Effect of Friction on Shear Connection in Composite Bridge Beams

In the design of new composite steel and concrete bridge beams, the shear connectors are assumed to transmit all of the longitudinal shear forces at the interface between the concrete slab and the steel beam. However, in practice, the forces on the shear connectors are modified by friction resistances at the interface. The effect of friction on the fatigue endurance of shear connectors is first illustrated through a specially developed finite-element analysis procedure. Then a simple mathematical assessment model is proposed that allows for the beneficial effect of friction on the fatigue endurance of shear connectors in composite steel and concrete bridge beams. This procedure can extend the design life of the shear connectors in existing composite bridge beams, as it can be used to estimate their remaining endurance and their remaining strength and, if necessary, to determine the effect of remedial work on increasing the endurance of the shear connectors.     

Static Behavior and Theoretical Model of Stud Shear Connectors

Stud shear connectors are the most widely used shear connectors in steel-concrete composite beams. The composite action of steel beam and concrete slab is effected by the stud shear properties directly. Thirty push-out tests on stud shear connectors were conducted to investigate the effects of stud diameter and height, concrete strength, stud welding technique, transverse reinforcement, and steel beam type on stud failure mode, load versus slip curve, and the shear bearing capacity. Based on the push-out test results, the stud shear mechanism was analyzed, a new expression of stud load-slip relationship was put forward, and a calculation model of stud shear bearing capacity was proposed taking into account the influences of stud diameter and height, material strength, and elastic modulus. Compared with existing models, the computed shear bearing capacities of the proposed calculation model had a better match with the experimental values.     

Field Evaluation of Decommissioned Noncomposite Steel Girder Bridge

Field tests conducted on a noncomposite steel girder bridge are described. Two separate 36.6 m (120 ft) units, each three-span continuous, were subjected to increasing static loads by means of a trailer and concrete barriers. Results show that the girders acted as partially composite sections in the positive moment region up to the onset of yield. Due to curb participation and the transverse location of the applied load, exterior girders exhibited a higher degree of partially composite action. In the negative moment region, partially composite action was evident only in the exterior girders. As a result of partially composite action and curb participation, the yield load was about 7% higher than predicted. Bearing restraint is shown not to have a significant impact on the behavior of the tested bridges. In addition, the stiffness of the interior girders, as measured under the constant weight of a dump truck, are shown to be virtually unaffected by the heavy trailer loads. More significant changes in girder stiffness were observed between different transverse load positions of the dump truck.     

Behavior of Composite Bridge Decks with Alternative Shear Connectors

An experimental study of composite bridge decks with alternative shear connectors has been performed. The alternative shear connector consists of concrete filled holes located in the webs of grid main bars and friction along the web embedded in the slab, which enables shear transfer between the concrete slab and steel grid. Results of static and fatigue tests on full-scale prototype decks indicated that composite action between the concrete slab and steel grid is maintained well above the service load range even after fatigue loading, the eventual loss of composite action at overload is gradual, failure was controlled by punching shear of the concrete slab and was unaffected by the shear connectors, and no significant change in behavior was observed due to fatigue loading. Further, the measured stress range at the shear connection location would not control the fatigue behavior of the deck in positive bending, and no fatigue cracking of the steel grid was observed in negative bending.     

Stiffness and Strength of Metal Bridge Deck Forms

Light gauge metal sheeting is often utilized in the building and bridge industries for concrete formwork. Although the in-plane stiffness and strength of the metal forms are commonly relied upon for stability bracing in buildings, the forms are generally not considered for bracing in steel bridge construction. The primary difference between the forming systems in the two industries is the method of connection between the forms and girders. In bridge construction, an eccentric support angle is incorporated into the connection details to achieve a uniform slab thickness along the girder length. While the eccentric connection is a benefit for slab construction, the flexible connection limits the amount of bracing provided by the forms. This paper presents results from the first phase of a research study investigating the bracing behavior of metal bridge deck forms. Shear diaphragm tests were conducted to determine the shear stiffness and strength of bridge deck forms, and modified connection details were developed that substantially improve the bracing behavior of the forms. The measured stiffness and strength of diaphragms with the modified connection often met or exceeded the values of diaphragms with conventional noneccentric connections. The experimental results for the diaphragms with the modified connection details dramatically improve the potential for bracing of steel bridge girders by metal deck forms.     

