Journal of Structural Engineering

This paper presents the current state of the art of nonlinear analysis of steel-concrete composite structures. The focus is on frame elements, which are computationally faster than continuum finite element models. First, section models are presented, with a review of resultant and fiber models and a discussion of possible practical applications. The presentation of frame elements follows. Models with lumped and distributed inelasticity, as well as models with perfect and partial connections are covered. Rigid and partially restrained joints are then reviewed and discussed at length. A discussion of the analysis of structural walls completes the presentation of the models. Modeling applications to the analysis of composite frames are also presented. This state-of-the-art review focuses on developments that have stemmed from the recently completed National Science Foundation sponsored U.S.-Japan program on composite and hybrid structures.

The behavior of high strength square concrete filled steel tube (CFT) beam columns subjected to constant axial load and cyclically varying flexural loading was investigated experimentally. The effects of the width-to-thickness $(b/t)$ ratio, the yield stress $\left({\sigma }_y\right)$ of the steel tube, and the level of axial load on the behavior (stiffness, strength, ductility, and energy dissipation) of high strength square CFT beam columns were studied. Eight cyclic beam-column specimens were tested. Crushing of concrete and cyclic local buckling in the steel tube affected the cyclic strength and stiffness of the CFT specimens. The elastic section flexural stiffness under cyclic loading decreases rapidly due to tension cracking of the concrete infill and local buckling of the steel tube. The total energy dissipated by the cyclic specimens is dominated by the flexural energy dissipated in the failure segment (plastic hinge region). Both the current American Concrete Institute code provisions and the modified Architectural Institute of Japan method predict the moment capacities of the high strength square CFT specimens accurately. The cyclic curvature ductility $\left({\mu }_{\varphi -c}\right)$ decreases significantly with an increase in the axial load level. At higher axial load levels, the steel tube $b/t$ ratio and ${\sigma }_y$ have a small influence on ${\mu }_{\varphi -c}\text{.}$ However, at lower axial load levels, increasing the steel tube $b/t$ ratio or ${\sigma }_y$ reduces ${\mu }_{\varphi -c}\text{.}$

A 5 year research on concrete-filled steel tubular (CFT) column system was carried out as a part of the fifth phase of the U.S.–Japan Cooperative Earthquake Research Program, and the tests of centrally loaded short columns were finished. The objectives of these tests were to clarify the synergistic interaction between steel tube and filled concrete, and to derive methods to characterize the load–deformation relationship of CFT columns. A total of 114 specimens was fabricated and tested in the experimental phase of investigations on centrally loaded hollow and CFT short columns. Parameters for the tests are as follows: (1) tube shape, (2) tube tensile strength, (3) tube diameter-to-thickness ratio, and (4) concrete strength. In determining the range of parameters, the emphasis was placed on obtaining a wide range of test data for establishing a generally applicable design method of CFT column systems. Design formulas to estimate the ultimate axial compressive load capacities are proposed for CFT columns with both circular and square sections based on tests results described in this paper.

The behavior of concrete-filled circular and square steel tubular (CFT) beam columns with a variety of material strengths was investigated experimentally. The main test parameters are steel strength (400, 590, and 780 MPa), diameter- (width-) to-thickness ratio of steel tube, and concrete strength (40 and 90 MPa). The interior beam-column models were tested under constant axial compression and cyclic horizontal load with incrementally increasing lateral deformation to clarify the effects of the test parameters on the behavior. The exterior beam-column models were laterally loaded under variable axial load, including axial tension. The behavior of square beam columns subjected to biaxial bending was also investigated. The test results show that higher strength and thicker steel tubes give better overall behavior of the beam column, while higher strength concrete has an adverse effect on the behavior. Furthermore, computer simulations for the hysteretic behavior of CFT beam-column specimens are presented here. The analytical results show excellent agreement with the test results except for a few cases.

