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 beamcolumn 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. INTRODUCTION The National Institute of Standards and Technology (NIST) has initiated a research program to study the behavior of structures that, when exposed to abnormal loads, might lead to progressive collapse. At present, design and evaluation of structures for progressive collapse potential are typically based on acceptance criteria obtained from seismic research (e.g., FEMA 350, 2000). As will be shown in this paper, using this approach to predict the response to monotonic loading similar to that expected during progressive collapse underestimates the rotational capacities of the connections. To understand the behavior of structural systems near their ultimate strength limit states and to develop reliable tools to quantify the reserve capacity and robustness of structural systems, the NIST study involves analysis of three-dimensional models of structures with various materials and systems to assess the vulnerability of different types of structural systems to progressive collapse. The three-dimensional analyses use experimentally validated subsystem models of the various components and connections of the structure. The study reported herein covers the development of finite element models of steel moment resisting connections with experimental validation. This paper describes two tests of steel beamcolumn assemblies with selected moment resisting connections under vertical displacement of a center column, representing a column removal scenario. These tests help fill the gap in defining the response characteristics of these connections under monotonic loading, and also contribute to establishing a database of connection behavior that can be used to assess the robustness of structural systems. Finite element models of the tested assemblies are developed and validated with the purpose of understanding the response characteristics and providing input to threedimensional models of complete structural systems to be analyzed in future studies. DESCRIPTION OF BUILDING DESIGNS Prototype steel framed buildings were designed in the NIST study for the purpose of examining their vulnerability to progressive collapse. The buildings are 10-story office buildings with plan dimensions of 100 ft x 150 ft (30.5 m x 45.7 m). The buildings were designed and detailed for two Seismic Design Categories (SDC) to examine the effectiveness of seismic design and detailing in resisting progressive collapse. One building was designed for SDC C, which resulted in a design using intermediate moment frames (IMFs) for the lateral load resisting system and the other for SDC D, which resulted in a design using special moment frames (SMFs) as defined in the American Institute of Steel Construction (AISC) Seismic Provisions (2002). Moment frames, located around the perimeter of both buildings, provided the lateral load resistance. Connections used in the moment frames were selected from the prequalified steel connections specified in FEMA 350 (2000): (1) Welded Unreinforced Flange-Bolted Web (WUF-B) connections for the IMFs in the SDC C building, and (2) Reduced Beam Section (RBS) connections for the SMFs in the SDC D building. Beam-column assemblies consisting of two-span beams connected to three columns (see Figure 2) were selected from the second floor of the moment resisting frames of each of the two buildings for the experimental and computational studies presented herein. The beams had a span length (center to center of columns) of 20 ft (6.10 m). The beams selected from the building in the SDC C zone were W21x73 sections, and were connected to W18x119 columns using WUF-B connections. The beams selected from the building in the SDC D zone were W24x94 sections, and were connected to W24x131 columns using RBS connections. ASTM A992 structural steel (Fy = 50 ksi, 345 MPa) was used in all beams, columns, and doubler plates in the panel zone. ASTM A36 steel (Fy = 36 ksi, 248 MPa) was used for the shear tabs and continuity plates at connections. ASTM A490 high strength bolts were used for the bolted connections, and welding requirements followed the recommendations in FEMA 353 (2000). BEAM-COLUMN ASSEMBLY WITH WUF-B CONNECTIONS Description of WUF-B Connection The WUF-B connection is similar to the connection commonly used prior to the 1994 Northridge earthquake. After significant research, it was determined that, with several improvements and appropriate quality assurance, this connection can perform reliably. FEMA 355D (2000) provides extensive information on the testing and performance of the WUF-B connections under seismic loading. The acceptable values for inter-story drift angle or rotation capacity of the WUF-B connection was specified in FEMA 350 (2000) based on a statistical analysis of the results from cyclic tests of full-scale connections. The rotation capacity, in radians, corresponding to collapse prevention, characterized by the inability of the connection to maintain its integrity under gravity loading, was estimated to be U = 0.060 0.0006db, where db is the beam depth in inches. For the W21x73 section used with the WUF-B connection, U = 0.047 rad. The WUF-B connection used in this study is shown in Figure 1. As shown, the beam web is connected to the column flange using a shear plate (shear tab), which is fillet welded to the column using 5/16 in (8 mm) weld and bolted to the beam web using three 1 in (25 mm) diameter, high strength bolts. The bolt holes are standard holes with an edge distance of 2.75 in (70 mm). The beam flanges are joined to the column flange using complete joint penetration (CJP) groove welds. Weld access holes are cut from the beam flanges per the recommendations of FEMA 350 (2000). Continuity plates are provided for both interior and exterior columns as shown in Figure 1. No doubler plates were required for either column.