DEVELOPING COMPOSITE ACTION IN EXISTING NON-COMPOSITE STEEL GIRDER BRIDGES

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Nov 25, 2013 (3 years and 6 months ago)

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DEVELOPING COMPOSITE ACTION IN EXISTING NON-COMPOSITE
STEEL GIRDER BRIDGES

Gunup Kwon
Department of Civil, Architectural, and Environmental Engineering
University of Texas at Austin, Austin, Texas, USA
gunup@mail.utexas.edu

Michael D. Engelhardt
Department of Civil, Architectural, and Environmental Engineering
University of Texas at Austin, Austin, Texas, USA
mde@mail.utexas.edu

Richard E. Klingner
Department of Civil, Architectural, and Environmental Engineering
University of Texas at Austin, Austin, Texas, USA
klingner@mail.utexas.edu


ABSTRACT
This paper describes the results of a study investigating methods to strengthen existing non-
composite steel bridge girders using post-installed shear connectors. Four non-composite steel-
concrete beams representing existing non-composite bridge girders were strengthened using
three types of post-installed shear connectors. The full-scale, retrofitted partially composite
beams were tested under static loading to evaluate their strength, stiffness, and ductility. One
non-composite beam was also tested as a baseline specimen. Results of the study indicate that
the addition of a relatively small number of post-installed shear connectors can increase the
load-carrying capacity of non-composite girders by more than 50 percent. Based on the results
of this study, a design approach based on partial composite action was developed for
strengthening existing steel bridge girders by using post-installed shear connectors.


INTRODUCTION
Composite design for steel-concrete bridge girders is common for new construction. A number
of older bridges, however, were constructed with floor systems consisting of a non-composite
concrete slab over steel girders. A number of these bridges have inadequate load ratings and
may require replacement or strengthening. A potentially economical means of strengthening
these floor systems is to connect the existing concrete slab and steel girders to permit the
development of composite action.
To achieve the benefits of composite action, the existing steel girder must be connected to the
existing concrete slab to transfer shear at the steel-concrete interface. For new bridges,

composite action is achieved by welding shear studs to the top of the steel girder prior to
casting the concrete slab. For existing bridges, however, this approach is not possible, since the
slab is already in-place. Therefore, the objective of the research study described here was to
identify structurally efficient and practical ways to post-install shear connectors in existing
bridges, and to develop performance data and construction methods for girder strengthening
using post-installed shear connectors.
This study represents the final phase of a multi-phase research program on post-installed shear
connectors. An extensive series of tests on various types of post-installed shear connectors
were conducted under static, high-cycle fatigue, and low-cycle fatigue loads to identify
connectors with advantageous structural performance characteristics (Hungerford 2004,
Schaap 2004, Kayir 2006). In this study, full-scale beam tests were performed to evaluate
system performance of beams retrofitted with post-installed shear connectors.

COMPOSITE BEAMS AND SHEAR CONNECTORS
Mechanical shear connectors are commonly used in composite construction to connect the
concrete slab and steel girders to permit transfer of horizontal shear force at the steel-concrete
interface. In the AASHTO LRFD Bridge Design Specifications (2007), the required number of
shear connectors is determined based on criteria: static strength and fatigue. Shear connectors
are first designed for fatigue loads due to moving vehicles and then checked for static ultimate
strength. The girder is checked for static ultimate strength assuming full composite action, that
is assuming that the number of shear connectors at the steel-concrete interface is enough to
transfer the shear force developed when the steel girder is fully yielded or when the concrete
slab reaches its full compression capacity. In buildings, composite beams are often designed as
partially composite. In bridges, however, AASHTO LRFD Specifications require that composite
beams be designed as fully composite under static loading.
This difference in practice is important, because partially composite design is particularly useful
for strengthening existing bridges. Post-installed shear connectors are expensive to install, and
it is preferable to use a relatively small number of connectors. For a partially composite beam,
the amount of shear force which can be transferred at the steel-concrete interface is limited by
the strength of the shear connectors. Thus, the ultimate strength of the girder is controlled by
the strength of the shear connectors. The shear connection ratio,
f
NN/
, can be defined as the
ratio of the number of shear connectors at the steel-concrete interface,
N
, to the number of
shear connectors required for fully composite design,
f
N
.
For either fully or partially composite design, slip at the steel-concrete interface is unavoidable.
Slip occurs due to local crushing of the concrete around the lower shank of the shear connector
and due to bending of the shear connector (Viest et al. 1997). Ollgard et al. (1971) developed
Eq. 1 to predict load-slip behavior of welded shear studs. According to the equation, load in a
shear connector,
Q
, is 99 percent of the ultimate strength,
u
Q
, at 5.0-mm slip.

