Yihai Bao

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,...

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.

Evaluation of Structural Robustness against Column Loss: Methodology and Application to RC Frame Buildings

A computational methodology is presented for evaluating structural robustness against column loss. The methodology is illustrated through application to reinforced concrete (RC) frame buildings, using a reduced-order modeling approach for three-dimensional RC framing systems that includes the floor slabs. 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 the reduced-order modeling approach, and an energy-based procedure is employed to account for the dynamic effects associated with sudden column loss. Results obtained using the energy-based approach are found to be in good agreement with results from direct dynamic analysis of sudden column loss. A metric for structural robustness is proposed, calculated by normalizing the ultimate capacities of the structural system under sudden column loss by the applicable service-level gravity loading and by evaluating the minimum value of this normalized ultimate capacity over all column removal scenarios. 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). The SMF building, with its more stringent seismic design and detailing, is found to have greater robustness.

Alternative Load Path Analysis of a Prototype Reinforced Concrete Frame Building

This paper presents alternative load path analysis of a 5-bay by 5-bay, 10-story prototype reinforced concrete moment frame building under column loss scenarios. This prototype building is used for example analysis problems in the Alternative Load Path Analysis Guidelines, which are being developed by the Disproportionate Collapse Technical Committee of the Structural Engineering Institute, and the analyses presented in this paper have been submitted for inclusion in these guidelines as part of the chapter on advanced numerical modeling. The prototype building was designed for seismic design category C, for a location in Atlanta, GA. Intermediate moment frames were selected as the lateral force-resisting system and were designed in accordance with the requirements of the American Concrete Institute 318 Building Code. A twoway concrete slab was used at each floor level, including the roof. Using a reducedorder finite-element modeling approach, full-building analyses are presented under three different first-story column loss scenarios, including a corner column, an edge column, and an interior column. Results from nonlinear dynamic analysis (i.e., sudden column removal) are compared with results from nonlinear static analysis (i.e., pushdown analysis) using an energy-based procedure to account for the dynamic effects of sudden column loss. The robustness index for the building is also evaluated based on its ultimate capacity under the same set of sudden column loss scenarios.