Shear Capacity of Composite Steel Girder at Simple Support Virtis/Opis User Group Conference Nashville, TN, August 3-4, 2010 George Huang, PhD, PE California

Shear Capacity of Shear Capacity of Composite Steel Composite Steel Girder at Girder at SimpleSimple

Support Support Virtis/Opis User Group Virtis/Opis User Group

ConferenceConference

Nashville, TN, August 3-4, 2010Nashville, TN, August 3-4, 2010

George Huang, PhD, PEGeorge Huang, PhD, PE

California Department of California Department of TransportationTransportation

OutlineOutline

BackgroundBackground AASHTO Specification ReviewAASHTO Specification Review Concrete Deck Lab Test by Concrete Deck Lab Test by

Shanmugam Shanmugam Bridge Field Test By AuBridge Field Test By Au Proposed Capacity Calculation Proposed Capacity Calculation

MethodMethod Proposed Virtis EnhancementProposed Virtis Enhancement

BackgroundBackground

Many composite steel bridges Many composite steel bridges designed before 70’s were re-rated designed before 70’s were re-rated with LFR. with LFR.

Some bridges have much smaller Some bridges have much smaller ratings due to shear deficiency at ratings due to shear deficiency at support. support.

Based on new rating results, permit Based on new rating results, permit vehicles would often not be allowed vehicles would often not be allowed on these bridges.on these bridges.

BackgroundBackground

Traffic histories show that permit Traffic histories show that permit vehicles have travelled on these vehicles have travelled on these bridges for over 40 years.bridges for over 40 years.

Bridge field inspections found there Bridge field inspections found there was no distress on the steel girder was no distress on the steel girder or concrete deck near support for or concrete deck near support for most bridges .most bridges .

What is the correct rating?What is the correct rating?

Bridge Example Bridge Example

Bridge Number: 50-0316Bridge Number: 50-0316 Bridge Name: Route 46/5 SeparationBridge Name: Route 46/5 Separation Year Built: 1967Year Built: 1967 Bridge width = 44’ ; depth = 4’-11”Bridge width = 44’ ; depth = 4’-11” Spans: 83’, 90’, 90’, and 83’Spans: 83’, 90’, 90’, and 83’ Super-Structure: Simple Span Super-Structure: Simple Span

Composite Weld Steel Plate Girders Composite Weld Steel Plate Girders (4) spacing@12’(4) spacing@12’

Design Live Load: HS 20-44 Design Live Load: HS 20-44

Br. No. 50-0316Br. No. 50-0316

Br. No. 50-0316Br. No. 50-0316

General PlanGeneral Plan

Typical Section & Girder Typical Section & Girder LayoutLayout

Girder Details at SupportGirder Details at Support

Changes in RatingsChanges in Ratings Working Stress Rating (1974)Working Stress Rating (1974)

HS 20 Inventory Rating Factor = 1.12 HS 20 Inventory Rating Factor = 1.12 P13 Operating Rating Factor = 1.27P13 Operating Rating Factor = 1.27

Control Case: Interior Girder, Moment at Control Case: Interior Girder, Moment at middle of span 2 (Shear was not rated)middle of span 2 (Shear was not rated)

Load Factor Rating (2010)Load Factor Rating (2010)HS 20 Inventory Rating = 0.75 HS 20 Inventory Rating = 0.75

P13 Operating Rating Factor = 0.62P13 Operating Rating Factor = 0.62Control Case: Interior Girder, Shear at Control Case: Interior Girder, Shear at supports of span 1supports of span 1

California Permit TrucksCalifornia Permit Trucks

Reason for Shear Reason for Shear DeficiencyDeficiency

Original Design Error?Original Design Error? Design Code Changes?Design Code Changes? If shear at simple support is ignored, If shear at simple support is ignored,

the inventory rating factor will be the inventory rating factor will be greater than 1.0greater than 1.0

General Structure Details General Structure Details Near SupportNear Support

Changes in Design Changes in Design SpecificationSpecification

Before AASHTO introduced LF for steel Before AASHTO introduced LF for steel structure in 1973, bridges were structure in 1973, bridges were designed with Working (Allowable) designed with Working (Allowable) Stress method.Stress method.

