s6 loadings-chbdc

April 5, 2006 CHBDC-S6 Bridge Loading 1

Loading Summary for a Slab on Girder

Bridge According to the CAN/CSA-S6

Presented By: Andrew Chad

2006


Outline

Introduction

Refresher: Limit States

Load Combinations

Introduce Example Bridge

Simplified Method of Analysis

Typ. Formatted Spreadsheet Layout

Load Descriptions and Design Values

Conclusion

Basically: A comprehensive load summary, takedown and analysis procedure for a new highway bridge according to CAN/CSA-S6


Limit States

S6 Limit States Criteria:

Ultimate Limit States (ULS)

Fatigue Limit States (FLS)

Serviceability Limit States (SLS)

The chief advantages of LS Design

Method are:

The recognition of the different

variabilities of the various loads, for

the Working Stress Method

(AASHTO) encompassed both in the

same factor of safety;

The recognition of a range of limit

states

The promise of uniformity by the use

of statistical methods to relate all to

the probability of failure.


Limit States

Disadvantages:

Necessity to choose an acceptable

risk of failure; for example, to

quantify the acceptability of some

risk that involves only structural

collapse, with a risk that leads to

loss of life.

The probability of failure must be

applied to the number of events

that may occur during the life of the

structure. There is an essential

difficulty in predicting an event that

may not occur until 75-100 years

from the point of design.


Bridge Load Types

Dead Loads (D)

Earth & Hydrostatic Pressure (E)

Secondary Prestress (P)

Live Loads (L)

Strains, Deformations and Displacement Associated Loads (K)

Wind Load on Structure (W)

Wind on Traffic (V)

Load due to Differential Settlement (S)

Earthquake Loads (EQ)

Stream and Ice Pressure, Debris Torrents (F)

Ice Accretion Load (A)

Collision Load (H)


Load Types: Superstructure Only

Dead Loads (D)

Live Loads (L)

Wind Load on Structure (W)

Wind on Traffic (V)

Earthquake Loads (EQ)


Load Combinations

Load Factors based on a service

life of 75 yrs

Based on minimum reliability

index of 3.75


Load Combinations


Design Example

A “Simple” Bridge:

2 span, 4 lane bridge

225mm R/C Slab, on 5 continuous

steel girders

Span length 20m x 2

Typical highway overpass structure

Superstructure only!

A-A

A-A

3.5m


Formatted Spreadsheet

S



Simplified Method of Analysis:

The bridge width is constant

The support conditions are closely equivalent to line support, both at the ends of the bridge and, in the case of multispan bridges, at intermediate supports

For slab and slab on girder bridges with skew, the provisions of A5.1(b)(i) are met

For bridges that are curved in plan, the radius of curvature, span, and width satisfy the relative requirements of A5.1(b)(ii)

A solid or voided slab is of substantial uniform depth across a transverse section, or tapered in the vicinity of a free edge provided that the length of the taper in the transverse direction does not exceed 2.5m



Simplified Method of Analysis:

For slab-on-girder bridges, there shall be at least three longitudinal girders that are of equal flexural rigidity and equally spaced, or with variations from the mean of not more than 10% in each case

For a bridge having longitudinal girders and an overhanging deck slab, the overhang does not exceed 60% of the mean spacing betweeen the longitudinal girders or the spacing of the two outermost adjacent webs for box girders, and, also, is not more than 1.8m

For a continuous span bridge, the provisions of A5.1(a) shall apply

In the case of multispine bridges, each spin has only two webs. Also, the conditions of Cl. 10.12.5.1 shall apply for steel and steel-composite multispine bridges.

CON’T


Dead Load

225mm

If bridge satisfies Cl.5.6.1.1 use “Simplified Method of Analysis”

The Beam Analogy Method: “it is permitted to the whole of the

bridge superstructure, or of part of the bridge superstructure contained between two parallel vertical planes running in the longitudinal direction, as a beam”

Take 3 interior girders & associated T.W., 9” R/C Concrete Typ.

Take 2 exterior girders & associated T.W., 9” R/C Concrete Typ.

