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 Department of Transportation
Outline
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Background
AASHTO Specification Review
Concrete Deck Lab Test by Shanmugam
Bridge Field Test By Au
Proposed Capacity Calculation Method
Proposed Virtis Enhancement
Background
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Many composite steel bridges designed before
70’s were re-rated with LFR.
Some bridges have much smaller ratings due to
shear deficiency at support.
Based on new rating results, permit vehicles
would often not be allowed on these bridges.
Background
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Traffic histories show that permit vehicles have
travelled on these bridges for over 40 years.
Bridge field inspections found there was no
distress on the steel girder or concrete deck
near support for most bridges .
What is the correct rating?
Bridge Example
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Bridge Number: 50-0316
Bridge Name: Route 46/5 Separation
Year Built: 1967
Bridge width = 44’ ; depth = 4’-11”
Spans: 83’, 90’, 90’, and 83’
Super-Structure: Simple Span Composite Weld
Steel Plate Girders (4) spacing@12’
Design Live Load: HS 20-44
Br. No. 50-0316
Br. No. 50-0316
General Plan
Typical Section & Girder Layout
Girder Details at Support
Changes in Ratings
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Working Stress Rating (1974)
HS 20 Inventory Rating Factor = 1.12
P13 Operating Rating Factor = 1.27
Control Case: Interior Girder, Moment at middle of
span 2 (Shear was not rated)
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Load Factor Rating (2010)
HS 20 Inventory Rating = 0.75
P13 Operating Rating Factor = 0.62
Control Case: Interior Girder, Shear at supports of
span 1
California Permit Trucks
Reason for Shear Deficiency
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Original Design Error?
Design Code Changes?
If shear at simple support is ignored, the
inventory rating factor will be greater than 1.0
General Structure Details Near
Support
Changes in Design Specification
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Before AASHTO introduced LF for steel
structure in 1973, bridges were designed with
Working (Allowable) Stress method.
In 1973 AASHTO (11th Ed.) Standard
Specifications, shear capacities at interior and
first panel locations were the same for both WS
and LF. The equation is similar to the one used
for an interior panel.
Changes in Design Specification
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In 1977 AASHTO 12th ed., a lower shear
capacity equation was introduced at the first
panel location for WSD;
In the 1978 AASHTO Interim Specifications, a
lower shear capacity equation was introduced at
the first panel location for LFD;
Changes in Design Specification
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In the 1983 AASHTO 13th ed., chapter layout
becomes similar to the current Standard Spec.
In the1984-1986 Interim Specification, the
current shear capacity equation at the first panel
was introduced for the Load Factor method
Shear Capacity Equations (LF)
Other than the first panel:

Vu  V p C 

0 . 87 (1  C ) 