Beam columns of concrete-filled steel tubular (CFT) columns are designed using a design formula established by the Architectural Institute of Japan (AIJ). However, the scope of the formula in the current AIJ standard is limited to ordinary material strengths, and a rational mathematical model which makes it possible to reproduce load-deformation behavior in CFT columns has not yet been established. The main objectives of this research were to investigate the effect of higher material strengths on the flexural behavior and to create mathematical models of the steel tube and filled concrete. Adequate study of the effects of the section shape, diameter (width)-to-thickness ratio, and the combination of strengths of the steel tube and filled concrete was included in the scope of this investigation. Based on the experimental results of 65 eccentric compression tests, an analytical model for the restoring force characteristics of the flexural behavior of CFT columns is proposed, and is found to reproduce experimental results such as the synergistic interaction between the steel tube and filled concrete with good accuracy.

As part of the U.S.-Japan Cooperative Research Program on Composite/Hybrid Structures (1992), investigations were carried out at the University of Nebraska-Lincoln and University of Illinois, to develop moment connection details and accompanying design provisions for connecting steel beams to circular concrete filled tube (CFT) columns. This paper provides an overview of the work carried out. Work at the University of Illinois consisted of evaluating the inelastic response of six possible connection details. Six large-scale connections were tested to failure using the quasi-static test method. Connections welded to the skin of the tube created large distortions of the tube wall, and were susceptible to weld, flange, or tube wall fracture. External diaphragm and continuous web details exhibited more favorable inelastic behavior, but the flexural strength of these connections began to deteriorate relatively early in the imposed deformation history. Continuing the flanges through the composite column showed adequate strength, however due to excessive slipping this connection did not dissipate significant inelastic dynamic energy. One of the details tested at the University of Illinois was passing the girder section through the CFT columns. This through beam detail provided the most effective detail to achieve the ideal rigid connection condition. Work at the University of Nebraska-Lincoln concentrated on comprehending the behavior of the through beam connection detail. The behavior of CFT columns under seismic loads was studied through testing six columns which were subjected to a constant axial load in addition to a cyclic lateral load. Failure modes for through beam connection detail were identified through testing seven, two-thirds scale connection specimens. These tests were used to comprehend the force transfer mechanisms between steel beams and CFT columns and develop design provisions that could estimate the capacity of the various elements of the through beam connection detail.

The results of an experimental research study involving the testing of ten full-scale moment resisting connections under simulated seismic loading conditions are presented. Each test specimen modeled the interior joint of a moment resisting frame consisting of square concrete filled steel tube columns and wide flange steel girders, where energy dissipation was designed to occur either primarily in the beams or in the connection panel zone during a severe earthquake. The results of the study indicate that moment resisting connection details can be economically designed that enable more than 0.045 rad of inelastic story drift to develop under cyclic loading. These details include split-tee connections without a shear tab, weak panel zone connections, and extended tee connections. Panel zone shear yielding and local buckling are shown to be a ductile mode of response, with minimal strength deterioration occurring in the connection. The use of interior diaphragms in the column is shown to locally stiffen the joint, but also lead to strain concentrations and fracture of the beam flanges at their weld access holes. Furthermore, strain concentrations develop at connection details that lack a gradual transition in geometry and result in a reduction in connection ductility.

Composite concrete filled tube (CFT) columns are advantageous for use in seismic resistant braced frame construction since these CFT columns are relatively inexpensive, stiff and strong. However, to utilize the composite behavior of the columns, the vertical component of brace force must be transferred through the beam-brace-column connection and distributed to both the concrete and steel in the column. Beam-brace-column connection details are critical for development of this composite action. This paper describes analytical and experimental studies carried out to better understand the transfer and distribution of force to the joint. A range of beam-brace-column gusset plate connections were considered. It is shown that the majority of force transferred from the steel into the concrete occurs by bearing rather than by friction. Steel gusset plates with horizontal ribs, or gusset plates with holes, allow more force transfer and have more composite action than plain gusset plates. Slip deformations between the steel and concrete are likely to be too small to mobilize the strength of shear studs in these connections.

