Guide for Roadway Lighting apply. The criteria recommended in

ANSI/IES RP‐8‐05 Standard Practice for Roadway Lighting is shown in Table 7‐2 and

Table 7‐3.

Table 7‐2. Illuminance Method—Recommended Values

Road

Pedestrian

Conflict

Area

Pavement Classification

(minimum maintained average values) Uniformity

Ratio

Eavg/Emin

Veiling

Luminance

Ratio

Lmax/Lavg

R1

(lux/fc)

R2 & R3

(lux/fc)

R5

(lux/fc)

Freeway Class A 6.0/0.6 9.0/0.9 8.0/0.8 3.0 0.3

Freeway Class B 4.0/0.4 6.0/0.6 5.0/0.5 3.0 0.3

Expressway High 10.0/1.0 14.0/1.4 13.0/1.3 3.0 0.3

Medium 8.0/0.8 12.0/1.2 10.0/1.0 3.0 0.3

Low 6.0/0.6 9.0/0.9 8.0/0.8 3.0 0.3

Major High 12.0/1.2 17.0/1.7 15.0/1.5 3.0 0.3

Medium 9.0/0.9 13.0/1.3 11.0/1.1 3.0 0.3

Low 6.0/0.6 9.0/0.9 8.0/0.8 3.0 0.3

Collector High 8.0/0.8 12.0/1.2 10.0/1.0 4.0 0.4

Medium

6.0/0.6

9.0/0.9

8.0/0.8

4.0

0.4

Low 4.0/0.4 6.0/0.6 5.0/0.5 4.0 0.4

Local High 6.0/0.6 9.0/0.9 8.0/0.8 6.0 0.4

Medium 5.0/0.5 7.0/0.7 6.0/0.6 6.0 0.4

Low 3.0/0.3 4.0/0.4 4.0/0.4 6.0 0.4





June 2010 7‐3

Table 7‐3. Luminance Method—Recommended Values

Road

Pedestrian

Conflict

Area

Average

Luminance

Lavg

(cd/m

2

)

Uniformity

Ratio

Lavg/Lmin

(max

allowed)

Uniformity

Ratio

Lmax/Lmin

(max

allowed)

Veiling

Luminance

Ratio

Lmax/Lavg

(max

allowed)

Freeway Class A 0.6 3.5 6.0 0.3

Freeway Class B 0.4 3.5 6.0 0.3

Expressway High 1.0 3.0 5.0 0.3

Medium 0.8 3.0 5.0 0.3

Low 0.6 3.5 6.0 0.3

Major High 1.2 3.0 5.0 0.3

Medium 0.9 3.0 5.0 0.3

Low 0.6 3.5 6.0 0.3

Collector High 0.8 3.0 5.0 0.4

Medium 0.6 3.5 6.0 0.4

Low 0.4 4.0 8.0 0.4

Local High 0.6 6.0 10.0 0.4

Medium 0.5 6.0 10.0 0.4

Low 0.3 6.0 10.0 0.4

The American Association of State Highway and Transportation Officials (AASHTO)

also have recommendations in the AASHTO Roadway Lighting Design Guide 2005

which,

in

general,

are

in

agreement

with

the

IES

and

are

shown

in

Table

7‐

4.

Based on these recommendations considering the bridge roadway to be classified as

a freeway, a average illuminance of 6 to 12 lux (.6 to 1.1 footcandles) and average

luminance of 0.4 to 1.0 candelas/square meter (cd/m2) would be suitable design

ranges.

For the pedestrian area adjacent to the roadway, the lighting criteria described in IES

DG‐5‐94 Recommended Lighting for Walkways and Class 2 Bikeways is most suitable.

RP‐8 has a pedestrian area criteria but it applies to sidewalks and walkways adjacent

to the roadway where pedestrian conflicts can occur. The bridge pedestrian walkway

is separated and protected from the roadway so the recommendations of DG‐5 are

more applicable.

Using Table 2 of this guide and classifying the bridge walkway as a pedestrian

overpass an average horizontal value of 2 lux (0.2 footcandles) and a vertical

illuminance values of 5 lux (0.5 footcandles) should be used.





June 2010 7‐5

Table 7‐5. Recommended Illumination (Values in lux)

Average Conditions Special Conditions1

Avg Maintained

Illuminance

Levels

Horizontal Levels (Eavg)2

Horizontal

Avg to

Min

Avg

Min

Maintained

Avg Vertical

Levels (Eavg)3

Avg to

Min

Ratio

Sidewalks along streets by area classifications4

Commercial 10 4:1 20 5:1

Intermediate 5 4:1 10 5:1

Residential 2 10:1 5 5:1

Park walkways and class I bikeways 5 10:1 5 5:1

Pedestrian tunnels 20 4:1 55 5:1

Pedestrian overpasses 2 10:1 5 5:1

Pedestrian stairways 5 10:1 10 5:1

7.3 Lighting Alternatives Analysis

7.3.1 Source Selection

Based on the direction of the Visual Quality Manual for the use of a “white” light

source like metal halide, a review of possible sources meeting this criteria was

evaluated. The types of lamps which were considered were:

Metal Halide

Fluorescent

Induction

LED

Each of these sources has unique operating characteristics in terms of efficiency,

lamp life, temperature impacts, and lumen depreciation, so a description of each is

included below with an expanded description of LED’s. Because LED’s are a solid

state device and not a conventional lamp type, more information is provided

including some LED “basics”.

Metal Halide

Metal halide lamps are high intensity discharge lamps which operate by vaporizing

various materials at extremely high voltages, stabilizing after ignition. Metal halide

lamps are available in various color temperatures and are fairly efficient at about 70‐

80 lumens per watt. The life of a metal halide lamp will range from about 12,000 to

20,000 hours depending on the lamp wattage used (rated life is determined as the

time when 50% of the lamps have failed). Metal halide lamps do experience some

color shift during their life and generally need to be in enclosed luminaires. The

lamps are prone to shortened life in conditions encountering sustained vibration

because of arc tube failure and mechanical failure of the lamp components.





7‐6 June 2010

Fluorescent / Induction

Fluorescent sources operate by generating an ultraviolet arc spanning over two ends

of the lamp. As the ultraviolet energy comes in contact with phosphors it produces

visible light. Induction technology uses coils, external to the lamp to generate an

electric field to perform this function. The use of these external coils, operating at

high frequency,

help

extend

the

lamp

life

of

this

source

to

over

100,000

hours.

Linear fluorescent lamps are sometimes considered for bridge lighting because of

their long life and efficiency. Typical arrangements are as linear rail lights or accent

lighting. A T8 fluorescent lamp with reduced mercury content, operating in

conjunction with a programmed start ballast will provide a lamp life of over 40,000

hours and a system efficiency of approximately 110 lumens per watt. The lamps with

electronic ballasts are durable in high vibration environments when used in properly

designed fixtures. Temperature however does effect lamp output. Typically

optimized for a bulb wall temperature of 77 degrees F, the lamps output can be

reduced by over half in very cold environments.