Seismic Retrofit of State Street Bridge on Interstate 80

The State Street Bridge, in Salt Lake City, was designed and built in 1965 according to the 1961 AASHO specifications; the design did not include earthquake-induced forces or displacements since only wind loads were considered. The bridge consists of four reinforced concrete (RC) bents supporting composite welded steel girders; the bents are supported on cast-in-place concrete piles and pile caps. A vulnerability analysis of the bridge was conducted that determined deficiencies in (1) confinement of column lap splice regions, (2) anchorage of longitudinal column bars in the bent cap, (3) confinement of column plastic hinge zones, and (4) shear capacity of columns and bent cap–column joints. Seismic retrofit designs using carbon-fiber-reinforced-polymer (CFRP) composites and steel jackets were performed and compared for three design spectra, including the 10% probability of exceedance in 250 years earthquake. The CFRP composite design was selected for implementation and application of the composite was carried out in the summer of 2000 and 2001, while the bridge was in service. The paper describes the CFRP composite design, which, in addition to column jackets, implemented an “ankle wrap” for improving joint shear strength and a “U-strap” for improving anchorage of column bars in the bent cap; other retrofit measures were implemented, such as bumper brackets and a deck slab retrofit. A capacity versus demand evaluation of the as-built and retrofitted bents is presented.     

Repair of Damaged Steel-Concrete Composite Girders Using Carbon Fiber-Reinforced Polymer Sheets

The aging infrastructure of the United States requires significant attention for developing new materials and techniques to effectively and economically revive this aging system. Damaged steel-concrete composite girders can be repaired and retrofitted by epoxy bonding carbon fiber-reinforced polymer (CFRP) laminates to the critical areas of tension flanges. This paper presents the results of a study on the behavior of damaged steel-concrete composite girders repaired with CFRP sheets under static loading. A total of three large-scale composite girders made of W355×13.6 A36 steel sections and 75-mm-thick by 910-mm-wide concrete slabs were prepared and tested. One, three, and five layers of CFRP sheet were used to repair the specimen with 25, 50, and 100% loss of the cross-sectional area of their tension flange, respectively. The test results showed that epoxy bonded CFRP sheet could restore the ultimate load-carrying capacity and stiffness of damaged steel-concrete composite girders. Comparison of the experimental and analytical results revealed that the traditional methods of analysis of composite beams were conservative.     

Comparison of Three Flexural Retrofit Systems under Monotonic and Fatigue Loads

The majority of experimental work involving the flexural retrofit of concrete bridge girders has been conducted on beam specimens with adhesive-applied, soffit-mounted, fiber-reinforced polymer (FRP) composite systems, referred to in this study as conventional adhesive application (CAA). It has been observed that the performance of such girders is often controlled by the quality of the bond between the FRP and the concrete substrate and the substrate’s ability to transfer stress from the steel to the FRP. With the goal of improving the performance of bonded FRP in mind, two additional soffit-mounted retrofit schemes are investigated: near-surface mounted (NSM), where the FRP strips are embedded in adhesive within slots cut into the substrate concrete, and, powder-actuated fastener-applied (PAF) FRP, which uses a powder-actuated nail gun to install mechanical fasteners through predrilled holes in the FRP into the concrete substrate, “nailing” the FRP in place. The PAF application is a recent development, and little work has been done on it other than by the proprietors of the system. This study reports on a comparative study of the static and fatigue performance of reinforced concrete beams retrofitted with CAA, NSM, and PAF FRP retrofit systems. Ten medium-scale beams were tested: six strengthened specimens, two per retrofit method, were tested under cyclic loading conditions, and four specimens, one per retrofit method and one control specimen, were tested monotonically to failure. The results of this study indicate that although all three methods of FRP application result in significant strength increases over the control specimen under monotonic loading conditions, the CAA method is outperformed by the other methods under cyclic conditions. A number of other relevant detailed conclusions with respect to performance and practical application issues are presented for each of the methods of retrofit examined in this study. Significantly, clear evidence of FRP debonding in the midspan region prior to specimen failure is presented.     

