bridge alternate type study - stream crossing (09-04-12)v2

Structure Type Studies for

Local Infrastructure Managers

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Structure Type Study Reference Paper

Purpose of the Paper

To give local infrastructure managers a guide and

reference to use when scoping and evaluating a site

for a bridge project.

Ensures all costs and factors are considered,

including life cycle costs.

Real world sites and examples.

First of a 3 Part Series. Eventually 2 more sites will

be investigated.

Engineers Club of Memphis - 8.22.2011

Why was THIS bridge built here?

Existing Site, Bridge & Conditions

Situated in a low-land area

Rural character (narrow 10 ft. lanes, no shoulders)

Low Volume ~ 350 vehicles per day (vpd)

Loosely consolidated sandy-silt, silt and clay

Bedrock not encountered in the soil borings

Existing 3-span beam bridge (simple spans)

Steel floor with asphalt fill and wearing surface

Frame-bent piers from steel caps on piles

Built as a temporary structure 30 years prior

Bridge in poor condition and closed

Hydraulic Conditions


Channel was wooded & unmaintained

Woody debris on the piers a maintenance problem

Flooding of approach roadways

Flood level at, or nearly at low-steel elevation

Channel was constricting, as velocities through

channel were 5x greater than up- or downstream

Existing bridge

Surrounding low land

Utilities

Rural roadway & character

Farm drive

DETOURED! Subsurface Conditions

Entrenched streamWooded area

Woody debris

What goes into Scoping a project

What’s important to consider?

Funding strings? or no funding strings?

What are the goals of the project?

How easy is it going to be to build?

Maintenance - What am I going to be stuck

with?

How long will this bridge be here?

What else is important?

Goals for the Project

Provide adequate bridge width for facility

Provide adequate span to allow hydraulic clearance

Minimized impacts to profile grade

Minimized in-stream work

Minimize impacted R/W & construction footprints

Minimize detour (closure duration)

Minimize adjacent road damage

Minimize construction risk

Minimize construction costs

Scope of Proposed Work

Projected Traffic: 500 vpd.

Bridge Width: 11ft lanes plus 4 ft shoulders = 30 ft rail/rail

Approach slabs, guardrail, embankment & pavement

Design Year Flood: 10-year, Low steel = EL 1028.0 ft

Ordinary High Water (OHW) = EL 1024.0 ft.

Increase hyd. opening using 2:1 slopes & spill-thru

abutments

Deep foundations using friction resistance CIP/steel pipe

piles

Over-the-side drainage with splash guards

120 ft. (+/- ) span range

New rock channel protection along end slopes & aprons

Taper improvements into existing

What’s left?

Alternates of span configurations?

Alternates of structure types?

Remaining Variables

Structure Depth

Profile Grade

Impacted Footprint

10-year EL = 1028.0

100-year EL = 1029.6

Exist Span = 90 ft (+/-)

Proposed Span ~ 120 ft

STRUCTURE DEPTH

OHW EL = 1024.0

Normal EL = 1020.0

Case Study No. 1

Structure Type Study Narrative (21

Pages)

Alternate Descriptions

Life Cycle Cost Analysis

Evaluation Matrix

Evaluation of Alternatives

Evaluation of NPV of Alternatives

Conclusion & Summary

6 Appendices with drawings, cost

estimates, etc.

Alternatives

Alt. 1 – 3-Span, Cast-in-Place Slab

Alt. 2 – Single Span, P/S AASHTO Girders

Alt. 3 – Single Span, Steel Low (Pony) Truss

Alt. 4 – 3-Span, Composite P/S Concrete Box

Beams

Basis for Comparisons

Initial Construction

Engineering costs (15% of construction)

Roadway pay items (Appendices A & B)

Bridge pay items (Appendices A & B)

Right-of-way costs

Basis for Comparisons

Life Cycle Costs

Initial cost to construct

Annual maintenance and inspections

Annual work activities

Minor rehabilitation projects (20 years)

Major rehabilitation projects (50 years)

Residual Values

Net Present Value (NPV)

Annual Work Activities

Debris Removal

Bridge Inspection

10 Year Work Activities

Silane Sealer

Light Patching

50 Year Work Activities

Deck slab replacement

Deck edge replacement

Superstructure replacement

or reconditioning

… and also qualitative measures

Temporary construction

Environmental permitting

Ease of delivery

Construction equipment

Life Cycle Costs: Net Present Values

Cost basis = 2002 State Bid Tabulations

Design Life Term = 75 years

Interval timeline = 10 years; 20 years; 50 years

Inflation (2002 – 2010) = 135%

Discount rate = 2.7% per year n

NPV = Σ RCFt / (1+i)t

t=0

where:

RCFt

i

n

=

=

=

Real Cash Flow

Annual Discount Rate

term

to calculate NPV:

#1 3-Span Concrete Slab Bridge

36 ft - 45 ft - 36 ft spans c/c of bearings

Bridge Limits = 118.3 ft

Project Limits = 450 ft

Structure depth = 2.07 ft (Slab thickness = 22 in.)