In 1973 AASHTO (11th Ed.) StandardIn 1973 AASHTO (11th Ed.) Standard Specifications, shear capacities at Specifications, shear capacities at interior and first panel locations were interior and first panel locations were the same for both WS and LF. The the same for both WS and LF. The equation is similar to the one used for equation is similar to the one used for an interior panel.an interior panel.


In 1977 AASHTO 12In 1977 AASHTO 12thth ed., a lower ed., a lower shear capacity equation was shear capacity equation was introduced at the first panel location introduced at the first panel location for WSD;for WSD;

In the 1978 AASHTO Interim In the 1978 AASHTO Interim Specifications, a lower shear Specifications, a lower shear capacity equation was introduced at capacity equation was introduced at the first panel location for LFD; the first panel location for LFD;


In the 1983 AASHTO 13In the 1983 AASHTO 13thth ed., ed., chapter layout becomes similar to chapter layout becomes similar to the current Standard Spec.the current Standard Spec.

In the1984-1986 Interim In the1984-1986 Interim Specification, the current shear Specification, the current shear capacity equation at the first panel capacity equation at the first panel was introduced for the Load Factor was introduced for the Load Factor method method

Shear Capacity Equations Shear Capacity Equations (LF)(LF)

Other than the first panel:Other than the first panel:

(10-(10-114)114)

At the first panel:At the first panel:

(10-(10-119)119)

Where :VWhere :Vpp= 0.58F= 0.58FyyDtDtww

C = (buckling shear stress)/(shear C = (buckling shear stress)/(shear yielding stress)yielding stress)

2)/(1

)1(87.0

Dd

CCVV

o

pu

pu CVV

Resistance Due to Post-Resistance Due to Post-BucklingBuckling

The second term in Eq. (10-114) is The second term in Eq. (10-114) is the additional shear capacity the additional shear capacity provided by post-buckling resistance provided by post-buckling resistance due to web tension-field action. This due to web tension-field action. This additional shear capacity is ignored additional shear capacity is ignored at the first panel location.at the first panel location.

Cause of Shear Cause of Shear DeficiencyDeficiency

The deficiency is due to the changes The deficiency is due to the changes in design specification for shear in design specification for shear capacity reduction at the first panel capacity reduction at the first panel in 1977 (WSD) and 1978 (LFD) .in 1977 (WSD) and 1978 (LFD) .

How to Solve the How to Solve the “Deficiency”“Deficiency”

Retrofit the StructureRetrofit the Structure

oror Modify the Shear Capacity Modify the Shear Capacity

Calculation Equation for Rating Calculation Equation for Rating AnalysisAnalysis

Modify Shear Capacity Modify Shear Capacity EquationEquation

Are the current shear capacity Are the current shear capacity calculation equations too calculation equations too conservative (for rating conservative (for rating analysis)?analysis)?

What’s the real shear capacity?What’s the real shear capacity?

Assumption for Current Assumption for Current EquationEquation Capacity of girder flange is ignored;Capacity of girder flange is ignored; Additional shear stiffener (extra Additional shear stiffener (extra

panel) is required to develop post-panel) is required to develop post-buckling tension field in web;buckling tension field in web;

Capacity of concrete deck is ignored. Capacity of concrete deck is ignored.

In Real ConditionIn Real Condition

Girder flanges do have stiffness, and Girder flanges do have stiffness, and the composite top flange is much the composite top flange is much stiffer.stiffer.

Even without extra panel, flange Even without extra panel, flange should provide some anchorage to should provide some anchorage to develop some tension effect in the develop some tension effect in the first panel.first panel.

Concrete deck does have some shear Concrete deck does have some shear capacity.capacity.

Deck Capacity from Lab Deck Capacity from Lab TestTest

Lab tests were conducted for Lab tests were conducted for composite plate girder. Testing composite plate girder. Testing results were published by results were published by Shanmugam and Baskar in ASCE Shanmugam and Baskar in ASCE Journal of Structural Engineering, Journal of Structural Engineering, Sept. 2003Sept. 2003

Concrete deck:Concrete deck:width = 1000 mm (39.4 in)width = 1000 mm (39.4 in)thickness = 150 mm (5.9 in)thickness = 150 mm (5.9 in)f’c = 400 MPa (5.8 Ksi)f’c = 400 MPa (5.8 Ksi)

Typical Test SpecimenTypical Test Specimen

Instruments LayoutInstruments Layout

Test of Steel GirderTest of Steel Girder

Test of Composite GirderTest of Composite Girder

Description of Test Description of Test GirdersGirders

Spg1 and 2 are Spg1 and 2 are steel girders only.steel girders only.

cpg1, 2, 3, 4 are cpg1, 2, 3, 4 are composite steel composite steel girders with girders with reinforced reinforced concrete decks.concrete decks.

cpg3 and 4 have cpg3 and 4 have additional shear additional shear bars in the deck.bars in the deck.