Induction lamps have a lower efficiency than some linear fluorescent lamps,

operating at approximately 75‐80 lumens per watt. Lamp life is very long at

approximately 100,000 hours. The end of life of an induction system is typically

when the high frequency driver fails to operate. LED

(parts of the text included in this section are excerpted from the US Department of Energy documents at www.netl.doe.gov) LEDs differ from traditional light sources in the way they

produce light. In an incandescent lamp, a tungsten

filament is heated by electric current until it glows or emits

light. In a fluorescent lamp, an electric arc excites mercury

atoms, which emit ultraviolet (UV) radiation. After striking

the phosphor coating on the inside

of glass tubes, the UV radiation is

converted and emitted as visible

light.

An LED, in contrast, is a

semiconductor diode.

It

consists

of

a chip of semiconducting material

treated to create a structure called

a p‐n (positive‐negative) junction.

When connected to a power

source, current flows from the p‐

side or anode to the n‐side, or cathode, but not in the reverse direction. Charge‐

carriers (electrons and electron holes) flow into the junction from electrodes. When

Figure 7‐1. Typical LED

Figure 7‐2. Schematic of LED Operation





June 2010 7‐7

an electron meets a hole, it falls into a lower energy level, and releases energy in the

form of a photon (light).

The specific wavelength or color emitted by the LED depends on the materials used

to make the diode.

Red LEDs are based on aluminum gallium arsenide (AlGaAs). Blue LEDs are made

from indium gallium nitride (InGaN) and green from aluminum gallium phosphide

(AlGaP). “White” light is created by combining the light from red, green, and blue

(RGB) LEDs or by coating a blue LED with yellow phosphor.

Color Characteristics Unlike incandescent and fluorescent lamps, LEDs are not inherently white light

sources. Instead, LEDs emit light in a very narrow range of wavelengths in the visible

spectrum, resulting in nearly monochromatic light. This is why LEDs are so efficient

for

colored

light

applications

such

as

traffic

lights

and

exit

signs.

However,

to

be

used

as a general light source, white light is needed.

White light can be generated from LED’s by either coating a blue LED with a yellow

phosphor, or by using monochromatic red, green, and blue (RGB), LED’s and

operating them at different levels to “mix” and create white, or any other color light.

For most applications were the light source is visible, white (coated) LED are used.

For RGB configurations, unless using an intermediate optic material, when viewing

the sources directly you can see the individual red, green and blue LED’s and the

source does not appear white. When applied to a surface however the mix of the

colors creates a white surface.

LED’s with a high correlated color temperature (which are “cooler” or bluer in

appearance) tend to be the more efficient LED’s in terms of lumens per watt of

power consumed. “Warmer” LEDs are available but have a slightly lower efficiency..





7‐8 June 2010

Photometric Characteristics LED’s are available in different emitting patterns. An example of a typical pattern

(from Phillips Luxeon K2) is shown in Figure 7‐3 and Figure 7‐4.

Figure 7‐3. Typical Representative Spatial Radiation Pattern for White Lambertian

Figure 7‐4. Typical Polar Radiation Pattern for White Lambertian





June 2010 7‐9

Effect of Electrical Variations The light output of an LED is related to the amount of current it is being operated.

The greater the current, the higher the light output of the LED. These variations in

current also alter the useable life and efficiency of the LED. It is important to look at

the performance data provided for particular LED’s and the current that they will be

actually operated.

Table 7‐6. Flux Characteristics for LUXEON K2 with TFFC Junction and Case Temperature = 25 C

Minimum Performance at Test Current Typical Performance at Indicated Current

Color Part Number

Minimum

Luminous Flux

(lm) at 100 mA

Typical Luminous Flux (lm)

at 1600 mA at 700 mA at 350 mA

Cool White LXK2‐PWC4‐0200 200 275 170 95

LXK2‐PWC4‐0180 180 250 150 85

LXK2‐PWC4‐0160 160 220 135 75

Figure 7‐5. Typical Luminous Flux





7‐10 June 2010

LED’s also require a driver, similar to the way a fluorescent and high‐intensity

discharge (HID) light sources cannot function without a ballast, which provides a

starting voltage and limits electrical current to the lamp. The driver converts line

power to the appropriate voltage (typically between 2 and 4 volts DC for high‐

brightness LEDs) and current (generally 200‐1000 milliamps or mA), and may also

include dimming

and/or

color

correction

controls.

Currently available LED drivers are typically about 85% efficient. So LED efficacy

should be discounted by 15% to account for the driver.

Effect of Temperature The luminous flux figures cited by LED manufacturers are based on an LED junction

temperature (Tj) of 25°C. LEDs are tested during manufacturing under conditions

that differ from actual operation in a fixture or system. In general, luminous flux is

measured under instantaneous operation (perhaps a 20 millisecond pulse) in open

air. Tj will always be higher when operated under constant current in a fixture or

system.

LEDs

in

a

well‐

designed

luminaire

with

adequate

heat

sinking

will

produce

10%‐15% less light than indicated by the “typical luminous flux” rating.

Source: Philips Luxeon K2—(10/07) Figure 7‐6. Typical Light Output Characteristics Over Temperature

(Cool‐White at Test Current)

Estimated Life LED’s, much like the mercury lamp, will continue to operate for long periods of time

but at a steadily deteriorating light output. For this reason the estimated life of an

LED cannot be based on how long it is still operating but how long it is still being an

effective lighting source. Research (ref. Mark Rea, Lighting Research Center, 2000)

has shown that for general lighting, a majority of occupants can accept a lighting

level reduction of up to 30%, when done gradually. For this reason, as well as for





June 2010 7‐11

standardization, the life of an LED is established to be the time when its’ light level

has been reduced to 70% of its initially rated value. This is called an L70 life.

Source: Adapted from Bullough, JD. 2003. Lighting Answers: LED Lighting Systems. Troy, NY. National Lighting Product Information Program, Lighting Research Center, Rensselaer Polytechnic Institute.

Figure 7‐7. Typical Lumen Maintenance Values for Various Light Sources

By this

method,

most

currently

available

LED’s

are

rated

between

40,000

and

60,000

hours of life at rated current. Much longer operating life can be obtained by

operating the LED’s at a lower forward current and just using more LEDs. LED life can

be extended 120,000 hours or higher by driving the LEDs at reduced current.

Because LEDs are solid state devices, without arc tubes, they are very stable in high

vibration environments such as bridges assuming proper construction of the sold

state elements and circuit boards.

The advancement of LED technology is exponentially faster than conventional lamp

sources. Based on current advancements and expected continuation, LEDs should

surpass all other sources in terms of operating life and energy efficiency within the

next 2 years.

When

looking

at

current

technology

and

the

future

potential,

the

use

of LEDs seem appropriate for the St. Croix River Crossing if used with lower drive

currents and lower color temperatures to better compliment the proposed finishes

of the bridge and eliminate the environmental impacts of higher color temperature

sources.

7.4 Architectural Lighting The architectural lighting options

for the bridge ranged from no

accent lighting to either

floodlighting or

using

point

sources to accent most of the

structure and cables. In this

section of the report we will

discuss the progression of each

option as designed and presented

to the Visual Quality Committee and where consensus was reached.

Figure 7‐8. Base Architectural Lighting Option





7‐12 June 2010

The base architectural lighting option looked at no accent lighting for the bridge

structure using only required lighting for basic motorist safety. The roadway was

illuminated to the required lighting levels discussed in the criteria section using

metal halide lamps in standard cobra head style full cutoff luminaires on 35’ davit

arm style pole with 8’ arms. The walkway area met the minimum required horizontal

values.

