Joseph A. Main

Testing and Analysis of Steel and Concrete Beam-Column Assemblies under a Column Removal Scenario

This paper presents an experimental and computational assessment of the performance of steel and reinforced concrete beam-column assemblies under monotonic vertical displacement of a center column, simulating a column removal scenario. The assemblies represent portions of structural framing systems designed as intermediate moment frames (IMFs) and special moment frames (SMFs) for Seismic Design Categories C and D, respectively. The steel IMF and SMF assemblies were designed in accordance with ANSI/AISC 341-02 by using prequalified moment connections specified in FEMA 350. The concrete IMF and SMF assemblies were designed and detailed in accordance with ACI 318-02 requirements. Each full-scale assembly comprises two beam spans and three columns, and downward displacements of the center column are imposed until failure. The study provides insight into the behavior and failure modes of the assemblies, including the development of catenary action. Both detailed and reduced finite-element models are developed,...

Performance of Steel Moment Connections under a Column Removal Scenario. I: Experiments

AbstractThis paper presents an experimental study of two full-scale steel beam-column assemblies, each comprising three columns and two beams, to (1) define their response characteristics under a column-removal scenario, including the capacity of the beams and their connections to carry loads through catenary action, and (2) provide experimental data for validation of beam-to-column connection models for assessing the robustness of structural systems. The assemblies represent portions of the exterior moment-resisting frames of two 10-story steel-frame buildings. One test specimen had welded unreinforced flange, bolted web connections, and the other had reduced beam–section connections. When subjected to monotonically increasing vertical displacement of the unsupported center column, both specimens exhibited an initial elastic response dominated by flexure. With increased vertical displacement, the connections yielded, and axial tension developed in the beams. The axial tension in the beams increased until...

Performance of Steel Moment Connections under a Column Removal Scenario. II: Analysis

AbstractThis paper presents a computational investigation of the response of steel beam-column assemblies with moment connections under monotonic loading conditions simulating a column removal scenario. Two beam-column assemblies are analyzed, which incorporate (1) welded unreinforced flange bolted web connections, and (2) reduced beam section connections. Detailed models of the assemblies are developed, which use highly refined solid and shell elements to represent nonlinear material behavior and fracture. Reduced models are also developed, which use a much smaller number of beam and spring elements and are intended for use in future studies to assess the vulnerability of complete structural systems to disproportionate collapse. The two modeling approaches are described, and computational results are compared with the results of the full-scale tests described in the companion paper. Good agreement is observed, demonstrating that both the detailed and reduced models are capable of capturing the predominan...

Composite Floor Systems under Column Loss: Collapse Resistance and Tie Force Requirements

AbstractThis paper presents a computational assessment of the performance of steel gravity framing systems with single-plate shear connections and composite floor slabs under column loss scenarios. The computational assessment uses a reduced modeling approach, while comparisons with detailed model results are presented to establish confidence in the reduced models. The reduced modeling approach enables large multibay systems to be analyzed much more efficiently than the detailed modeling approaches used in previous studies. Both quasi-static and sudden column loss scenarios are considered, and an energy-based approximate procedure for analysis of sudden column loss is adopted after verification through comparisons with direct dynamic analyses, further enhancing the efficiency of the reduced modeling approach. Reduced models are used to investigate the influence of factors such as span length, slab continuity, and the mode of connection failure on the collapse resistance of gravity frame systems. The adequ...

Wind Effects on Low-Rise Metal Buildings: Database-Assisted Design versus ASCE 7-05 Standard Estimates

Peak bending moments are compared for a set of steel portal frames of industrial buildings in an open terrain calculated using database-assisted design (DAD) techniques and ASCE 7-05 Standard plots. The comparisons indicate that, depending on the building dimensions, the peak bending moments at the knee based on DAD techniques are generally larger by 10–30% than their counterparts based on the ASCE 7-05 plots. (In one case with a relatively steep roof slope of 26.6° the discrepancies exceed 70%.) For the buildings considered, the discrepancies increase with increasing roof slope and with increasing eave height.

Modeling and Analysis of Single-Plate Shear Connections under Column Loss

AbstractThis paper presents a computational assessment of the behavior of single-plate shear (shear tab) connections in gravity frames under column-loss scenarios. Two-span beam assemblies without floor slabs are considered under push-down loading of the unsupported center column. Both detailed and reduced modeling approaches are used in the computational assessment, and comparisons with experimental data are presented to establish confidence in the models. The models are used to investigate the influence of factors such as end support conditions, span length, connection strength, and postultimate connection behavior on the collapse resistance of gravity framing systems. Rotational capacities of single-plate shear connections under column-loss scenarios are, in some cases, less than half the values based on seismic test data, owing to the axial extension imposed on the connections in addition to rotation.

Testing and Analysis of Steel Beam-Column Assemblies under Column Removal Scenarios

This paper presents an experimental and analytical assessment of the performance of steel beam-column assemblies with two types of moment-resisting connections under vertical displacement of a center column, simulating a column removal scenario. The connections considered include (1) a welded, unreinforced flange, bolted web connection and (2) a reduced beam section connection. The test configuration in both cases consisted of two beam spans and three columns. The two exterior column bases were anchored to the strong floor of the testing laboratory, and two diagonal braces were rigidly attached to the top of each exterior column. A downward vertical displacement of this column was imposed using a hydraulic ram, to simulate a column removal scenario. Load was applied under displacement control until connection failure occurred. The study provides insight into the behavior and failure modes of the connections, including their ability to carry tensile forces that develop in the beams. Both detailed and reduced finite element models of the connections are developed that capture the primary response characteristics and failure modes. The detailed models are capable of resolving the nonlinear behavior and failure in great detail, while analyses with the reduced models can be executed much more rapidly, facilitating implementation in models of entire structural systems. Good agreement is observed between the experimental and computational results, providing validation for the detailed and reduced models.

