A Study on the Collapse Control Design Method for High-Rise Steel Buildings

A Study on the Collapse Control Design Method for High-rise Steel Buildings by Akira Wada 1, Kenichi Ohi 2, Hiroyuki Suzuki 3, Mamoru Kohno 4 and Yoshifumi Sakumoto 5 ABSTRACT 1. INTRODUCTION Two direct causes led to the collapse on The collapse of the World Trade Center towers September 11, 2001 of the World Trade Center (WTC1 and WTC2) was the direct result of towers: column damage caused by aircraft crash column damage and large-scale fires caused by and the resulting large-scale fires. In spite of this airplane crashes. In spite of this, WTC1 and damage, the towers remained standing after the WTC2 remained standing for 102 minutes and crashes for 102 and 56 minutes, respectively, 56 minutes respectively, during which many during which many lives were saved. The lives were saved. The fact that so many lives collapse of the WTC towers, however, may be were saved is reportedly due to the large taken as an alert that local failures can trigger a deformation capacity or load redistribution progressive collapse. It was also a landmark capacity inherent in steel structures [1]. From event in that it alerted construction engineers to this, it can be understood that the tower the importance of preventing progressive structures of the World Trade Center (hereinafter collapse in similar structures. referred to as “WTC”) had a certain degree of redundancy. Nevertheless, the WTC collapse Prevention of progressive collapse requires the serves as a warning about progressive collapse development of design technologies for frames triggered by a local collapse that causes an that have high redundancy. The Japan Iron and entire building collapse. It was a landmark event Steel Federation together with the Japanese that alerted construction engineers to the Society of Steel Construction established the importance of preventing progressive collapse in committee on “The Study on Redundancy of other similar buildings. High-Rise Steel Buildings” in June 2002 in an attempt to study and provide a better The British Standards and Building Standards understanding on progressive collapse by [2] were the first to incorporate the prevention collaboration with Council on Tall Buildings & of progressive collapse in design standards. The Urban Habitat. This paper presents a new incorporation of measures against progressive collapse control design method for high-rise collapse was based on proving through steel building structures. The basic concept of experience and was made to prevent the kind of the present collapse control design methods is to progressive collapse attributed to a gas save human lives. Therefore, the method explosion in 1968 in a 22-story high-rise presented here to prevent progressive collapse residential building in Ronan Point, United until the completion of evacuation makes Kingdom. Further, in the Building Standards of assumptions about which structural members are 1 Professor, Tokyo Institute of Technology, Ookayama likely to be lost and proposes the idea of ‘key 2-12-1, Meguro, Tokyo 152-8550, Japan elements’ that are linked with a building’s core 2 Professor, Kobe University, Rokkodai-machi 2-1, Nada, section to serve as the evacuation route and Kobe 657-8501, Japan consist of structural members indispensable for 3 Professor, University of Tsukuba, Tenodai 1-1-1, supporting redistributed vertical loads. Tsukuba 305-8573, Japan 4 Head, Fire Standards Division, Building Department, KEYWORDS: Collapse Control Design, Key National Institute for Land and Infrastructure Element, Progressive Collapse Management (NILIM), Tsukuba 305-0802, Japan 5 General Manager, Nippon Steel Corporation, Otemachi 2-6-3, Chiyoda, Tokyo 100-8071, Japan New York City (NYC Standards) established in unexpected loads or to accident and where February 2003, the following recommendation vertical load supporting members lose was made regarding the prevention of functionality due to large-scale fire, it is progressive collapse such as that seen in the important to provide measures whereby local WTC collapse. collapse does not lead to entire collapse. To achieve this goal, it is necessary to increase “Recommendation 1: Publish structural design vertical load redistribution capacity by providing guidelines for optional application to ensure back-up systems for multiplying the number of robustness and resistance to progressive loading routes, as shown in Table 1. Further, it is collapse.” necessary to secure the plastic deformation capacity and fire resistance of individual steel Meanwhile, studies are now underway along members and joints between them. with extensive discussions in a variety of related fields regarding the development of a simple, High-rise steel buildings constructed in Japan practical design method. In order to suppress using seismic-resistant design have surplus progressive collapse, it is necessary to develop a capacity vis-à-vis stationary vertical loads and technology for designing frames with high employ connections with appropriate redundancy. With this in mind, the Japan Iron load-bearing capacity for the joints. Because of and Steel Federation established the Committee this, it is believed that vertical load to Study the Redundancy of High-Rise Steel redistribution capacity can be increased with Buildings within the Japanese Society of Steel minimal added cost. Further, as stated in the Construction; this committee has carried out the following, the application of SN steel (low following studies aimed at improving the safety yield-point high performance steel), of high-rise buildings: fire-resistant (FR) steel and concrete-filled steel ・ A study of collapse control design methods tube (CFT) structures facilitates improved based on seismic- and fire-resistant plastic deformation capacity in remaining technologies used in Japan, and members when some columns are lost and ・ A study to quantify the redundancy of during fire. high-rise steel buildings in Japan aimed at producing a frame with high redundancy. 3. ASSESSMENT METHOD In this paper, findings obtained from the 3.1 Setting Targets collapse of the WTC are described and a method to prevent progressive collapse is examined. Fig. 2 shows the difference between the Further, a collapse control design method that concepts employed in the present collapse can prevent the occurrence of progressive control design (right) and those found in collapse is outlined. conventional structural and fire-resistant designs (left). 2. FINDINGS FROM WTC COLLAPSE Generally, it is difficult and uneconomical to In order to structure a progressive collapse conduct structural design by assuming control design method for high-rise buildings accidental loads due to extreme events. with higher redundancy, the Committee to Study Accordingly, in contrast to conventional the Redundancy of High-Rise Steel Buildings methods, the present design method assesses and organized the causes of the WTC collapse with improves the redundancy of buildings by reference to the available literature [1] and then assuming the loss of structural members such as outlined its findings. Fig. 1 shows the study columns and beams due to accidents and results for the cause of the WTC collapse. From assessing how many members might be lost and this figure, it is understood that in cases where the probability of entire collapse occurring. vertical load supporting members are lost due to Because it is fair to expect that fire separations progressive collapse, the present design method will break and that fire will spread not only aims to compensate for loss or decline in the horizontally but also vertically, it is necessary yield strength of members that support vertical when estimating member loss to pay attention to loads. In the initial design stage, structural the effect (increasing the degree of loss) that fire designers judge whether or not to apply the will have. collapse control design method, taking into account the risk of explosions and airplane Based on the above, designers discuss whether crashes in the building under consideration. or not a structure is designed both in terms of Buildings exposed to limited risks may not structure and fire resistance to compensate for require a collapse control design method; only a the loss of members and whether or not collapse conventional design method will be selected in control design is to be applied. When collapse these cases. control design is used, the key-element members are specified in the frame design according to an Further at this stage of design, the potential scale assessment flow as described in the next section. of column member loss is assumed by taking Priority is given to protecting the key-element into account the degree of risk involved and the members so as to improve building redundancy. importance of the building, i.e. the effect it would have in the case of collapse. The British 3.2 Assessment Flow Standards and Building Standards [2] prescribe the prevention of progressive collapse even in Fig. 3 shows an outline of assessment flow. In the case of one column being lost. In cases when the following, the present collapse control the design of a building requires more design method is explained in terms of appropriate redundancy, it is desirable to assessment flow. determine the number of columns to be lost in the design. More practical determination of the 3.2.1 Assessing Risk and Judging Whether or members to be lost can be made after fixing the Not to Use Collapse Control Design sectional dimensions of the members by means When considering the probability of explosions

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