In the design of beam-to-column subassemblies in concrete-filled steel tubular column systems, a design formula established by the Architectural Institute of Japan (AIJ) is applied. However, in the current AIJ standard, the scope of the design formula is limited to ordinary material strengths; i.e. up to an unconfined compression strength of 36 MPa for concrete and tensile strength of 490 MPa for structural steel tubes. So, the effect of higher material strengths on subassembly behavior is studied in this investigation. The experimental results presented in this paper verified that the above-mentioned AIJ design formula is applicable up to an unconfined compression strength of 110 MPa for concrete and tensile strength of 809 MPa for structural steel tubes. An analytical model for the restoring force characteristics of the shear panel in the subassemblies was also proposed, and was found to reproduce the experimental results with good accuracy.

A key factor behind successful performance of outrigger beam–core wall connections in hybrid structures is the adequacy of headed studs that are typically used to connect a stud plate, onto which the outrigger beam is connected through a shear tab, to the wall. In an effort to better understand cyclic behavior of stud groups under combined action of gravity shear and cyclic diaphragm forces, the research reported was undertaken. Six 1/3-scale subassemblies that involve a portion of the wall, connection, and outrigger beam were fabricated and tested. The test results suggest that a design method which closely follows available equations for computing stud capacity results in connections that reach the design loads; however, the mode of failure is stud pullout. This mode of failure does not have any ductility and lacks energy dissipation capabilities. Moreover, the available design equations fail to capture the substantial increase in strength due to wall boundary element reinforcement around the studs. The boundary element also alters the mode of failure from stud pullout to stud fracture. A new design technique based on dissipating the input energy through yielding and eventual fracture of the shear tab was developed. The performance of a test specimen designed based on this methodology was found to be satisfactory in terms of strength, ductility, and energy dissipation characteristics. Two simple adjustments to available equations were also identified to account for the effects of confinement, by wall boundary element reinforcement, around headed studs.

Adequate strength of headed studs under cyclic loads is a prerequisite for satisfactory performance of outrigger beam–core wall connections that commonly use multiple headed studs. Previous studies have typically not investigated the effects of reinforcement around studs, and have not accounted for the effects of cracking, damage, and yielding of reinforcement on the strength of studs. To remedy these deficiencies, two 1/4-scale subassemblies that contain a cantilever wall and two outrigger beams with and without floor diaphragms were subjected to cyclic loading. The wall reinforcement around the connection was selected according to the anticipated level of cracking and plastic hinge formation. The design methodology followed in this research resulted in connections that could develop and exceed the design forces despite extensive cracking and yielding of wall reinforcement around the headed studs. The presence of heavily confined wall boundary elements around headed studs increases the capacity. Simple methods to account for the influence of cracks and strengthening effects of boundary elements were developed. The resulting analytical model was able to accurately establish the expected mode of failure and capacity of outrigger beam–wall connections. For the outrigger beam–wall connection detail selected, the outrigger beam was found to transfer the majority of diaphragm forces directly to the core wall with negligible participation by the floor. Therefore, floor slab–wall connections can be based on simple details that resist only gravity loads unless the connections are specifically designed to transfer the diaphragm forces to the wall.

Shear wall systems are one of the most commonly used lateral-load resisting systems in high-rise buildings. The composite shear wall system studied herein consists of a steel plate shear wall with a reinforced concrete wall attached to one side of it using bolts. In this paper, experimental studies of three-story composite shear wall specimens are presented and test results are discussed. Two half-scale specimens were tested and both showed highly ductile behavior and stable cyclic postyielding performance. The specimens were able to tolerate 33 cycles of shear displacements and reach maximum interstory drift of more than 0.05. Here the interstory drift is defined as lateral movement of the floor over the story height. The bolts connecting the reinforced concrete walls to steel plate shear walls were able to ensure the composite action by bracing the steel plate shear wall to the reinforced concrete shear wall and preventing the overall buckling of steel plates. During late cycles and after shear yielding of the steel plate, inelastic local buckling of the steel plate shear wall occurred in the areas between the bolts. The experimental results and their implication in seismic design are summarized and discussed.