Profile grade impacts: similar to existing

Engineering = $113,000

Construction = $753,000

Maintenance: 7 projects in 75 yrs @ $1,332,800

Life Cycle Costs (NPV) = $1,124,115

#1 3-Span Concrete Slab Bridge

#1 – Three Span Concrete Slab Bridge


#2 Single Span Prestressed I-Beam Bridge


114 ft span c/c of bearings


Project Limits = 1,350 ft

Structure depth = 7.07 ft (AASHTO Type IV = 72 in.)

Profile grade impacts: +3.3 ft





#3 Single Span Steel Half-Through Truss

116 ft span c/c of bearings



Structure depth = 3.56 ft (W30 FB + 10 ½” Slab)






#3 Single Span Steel Half-Through Truss

#4 3-Span Composite Box Beam Bridge

36 ft - 45 ft - 36 ft spans c/c of bearings



Structure depth = 2.16 ft (CB17 + 6” Topping)






#4 3-Span Composite Box Beam Bridge

Comparison MatrixComparisons Alternate 1

(3-Span

CIP Slab)

Alternate 2

(1-Span

P/S Girder)

Alternate 3

(1-Span Steel Half

Truss, DOT)

Alternate 4

(3-Span Comp.

Box Beam)

Construction

Cost

(+/- % Min.)

$876,000(3.2%)

$1,143,000(34.7%)

$949,400(11.9%)

$848,400(0.0%)

Life Cycle Cost

(+/- % Min.)$1,124,000

(0.0%)

$1,432,000(27.4%)

$1,212,000(7.8%)

$1,189,000(5.8%)

Total Cost/

SF of Bridge$247/sf $325 /sf $264 /sf $239 /sf

Spans:

Bridge Limits:

Work Limits:

3 spans

118.32 ft

450 ft

1 span

117.16 ft

1,350 ft

1 span

120.0 ft

450 ft

3 spans

118.50 ft

450 ft

Embankment

Rise0.07 ft 3.35 ft -0.09 ft 0.07 ft

Comparison Matrix, cont’dComparisons Alternate 1

(3-Span

CIP Slab)

Alternate 2

(1-Span

P/S Girder)

Alternate 3

(1-Span Steel Half

Truss, DOT)

Alternate 4

(3-Span Box

Beam)

Hydraulic

Clearance

(10 yr flood)

1.9 ft 0.3 ft 0.3 ft 1.8 ft

Substructure

Units4 2 2 4

Deep

Foundations

4,680 ft

Driven

5,760 ft

Driven

3,960 ft

Driven

4,770 ft

Driven

Environmental

Impacts

ACOE 404 Permit

for pier const.

ACOE 404 temp.

work areaNo work in stream

ACOE 404 Permit

for pier const.

R/W

Impacts

Strip take on 1 side

(ditch)

Strip take on 2

sides (embank &

ditch)


(ditch)


(ditch)

Comparison Matrix, cont’dConstruction

Comparisons

Alternate 1

(3-Span

CIP Slab)

Alternate 2

(1-Span

P/S Girder)

Alternate 3

(1-Span Steel Half

Truss, DOT)

Alternate 4

(3-Span Box

Beam)

Temporary

Works or Areas

Causeway

& form shoring

Work area for lifting

beams

(60T ea.)

noneCauseway to

construct piers

Flood Risk

during Const.High Medium Low Medium

Utilities n/a n/a n/a n/a

Equipment

NeededConcrete pump

500T Crane;

Concrete pump

250T Crane;

Concrete pump

250T Crane;

Concrete pump

Other

Factors

In-stream piers are

susceptible to

debris

Delivery of long

beams; road

damage

Smaller crane lifts;

crane lifts from

roadway

In-stream piers are

susceptible to

debris; smaller

crane lifts

Conclusions

Based on the owner preference for a clear-span

bridge, the lowest cost one-span bridge is the steel

low-truss bridge. This is, in fact, what the owner

selected, and the structure was constructed in 2002.

The premium to do the selected alternative over

least cost alternative is $101,000 (initial costs) which

is reduced over time to a Net Present Value premium

of only $23,000 (75 years).

bridge alternate type study - stream crossing (09-04-12)v2

Documents

bridge site

narrow bridge

bridge project

bridge conditionssituated

low steel

existing site

ft lanes

site photo