(
)
5/2
18
1
Δ−
−= eQQ
u
(1)
where,
Δ
= Slip of shear connector, in.
Oehlers and Sved (1995) developed equations to predict maximum slip at the steel-concrete
interface for a simply supported beam at maximum load. It is assumed in the analysis that the
steel beam and concrete slab remain linear elastic and the shear connectors are plastic. A
general equation to predict maximum slip at the steel-concrete interface,
max
s
, is shown in Eq. 2.

M
max
P
A
m

(a) Beam with a concentrated load (b) Moment diagram
(c) Interface shear force diagram with (d) Interface shear force diagram with
uniformly distributed shear connectors shear connectors concentrated at left support
Figure 1 – Predicting maximum slip at the steel-concrete interface


21max
KAKAs
shm

=
(2)
where,
m
A
= Area under moment diagram in a shear span
sh
A
= Area under interface shear force diagram in a shear span
( ) ( )
cs
cs
EIEI
hh
K
+
+
=
1
,
(
)
( ) ( )
( )
( )
cscs
cs
EAEAEIEI
hh
K
11
2
2
++
+
+
=

s
h
= Distance from centroid of steel beam to the steel-concrete interface
c
h
= Distance from centroid of concrete slab to the steel-concrete interface
( )
s
EI
= flexural rigidity of steel beam
(
)
c
EI
= flexural rigidity of concrete slab
( )
s
EA
= Axial rigidity of steel beam
(
)
c
EA
= Axial rigidity of concrete slab
The interface shear force at a point along the beam can be obtained as the sum of the shear
connector strength from the support to the point under consideration. Note that Eq. 2 assumes
fully loaded shear connectors along the span of the composite beam.
Equation 2 developed by Oehlers and Sved (1995) indicates that slip at the steel-concrete
interface can be reduced if shear connectors are moved toward the supports. Figure 1 shows a
simply supported beam with a concentrated load at midspan. The moment diagram is plotted in
Fig. 1(b). When the shear connectors are uniformly distributed along the span, the interface
shear force diagram is shown in Fig. 1(c). If all of the shear connectors are moved to the
supports, the interface shear force diagram doubles in size (See Fig. 1(d)), doubling the second
parameter in Eq. 2 and significantly decreasing the maximum slip at the steel-concrete
interface. It is of course not possible to place all shear connectors at the support. This analysis
suggests, however, that it is advantageous to locate the shear connectors closer to the
supports, rather than spacing them uniformly along the length of the beam. This concept was
used to advantage in this study, as described later.
ΣQ
u
A
sh
A
sh
ΣQ
u

Bridge Deck
Girder Flange
Grout
Bridge Deck
Girder Flange
Grout
Bridge Deck
Girder Flange
a) Double Nut Bolt (DBLNB) b) High-Tension Friction-Grip Bolt (HTFGB) c) Adhesive Anchor
(HASAA)
Figure 2 – Post-installed shear connectors

STUDY ON POST-INSTALLED SHEAR CONNECTORS
In the previous phases of this study, 11 types of post-installed shear connectors were
developed and the performance of individual connectors was evaluated under static, high-cycle
fatigue, and low-cycle fatigue loading (Hungerford 2004, Schaap 2004, Kayir 2006). Connectors
included conventional headed shear studs, concrete anchors, high-strength bolts and rods, and
adhesive anchors.
To evaluate the global performance of retrofitted bridges, 3 types of post-installed shear
connectors were selected for full-scale beam tests. The selected shear connectors are shown in
Fig. 2 along with their designations in this study. Kayir (2006) and Kwon (2008) proposed
design equations for these shear connectors for static and fatigue loadings.
For static strength, Eq. 3 was proposed to predict the ultimate strength of post-installed shear
connectors,
u
Q
, under static loading (Kayir 2006).