7.4.1 Option 1A From the base option of just

using the roadway lighting, the

roadway lighting luminaires were

replaced with LED luminaires

having increased

distribution/output to the front

side of the luminaire. This

allowed more lighting to be

placed on

the

cables

from

the

roadway lighting equipment.

The roadway lighting however is

mounted on the median which is

not centered between the cables

due to the addition of the

walkway on one side of the

bridge. Because of this, the

lighting on the cables is different

depending on which direction

the bridge

is

viewed.

Looking

south,

the

cables

are

mostly

illuminated.

Looking

north, the illuminated section of the cables is not as high due the increased distance

to the luminaire so the effect is less pronounced.

Floodlights were added at the

base of the piers which light the

inside surface of the split pier.

Four floodlights were used for

each pier consisting of narrow

beam LED floodlights. These

floodlights were also RGB color

change floodlights

allowing

the

lower piers to change color for

the purpose of special occasions

or significant events. The RGB

LED arrangement can be tuned to produce virtually any color. In addition, each

group of floodlights on each pier and be individually controlled and set to different

preset colors.

Figure 7‐9. Lighting Option 1a

Figure 7‐10. Lighting Option 1b





June 2010 7‐13

7.4.2 Option 2A

The next lighting alternative

investigated was to add LED

floodlights to the upper portion

of the towers. The intent of the

floodlighting alternatives

analysis is to keep adding layers

of light in an attempt to reach

the proper balance and

perspective.

Two LED floodlights were used

at the top portion of the pier

directed down the face of the

tower. These additional

floodlights added the effect of

visually lengthening

the

perceived height of the towers.

It also has the effect connecting

the bottom to the top of the

towers.

The negative effects of these

lights include the addition of

more lighting into the night

environment at the bridge and it

did somewhat compete with the cable lighting. Because the inside surface of the

cables are

illuminated,

an

observer

sees

the

cable

lighting

on

the

opposite

side

of

the bridge. Unless directly lined up with the bridge, the illuminated tower would not

usually line up with the illuminated cables causing some clutter and potentially an

unbalanced appearance.







7‐14 June 2010

7.4.3 Option 2AA The next progression of surface

floodlighting included another

group of floodlights mounted at

the top

of

the

towers

to

light

the

inside face of the towers. These

additional floodlights would add

symmetry to the illuminated

surfaces of the bridge and also

increase the visual interest to the

motorist passing over the bridge.

When using the white light the

floodlights did seem to more

evenly balance the bridge. From

the roadway

the

lighting

also

helped accentuate the full tower

height. The color change option

however did not produce the

same results. The roadway

lighting, particularly with the

higher angle forward distribution

tended to wash out the color

impact of the floodlights and

make the effect somewhat

muddy and inconsistent.

Figure 7‐13. Lighting Option 2aa

Figure 7‐14. Lighting Option 2bb





June 2010 7‐15

7.4.4 Option 3 Option 3 investigated the use of

a conventional method of cable

lighting. Cable lighting for

bridges can

be

done

in

two

ways

resulting in different effects. The

most often used method is

floodlights mounted at the base

of the cable aimed along its

length towards the tower. This

type of lighting tends to

uniformly light the cables

regardless of the viewing

direction. Cable grazing is

another lighting method where

the floodlights

are

arranged

along the bridge and aimed

vertically, grazing the cables.

With this type of arrangement

the cables appear more brightly

lit on the side of the tower

where the observer is located.

When the observer changes, so

does the location of perceived

brightness for the cables. For

this alternative

however

the

first

method was used aiming along

the cables. This option can be

done with standard metal halide

floodlights or with LED

floodlights.

While this arrangement does give the towers a strong visual appearance from a

driver’s perspective it ignores the lower portion of the towers and front faces by

only illuminating the outside face.

Figure 7‐15. Lighting Option 3





7‐16 June 2010

7.4.5 Option 4 The last option investigated was

various arrangements of direct

view sources. In this type of

option, lower

wattage

sources

using globes or opaque lenses

are used to “dot” or line the

structure with points of light.

The most commonly used

application of this type of

lighting is the a necklace lighting

system often used on

suspension bridges.

While useful as necklace lighting

where the

curving

arch

of

the

main cables are highlighted by

the points of light, it only

seemed to detract from the

strong structural elements that

make up the St. Croix Bridge

design.

7.4.6 Preferred Option As a result of discussing the

various options and considering

the long

standing

concerns

about environmental impacts,

spill lighting, conflict with the

natural beauty of the river and

surrounding area, the simple

and subtle approach seemed to

be Option 1A. Also from a bridge

aesthetic standpoint, the

organic and graceful design of

the bridge seemed better

complemented by

a more

natural “moonlit” glow than a

strong “feature” type of

lighting system.



Figure 7‐18. Preferred Option—View from

Sunnyside Marina

Figure 7‐19. Preferred Option—View from Stillwater





June 2010 7‐17

This option using lower pier LED

floodlights as the only added

lighting to the roadway and

pedestrian lighting was generally

agreed to be the correct

approach. To

arrive

at

the

proper balance of lighting in this

situation and environment will

require a strict criteria and

explicit specifications in the RFP

documents to achieve the

desired results. This can be done

by using the calculated results of the current design as the light level criteria for

various surfaces of the bridge and specifying fixture output and distribution required

for installation.

The lower

pier

lighting

would

also

be

RGB

color

change

lighting

programmed

with

preset lighting colors which can be activated remotely.

7.5 Roadway Lighting

The roadway lighting, as partially

described in the architectural

lighting section, is made up of

median mounted 35’ poles, with

8’ davit arms, spaced at 160’

centers. The pole spacing is

arranged

to

coincide

with

the

80’ tower spacing. The

luminaires are 292 watt LED

luminaires. For this initial

analysis the luminaire used was

a Beta LED 12 bar unit similar to

the luminaire currently used on

the St. Anthony Falls Bridge. It

should also be noted that the

lighting calculations were

performed at the rated output

and drive

current

of

the

luminaires and is therefore

higher than required.

In order to extend the

maintenance life of the LED’s a lower drive current will be specified in the

design/build RFP so that the lighting levels are lowered to the criteria levels and the

associated wattage will be reduced.

Figure 7‐20. Preferred Option—View Looking North

Figure 7‐21. Roadway Lighting





7‐18 June 2010

7.6 Loop Trail Lighting

The loop trail lighting consists of 40 watt LED linear luminaires, integrated into the

bridge railing. The lighting levels with this added lighting meet the requirements of

the IESNA for both standard and

enhanced security by providing 5

lux minimum vertical illuminance. By providing this

vertical illuminance, facial

recognition is enhanced. This

improves the pedestrian’s

comfort level and sense of

security while traveling over the

bridge. It also helps increase

object contrast in the walkway

area which will aid cyclists in

seeing unexpected

obstacles.

7.7 Navigation and Obstruction Lighting

Even though there is no defined navigable channel by the Coast Guard in the area of

the proposed bridge, concerns were raised about the visibility of the bridge piers by

boaters. The current option includes pier accent lighting but those lights may not be

in operation during all the hours of darkness. As a

result, bridge navigation lights are proposed to be

placed near the bottom of each of the piers, one on

each side. These lights would utilize LED sources to

increase

their

operating

life.