Takes less Dead load, more live load due to deck support conditions

α Varies with different materials 1.5 for wearing surfaces

1.1 for steel girders



S


Live Load

Originally used Live Loads specified in AASHTO, changed in 1979 to maximum legal limits observed loads in all provinces.

Ontario uses maximum observed loads (MOL) vs. Canadian Legal Limits in other provinces

Load based on CL-W Loading CL-W Truck as specified in Cl. 3.8.3.1

Not less than CL-625 (kN) for national highway network.

Weight to 625kN in 2000, LL factor increased to 1.7 max

CL-W Lane Load as specified in CL. 3.8.3.2 9kN/m based on work done by Taylor at

Second Narrows Bridge

80% Truck load included in analysis

Dynamic Load Allowance Factors to account for more concentrated loading Vary with amount of truck being used, size

of bridge feature


Live Load

Load Cases:

3 Load Cases ULS

Worst case of truck load, lane

load including DLA

Pedestrian loads, maintenance

+ sidewalk loads omitted

2 Load Cases SLS

1 Load Case FLS

2 lines of wheel loads in 1 lane

Multi-lane loading modification factor

When >1 lane is loaded, reduce

loads per Table 3.8.4.2

1 lane = 1.0

2 lane = 0.9

3 lane = 0.8


Live Load: Analysis

Longitudinal Moment Mg = Fm * Mgavg

Where: Fm =Amplification Factor to account

for tranverse variation in max moment intensity

Mgavg = Average moment per girder by sharing equally the total moment, including multiple lane load factor

Longitudinal Moment FLS: Loaded with 1 truck at center of 1

lane

Mg = Fm * Mgavg

Where: Fm =Amplification Factor to account

for tranverse variation in max moment intensity

Mgavg = Average moment per girder by sharing equally the total moment

Shear is Found in Similar Manner



S


Cl.-3.10 Wind Loads

“Superstructure shall be designed for wind induced vertical and horizontal drag loads acting simultaneously”

Fh=qCeCgCh

Fv=qCeCgCv

Where: q = reference wind pressure

1/50 for L<125m

Ce = Exposure Factor (.1H)2

Cg = Gust Effect Coefficient 2.0 for L < 125m, 2.5 for more slender

bridges/structures

Ch,Cv = Horizontal, Vertical drag coefficients

Bridge type not typically sensitive to wind Not: Flexible, Slender, Lightweight, Long

Span, or of Unusual Geometry.


Cl.-3.10 Wind Loads


Exceptional Loads

Low Frequency/Probability of

Occurrence

Earthquake

Collision

Stream and Ice Pressure/Debris

Ice Accretion


Earthquake Loads

For a “Lifeline”, Slab on Girder,

L<125m, located in Seismic Zone 4:

Minimum Analysis = Multi Mode

Spectral (MM) Analysis

No analysis necessary for SOG

single span bridges

Not performed due to scope

Same principles as a multi-degree of

freedom structure would apply

Structure analyzed in 2 principal

directions

Find principal modes, modal

mass, modal participation,

combine to 90% mass

participation (SRSS, CQC)

Vertical motions taken by including

dead load factor in ULS

CAN/CSA-S6 Section 4

Prescribes Analysis based on:

Bridge Geometry

Type

Location

Importance

Regular vs. Irregular


Collision Loads

Superstructures to be design for

“Vessel Collision”

Substructure to be designed for

vehicle collision load, Vessel

Collision

Not to be included in

spreadsheet, see S6-3.14


Conclusions

C.H.B.D.C. based on O.H.B.D.C.

which was revolutionary in its use of

LSD and design vehicle based on

legal limits

C.H.B.D.C. complicated but well

written code

Many loads were omitted for this

“simple” bridge, only a basic

design/analysis was performed

Easy to get confused, make “small”

mistakes

Simplified methods of design are a

good start, although still somewhat

tricky.


Conclusions

QUESTIONS?

s6 loadings-chbdc

Documents

chbdcs6 bridge loading

bridge width

simple bridge

lane bridge

wind load

limit states load combinations

load combinations load

new highway bridge