Service Load Effective Compression Flange Width in Fiber Reinforced Polymer Deck Systems Acting Compositely with Steel Stringers

This paper reports on the field study of a steel stringer-fiber reinforced polymer (FRP) deck composite bridge in Pennsylvania. The objective of the study is to assess the effective compression flange width in the FRP deck and floor systems when they act compositely with underlying steel girders at service conditions. The research results reported herein support the notion of employing a design approach, for both interior and exterior girders of a composite floor system, that is philosophically consistent with current practice related to steel girders acting compositely with concrete decking. It appears from the results presented herein that FRP decks and floors acting compositely with underlying steel girders exhibit an effective width that is close to the actual girder spacing for interior beams, and approximately one-half this value for exterior beams.     

Designing and Testing of Concrete Bridge Decks Reinforced with Glass FRP Bars

In addition to their high strength and light weight, fiber-reinforced polymer (FRP) composite reinforcing bars offer corrosion resistance, making them a promising alternative to traditional steel reinforcing bars in concrete bridge decks. FRP reinforcement has been used in several bridge decks recently constructed in North America. The Morristown Bridge, which is located in Vermont, United States, is a single span steel girder bridge with integral abutments spanning 43.90 m. The deck is a 230 mm thick concrete continuous slab over girders spaced at 2.36 m. The entire concrete deck slab was reinforced with glass FRP (GFRP) bars in two identical layers at the top and the bottom. The bridge is well instrumented at critical locations for internal temperature and strain data collection with fiber-optic sensors. The bridge was tested for service performance using standard truck loads. The construction procedure and field test results under actual service conditions revealed that GFRP rebar provides very good and promising performance.     

Fracture Model for Flexural Failure of Beams Retrofitted with CARDIFRC

A new retrofitting technique based on a material [Cardiff Fiber Reinforced Concrete (CARDIFRC)] compatible with concrete has been developed at Cardiff University. It overcomes some of the problems associated with the current techniques based on externally bonded steel plates and fiber-reinforced plastic laminates which are due to the mismatch of their tensile strength and stiffness with that of the concrete structure being retrofitted. CARDIFRC is characterized by high tensile/flexural strength and high energy-absorption capacity (i.e., ductility). The special characteristics of CARDIFRC make it particularly suitable for repair, remedial and upgrading activities (i.e., retrofitting) of existing concrete structures. It has been shown that damaged reinforced concrete beams can be successfully strengthened and rehabilitated in a variety of different retrofit configurations using precast CARDIFRC strips adhesively bonded to the prepared surfaces of the damaged beams. To predict the moment resistance of the beams retrofitted in this manner an analytical model is introduced in the present paper. This model takes a fracture mechanics approach and follows the initiation and growth of the flexural crack that eventually leads to the failure of the retrofitted beams. The results of this analytical model are found to be in very good agreement with the test results.     

Fatigue Behavior of Carbon Fiber Reinforced Polymer-Strengthened Reinforced Concrete Bridge Girders

This study examines the effects of one-dimensional fiber-reinforced polymer (FRP) composite rehabilitation systems on the flexural fatigue performance of reinforced concrete bridge girders. Eight 508?mm deep and 5.6?m long reinforced concrete T-beams, with and without bonded FRP reinforcement on their tensile surfaces, were tested with a concentrated load at midspan under constant amplitude cyclic loading. The objective of this investigation is to establish the effect that these repair systems have on the fatigue behavior and remaining life of the girders. Results indicate that the fatigue behavior of such retrofit beams is controlled by the fatigue behavior of the reinforcing steel. The fatigue life of a reinforced concrete beam can be increased by the application of an FRP retrofit, which relieves some of the stress carried by the steel. The observed increase in fatigue life, however, is limited by the quality of the bond between the carbon FRP and concrete substrate. Debonding, initiating at midspan and progressing to a support, is common and is driven partially by the crack distribution and shear deformations of the beam.     