Test Loads: Steel VS Test Loads: Steel VS Composite Composite (d/t = 250)(d/t = 250)

Test Loads: Steel VS Test Loads: Steel VS Composite Composite (d/t = 150)(d/t = 150)

Summary of Lab Test Summary of Lab Test

The paper concluded the concrete The paper concluded the concrete deck did provide additional shear deck did provide additional shear capacity;capacity;

Without shear bars, concrete deck Without shear bars, concrete deck had a sudden failure mode;had a sudden failure mode;

With shear bars, concrete deck had With shear bars, concrete deck had a ductile failure mode.a ductile failure mode.

Discussion of Lab TestDiscussion of Lab Test

The difference between the The difference between the maximum elastic shear capacities of maximum elastic shear capacities of cpag1 and spag1 is the same as the cpag1 and spag1 is the same as the difference between spg2 and cpg2 difference between spg2 and cpg2 (about 200 KN). This may due to the (about 200 KN). This may due to the same concrete deck dimensions used same concrete deck dimensions used for both cpag1 and cpag2.for both cpag1 and cpag2.

Discussion of Lab TestDiscussion of Lab Test

In the load-deflection plot for d/t = In the load-deflection plot for d/t = 250, the initial elastic stiffness for 250, the initial elastic stiffness for cpag1and spag1 are about the same. cpag1and spag1 are about the same. This may imply that the concrete This may imply that the concrete deck is not effective until the steel deck is not effective until the steel girder behaviors nonlinearly (or girder behaviors nonlinearly (or steel web starts to yield and buckle).steel web starts to yield and buckle).

Bridge Field Testing Bridge Field Testing

Au, Lam and Tharmabala (the Bridge Au, Lam and Tharmabala (the Bridge Office of the Ministry of Office of the Ministry of Transportation of Ontario) published Transportation of Ontario) published “Investigation of shear resistance of “Investigation of shear resistance of steel bridge girders by load testing steel bridge girders by load testing and monitoring of load response and monitoring of load response data under highway traffic data under highway traffic conditions” in Canadian Journal of conditions” in Canadian Journal of Civil Engineering, 2009. Civil Engineering, 2009.

Reason for the TestingReason for the Testing

During rehabilitation, a strength During rehabilitation, a strength evaluation revealed a significant evaluation revealed a significant deficiency in the shear resistance deficiency in the shear resistance of existing girders at support of existing girders at support locations.locations.

Bridge girders showed Bridge girders showed no signs no signs of distressof distress

Scope of Testing Scope of Testing ProgramProgram

Monitor real stresses in end panels of Monitor real stresses in end panels of two selected girders when subjected to two selected girders when subjected to ((ii) a test truck with known axle loads ) a test truck with known axle loads and and ((iiii) normal highway traffic loading) normal highway traffic loading

Calibrate observed stresses against Calibrate observed stresses against theoretically expected responses in theoretically expected responses in girdersgirders

Calculate the live load capacity factor Calculate the live load capacity factor using shear data derived from field using shear data derived from field measurementsmeasurements

Traffic Lane Layout – Traffic Lane Layout – Span KSpan K

Transverse Section –Span Transverse Section –Span KK

Bridge Instrumentation Bridge Instrumentation detailsdetails

Bridge Instrumentation Bridge Instrumentation detailsdetails

Testing Truck and Testing Truck and LocationLocation

Canadian Highway Bridge Canadian Highway Bridge Design Code (CHBDC)Design Code (CHBDC)

Based on CHBDC Based on CHBDC

Total shear capacity = 1.01x1071Total shear capacity = 1.01x1071 = 1081.71 KN= 1081.71 KN

Available live load shear capacityAvailable live load shear capacity=1.01x1071 – 988=1.01x1071 – 988=93.71 KN or 94 KN=93.71 KN or 94 KN

Un-factored dead load shear can be Un-factored dead load shear can be calculated as 861 KNcalculated as 861 KN