This paper presents an investigation of the seismic behavior of hybrid coupled wall systems, in which steel beams are used to couple reinforced concrete shear walls. System response is studied using transient finite element analyses that account for the effect of concrete cracking, crushing, and steel yielding. Shear stresses in the concrete constitutive model are handled separately from normal stresses, which allows concrete to undergo high shear deformations without causing premature concrete crushing in the model. The developed finite element models are validated through comparisons with more refined models and test data. Suites of analyses are conducted to investigate pertinent parameters including hazard level, earthquake record scaling, dynamic base shear magnification, interstory drift, shear distortion, coupling beam plastic rotation, and wall rotation. The analyses show that hybrid coupled walls are particularly well suited for use in regions of high seismic risk.

This paper describes an analytical investigation of a new type of hybrid coupled wall system for seismic regions. Coupling of concrete walls is achieved by posttensioning steel beams to the walls using unbonded posttensioning tendons. Different from conventional hybrid coupled walls, the coupling beams of the new system are not embedded into the walls. The effect of structural design parameters such as the amount of posttensioning, beam properties, and wall properties on the behavior of multistory coupled walls under lateral loads, including the amount of coupling, energy dissipation, and displacement capacity is investigated. Systems with precast concrete walls as well as monolithic cast-in-place reinforced concrete walls are considered. The behavior of posttensioned coupled wall systems is compared with the behavior of systems with embedded steel coupling beams and systems without coupling. Design tools to estimate the nonlinear lateral load-displacement behavior of the walls are developed by quantifying selected limit states for the walls. The results indicate that posttensioned hybrid coupled walls with initial stiffness similar to walls with embedded steel coupling beams can be designed to provide stable levels of resistance under lateral loads over large nonlinear cyclic deformations. The degree of coupling between the walls can be controlled by changing the amount of posttensioning in the beams, as well as other beam and wall properties.

In this paper, results are presented from tests of four reinforced concrete (RC) column-steel beam (RCS) subassemblies under large displacement reversals and dynamic analyses of RCS systems under various ground motions. The test specimens were designed following a strong column-weak beam philosophy and a deformation-based capacity design method for the connections. The effect of joint deformations on overall behavior of RCS frame systems was studied through a series of inelastic dynamic analyses performed on a six-story RCS frame with various joint modeling and design parameters. Results from this research show that RCS frame systems perform satisfactorily under seismic excitations. The test specimens showed a good strength and stiffness retention capacity with excellent energy dissipation up to displacement levels of approximately 5.0% drift. The joint deformation-based capacity design procedure was effective in controlling damage in the connection region, and thus forcing most of the inelastic activity to concentrate in the beam ends. From the analytical studies, it was found that joint deformations have a significant influence on maximum interstory drift of RCS frames. For the ground motions considered, an increase of up to 35% in maximum story drift was obtained when connection flexibility was considered in the analyses.

Partially restrained connections and their interaction with the floor slab have received recent attention due to numerous failures of welded steel moment frames during the 1994 Northridge and 1995 Kobe earthquakes. After the earthquakes, it was apparent that the “simple” connections, including the slab, served as a “backup” structural system after the full moment connections failed. These connections significantly influenced the behavior of the lateral load resisting system. This paper reports on the results of a unique, full-scale bidirectional test on a composite connection. The connection behaved very well up to a uniaxial drift of about 2%. After that drift level, composite action began to deteriorate and the connection strength and stiffness declined. The results indicate that two previously unreported failure modes may need to be considered in the design of composite buildings if large cyclic bidirectional forces are present. One is a “punch through” of the column web due to the force transfer from the “pinned” side of the connection. The other is the formation of a concrete wedge as a result of the large bearing forces between the slab and steel column flange. As a result of this observation, a suggestion that the full depth of the slab be maintained for a region 150 mm (6 in.) around the joint has been incorporated in the commentary to Part II of the 2002 AISC Seismic Specification.