FAA obstruction lights will be required at the top of

the towers. Two of these lights would be used on

each of the towers. These lights are omni‐direction,

red obstruction lights and would also use LED

technology to lengthen their service life.

7.8 Light Trespass / Glare / Environmental Impacts

Because of the concerns contained in the Visual Quality Manual and from the

meetings discussing the project, minimizing light trespass, glare, and sky glow were

key design

elements.

The

use

of

LED

luminaires

allows

for

precise

control

of

lighting

both in the beam size but also by controlling the output of the LED’s. For this

concept design the following approaches were used to minimize environmental

impacts:

Lighting was designed to meet the minimum recommended values of the IESNA.

Lighting for the preferred option has limited uplight. The floodlights used at the

bottom of the piers are narrow distribution LEDs with most light contained, and

blocked, by the bridge deck.

Figure 7‐22. Loop Trail Lighting Using Linear

Luminaires

Figure 7‐23. Navigation and

Obstruction Lights





June 2010 7‐19

The proposed control system for the lighting would leave the roadway lighting

system on, but turn off both the architectural lighting and the pedestrian lighting

at a predetermined time (possibly 12AM). By using the navigation lighting the

pier lighting is not required throughout the night. For the trail lighting the basic

horizontal illuminance requirements for the sidewalk can be met with the

roadway lighting

system.

The

enhanced

vertical

illumination

however

will

not

be

met but it is anticipated that pedestrian volumes will be very low and therefore

unnecessary.

Veiling luminance calculations have been performed to make sure the lighting

system provided limits disability glare to the prescribed limits of the IES.

7.9 System Maintenance

Maintenance was a strong consideration when evaluating proposed lighting systems

and source types. The use of LED technology minimizes the relamping requirements.

Also the use of reduced current operation extends the life and overall efficiency of

the LEDs

considerably.

For

the

systems

proposed

we

anticipate

a minimum

of

a 10

year source life. After this time the LEDs and drivers would need to be replaced, with

the next generation of LEDs gaining additional life expectancy and efficiency. Some

key considerations when reviewing the proposed system include:

The lower pier floodlights and pier navigation lights will need to be maintained

from the water level. That would mean the use of a boat or small barge with

ladders/scaffolds when the LEDs have reached their end of life. The control

system proposed and short “on” duration will likely yield a much longer than 10

year service cycle but maintenance predictions included in this report have been

kept conservative.

The obstruction lights on the tower would be maintained by access through the

towers

The roadway lighting would be maintained by conventional means, most likely

requiring a lane closure on both sides of the roadway

The trail pedestrian lighting system would require replacement of the LED strip

after the 10+ year expected life. Again the life is a

very conservative estimate based on the proposed

control system and hours of operation.

7.10 Box Section Inspection Lighting

The interior of the box section of the bridge will be

accessed for

periodic

inspection.

In

order

to

support

these inspections, a lighting and control system, as well

as maintenance receptacles, is used within the box. For

this project, it is proposed to use LED utility lights within

the box, controlled by 3 way switches and timer and/or

occupancy sensors. The timer/sensors are used for turning the lighting off, if it has

inadvertently been left illuminated after an inspection.

Figure 7‐24. LED Utility

Light





7‐20 June 2010

7.11 Signing

The signing limits for the St

Croix River Crossing will

include the entire bridge and

approximately 100 feet beyond

the ends of the bridge. The

purpose of the signing is to

convey the necessary

information in a

comprehensible manner

allowing the driver adequate

time to initiate any allowable

action required in a safe

manner.

7.11.1 Review of the Visual

Quality Manual

The location of signs will be based on current sign placement standards adopted by

Mn/DOT and WisDOT and where possible, will be placed in the roadside consistent

with the VQM. Mn/DOT has additionally stated that they did not want any large

overhead signs mounted in the median of the bridge.

Where overhead sign structures are required, they should be a neutral gray in color

in accordance with the VQM. However, Mn/DOT, in contrast to the VQM, has

specifically required that all overhead sign structures use the standard truss style

configuration mounted onto a standard sign post or where possible connected

directly

to

the

pier

columns

in

an

unobtrusive

acceptable

manner.

The

exact

elevation of the sign truss shall be consistent with adopted Mn/DOT standards for

overhead signing and will be a function of the sign panel sizes being accommodated.

The truss should also be mounted so as to allow for the possibility of adding a future

walkway and sign lighting system.

Where it is required that signs be mounted on a bridge overpass to serve the

motorists on the under passing road, the signs should be attached in an unobtrusive

manner consistent with the VQM. Where possible, that shall mean the top of the

sign should not extend above the bridge rail and the bottom of the sign should not

extend below the bottom of the structure.

The current

signing

concept

only

depicts

one

sign

to

be

mounted

on

each

sign

truss

at each location. As a result there is no contradiction with the VQM requirement

that all signs on the same structure have the same vertical dimension and are

mounted at the same elevation.

7.11.2 Additional Concept Refinements

All other signing beyond the limits of the project or not directly attached to the

bridge structure itself shall be provided for by others under a separate contract.

Figure 7‐25. Typical Inspection Lighting





June 2010 7‐21

Additional signing for delineators, markers, speed limits, route confirmation signs,

and trail signs shall be located as need and defined by the Contractor and approved

by Mn/DOT.

As a result of eliminating the median mounted overhead sign structures, the

structure for the begin and end junction signs have not been defined at the time of

this writing

as

to

if they

will

be

mounted

overhead

or

mounted

off

to

the

side

of

the

bridge or ground mounted. Further discussion is needed to finalize this element.





June 2010 8‐1

8 Visual Quality

8.1 Introduction

The visual appearance of the St. Croix River Crossing and the context of the bridge

environs are a critical factor in the design development of the bridge. As the SFEIS

was developed

between

2004

and

2006,

the

extradosed

bridge

type

was

select

for

the main river crossing through an extensive stakeholder process that involved local

state and federal government agencies, as well as, local and national citizen

organizations. In parallel with the SFEIS process, a VQM was developed to outline

the aesthetic values for the project. A VQRC with member participation from all of

the stakeholder groups was a key part of the visual quality process. In addition the

VQM process included a public open house to gather public input for the aesthetic

development of the bridge.

Through the VQM process, a preferred architectural treatment was selected from a

number of options. The preferred concept is called “Organic” to compliment the

natural forms

of

the

setting.

The

VQM

describes

the

Organic

concept

as

“characterized by curved planes, tapered forms, smooth surfaces, and expressed joints between parts.” In the preliminary engineering phase of the project, the stakeholder involvement has

been continued with the formation of a VQAC. The VQAC is made up of a subset of

the VQRC member organizations, and includes the following members:

City of Bayport, MN

City of Oak Park Heights, MN

City of Stillwater, MN

Town of St. Joseph, WI

Minnesota State Historic Preservation Office

National Park Service

The purpose of the VQAC is to:

Perpetuate the work that has been accomplished through the Environmental

Process and Visual Quality Process

Assist the Project Team through the interpretation of the Visual Quality Manual

in areas where the intent is not fully defined.