考虑楼板效应的外环板式梁柱节点抗弯承载力

楼板的存在对梁柱节点的局部受力影响显著, 在梁柱节点设计中, 若仅仅把楼板与钢梁的组合效应作为安全储备, 可能会产生结构由"强柱弱梁"转变成"强梁弱柱"的颠覆性结果, 因此忽略混凝土楼板对节点承载力及刚度的影响是造成破坏的重要原因.基于已完成的带楼板的T型梁柱节点低周往复荷载试验, 建立了非线性有限元分析模型.为了更加全面地了解钢梁-楼板组合节点的工作机制, 进一步补充完善试验研究的不足, 模型考虑了楼板与钢梁之间的栓钉连接以及材料非线性等因素, 模型的计算结果与试验结果具有高吻合度.在此基础上, 通过有限元参数分析, 详细分析了构件尺寸效应、轴压比、楼板厚度、楼板强度和柱宽厚比共五个参数对考虑楼板影响的外环板式梁柱节点抗震性能的影响.结果表明尺寸效应、轴压比对梁端抗弯承载力及刚度的影响小到可以忽略, 楼板厚度、楼板强度和柱宽厚比对梁端抗弯承载力有显著影响.结合理论分析进一步提出了考虑楼板影响的外环板式梁柱节点梁端抗弯承载力计算公式, 通过对比公式计算结果与试验、有限元分析结果可得, 该计算公式可较好的计算带楼板外环板式梁柱节点梁端抗弯承载力.      

Retrofitting of Reinforced Concrete Beams with CARDIFRC

A new retrofitting technique based on a material compatible with concrete is currently under development at Cardiff University. It overcomes some of the problems associated with the current techniques based on externally bonded steel plates and FRP (fiber-reinforced polymer) laminates which are due to the mismatch of their tensile strength and stiffness with that of the concrete structure being retrofitted. This paper will describe briefly the technology necessary for preparing high-performance fiber-reinforced concrete mixes (HPFRCC), designated CARDIFRC. They are characterized by high tensile/flexural strength and high energy-absorption capacity (i.e., ductility). The special characteristics of CARDIFRC make them particularly suitable for repair, remedial, and upgrading activities (i.e., retrofitting) of existing concrete structures. The promising results of several studies using CARDIFRC for retrofitting damaged concrete flexural members will be presented. It will be shown that damaged reinforced concrete beams can be successfully strengthened and rehabilitated in a variety of different retrofit configurations using precast CARDIFRC strips adhesively bonded to the prepared surfaces of the damaged beams. To predict the moment resistance and load-deflection response of the beams retrofitted in this manner an analytical model will be introduced, and the results of the computations will be compared with the test results to evaluate the accuracy of the model.     

Strengthening of Reinforced Concrete Bridge Decks Using Carbon Fiber-Reinforced Polymer Composite Materials

This paper presents the results of an investigation of the monotonic and fatigue behavior of one-way and two-way reinforced concrete slabs strengthened with carbon fiber-reinforced polymer (CFRP) materials. The five one-way slab specimens were removed from a decommissioned bridge in South Carolina. Three of the slabs were retrofitted with CFRP strips bonded to their soffits and the other two served as unretrofit, control specimens. Of the five one-way slab specimens, one unretrofit and two retrofit slabs were tested monotonically until failure. The remaining two specimens, one unretrofit and one retrofit, were tested under cyclic (fatigue) loading until failure. In addition, six half-scale, two-way slab specimens were constructed to represent a full-scale prototype of a highway bridge deck designed using the empirical requirements of the AASHTO LRFD Bridge Design Manual. Of the six square slabs, two were unretrofitted and served as the control specimens, two were retrofitted using CFRP strips bonded to their soffits making a grid pattern, and two were retrofitted with a preformed CFRP grid material bonded to their soffit. Three slabs, one unretrofit, one CFRP strip, and one CFRP grid retrofitted, were tested monotonically until failure and the remaining three slabs were tested under cyclic (fatigue) loading until failure.     

Modeling of Steel-Concrete Composite Beams under Negative Bending

Negative bending moments acting in the support regions of continuous composite beams generate tensile stresses in the concrete slab and compressive stresses in the lower steel profile. As a result the mechanical behavior of these beams is strongly nonlinear even for low stress levels, due not only to the slip at the beam-slab interface, but also to cracking in the slab. Therefore, an adequate theoretical modeling should take account of the interactions between the structural steel and the concrete slab by shear connectors and also between steel rebars and concrete in tension by bond phenomenon. In this paper a model of steel and concrete composite beams subjected to negative bending is presented. It accounts for the slip occurring at both the beam-slab interface and the steel reinforcement-concrete interface. Some numerical results, obtained using a suitable numerical procedure, are discussed to show the capacity of the model.     