uscu
FAQ 5.0
=
(3)
The effective shear area,
sc
A
, of shear connectors with threads in the shear plane can be
calculated as 80% of the gross area of the unthreaded connectors and
u
F
is the ultimate tensile
strength of the connector material. Current design equations for conventional welded shear
studs and concrete anchors (AASHTO 2007, ACI 2005) did not provide a conservative
prediction of the ultimate strength of post-installed shear connectors measured in tests.
For fatigue strength, Eq. 4 was recommended for the fatigue endurance limit for the DBLNB
connectors (ASTM A193 B7 threaded rods), and the HTFGB connectors (ASTM A325 bolts with
threads not in the plane of shear) (Kwon 2008).

scr
AMPaZ
×
=
240
(4)
Where,
r
Z
= allowable range of shear force, in
N

Slutter and Fisher (1966) and Lehman et al. (1965) report that concrete strength does not
significantly affect fatigue endurance of shear connectors. Therefore, ASTM 193 B7 threaded
rods were used for the HASAA connectors in this study for full-scale beam tests.

TEST AND ANALYSIS PROGRAM
Test specimens
Five full-scale non-composite beams were built and tested under static loading. One reference
specimen was not retrofitted, and the remaining four were retrofitted with post-installed shear

connectors. All specimens were an 11.6-m long simply supported beam, with a concentrated
load applied at midspan. For all specimens, the steel beam was a W30x99 section of ASTM
A922 steel. The reinforced concrete slab was 2130-mm wide and 178-mm thick, with a specified
concrete compressive strength of 20.7 MPa. Details are shown in Fig. 3.
Figure 4 shows the computed load-carrying capacity of the test specimens with respect to the
shear connection ratio from simple plastic analysis. These beam strength values were
calculated using the minimum specified concrete strength (
c
f'
= 20.7 MPa) and the minimum
specified yield stress of the steel (
y
F
= 345 MPa). The contribution of the longitudinal
reinforcing bars was neglected in the strength calculation. The minimum specified tensile
strength of ASTM 193 B7 threaded rod for the DBLNB and HASSAA connectors and ASTM
A325 high-strength bolt for the HTFGB connectors is 862 MPa and 827 MPa, respectively.
Based on these calculations, the non-composite beam has a capacity of 609 kN. As shown in
Fig. 4, adding shear connectors significantly increases the computed load capacity of this beam,
even with low values of shear connection ratio. Based on this analysis, it was decided to design
the four partially composite beam specimens with a 30-percent shear connection ratio, resulting
in a predicted increase of 48 percent in load-carrying capacity compared to the non-composite
beam. To achieve a 30-percent shear connection ratio requires 16 shear connectors for each
shear span (a total of 32 shear connectors for a beam). The shear connectors were 22 mm (7/8
in.) in diameter, and their static strength was predicted using Eq. 3.
The specimen designations used below begin with the connection method, followed by the
shear connection ratio in percentage. “BS” stands for Beam Static test.
2134
178
#4@305
305
#5 & #4 @152
W30x99
Unit: mm

600
800
1000
1200
0 0.2 0.4 0.6 0.8 1
Load (kips)
Shear connection ratio (N/N
f
)
609 kN
901 kN
30%
1048 kN


Material properties
Mechanical properties of structural components used in the test specimens are shown in Table
1. Concrete for the specimens was delivered by ready-mix truck. The specified compressive
strength was 20.7 MPa with 19-mm river aggregate. Five Star
®
Highway Patch, a fast-setting
high-strength grout, was used to fill the hole in the concrete slab after installation of the DBLNB
and HTFGB connectors. Hilti HY 150 adhesive was used for the HASAA connectors.
The steel beams used for the full-scale beam tests were of ASTM A992, which has a minimum
specified yield stress of 345 MPa. Steel coupons were taken from both the beam web and the
flange. The Grade 60, #4 longitudinal reinforcing bars used in the specimens were also tested in
tension to evaluate mechanical properties. The ultimate tensile strength of the ASTM A193 B7
Figure 3 – Details of specimen cross section
Figure 4 – Predicted load capacity of test
specimens versus shear connection


