S6 Loadings

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

IntroductionRefresher: Limit StatesLoad CombinationsIntroduce Example Bridge Simplified Method of AnalysisTyp. Formatted Spreadsheet LayoutLoad Descriptions and Design ValuesConclusion

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-A3.5m


Formatted Spreadsheet

S


Simplified Method of Analysis

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

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 provincesLoad 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

coefficientsBridge 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

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