Field MeasurementField Measurement

Prorated from the measurement, the Prorated from the measurement, the factored live load is estimated at 437 factored live load is estimated at 437 KN;KN;

Based on the maximum vertical Based on the maximum vertical shear strain measured under normal shear strain measured under normal traffic, the largest shear force under traffic, the largest shear force under live load is estimated at 606 KN live load is estimated at 606 KN

Summary of Live Load Summary of Live Load CapacityCapacity

Conclusion of the PaperConclusion of the Paper

The actual steel girder shear capacity The actual steel girder shear capacity at simpleat simple support is larger than that support is larger than that calculated by design code (CHBDC).calculated by design code (CHBDC).

The actual live load in the steel girder The actual live load in the steel girder is smaller than that calculated by is smaller than that calculated by design code.design code.

The bridge has enough shear capacity The bridge has enough shear capacity (F=1.39) to carry the design live loads.(F=1.39) to carry the design live loads.

Total Steel Shear Total Steel Shear CapacityCapacity

Paper suggested the total shear capacity Paper suggested the total shear capacity of the steel girder was 1594 KN, which of the steel girder was 1594 KN, which was the sum of measured live load force was the sum of measured live load force (606 KN) and the FACTORED shear (606 KN) and the FACTORED shear force (988 KN) due to existing dead force (988 KN) due to existing dead load;load;

And in order to reach this 1594 KN And in order to reach this 1594 KN based on the CHBDC, 38% of post-based on the CHBDC, 38% of post-buckling shear component had to be buckling shear component had to be included.included.

Discussion of Total Shear Discussion of Total Shear CapacityCapacity

Deck could carry some loads. Deck could carry some loads. However, since there was no However, since there was no distress, it might assumed that most distress, it might assumed that most dead load was taken by steel girder;dead load was taken by steel girder;

Only the non-factored dead load Only the non-factored dead load (861 KN) should be included;(861 KN) should be included;

The total least shear capacity might The total least shear capacity might be 1467 KN (not 1594 KN).be 1467 KN (not 1594 KN).

AASHTO VS CHBDC AASHTO VS CHBDC

At first panel:At first panel: AASHTO: Vu = 1135 KN (255.2 Kips)AASHTO: Vu = 1135 KN (255.2 Kips) CHBDC: Vu = 1.01x1071 KN CHBDC: Vu = 1.01x1071 KN =1082 KN=1082 KN AASHTO/CHBDC = 1.05AASHTO/CHBDC = 1.05

At interior panel: At interior panel: AASHTO: Vu = 2559 KN (575.3 Kips)AASHTO: Vu = 2559 KN (575.3 Kips) CHBDC: Vu = 1.01x2436 KN = CHBDC: Vu = 1.01x2436 KN = 2460 KN2460 KN AASHTO/CHBDC = 1.04 AASHTO/CHBDC = 1.04

AASHTO VS CHBDC AASHTO VS CHBDC

Equivalent dead load factor Equivalent dead load factor

CHBDC = 1.15, AASHTO = 1.3CHBDC = 1.15, AASHTO = 1.3 Live load factorLive load factor

CHBDC = 1.42, AASHTO = 1.3CHBDC = 1.42, AASHTO = 1.3 Rating factor for live loadsRating factor for live loads

CHBDC = 0.10, AASHTO = CHBDC = 0.10, AASHTO = 0.020.02

Need New Approach to Need New Approach to Calculate Shear Capacity Calculate Shear Capacity

Based on lab testing, field Based on lab testing, field testing results, and bridge testing results, and bridge ratings and field inspections of ratings and field inspections of several bridges in California, several bridges in California, there is a need for a new there is a need for a new approach.approach.