Two frame specimens with reinforced concrete columns and steel beams, which had different beam-column joint detailing, were analyzed using the nonlinear three-dimensional finite element method (FEM). One specimen had through-beam-type beam-column joints with face bearing plates, and the failure mode was beam yielding. The other specimen had through-beam-type beam-column joints with cover plates, and the failure mode was joint shear failure. Analysis of the latter was conducted before testing to predict the behavior of the specimen. FEM analysis that took into consideration the interaction between the reinforcement and concrete and that between the steel and concrete simulated the behavior of the frame specimen well. The failure process and shear resisting mechanisms of differently shaped beam-column joints (interior, exterior, interior top, and corner) were understood by analytical results of the stress–strain relationship among concrete elements, shear force of the beam-column joints, the contribution of shear resisting elements, and deformation components, which were not obtained in the experiment.

Seismic design guidelines for composite reinforced concrete and steel (RCS) buildings consisting of reinforced concrete columns and structural steel beams were proposed in Japan as a practical outcome of the U.S.–Japan Cooperative Earthquake Research Program on Composite and Hybrid Structures. A design method satisfying the most prevalent seismic provisions used in Japan, which are based on the working stress design method, is described. At the same time, an alternative method, which is suitable for use in the context of the capacity spectrum method, is also briefly introduced. This paper outlines the design guidelines, giving priority to an explanation of the design method for the working stress seismic provisions. In this method, the weak beam–strong column mechanism is used as the design basis for buildings under severe earthquake. Therefore, moment magnification factors for both columns and joint panels are provided to secure the mechanism. Design equations for the ultimate shear strength of joint panels and associated hysteresis models are also included in the guidelines, and can be applied to a total of twelve details of RCS beam–column joints, including both the through beam and the through column types.

A mechanical modeling approach for the simulation of partially restrained (PR, or semirigid) steel and composite connections subjected to any loading regime is described, and verification studies utilizing this approach are presented. This paper first presents a description of the advantages and drawbacks of a previously published component model, which served as the basis for developments discussed here. An original mechanical model is then introduced and described in detail. This model is capable of taking into account the influence of all the main deformation components, including slip in the bolts, partial interaction between the concrete slab and steel girder, shear deformation of the panel zone, and cracking and crushing of the slab. The model also evaluates the energy dissipated by each component, leading to a clear understanding of the connection behavior. Preliminary validation analyses are then presented and discussed. The results indicate that this approach can predict the complex behavior of PR composite connections well, and that the computation effort is minimal when compared to more sophisticated approaches.

A total of three beam-to-column joints for composite structural systems consisting of reinforced concrete columns and steel (RCS) beams, which are referred to as “composite RCS systems,” were tested to investigate the structural performance and the stress transferring mechanisms as a part of the studies on “Composite and Hybrid Structures” of the U.S.–Japan cooperative earthquake research program, which started in April 1993 as a five-year research program. The joint details examined were “through-column type,” of which the joints are reinforced by cover plates and horizontal stiffeners without beam flanges. This paper indicates that the use of thicker cover plates and extended face bearing plates are effective to enhance the structural performance of the joints. Stress transferring mechanisms in the joints are also discussed using three macromodels.

Extensive research over the past 15 years has led to improved models and criteria for seismic design of composite steel–concrete moment frames and their components. Much of this research has been conducted through the U.S.–Japan Cooperative Research Program on Composite and Hybrid Structures. Research on reinforced concrete and steel moment frame structures includes (a) tests and analyses of over 50 composite beam–column connections, (b) tests and analyses of two composite frames, and (c) performance-based seismic design and simulation studies of prototype composite frame buildings. These projects, together with prior research, demonstrate that composite frames can be designed with seismic deformation capacity and toughness comparable to traditional steel or reinforced concrete construction.