Advise the Project Team regarding refinements to the conceptual design during

the preliminary engineering phase

Assist

the

Project

Team

in

evaluating

how

newly

discovered

aspects

discovered

during preliminary engineering relate to the previously developed concept

design.

Attend VQAC Meetings to offer input and comment on visual quality issues

related to the preliminary engineering.

Inform the agencies or organizations that are represented of information

presented or discussed with regard to the relationship between the conceptual





8‐2 June 2010

design documented in the Visual Quality Manual and the preliminary

engineering design.

In addition, public involvement was continued through two open houses, where the

concept refinement was presented for public input and comment.

8.2 Objectives

The specific objectives of the concept refinement through the preliminary

engineering phase include:

Collaborate with the structural discipline to study and execute refinements of

the concept design using the VQM for guidance.

Develop architectural drawings that support proposed structural design concepts

and compliment the intent described in the VQM including:

o 2D drawings showing the various concept refinements.

o 2D lighting drawings showing the river bridge at night, on the river up and

down stream and on the roadway from the Minnesota and Wisconsin sides.

o 2D drawings

to

describe

the

design

intent

to

both

the

professional

and

the

layman.

o 2D CAD drawings indicating accurate scale and proportion to be used for

discussion purposes with the Design Team and the VQAC

o 2D CAD drawings developed for formal presentations to the DOT’s, other

agencies, and the public at large.

o Refinements to the concepts presented in the VQM, such as further design

refinement of the two‐column vs. three‐column piers, to be presented to

obtain concurrence of the Design Team and input from the VQAC on the

refinements.

Provide

input

to

the

Mn/DOT

Visualization

Unit

for

their

preparation

of

photo

‐realistic renderings of the bridge in both daytime and nighttime simulations.

8.3 Refinement Process

The VQM presents a concept design and visual, functional, and engineering guidance

for the further development of the concept design. The VQM defines the selected

“Organic” concept as:

The parts look as if they were found in nature, or shaped by natural forces.

The vertical pier forms are reed‐like; the girders are rounded and tapered like

bones or tree branches; and walls, barriers and railings are curved and blended

into

the

larger

forms.

Transitions are gradual and smooth; edges are soft and curved; and colors are

unified and natural expressions of their materials.

In preparation for the concept refinement and as part of the preliminary

engineering, the concept design was reviewed and evaluated to more fully

understand these defining principles of the concept. The refinement of the concept

required close collaboration between the architectural discipline and the structural





June 2010 8‐3

discipline, so that the aesthetic values outlined in the VQM are coordinated with the

structural requirements of a major river crossing. The outward appearance of the

bridge and its relationship to the bluffs, wetlands, and river are of utmost

importance, but the structural integrity of the bridge is the backbone that supports

that visual experience. The bridge structure must be functional and economical

while expressing

the

intended

context

sensitive

design.

With these values in mind, the focus of the Visual Quality effort was:

Pier Concept Refinement: The concept pier is a three‐column pier with two

exterior columns that extend above the roadway to provide an attachment for

the stay cables, while the interior column is under the bridge girders along the

centerline of bridge. During development of the VQM, the preference of the

VQRC was to have a two‐column pier, but a three‐column pier was included in

the concept until structural investigation was performed to verify the feasibility

of the two‐column pier. The “Organic” concept represents the light and elegant

character desired of the river bridge, and the two‐column conveys these

attributes to

the

fullest

extent.

Through technical analysis of the structure it was possible to confirm that the

number of columns can be reduced from three to two. This was achievable without

changing the size or form of the column while retaining the preferred shape and

proportions. While the depth and width of the cross girder was also retained, it

became necessary to modify the lower edge of the cross girder to provide greater

contact area of the girder with the tower legs. The original concept of a curved outer

edge along the cross girder was also retained though the radius of the curved edge

had to be shortened to increase the necessary surface area. This has resulted in a

slightly different but acceptable visual appearance of the cross girder.

Figure 8‐1. Section View Illustrates Curved Outside Edges of Cross Girder

Pier Column Shape Refinement: The pier column shape depicted in the VQM has

variable dimensions from top to bottom of column creating a tapered cylindrical

cross‐section that is slightly canted toward the bridge centerline as the column

extend upward from the water. The Cost Risk Assessment and Value Engineering





8‐4 June 2010

(CRAVE) study undertaken in late 2008, recommended that the pier column

shape be simplified to improve constructability and provide cost savings.

A number of refinements were explored but the one chosen retains the visual

character of the VQM column while providing a greater level of constructability. The

chosen refined design consists of a uniform sloping and canted exterior on three

outside column

faces

while

the

inboard

face

is

vertical

and

uniformly

tapered.

To

achieve this, short tangent segments were introduced into the column section on all

four sides so that transitional blocking could be more easily introduced into the

formwork. The interior face of the twin column sections above the roadway deck

line where closed with a single tangent. This was in part to enhance constructability

and to reduce cost.

Near the base of the river columns, a sloped concrete fill is proposed. This fill area is

to be located at the surface water elevation and slope upward closing the hollow

column form. The purpose for the fill is to prevent unauthorized entry of small

vessels or persons. It will also protect the columns from ice intrusion and reduce

lodging of

drift.

Figure 8‐2. Tangent Segments Have Been Introduced Along Vertical Faces of Tower Forms

for Ease

of

Construction

Pedestrian Trail Location: In the VQM concept, the pedestrian trail cantilevers from

the north edge of the bridge girder outboard of the north plane cable stays

(outboard scenario). This requires the trail to curve outward and around the north

column at each of the piers. The CRAVE study recommended that the pedestrian

trail be relocated onto the bridge girder, placing the trail inboard of the north cable

plane. The recommendation is based on improved constructability, improved





June 2010 8‐5

operations, maintenance and inspection, and potential cost savings. With the trail

inboard of the cables, interference between pedestrian/bicycle travel and the cables

is eliminated, and pedestrians can be removed from the through traffic of the trail

by placing overlooks at the piers.

Both the outboard and inboard trail locations were evaluated for constructability,

safety, operations

and

maintenance,

cost

savings

and

visual

characteristics.

The

two

scenarios were modeled and compared and contrasted. The analysis concluded that

the inboard trail concept provides superior safety, operations and maintenance

characteristics. These include ease of access for safety, emergency, snow removal

and maintenance vehicles. Pedestrian movements are linear providing greater user

safety as the alignment affords trail users a clear sight lines along the entire length

of the bridge. Lighting is more easily and efficiently accomplished as roadway

lighting systems are adequate to provide pedestrian lighting needs for certain use

conditions and in times of higher use, it is easily supplemented with low level

lighting located within the hand rail. The inboard trail separates users from the cable

systems of

the

bridge

yet

still

provides

ample

viewing

of

the

structure

and

its’

components as well as views to the river beyond.

Figure 8‐3. View along pedestrian trail located inside of tower line





8‐6 June 2010

Figure 8‐4. Section view of trail located on north edge along west bound travel lanes

Pedestrian Trail Overlook: The VQM concept does not have specific locations

designated for pedestrians to stop along the bridge and view the river. The

inboard trail provides the opportunity to use the space adjacent to the north

column at each pier as an overlook. This will provide space off of the through

trail alignment for pedestrians to enjoy the scenic river valley, while separated

from bicycle traffic.

