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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
April 5, 2006 CHBDC-S6 Bridge Loading 2
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
April 5, 2006 CHBDC-S6 Bridge Loading 3
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.
April 5, 2006 CHBDC-S6 Bridge Loading 4
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.
April 5, 2006 CHBDC-S6 Bridge Loading 5
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)
April 5, 2006 CHBDC-S6 Bridge Loading 6
Load Types: Superstructure Only
Dead Loads (D)
Live Loads (L)
Wind Load on Structure (W)
Wind on Traffic (V)
Earthquake Loads (EQ)
April 5, 2006 CHBDC-S6 Bridge Loading 7
Load Combinations
Load Factors based on a service
life of 75 yrs
Based on minimum reliability
index of 3.75
April 5, 2006 CHBDC-S6 Bridge Loading 8
Load Combinations
April 5, 2006 CHBDC-S6 Bridge Loading 9
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
April 5, 2006 CHBDC-S6 Bridge Loading 10
Formatted Spreadsheet
S
April 5, 2006 CHBDC-S6 Bridge Loading 11
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
April 5, 2006 CHBDC-S6 Bridge Loading 12
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
April 5, 2006 CHBDC-S6 Bridge Loading 13
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
April 5, 2006 CHBDC-S6 Bridge Loading 14
Formatted Spreadsheet
S
April 5, 2006 CHBDC-S6 Bridge Loading 15
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
April 5, 2006 CHBDC-S6 Bridge Loading 16
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
April 5, 2006 CHBDC-S6 Bridge Loading 17
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
April 5, 2006 CHBDC-S6 Bridge Loading 18
Formatted Spreadsheet
S
April 5, 2006 CHBDC-S6 Bridge Loading 19
Formatted Spreadsheet
April 5, 2006 CHBDC-S6 Bridge Loading 20
Formatted Spreadsheet
April 5, 2006 CHBDC-S6 Bridge Loading 21
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.
April 5, 2006 CHBDC-S6 Bridge Loading 22
Cl.-3.10 Wind Loads
April 5, 2006 CHBDC-S6 Bridge Loading 23
Exceptional Loads
Low Frequency/Probability of
Occurrence
Earthquake
Collision
Stream and Ice Pressure/Debris
Ice Accretion
April 5, 2006 CHBDC-S6 Bridge Loading 24
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
April 5, 2006 CHBDC-S6 Bridge Loading 25
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
April 5, 2006 CHBDC-S6 Bridge Loading 26
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.
April 5, 2006 CHBDC-S6 Bridge Loading 27
Conclusions
QUESTIONS?