threaded rod for the DBLNB and the HASAA connectors and of the ASTM A325 high-strength
bolt for the HTFGB connector was determined from tension tests.
Table 1: Measured material properties ( MPa)
Specimen
Beam*
Concrete
**
Connector
**
Grout
**
Reinforcing
bar*
Flange Web
NON-00BS 392.6 (533.4) 419.8 (542.2) 43.12 - - 424.9 (713.8)
DBLNB-30BS 392.6 (533.4) 419.8 (542.2) 25.38 1014 52.21 424.9 (713.8)
HTFGB-30BS 379.1 (520.6) 408.8 (536.7) 28.02 1024 62.91 435.0 (707.1)
HASAA-30BS 392.6 (533.4) 419.8 (542.2) 24.90 1014 - 397.2 (683.6)
HASAA-30BS1 379.1 (520.6) 408.8 (536.7) 22.21 1014 - 435.0 (707.1)
* Yield strength, () ultimate strength, ** ultimate strength

Installation of shear connectors
The shear connectors in the laboratory specimens were installed using the procedures
described below, which were developed to be realistic for field installation. The DBLNB and
HTFGB connectors require access from both the top and the bottom of the slab. The HASAA
connection method, however, requires access only from the bottom of the slab.




(a) Coring concrete slab (b) drilling beam flange (c) Drilling and coring from the bottom
Figure 5 – Installation of shear connectors
Double-Nut-Bolt (DBLNB): Installation of the DBLNB connectors requires access from both the
top and the bottom of the slab. First, a 60-mm diameter hole was drilled through the top of the
concrete slab using a concrete coring machine as shown in Fig 5(a). Second, a 24-mm diameter
hole was drilled through the steel beam flange from the top side of the slab using a portable
magnetic drill. A hollow round bar was placed inside of the cored hole in the concrete to guide
the steel drill bit and to keep the inside surface of the concrete clean of cutting oil. Next, a 190-
mm long ASTM A193 B7 threaded rod was placed from the top to provide a 130-mm
embedment length, and the connector was tightened to a pretension of 170 kN using an impact
wrench. Finally, the hole was filled with grout.
High-Tension Friction-Grip Bolt (HTFGB): Installation of the HTFGB connectors requires more
steps than the DBLNB connectors. First, a 70-mm deep and 50-mm diameter hole was drilled
into the concrete from the top using a rotary hammer drill. Next, a 25-mm diameter hole,
concentric with the 50-mm diameter hole was drilled through the concrete slab from the top

Figure 6 – Test setup
using a coring machine. After cleaning the hole thoroughly, a 24-mm diameter hole was drilled
through the steel beam flange from the top side of the slab using a portable magnetic drill
(Figure 5(b)). A 190-mm long ASTM A325 high-strength bolt was then inserted from the top of
the slab into the hole. The connector was tightened to a pretension of 170 kN using an impact
wrench. Finally, the hole was filled with grout.
Adhesive Anchor (HASAA): The same 22-mm diameter ASTM A193 B7 threaded rods used for
the DBLNB connection method were also used for the HASAA shear connectors. First, a 24-mm
diameter hole was drilled through the steel flange from the bottom of the slab. A portable
magnetic base drill with an annular cutter was used to drill the hole (Fig. 5(c)). Second, a 130-
mm deep, 24-mm diameter hole was drilled into the concrete from the bottom using a rotary
hammer drill (Fig. 5(c)). Next, the hole was cleaned and partially filled with Hilti HIT HY 150
adhesive. An anchor rod was inserted in the hole. After the specified cure time of 50-min. at
68
F
o
, the nut was tightened to the specified torque (125 lb-ft) using a torque wrench.

Test setup and instrumentation
The test setup is shown in Fig. 6. The test
specimen was a simply supported beam, with
11.6-m long span. A concentrated load was
applied at the midspan of the beam with two 100-
ton capacity hydraulic rams. To prevent lateral
torsional buckling of the beam during concrete
casting and to provide for safety during testing of
the specimen, the beam was braced laterally at
midspan. Bracing was also provided at each end
of the beam for safety of the specimen. These
end braces were designed not to restrain
longitudinal movement of the concrete slab.
During each test, measurements were made of
applied load, vertical deflection, slip between the concrete slab and the steel beam flange, and
longitudinal strain in the steel beam. Displacement transducers at midspan and at quarter points
measured vertical deflection. Slip at the interface between the concrete slab and the steel beam
was also measured at the end and quarter points of the beam. Strain gages measured
longitudinal strain of the composite beam at midspan and at 152 mm from the midspan.