Influence of Reinforcement on the Behavior of Concrete Bridge Deck Slabs Reinforced with FRP Bars

This paper presents the results of an experimental study to investigate the role of each layer of reinforcement on the behavior of concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars. Four full-scale concrete deck slabs of 3,000?mm length by 2,500?mm width and 200?mm depth were constructed and tested in the laboratory. One deck slab was reinforced with top and bottom mats of glass FRP bars. Two deck slabs had only a bottom reinforcement mat with different reinforcement ratios in the longitudinal direction, while the remaining deck slab was constructed with plain concrete without any reinforcement. The deck slabs were supported on two steel girders spaced at 2,000?mm center to center and were tested to failure under a central concentrated load. The three reinforced concrete slabs had very similar behavior and failed in punching shear mode at relatively high load levels, whereas the unreinforced slab behaved differently and failed at a very low load level. The experimental punching capacities of the reinforced slabs were compared to the theoretical predictions provided by ACI 318-05, ACI 440.1R-06, and a model proposed by the writers. The tests on the four deck slabs showed that the bottom transverse reinforcement layer has the major influence on the behavior and capacity of the tested slabs. In addition, the ACI 318-05 design method slightly overestimated the punching shear strength of the tested slabs. The ACI 440.1R-06 design method yielded very conservative predictions whereas the proposed method provided reasonable yet conservative predictions.     

Evaluation of In-Service Performance of Tom’s Creek Bridge Fiber-Reinforced Polymer Superstructure

The Tom’s Creek Bridge is a small-scale demonstration project involving the use of fiber-reinforced polymer (FRP) composite girders as the main load-carrying members. The project is intended to serve two purposes. First, by calculating bridge design parameters such as the dynamic load allowance, transverse wheel load distribution, and deflections under service loading, the Tom’s Creek Bridge aids in modifying current American Association of State Highway and Transportation Officials bridge design standards for use with FRP composite materials. Second, by evaluating the FRP girders after exposure to service conditions, the project begins to answer questions about the long-term performance of these advanced composite material beams when used in bridge design. This paper details the in-service analysis of the Tom’s Creek Bridge. Five load tests, at 6-month intervals, were conducted on the bridge. Using midspan strain and deflection data gathered from the FRP composite girders during these tests, the aforementioned bridge design parameters have been determined. The Tom’s Creek Bridge was determined to have a maximum dynamic load allowance, IM, of 0.90, a transverse wheel load distribution factor, g, of 0.101, and a maximum deflection of L/490. Two bridge girders were removed from the Tom’s Creek Bridge after 15 months of service loading. These FRP composite girders were tested at the Structures and Materials Research Laboratory at Virginia Tech for stiffness and ultimate strength and compared to preservice values for the same beams. These measurements indicate that, after 15 months of service, the FRP composite girders have not significantly changed in stiffness or ultimate moment capacity.     

Experimental Study on Prefabricated Concrete Bridge Girder-to-Girder Intermittent Bolted Connections System

This paper reports on a new bridge deck slab flange-to-flange connection system for precast deck bulb tee (DBT) girders. In prefabricated bridge system made of DBT girders, the concrete deck slab is cast with the prestressed girder in a controlled environment at the fabrication facility and then shipped to the bridge site. This system requires that the individual prefabricated girders be connected through their flanges to make it continuous for live load distribution. The objectives of this study are to develop an intermittent bolted connection for DBT bridge girders and to provide experimental data on the ultimate strength of the connection system. This includes identifying the crack formation and propagation, failure mode, and ultimate load carrying capacity. In this study, three different types of intermittent bolted connection were developed. Four actual-size bridge panels were fabricated and then tested to collapse. The effects of the size and the level of the fixity of the connecting steel plates, as well as the location of the wheel load were examined. The developed joint was considered successful if the experimental wheel load satisfied the requirements specified in North American bridge codes. It was concluded that location of the wheel load at the deck slab joint affected the ultimate load carrying capacity of the connections developed. Failure of the joint was observed to be due to either excessive deformation and yielding of the connecting steel plates or debonding of the embedded studs in concrete.