2
1  ( d o / D ) 
(10-114)
At the first panel:
V u  CV p
(10-119)
Where :Vp= 0.58FyDtw
C = (buckling shear stress)/(shear yielding stress)
Resistance Due to Post-Buckling
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The second term in Eq. (10-114) is the
additional shear capacity provided by postbuckling resistance due to web tension-field
action. This additional shear capacity is ignored
at the first panel location.
Cause of Shear Deficiency
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The deficiency is due to the changes in design
specification for shear capacity reduction at the
first panel in 1977 (WSD) and 1978 (LFD) .
How to Solve the “Deficiency”
Retrofit the Structure
or
 Modify the Shear Capacity Calculation
Equation for Rating Analysis
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Modify Shear Capacity Equation
Are the current shear capacity calculation
equations too conservative (for rating
analysis)?
 What’s the real shear capacity?
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Assumption for Current Equation
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Capacity of girder flange is ignored;
Additional shear stiffener (extra panel) is
required to develop post-buckling tension field
in web;
Capacity of concrete deck is ignored.
In Real Condition
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Girder flanges do have stiffness, and the
composite top flange is much stiffer.
Even without extra panel, flange should provide
some anchorage to develop some tension effect
in the first panel.
Concrete deck does have some shear capacity.
Deck Capacity from Lab Test
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Lab tests were conducted for composite plate
girder. Testing results were published by
Shanmugam and Baskar in ASCE Journal of
Structural Engineering, Sept. 2003
Concrete deck:
width = 1000 mm (39.4 in)
thickness = 150 mm (5.9 in)
f ’c = 400 MPa (5.8 Ksi)
Typical Test Specimen
Instruments Layout
Test of Steel Girder
Test of Composite Girder
Description of Test Girders
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Spg1 and 2 are steel
girders only.
cpg1, 2, 3, 4 are
composite steel girders
with reinforced concrete
decks.
cpg3 and 4 have
additional shear bars in
the deck.
Test Loads: Steel VS Composite
(d/t = 250)
Test Loads: Steel VS Composite
(d/t = 150)
Summary of Lab Test
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The paper concluded the concrete deck did
provide additional shear capacity;
Without shear bars, concrete deck had a sudden
failure mode;
With shear bars, concrete deck had a ductile
failure mode.
Discussion of Lab Test
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The difference between the maximum elastic
shear capacities of cpag1 and spag1 is the same
as the difference between spg2 and cpg2 (about
200 KN). This may due to the same concrete
deck dimensions used for both cpag1 and cpag2.
Discussion of Lab Test
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In the load-deflection plot for d/t = 250, the
initial elastic stiffness for cpag1and spag1 are
about the same. This may imply that the
concrete deck is not effective until the steel
girder behaviors nonlinearly (or steel web starts
to yield and buckle).
Bridge Field Testing
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Au, Lam and Tharmabala (the Bridge Office of
the Ministry of Transportation of Ontario)
published “Investigation of shear resistance of
steel bridge girders by load testing and
monitoring of load response data under highway
traffic conditions” in Canadian Journal of Civil
Engineering, 2009.
Reason for the Testing
During rehabilitation, a strength evaluation
revealed a significant deficiency in the shear
resistance of existing girders at support
locations.
 Bridge girders showed no signs of
distress
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Scope of Testing Program
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Monitor real stresses in end panels of two
selected girders when subjected to
(i) a test truck with known axle loads and
(ii) normal highway traffic loading
Calibrate observed stresses against theoretically
expected responses in girders
Calculate the live load capacity factor using shear
data derived from field measurements
Traffic Lane Layout – Span K
Transverse Section –Span K
Bridge Instrumentation details
Bridge Instrumentation details
Testing Truck and Location
Canadian Highway Bridge Design
Code (CHBDC)
Based on CHBDC
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Total shear capacity = 1.01x1071
= 1081.71 KN
Available live load shear capacity
=1.01x1071 – 988
=93.71 KN or 94 KN
Un-factored dead load shear can be calculated as
861 KN
Field Measurement
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Prorated from the measurement, the factored
live load is estimated at 437 KN;
Based on the maximum vertical shear strain
measured under normal traffic, the largest shear
force under live load is estimated at 606 KN
Summary of Live Load Capacity
Conclusion of the Paper
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The actual steel girder shear capacity at simple
support is larger than that calculated by design
code (CHBDC).
The actual live load in the steel girder is smaller
than that calculated by design code.
The bridge has enough shear capacity (F=1.39)
to carry the design live loads.
Total Steel Shear Capacity
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Paper suggested the total shear capacity of the
steel girder was 1594 KN, which was the sum of
measured live load force (606 KN) and the
FACTORED shear force (988 KN) due to
existing dead load;
And in order to reach this 1594 KN based on
the CHBDC, 38% of post-buckling shear
component had to be included.
Discussion of Total Shear Capacity
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Deck could carry some loads. However, since
there was no distress, it might assumed that
most dead load was taken by steel girder;
Only the non-factored dead load (861 KN)
should be included;
The total least shear capacity might be 1467 KN
(not 1594 KN).
AASHTO VS CHBDC
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At first panel:
AASHTO: Vu = 1135 KN (255.2 Kips)
CHBDC: Vu = 1.01x1071 KN =1082 KN
AASHTO/CHBDC = 1.05
At interior panel:
AASHTO: Vu = 2559 KN (575.3 Kips)
CHBDC: Vu = 1.01x2436 KN = 2460 KN
AASHTO/CHBDC = 1.04
AASHTO VS CHBDC
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Equivalent dead load factor
CHBDC = 1.15, AASHTO = 1.3
Live load factor
CHBDC = 1.42, AASHTO = 1.3
Rating factor for live loads
CHBDC = 0.10, AASHTO = 0.02
Need New Approach to Calculate
Shear Capacity
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Based on lab testing, field testing results,
and bridge ratings and field inspections of
several bridges in California, there is a need
for a new approach.
Proposed New Shear Capacity Eq.
for Composite Plate Girder
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Total shear capacity includes both steel and
concrete deck
V u  V u , s  V u ,c
where
Vu ,s
 0 . 87 (1  C )
 CV p  m 
2
 1  ( d o / D )
V u ,c  n

V p

'
c
f bd t d
m and n are two proposed new parameters
Concrete Capacity Calibration
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Based on information from Shanmugam’s paper:
bc =1000 mm(39.37 in), tc =150 mm (5.9 in)
f ’c = 40 MPa (5801 psi),
estimated Vc = 200 KN (44961 lbs)
then
n = 2.54
to be conservative, use
phi = 0.85 with n = 2
Steel Capacity Calibration
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Based on information from Au’s paper:
Web depth D = 2438 mm (96”)
Web thickness tw = 9.53 mm (3/8”)
Trans. stiffener spacing d0 = 1534mm(60”)
Fy = 230 MPa (33 ksi)
Then
C = 0.37
Vp = 0.58FyDtw = 689 Kips
Steel Capacity Calibration
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Ignoring the deck and using the estimated least
shear capacity of 1467 KN (331.8 kips). Based
on
V u , s  CV
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p
 0 . 87 (1  C )
 m
 1  ( d o / D ) 2

V p

then m = 0.24
Since the girder was still in elastic, the actual m
should be larger than 0.24.
m = 0.25 may be used.
Steel Capacity Calibration
Please note:
 The higher measured steel capacity may be due
to the equation used to calculate buckling shear
stress being too conservative;
 The actual shear force in the steel girder could
be smaller than 1467KN, but the actual steel
girder capacity could be larger;
Rate Br. 50 -316 with Proposed
Method
Girder dimension:
 Top flange: 5/8” x 12”
 Web:
5/16” x 45”
 Bot. flange: 7/8” x 20”
 Spacing of shear stiffener: 34.7”
 calculated: C = 0.804 , Vp = 293.6 Kips
CVp = 236.1 Kips
with m=.25 Vu,s = 246.0 Kips
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Rate Br. 50 -316 with Proposed
Method
Minimum deck thickness: 8.25”
 Effective deck width: 99”
 f ’c = 3250 psi
with phi = 0.85 and n=2
Vu,c = 79.1 Kips
Total shear capacity
Vu = 246+79 = 325 Kips
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Rate Br. 50 -316 with Proposed
Method
Inventory Rating for HS20
Virtis: RF = 0.75
proposed: RF = 1.22
Operating Rating for Permit P13
Virtis: RF = 0.62
proposed: RF = 1.01
Proposed Virtis Enhancement
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If Virtis has the option for user to define
capacities at any point, user may use proposed
method to calculate composite steel plate girder
shear capacity near support and to replace the
shear capacity based on the AASHTO LFD
Specification.
This option may be used for locations, where
capacity has to be manually calculated, such as
hinge, splices, or structure damage.
Questions?