Finite Element Modeling of Test Specimens
In addition to the full-scale beam tests, the behavior of the strengthened beams was studied
using ABAQUS (2007), a general-purpose finite-element program that addresses geometric and
material nonlinearity.
To model the test specimens in ABAQUS, a 4-node conventional shell element (S4) was
selected for both the steel beam and the concrete slab. ABAQUS element Type S4 is a fully
integrated, finite-membrane-strain shell element. Connector elements were used to model shear
connectors. Among various types of connector elements, CARTESIAN connectors were used to
simulate the behavior of the shear connectors. This connector is a spring-like element defined in
a local Cartesian coordinate system. Connector failure can be specified with limit values for
force or relative displacement. If the specified failure criterion is reached, the connector is
removed and is no longer considered effective in the model (ABAQUS 2007).
Test specimen
End Bracing
Bracing
Hydraulic Ram
Support

The Hognestad (1951) stress-strain relationship was used to model the concrete stress-strain
curve for compression. A smeared cracking model was used to model concrete behavior in
tension (ABAQUS 2007). In this model, cracking is assumed to occur when the stress reaches
a failure surface. The concrete model does not track individual macro cracks. Instead, the
presence of cracks affects the stress and material stiffness of the corresponding integration
points (ABAQUS 2007). To model stress-strain response of the steel beams and reinforcing
bars, an elastic-perfectly plastic stress-strain relationship was used. Strain hardening was not
included in the material model. The load-slip relationship proposed by Ollgaard et al. (1971) was
adopted for shear connector behavior. The equation by Ollgaard et al. (1971) was developed for
conventional welded shear studs, not for post-installed shear studs. However, it was decided to
use this equation due to limited static test data for individual post-installed shear connectors.
0
200
400
600
800
1000
1200
0 50 100 150 200 250 300
Load (kN)
Deflection (mm)
NON-00BS
DBLNB-30BS
HTFGB-30BS
HASAA-30BS
max: 1142 kN
max: 726 kN
max: 1029 kN
max: 1065 kN


Figure 7 – Load-deflection relations Figure 8 – Deflected specimen after typical test

TEST RESULTS
Structural behavior of test specimens
During each test, in addition to electronically recording data, the specimens were visually
examined for cracks in the concrete slab, yielding and buckling in the steel beam and failure of
the post-installed shear connectors. Figure 7 shows the measured load versus midspan
deflection response for non-composite Specimen NON-00BS, as well as for the partially
composite Specimens DBLNB-30BS, HTFGB-30BS, and HASAA-30BS. These partially
composite beam specimens were provided with post-installed shear connectors that were
uniformly distributed along the span at a spacing of 725 mm. All partially composite beam
specimens showed significantly higher stiffness and strength than the otherwise identical non-
composite beam even with a low shear connection ratio of 30 percent.
For Specimens DBLNB-30BS and HASAA-30BS, a sudden strength drop was observed at
around 120-mm deflection due to the nearly simultaneous failure of multiple shear connectors.
Specimen HTFGB-30BS showed better deformation capacity than the other two partially
composite beams. It is believed that the oversized hole in the concrete associated with the
HTFGB connectors gave them a large slip capacity, resulting in a large deformation capacity for
Specimen HTFGB-30BS. It is, however, noteworthy that 120-mm deflection before shear
connector failure for Specimens DBLNB-30BS and HASAA-30BS is significant and the
specimens resisted significant load even after shear connector failure. Figure 8 shows the
deflected test specimen at the end of the test for Specimen NON-00BS.

0
10
20
30
40
50
0 50 100 150 200 250 300
End Slip (mm)
Deflection (mm)
NON-00BS
DBLNB-30BS
HTFGB-30BS
HASAA-30BS

0
100
200
300
400
500
600
700
0 50 100 150 200 250 300
Neutral axis height (mm)
Deflection (mm)
NON-00BS
DBLNB-30BS
Neutral axis for
non-composite beam
Plastic neutral axis for
partially composite beam

Figure 9 – Load-End slip relationships Figure 10 – Neutral axis locations of test specimens