The overlook concept is the result of moving the trail inboard of the tower line. The

overlook concept was developed to take advantage of the opportunity for non

motorized bridge users to have an unobstructed view of the river north of the

bridge. The overlooks are cantilevered platforms that extend out over the water

from the tower locations. The overlooks provide a resting and viewing area which is

out of the flow of non motorized traffic and away from the vehicular roadway and

provide a commanding view of the river and bluffs. Because the overlooks are not

part of the through trail facility, the option was explored to construct an overlook at

alternating tower lines. This further refinement of the overlook concept has resulted

in the recommendation to include four overlooks on the bridge. They are to occur at

three locations, pier lines 9, 11, and 13.

The underside of the overlook structure was also studied in detail. The design team

considered the platform underside as an opportunity to provide a dynamic daytime

element and to use lighting at night to accentuate this feature.





June 2010 8‐7

Figure 8‐5. Typical Pedestrian Overlook outside of tower on north elevation of bridge

Figure 8‐6. View of underside of Pedestrian overlook surrounding tower





8‐8 June 2010

Pedestrian Trail Railing: The VQM concept is a full height metal railing with a

short concrete curb. The pedestrian rail provides a number of opportunities for

input from the VQAC related to curb height and railing details including trail

lighting fixtures built into the railing.

The pedestrian railing design was refined to simplify the rail as much as possible

while still maintaining the principles identified in the VQM. Both a full height rail with short curb and a taller curb with shorter rail were evaluated. The taller rail was

complicated in design. The rail has to meet safety requirement for containment up

to a height of 27” with a 4” vertical bar spacing and then 6” minimum vertical bar

spacing thereafter. Both a 4” spaced rail and a combined 4” and 6” spaced rail were

modeled and illustrated. These designs resulted in complicated and or visually dense

appearance. Both of these designs impeded the view to the river and increased

construction cost. It was also noted that these full height rail options with small curb

also would be subjected to greater mechanical damage from snow removal

operations and other maintenance activities.

The chosen

rail

consists

of

a taller

concrete

curb

upon

which

the

metal

rail

is

attached. The added curb height of 21‐3/4 inches allows the metal railing to be

constructed solely with 6‐inch center to center separation which reduces the

amount of material, as well as visually “lightens” the railing making it more

transparent and more easily seen through. The design of the rail can easily be

fabricated in modules which will reduce cost, and enhance constructability and

maintenance operations.

To increase the levels of lighting on the trail, the railing posts have been designed in

paired arrangement which provides a nesting location for the small LED lighting

fixture. This design provides protection for the lighting while reducing the visual

prominence of

the

fixtures.

Figure 8‐7.

Higher

curb

height

with

6”

vertical

picket

spacing

and

integral

LED

light

fixtures

incorporated into post on pedestrian hand railing.





June 2010 8‐9

Cable Anchorages: The VQM Concept has portrayed individual cable anchor

“pods” attached to the side of the box girder. This provides a certain texture to

the side of the bridge and exposes the supporting elements as part of the overall

visual appearance of the bridge. Refinement of the pods was performed to

confirm the structural dimensions needed to support the bridge and to maintain

the architectural

details

to

provide

a coordinated

and

smooth

transition

of

the

pod into the girders.

Two alternative cable connection details were studied. The exposed connection as

illustrated in the VQM and an alternative called a covered cable concept. The

structural requirements for the anchorages were determined and can be easily met

with either style. The motivation to evaluate alternative connection options was

largely as a result of visual consistency concerns of the design team. Both

alternatives were modeled and illustrated for the VQAC and Mn/DOT. The VQAC

unanimously chose the covered cable alternative because it supported the linear

form of the main bridge elements more effectively than the exposed cable

connection scenario.

There

are

also

some

construction,

maintenance

and

operations

benefits to the covered scenario as it easier to build, and provides a greater level of

protection for the cable terminations on the underside of the deck without limiting

inspection access to cable anchorages.

Figure 8‐8.

North

elevation

view

of

structure

with

covered

cable

connections





8‐10 June 2010

Minnesota approach Structures:

The VQM design of the Minnesota

Approach included a pier design

composed of tapered columns in

pairs. The columns are sloped at

1:33 and

the

pairs

are

separated

by

10’.

Two alternative column forms were

studied and modeled. This included

a paired column concept which

reduced the slope to 1:66 and the

separation between the pair to 5”

between columns. The other

concept was a single stem design

utilizing the 1:66 taper and reveals along the edges of the single column form. The

two alternatives

were

modeled

and

illustrated

from

various

view

points

along

the

Minnesota shore and adjacent river locations. It was determined that the single

tapered column form performed visually as well if not better than the pair

alternative and it was easier to construct. The VQAC and Mn/DOT agreed the single

column form was acceptable.

Figure 8‐10. View of Battered Abutment (Left) and Typical Single Stem Pier (Right) along

Minnesota Approach

Figure 8‐9. Enlarged view of covered cable

connections





June 2010 9‐1

9 Construction

9.1 Introduction

This section discusses what are considered to be some of the more critical

construction issues for both land based construction and construction of the river

spans that

will

need

to

be

addressed

for

the

successful

construction

of

the

project.

In addition, construction of major bridges requires that constructability be

investigated and evaluated at every stage of design development as well as during

construction as the successful Design‐Build (DB) Contractor plans the construction of

the work. This constructability analysis is performed to ensure that the completed

bridge will be fully compliant with the contract requirements and that acceptable

standards for construction means and methods have been met.

9.2 Construction Staging Areas

Construction staging areas will be required for the river crossing bridge work and for

the approach

structures.

Some

of

these

will

fall

within

the

construction

limits

while

some may be located nearby. There are very limited areas for construction staging

adjacent to the project area because of the complexity of the terrain, the wetlands,

the river way, and numerous man‐made features. Therefore it may be necessary for

the DB Contractor to make arrangements for additional staging areas as necessary

to support his construction needs. The Contract documents should make clear what

potential staging areas are available to the contractor as well as that he is

responsible for any additional staging areas that he deems may be required.

9.3 Casting Yard

In addition

to

construction

staging

areas,

if the

DB

Contractor

elects

to

use

precast

construction for the approach spans, the river spans, or both, it will be necessary to

utilize an existing casting yard facility or to set up a new facility specifically for this

project. Either way, the casting yard will need to have sufficient area for the precast

operations, storage of materials and storage of an inventory for precast segments. It

can be expected that for a project of this size, a parcel of land that is 20 acres or

more may be needed for a new casting yard facility. It is not unusual that storage of

approximately one third of the total number of segments may be required though

this is clearly dependent on the planned and actual schedule for precasting and

erection.

9.4 Precast vs.

Cast

-in

-place

Construction

The DB Contractor may elect to utilize either precast or cast‐in‐place construction

methods or a combination thereof perhaps differentiating between the approaches

and the river crossing. Both methods are considered viable options and the selection

will be made in consideration of the contractor’s experience and preference,

availability of equipment, as well as precast plant considerations including transport,

schedule and economics.





9‐2 June 2010

Some of the pros and cons of each construction method include the following.