Proposed New Shear Proposed New Shear Capacity Eq. for Composite Capacity Eq. for Composite

Plate GirderPlate Girder Total shear capacity includes both Total shear capacity includes both

steel and concrete decksteel and concrete deck

where where

m and n are two proposed new m and n are two proposed new parametersparameters

ddccu

p

o

psu

tbfnV

VDd

CmCVV

',

2,)/(1

)1(87.0

cusuu VVV ,,

Concrete Capacity Concrete Capacity CalibrationCalibration

Based on information from Based on information from Shanmugam’s paper:Shanmugam’s paper:

bbc c =1000 mm(39.37 in), t=1000 mm(39.37 in), tc c =150 mm (5.9 =150 mm (5.9 in)in)

f’f’c c = 40 MPa (5801 psi),= 40 MPa (5801 psi),

estimated Vestimated Vc c = 200 KN (44961 lbs)= 200 KN (44961 lbs)

then then n = 2.54 n = 2.54

to be conservative, useto be conservative, use

phi = 0.85 with n = 2phi = 0.85 with n = 2

Steel Capacity Steel Capacity CalibrationCalibration

Based on information from Au’s paper:Based on information from Au’s paper:Web depth D = 2438 mm (96”)Web depth D = 2438 mm (96”)Web thickness tWeb thickness tw w = 9.53 mm (3/8”)= 9.53 mm (3/8”)Trans. stiffener spacing dTrans. stiffener spacing d0 0 = = 1534mm(60”)1534mm(60”)Fy = 230 MPa (33 ksi)Fy = 230 MPa (33 ksi)ThenThen

C = 0.37 C = 0.37 Vp = 0.58FyDtVp = 0.58FyDtw w = 689 Kips= 689 Kips


Ignoring the deck and using the Ignoring the deck and using the estimated least shear capacity of estimated least shear capacity of 1467 KN (331.8 kips). Based on1467 KN (331.8 kips). Based on

then m = 0.24then m = 0.24 Since the girder was still in elastic, Since the girder was still in elastic,

the actual m should be larger than the actual m should be larger than 0.24. 0.24.

m = 0.25 may be used.m = 0.25 may be used.

p

o

psu VDd

CmCVV

2,)/(1

)1(87.0


Please note:Please note: The higher measured steel capacity The higher measured steel capacity

may be due to the equation used to may be due to the equation used to calculate buckling shear stress being calculate buckling shear stress being too conservative;too conservative;

The actual shear force in the steel The actual shear force in the steel girder could be smaller than girder could be smaller than 1467KN, but the actual steel girder 1467KN, but the actual steel girder capacity could be larger;capacity could be larger;

Rate Br. 50 -316 with Rate Br. 50 -316 with Proposed MethodProposed Method

Girder dimension:Girder dimension: Top flange: 5/8” x 12”Top flange: 5/8” x 12” Web:Web: 5/16” x 45” 5/16” x 45” Bot. flange: 7/8” x 20”Bot. flange: 7/8” x 20” Spacing of shear stiffener: 34.7”Spacing of shear stiffener: 34.7” calculated: C = 0.804 , Vp = 293.6 calculated: C = 0.804 , Vp = 293.6

KipsKips CVp = 236.1 KipsCVp = 236.1 Kips

with m=.25 Vu,s = 246.0 Kipswith m=.25 Vu,s = 246.0 Kips


Minimum deck thickness: 8.25”Minimum deck thickness: 8.25” Effective deck width: 99”Effective deck width: 99” f’c = 3250 psif’c = 3250 psi

with phi = 0.85 and n=2with phi = 0.85 and n=2

Vu,c = 79.1 KipsVu,c = 79.1 Kips

Total shear capacityTotal shear capacity

Vu = 246+79 = 325 KipsVu = 246+79 = 325 Kips


Inventory Rating for HS20Inventory Rating for HS20

Virtis: RF = 0.75Virtis: RF = 0.75

proposed: RF = 1.22proposed: RF = 1.22

Operating Rating for Permit P13Operating Rating for Permit P13

Virtis: RF = 0.62Virtis: RF = 0.62

proposed: RF = 1.01proposed: RF = 1.01

Proposed Virtis Proposed Virtis EnhancementEnhancement

If Virtis has the option for user to define If Virtis has the option for user to define capacities at any point, user may use capacities at any point, user may use proposed method to calculate composite proposed method to calculate composite steel plate girder shear capacity near steel plate girder shear capacity near support and to replace the shear capacity support and to replace the shear capacity based on the AASHTO LFD Specification. based on the AASHTO LFD Specification.

This option may be used for locations, This option may be used for locations, where capacity has to be manually where capacity has to be manually calculated, such as hinge, splices, or calculated, such as hinge, splices, or structure damage.structure damage.

Questions?Questions?

Documents

Shear Capacity of Composite Steel Girder at Simple Support Virtis/Opis User Group Conference Nashville, TN, August 3-4, 2010 George Huang, PhD, PE California