End slip and neutral axis locations
All of the test specimens showed an increase in slip at the interface between the concrete slab
and the steel beam as the deflection increased. Figure 9 shows the interface slip at the ends of
partially composite beams with uniformly distributed shear connectors along with the non-
composite beam specimen. The partially composite beams showed much less slip before any
shear connector failure. Behaviors of Specimens DBLNB-30BS and HASAA-30BS were similar
to that of Specimen NON-00BS after the multiple shear connector failures.
Specimens DBLNB-30BS and HASAA-30BS showed beam end slip values, at the point of
connector failure and sudden strength loss, of 5.8 mm and 6.9 mm, respectively. For Specimen
HTFGB-30BS, the first shear connector failure occurred at an end slip of 11.4 mm, much larger
than for the other two partially composite beam test specimens.
Composite action in the retrofitted partially composite beams can be further evaluated by
locating the neutral axis during the tests. Figure 10 shows the measured neutral axis location at
mid-span of the girder throughout the tests. Neutral axis locations were obtained by
interpolating strain data read from the strain gages on the beam section. For Specimen NON-
00BS, the neutral axis was located near mid-height of the steel section at most load levels, as
expected. At very low load levels at the start of the test, the neutral axis was located higher up
in the cross-section, suggesting some degree of initial composite action, probably due to bond
and/or friction between the steel and concrete. As indicated in Fig. 10, however, this composite
action occurred only at very low load levels. Once the load exceeded about 10% of the girder’s
full capacity, the girder subsequently behaved in an almost purely non-composite manner. For
Specimen DBLNB-30BS, the neutral axis stayed above mid-height of the steel section at all load
levels. All of the partially composite beam specimens showed almost full composite action in the
early stages of loading, likely due to friction at the steel-concrete interface. However, the
neutral axis moved down as the load increased, indicating partial composite interaction between
the steel beam and the concrete slab.

DEVELOPING INCREASED DEFORMATION CAPACITY
Analytical approach
The non-composite Specimen NON-00BS exhibited very high deformation capacity, as
indicated in Fig. 7. Specimens DBLNB-30BS and HASAA-30BS showed less deformation
capacity due to the limited slip capacity of the high-strength connectors and also due to the low
shear connection ratio. Specimen HTFGB-30BS showed significantly higher deformation

capacity in its overall load-deformation response due to the higher slip capacity of this
connector. It is believed that high slip capacity of the HTFGB connectors enabled the shear
connectors at the steel-concrete interface to redistribute interface shear among the connectors.
However, the HTFGB connectors are more difficult and time-consuming to install than the
HASAA and DBLNB connectors. Because of the easier installation characteristics of the HASAA
and DBLNB connectors, an approach was developed to increase the deformation capacity of
beams retrofitted with these connectors.
Oehlers and Sved (1995) indicate that concentrating shear connectors near zero moment
regions, resulting in the increase of
sh
A
in Eq. 2, can reduce slip at the steel-concrete interface
when the beam reaches its full capacity. This suggests that simply supported beams with shear
connectors concentrated near the supports can likely show higher deformation capacity than
beams with uniformly distributed shear connectors along the span.
Finite element analysis was used to evaluate the effect of moving shear connectors near the
zero moment regions. First, the full-scale beam test specimens were modeled with ABAQUS.
Figure 11 shows load-deflection relations for Specimen HASAA-30BS from the test along with
the finite element analysis results. As shown in the analysis result of Specimen HASAA-30BS,
the model used in this research could not simulate the behavior after shear connector failure.
However, the model is considered useful for predicting the behavior before shear connector
failure and the failure of the shear connectors. Figure 12 shows the longitudinal stress
distribution of Specimens HASAA-30BS from the FE analysis. As shown in the figure, neutral
axis of the beam moved toward the beam top flange due to composite action between the two
structural components.
Figure 11 also shows the analysis results of a partially composite beam with shear connectors
concentrated near the supports. In the FE model, shear connectors were moved near the
supports and were located at a 300-mm spacing. The total number of shear connectors was not
changed in the model. The analysis model with concentrated shear connectors was same with
the analysis model of Specimen HASAA-30BS except the shear connector locations. For
Specimen HASAA-30BS, the connectors were uniformly distributed along the length of the
beam, at a spacing of 725 mm. Compared to Specimen HASAA-30BS, the deformation capacity
of the partially composite beam with shear connectors relocated near the supports was
increased significantly. The increase of its deformation capacity can be attributed to the
decrease in slip at the steel-concrete interface as shown in Fig. 13. The partially composite
specimen with concentrated shear connectors near the support shows much less slip than the
specimen with uniformly distributed shear connectors in ABAQUS.
0
200
400
600
800
1000
1200
0 50 100 150 200 250 300
Load (kN)
Deflection (mm)
HASAA-30BS (Test)
HASAA-30BS (ABAQUS)
Specimen with 300mm
connector spacing (ABAQUS)