9.4.1 Precast Construction

With over 5000 linear feet of bridge deck there is an opportunity for cost

effective utilization of precast segmental construction. The river crossing will

have constant depth segments and the approaches will transition to a shallower

depth consistent

with

the

shorter

spans.

Maintaining

a constant

depth

simplifies

and reduces the cost of precast forms and allows for improved efficiency.

If the schedule allows, precast operations could be set up indoors to allow year

round production of segments.

One advantage of precast construction is that precasting can proceed concurrent

with construction of substructure elements to further shorten the schedule.

Dependent on the location of the casting facility, transport of segments to the

erection site by barge and positioning the segments for lifting could be very

efficient. The smaller approach segments would more likely be transported by

truck.

Precast segment

height

is

expected

to

be

in

a range

from

10

feet

to

16

feet,

approximately 10 feet long and up to 52 feet wide.

Precast segment weights are expected to be in the range of 145 to 160 tons

which would require use of special haulers. Haul routes from the proposed

casting yard to the jobsite must be checked for any special restrictions or permits

that may apply.

Utilizing precast segments would limit the amount of concrete to be delivered on

the river to the piers, towers and some limited cast‐in‐place superstructure

closure joints.

The repetitious nature of precasting segments in a “factory” style environment

tends to

lead

to

a higher

quality

of

finished

concrete

product.

On the downside the precast method requires the use of either an existing

precast facility or establishment of a new facility for the project.

Some existing casting yards known to exist within a 100 mile radius of the

project site that could be considered include three commercial facilities

constructing precast bridge beams and pipe products and one contractor owned

facility previously utilized for precast segments.

Even if the precast method is selected, some cast‐in‐place work would still be

required on approaches at the transition sections.

Erection can be ongoing at multiple locations and can share erection equipment.

Erection

in

winter

months

can

proceed

provided

heated

enclosures

are

used

and

provisions are made for curing of epoxy joints and cast‐in‐place joints. It can be

expected that there may be times when it is simply too cold to proceed with

erection even with such measures being taken.





June 2010 9‐3

9.4.2 Cast -in- Place Construction

Again, the large quantity of bridge deck construction offers efficiency if the DB

Contractor elects to use travelers for cast‐in‐place construction, particularly for

spans over the water.

Cast‐in‐place work on the approaches could, alternatively, be done on falsework

rather than

by

use

of

travelers.

The travelers can be easily shipped to site by barge or truck.

Cast‐in‐place deck construction over water would be more likely to be restricted

during severe low temperatures from mid‐December to mid‐March.

Both methods require the use of specialized equipment and previous experience in

this type of construction should be mandatory including PT tendon installation and

tensioning and grouting. It is customary to require at least two previous projects or a

minimum of 5 years experience. Given the limited construction of extradosed

structures in the US, it may not be practical to include a similar requirement for the

St Croix River Crossing, though, requiring supervisory personnel to have experience

with the

construction

of

at

least

two

segmental

or

cable

‐stayed

structures

in

the

last

10 years should be considered.

In order to provide for segment erection to proceed through winter months, special

attention must be paid to grouting of tendons. If allowed by the Specifications,

tendon grouting may be postponed in the winter months by the use of a corrosion

inhibitor until substrate temperatures rise again where freezing of freshly placed

grout is no longer a concern.

9.5 Permitting Requirements

The Minnesota side of the river bridge has environmental restrictions with a high

quality wetland

in

the

area,

very

limited

access,

and

other

bluff

impacts.

The

Wisconsin side of the river has bluff impacts and mussel beds adjacent to the shore.

It is essential that the permitting is established based on reasonable assumptions of

how the bridge construction is to be carried out. Provisions need to be included for

the delivery and site relocations of large pieces of equipment such as cranes,

travelers, haulers, dirt moving equipment. This will require temporary haul roads

throughout the project limits extending to the river from TH 36 and STH 35.

Consideration needs to be given at this stage for permanent access typically in the

form of a 12‐ to 15‐foot wide corridor paralleling each structure.

Access to Pier 14 on the Wisconsin bluff will be particularly challenging as the grade

drops approximately

200

feet

from

STH

35

to

the

river.

It

is

important

that

the

land

acquisition on the Wisconsin side provide for switch backing of the access road as

needed.

It is noted that the concept drawing titled “Wetland Impact Details” includes an

aerial view of the Minnesota approach spans area showing temporary and

permanent access roads as well as areas that are expected to be impacted less than

and greater than 6 months during construction. It is recommended that the





9‐4 June 2010

Contractor be given some flexibility to develop his own means and methods that

could affect those areas perhaps by defining a limiting impact assessment and the

permitting requirements by the appropriate agencies for any DB Contractor

proposed deviations from what has been indicated.

9.6 Erosion Control

and

Environmental

Compliance

Given the sensitivity of the wetland areas and watercourses that the project is to be

constructed over, it is essential that erosion control measures be employed during

construction. Erosion control must also be considered during design so that the

contract documents adequately spell out the minimum requirements and

restrictions with which the DB Contractor must comply. For the land based

construction, this will be primarily based on standard best management practices

(BMP) for erosion and sediment control, establishment of construction limits,

signage, spill prevention and control, training of the workforce in environmental

awareness and monitoring.

For work

over

water,

the

Contractor

must

plan

his

operations

and

control

the

work

to prevent contamination of the watercourse particularly during cofferdam

installation, excavation, trestle construction, barge movements and

spudding/anchoring, dredging, handling and disposal of drill shaft spoils, equipment

fueling and accidental spillage. It is recommended that there be at least one

individual on the contractor’s staff whose sole responsibility is environmental

compliance. Consideration should also be given to creating a line item for

environmental compliance items so that Mn/DOT and Wis/DOT can control and

exactly what is required in this area.

9.7 Foundation Construction Methods

A cofferdam is anticipated to be constructed at each pier for the construction of the

drilled shaft foundation cap since the top of the structural cap is assumed to be at

the mudline or approx 20 feet below waterline. The cofferdam could be installed

before or after installing the steel casings and drilled shafts. If built prior to the

drilling of the shafts, the cofferdam could serve as a template for shaft construction.

Once the cofferdam frame work and steel sheeting is in place with several bracing

tiers that act as a drilled shaft liner template, the drilled shaft operation can begin.

Each drilled shaft casing will be placed by a large crane on a barge and driven by a

vibratory hammer to just above the top of rock. All of the casings for each pier will

be placed

prior

to

any

drilling

of

the

shafts.

Subsequent

to

placing

the

pier

casings,

a drill rig and the appropriate support equipment such as a mud plant, drilling tools

and a scow will be mobilized to begin drilling the shafts and rock sockets.

Once the rock socket is complete, an airlift can be used for cleaning the shaft by

recycling the drilling fluid though the mud plant which separates the fluid from the

suspended solids and allows the clean fluid to pass back to the drilled shaft. The

placement of the reinforcing cage will follow the cleaning process and will require a





June 2010 9‐5

large crane due to the weight of the reinforcing which will likely be spliced in several

sections.

Tremies and CSL tubes will be placed prior to the concrete placement in each drilled

shaft. Due to the large quantity of concrete needed for each drilled shaft, a barge

mounted concrete batch plant may prove economically feasible.

