april 5, 2006chbdc-s6 bridge loading1 loading summary for a slab on girder bridge according to the...
TRANSCRIPT
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
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
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-A3.5m
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 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
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
coefficientsBridge 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?