Figure 11 – Test vs. FE Analysis results
Figure 12 – Longitudinal stress
distribution of a composite beam

0
5
10
15
20
25
30
0 50 100 150 200
End Slip (mm)
Deflection (mm)
HASAA-30BS (ABAQUS)
HASAA-30BS1 (ABAQUS)
HASAA-30BS (Test)

0
200
400
600
800
1000
1200
0 50 100 150 200 250 300
Load (kN)
Deflection (mm)
NON-00BS
HASAA-30BS1
HASAA-30BS
max: 1042 kN
max: 726 kN
max: 1065 kN

Figure 13 – Load-End slip relationships Figure 14 – Load-deflection curves for test specimens
Test Results for Partially Composite Beam with Concentrated Shear Connectors
Specimen HASAA-30BS1 was tested to verify the FE analysis results and evaluate the global
behavior of partially composite beams with post-installed shear connectors concentrated near
zero moment regions. Specimen HASAA-30BS1 had the same number of shear connectors as
Specimen HASAA-30BS. For Specimen HASAA-30SB1, the shear connectors were moved
towards the ends of the beam, and were spaced at 300 mm.
Figure 14 compares test results for Specimens HASAA-30BS1 and HASAA-30BS. The
deformation capacity of Specimen HASAA-30BS1 (concentrated connectors) is much greater
than that of Specimen HASAA-30BS (uniform connectors), and is comparable to that of
Specimen NON-00BS. Specimen HASAA-30BS1 showed first shear connector failure at a 170-
mm deflection, after which the applied load increased slightly before another shear connector
failure at 210 mm. The maximum load was 1043 kN at 197-mm deflection. It appears that
concentrating shear connectors near the supports not only decreases slip at the steel-concrete
interface, but also helps redistribute loads among shear connectors.
Test results from Specimen HASAA-30BS1 indicate that the stiffness and strength of non-
composite beams can be improved significantly by using relatively small number of post-
installed shear connectors without sacrificing deformation capacity.

SUMMARY
The study reported herein was the final phase of a research project to develop methods for
strengthening existing non-composite bridge girders using post-installed shear connectors.
Previous phases of this study identified possible post-installed shear connectors and evaluate
static and fatigue performance of those shear connectors. Based on this earlier works, three
types of post-installed shear connectors were selected for full-scale beam tests.
The number of post-installed shear connectors needed to strengthen an existing bridge girder is
determined based on the concept of partial composite design. Partial composite design is not
normally used for new composite bridge girders, because fatigue typically controls the required
number of shear connectors. Because of the superior fatigue characteristics of the post-installed
shear connectors tested in this study, however, fatigue is not likely to control the required
number of shear connectors, and partial composite design is therefore possible.
With partial composite design, 50 to 70 percent of the shear connectors normally needed for full
composite design can be eliminated, while still achieving a 40- to 50-percent increase in load-
carrying capacity in positive-moment regions of a girder. Based on the test data from single
shear connector tests, overall performance of girders retrofitted with post-installed shear

connectors was evaluated with a series of large-scale beam tests, supplemented with finite
element analysis. The results of this study suggest that strengthening existing non-composite
bridge girders using post-installed shear connectors may be an effective and economical
alternative to replacement of existing bridges.

ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support provided for this study by the Texas
Department of Transportation. The authors extend a special thanks to Jon Kilgore and Clara
Carbajal of the Texas Department of Transportation for their support, assistance and advice
throughout the entire course of this project. The authors also gratefully acknowledge the
contributions of Brad Schaap, Brent Hungerford, and Hulya Kayir in the earlier phases of this
research study. The experiments described herein were conducted at the Phil M. Ferguson
Structural Engineering Laboratory at the University of Texas at Austin.
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