Upon completion of the drilled shafts within the pier cofferdam, excavation for the

tremie seal can take place by using a water jet and an airlift coupled with a mud

plant and scow. Once excavated, concrete for the tremie seal will be placed up to an

elevation just short of the bottom of the foundation cap. It will be necessary to trim

the seal to remove laitance and humps from the tremie locations from protruding

into the foundation cap. When the trimming of the seal is complete, forming and

placement of the foundation cap and the bridge columns can then be constructed in

the dry.

9.8 Substructure Pier and Tower Construction

Access to the pier locations could be by barge as described above for drilled shaft

operations, by a series of floating pontoons/car barges or by trestle. Floating

pontoons would be quick to install and could be more easily reconfigured as work

progresses. Trestle construction would have the advantage of being off the water

and would not have to consider access and movements during winter months when

the river will be frozen. Either of these methods might be considered for perhaps

just one or two piers from either shore which would still leave the center spans open

for navigation. Whichever option is selected, it will be subject to USCG approval and

permitting.

The pier and tower construction will require delivery of rebar and concrete and will

require crane support. With the height of the towers exceeding 200 feet above the

water level, it may be efficient to utilize tower cranes supported on the permanent

pile caps. The tower cranes would also be able to service the towerhead assemblies,

stay anchor boxes, cable installation and tensioning.

The unique pier and tower cross section will pose some challenges in forming and

may limit the amount of reuse that each form can fulfill on a single column but

should permit reuse on other piers and towers.

It can be expected that the pier/tower construction will be made in approximately

ten lifts including three or four lifts for the towerhead incorporating the stay anchor

assemblies.

9.9 Reinforcing steel arrangements congestion and detailing

Since this is a Design‐Build project, it is to be expected that the DB Engineer of

Record’s (EoR) design drawings will be detailed such that rebar shop drawings are

not necessary. This would require that the detailer provide complete details and bar

bending diagrams for construction and to verify and resolve conflicts with stay





9‐6 June 2010

hardware, PT hardware and any other embeds required, such as ladders and

platforms or mechanical and electrical installations in the towers. The DB EoR should

also be responsible for identifying possible conflicts and for their resolution in the

development of the design drawings.

The design drawings should also identify any elements that are fracture critical such

as components

of

the

stay

anchor

assemblies

in

the

tower

heads

and

should

provide

clear requirements for all welds sizes, weld classification and NDE requirements.

9.10 Post -tensioning tendons and grouting

Complete details of the post‐tensioning system must be provided by the post‐

tensioning supplier. This should include records of all testing and material test

certificates that indicate compliance with the contract requirements. As stated in

the previous section, it is the responsibility of the EoR to ensure reinforcement

detailing is compatible with the post‐tensioning system and cable stay hardware and

that dimensional conflicts are resolved on the design drawings, not in the field.

Tensioning and grouting of the system should be performed by trained and

experienced personnel. American Segmental Bridge Institute certification for the

tensioning and grouting personnel is recommended. These operations also warrant

close inspection by QC personnel to ensure that the work is performed in

compliance with contract requirements to ensure that the life of the structure is not

compromised by lack of attention to detail. Consideration should be given to

inclusion of mock up tendon grouting to demonstrate adequacy of grouting

equipment and methods and for training of grouting crews.

9.11 Stay Installation

For an

extradosed

structure,

cable

tensioning

more

closely

resembles

post

‐

tensioning tendon stressing than the operations that are unique to cable stayed

bridge construction. The means and methods for the installation of the stay pipes

and for installation of the tendons do however need to be carefully planned and

carried out such that these critical elements are not damaged during construction.

9.12 Special architectural forming and finishing

It is noted that special architectural forming and finishing has been identified for this

structure. This relates to the form and shape of the piers, towers and the

superstructure. These are the result of the findings and recommendations contained

in the

Visual

Quality

Manual

(January

2007)

as

well

as

the

ongoing

concept

refinement process It should be recognized that there will be a premium cost

associated with the special architectural forming and finishing and the Contract

documents must be clear on what opportunity the DB Contractor may have to

propose alternate forms for any of these elements and the process for approval of

any such deviations.





June 2010 9‐7

9.13 Industry forum

An industry forum was held during the ASBI annual convention on Oct 25, 2009. At

the forum contractors were briefed on the conceptual details for the St Croix River

Crossing project and the schedule for procurement and for construction. This served

as a briefing to allow attending contractors to learn something about the Project. It

is recommended that a more formal forum be held prior to issuing the RFP to gather additional contractor input.





June 2010 10‐1

10 Maintenance and Inspection

10.1 Introduction

Maintenance and Inspection are very critical considerations for any bridge project

particularly so when the bridge may be outside of the typical bridge construction

practice as

is

the

case

for

the

St

Croix

River

Crossing

utilizing

extradosed

spans

that

are relatively uncommon in the US.

The extradosed river spans for the St. Croix River Crossing will require a specific

maintenance and inspection strategy. Access to critical components of the structure

for maintenance and inspection must be incorporated into the design development,

and specific maintenance and inspection procedures must be developed for the

long‐life expected for the structure.

10.2 Critical Elements

Operation, inspection, maintenance and access requirements for each of the

following items listed below must be addressed in the RFP documents. The

Contractor will be required to address each of those requirements during the final

design. Deliverables from the successful DB Contractor should also include an

Operation, Inspection and Maintenance Manual prepared specifically for the St Croix

River Crossing. The manual must include comprehensive details for each component

of the bridge including a list of components likely to require replacement within the

expected life of the structure.

Items that will require ongoing operation and maintenance include:

Anti‐icing system

Bridge Lighting

Navigation Lighting

Aviation Lighting

Lighting inside the bridge

Lighting inside the towers

Drainage

Signage

It is anticipated that these elements will be inspected at least once per year but may

require immediate attention should something cease to function at any time.

Life expectancy of these items is less than the structure life expectancy and will

likely require

full

replacement

a number

of

times

over

the

life

of

the

structure.

Items that will require inspection and periodic maintenance include:

Bearings

Expansion joints

Overlay

Painted surfaces




Concept Refinement Report It is recommended that these items be inspected every two years and in the event

that any unusual structure behavior or event has occurred.

It can be expected that these elements or components of them such as bearing

pads, will also have to be replaced a number of times over the expected life of the

structure.

Items that will require inspection and may require periodic maintenance include:

Cables and Protection Systems

Anchorage systems

Post‐tensioning systems

Concrete substructure elements

Concrete superstructure elements

Foundations including river piers below water

These are critical support elements of the structure and if constructed with care and

attention to detail should provide service for the life of the structure with little

maintenance. They

should

however

all

be

inspected

bi

‐annually

to

observe

any

changes in service and any indications of distress must be investigated and

addressed.

Access is a key aspect of any inspection program and it will be necessary to make

sure that equipment is available for regular inspection as indicated above. This will

be particularly the case for inspection of the exterior surfaces of the superstructure

elements of the river spans and the towers above deck. It will be important that

underbridge snoopers or manlifts that are currently in service will be able to reach

to those areas.

Substructure piers will require inspection from a manlift mounted on a barge.

For the Approach structures it will be important that permanent access be

constructed and maintained for future maintenance and inspection. This is

particularly important where there are wetlands adjacent to the bridge.