Robustness of Precast Concrete Frames: Experimental and Computational Studies

This paper describes both full-scale testing and detailed finite-element modeling of a precast concrete moment-frame assembly extracted from the perimeter moment frame of a 10-story prototype building. The assembly comprises two beam spans and three columns, and the unsupported center column is subjected to monotonically increasing downward displacement to simulate a column loss scenario. Failure of the assembly was due to non-ductile fracture of the bottom anchorage bars near the welded connection to the center column. Component-level testing of the welded connection detail revealed reductions in ductility of the anchorage bar in the heat-affected zone where the bar was welded to a connecting angle. Finite element analyses revealed that large bending moments, due to eccentricities in the welded connection details, also contributed to the premature fracture of the anchorage bars in the moment-frame assembly. Finite element analyses and comparisons with experimental measurements also provide insight into the load-carrying mechanisms of the precast concrete assembly, including initial flexural action followed by arching action.

Robustness Assessment of RC Frame Buildings under Column Loss Scenarios

A computational assessment of the robustness of reinforced concrete (RC) building structures under column loss scenarios is presented. A reduced-order modeling approach is presented for three-dimensional RC framing systems, including the floor slab, and comparisons with high-fidelity finite element model results are presented to verify the approach. Pushdown analyses of prototype buildings under column loss scenarios are performed using reduced numerical models, and an energy-based procedure is employed to account for the dynamic effects associated with sudden column loss. The load-displacement curve obtained using the energy-based approach is found to be in good agreement with results from direct dynamic analysis of sudden column loss. A measure of structural robustness is defined by normalizing the ultimate capacity under sudden column loss by the applicable service-level gravity loading. The procedure is applied to two prototype 10-story RC buildings, one employing intermediate moment frames (IMFs) and the other employing special moment frames (SMFs), and the SMF building, with its more stringent seismic design and detailing, is found to have greater robustness. INTRODUCTION Although computational and experimental studies on collapse resistance of RC beamcolumn subassemblies or planar frames were reported by researchers in recent years (Bao et al. 2008, Yi et al. 2008, Bao et al. 2012, and Lew et al. 2011), limited studies have been done on RC floor systems or realistic buildings. Previous studies of steel frame buildings have found that the floor slab contributes significantly to the collapse resistance of structures (Alashker et al. 2010). Experimental investigations on floor systems or realistic building structures are usually costly and [unrepeatable]. Numerical simulation provides an alternative approach. However, the challenge for numerical investigation is to develop [reliable model] that can be used in the analyses of large-scale structures with [affordable computational costs]. In this study, [experimentally calibrated models] for [beam-column frames] are used to develop reduced models for floor systems. The developed models are verified by comparing analysis results with high-fidelity finite element models. Using the [developed] reduced models, a collapse assessment approach is proposed and [descripted in great details]. An energy-based approximate procedure for analysis of sudden column loss, previously proposed by Powell (2003) and Izzuddin et al. (2008), is also considered and verified computationally, which enables the structural capacity under sudden column loss to be evaluated using the results of a single pushdown analysis. A metric for structural robustness is defined by normalizing the ultimate capacity under sudden column loss by the applicable service-level gravity loading. Two 10-story prototype buildings, which were designed for different seismic design categories, are evaluated for the potential loss of a first story column based on the proposed assessment approach. One building was designed for Seismic Design Category C (SDC C) and employs intermediate moment frames (IMFs), and the other was designed for Seismic Design Category D (SDC D) and employs special moment frames (SMFs). Full-scale beam-column assemblies from the prototype buildings have been tested to characterize the beam-to-column joint behavior (Lew et al. 2013) and to provide experimental data for validation of detailed and reduced numerical models (Bao et al. 2012). The results of the robustness assessment procedure show that the SMF building, with its more stringent seismic design and detailing, has greater robustness. FLOOR SYSTEM MODELING Two finite element models were developed to study the response characteristics of a two-bay by two-day prototype floor system. One is a detailed model with a total number of about 217,000 elements, including beam elements representing reinforcing bars and solid elements representing concrete. The other is a reduced model which consists of around 1700 shell elements representing slabs and 230 elements for beams. The plan view of the prototype floor system is shown in Fig. 1 and its reinforcement details are listed in Table 1. A bilinear stress-strain relationship is assumed for reinforcing bars with yield strength of 400 MPa and ultimate strength of 520 MPa. The corresponding plastic fracture strain is 15.6 %. Concrete compressive strength is assumed to be 25 MPa.

Development of 3D Models of Steel Moment-Frame Buildings for Assessment of Robustness and Progressive Collapse Vulnerability

Several prototype steel moment-frame buildings have been designed for the purpose of assessing their vulnerability to progressive collapse. The buildings were designed for moderate and high seismic regions. This paper summarizes the development of three-dimensional finite element models of these prototype buildings, with a focus on the modeling approach used for the connections and the composite floor system. Initial simulation results under a column removal scenario are presented to illustrate the model capabilities. Ongoing assessments of reserve capacity and progressive collapse vulnerability using these models are part of a larger study aimed at quantifying and comparing the relative robustness of different structural systems.