«R. SHANKAR NAIR R. Shankar Nair R. Shankar Nair, Ph.D., P.E., S.E. is a principal and senior vice president of Teng & Associates, Inc. in Chicago. In ...»
Both assumptions are reasonable for components in building frames subjected to earthquake shaking. However, neither assumption is valid for components in the immediate vicinity of air blast because such pressure loadings can produce gross damage and failure as shown in the numerical simulations of Figure 7. Further, the component models for earthquake simulation are based primarily on cyclic testing in the absence of significant axial load: testing conditions that are clearly inappropriate for components resisting progressive collapse.
The EDPs of Table 2 for performance-based blast engineering are virtually identical to those for earthquake engineering. Demand-to-capacity (D/C) ratios are useful when linear methods of analysis are employed but calibration of D/C ratios to Damage Measures (DMs) using nonlinear response-history simulation is required.
Values for the remaining EDPs could be output by nonlinear response simulations to develop DMs. Different conditional probabilities p( DM | EDP) will result from air-blast and progressive collapse type loadings. Much fullscale experimental testing will be required to both facilitate such calculations of conditional probabilities and calibrate existing component models for blast and progressive-collapse analysis (Crawford et al. 2001).
Decision Variables (DVs) in the form of performance objectives have been identified for use in performance-based earthquake engineering. Details are provided in Table 1. Table 3 provides similar information for performancebased blast engineering. The proposed performance levels, damage descriptions and downtime estimates are preliminary and mutable, and are presented only to kindle discussion on DVs for performance-based blast engineering.
SUMMARY REMARKSExtreme events such as blast loadings and severe earthquake shaking will generally induce nonlinear behavior in building frames and produce substantial nonstructural damage. Although the current prescriptive procedures for design against blast and earthquake loadings might produce buildings of acceptable safety, the procedures are indirect, of unknown reliability, and might result in inefficient and costly construction. Performance-based engineering should facilitate design and construction of buildings with a realistic and reliable understanding of the risk of loss (physical, direct economic and indirect economic) that might occur as a result of future blast attack, earthquake shaking (or both).
The second-generation performance-based earthquake engineering methodology, which is being adopted for the ATC-58 project, is applicable conceptually to performance-based blast engineering. Sample intensity measures, engineering demand parameters, and performance levels for use in performance-based blast engineering were presented to foster discussion. Key similarities and differences between performance approaches for blast and earthquake engineering were identified.
ACKNOWLEDGEMENTSThe authors wish to acknowledge the contributions of Mr. Craig Comartin, S.E., of Comartin-Reis, Stockton, CA;
Messrs. Donald Dusenberry, S.E., and Dominic Kelly, S.E., of Simpson, Gumpertz and Heger, Boston, MA; Mr.
John Crawford, P.E., of Karagozian & Case, Glendale, CA; Mr. Jesse Karns, S.E., of Myers, Houghton & Partners, Long Beach, CA; and Drs. Charles Welch and Paul Mlakar of the U.S. Army Engineer Research and Development Center, Vicksburg, MS; to the preparation of this paper. These contributions, both direct and indirect, are gratefully acknowledged.
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This paper provides an overview of key issues related to the survivability of steel buildings subjected to explosive load incidents, and an outline of required research to address some of the problems that were identified in previous studies.
Explosive loads associated with high explosive devices are expected to induce significant localized structural damage that could evolve into massive structural collapse. Recent numerically simulated responses of individual structural steel elements and connections to such loads have raised serious concerns about their ability to survive explosive loading incidents. Blast resistant structural systems are designed according to various guidelines, some of which are based on simplified assumptions whose suitability might be questioned. Furthermore, the relationships between localized structural damage and numerically-simulated progressive collapse have highlighted very complicated nonlinear dynamic phenomena. These phenomena require further investigation using more realistic representations of the corresponding issues.
INTRODUCTION AND BACKGROUND
Explosive loading incidents have become a serious problem that must be addressed quite frequently. Many buildings that could be loaded by explosive incidents are moment resistant steel frame structures, and their behavior under blast loads is of great interest. Besides the immediate and localized blast effects, one must consider the serious consequences associated with progressive collapse that could affect people and property in an entire building. Progressive collapse occurs when a structure has its loading pattern, or boundary conditions, changed such that structural elements are loaded beyond their capacity and fail. The residual structure is forced to seek alternative load paths to redistribute the load applied to it. As a result, other elements may fail, causing further load redistribution. The process will continue until the structure can find equilibrium either by shedding load, as a by-product of other elements failing, or by finding stable alternative load paths. In the past, structures designed to withstand normal load conditions were over designed, and have usually been capable of tolerating some abnormal loads. Modern building design and construction practices enabled one to build lighter and more optimized structural systems with considerably lower over design characteristics. Progressive collapse became an issue following the Ronan Point incident (HMSO, 1968), when a gas explosion in a kitchen on the 18 th floor of a precast building caused extensive damage to the entire corner of that building, as shown in Figure 1. The failure investigation of that incident resulted in important changes in the UK building code (HMSO, 1976). It requires to provide a minimum level of strength to resist accidental abnormal loading by either comprehensive ‘tying’ of structural elements, or (if tying is not possible) to enable the ‘bridging’ of loads over the damaged area (the smaller of 15% of the story area, or 70 m 2), or (if bridging is not possible) to insure that key elements can resist 34 kN/m 2. These guidelines have been incorporated in subsequent British Standards (e.g., HMSO 1991, BSI 1996, BSI 2000, etc.). Although many in the UK attribute the very good performance of numerous buildings subjected to blast loads to these guidelines, it might not be always possible to quantify how close those buildings were to progressive collapse.
Recent developments in the efficient use of building materials, innovative framing systems, and refinements in analysis techniques could result in structures with lower safety margins. Both the Department of Defense (DoD) and the General Services Administration (GSA) have issued clear guidelines to address this critical problem (DoD 2002, GSA 2003).
Nevertheless, these procedures contain assumptions that may not reflect accurately the actual post attack conditions of a damaged structure, as shown in Figures 2a and 2b, that highlight the very complicated state of damage that must be assessed before the correct conditions can be determined. The structural behavior associated with such incidents involves highly nonlinear processes in both the geometric and material domains. One must understand that various important factors can affect the behavior and failure process in a building subjected to an explosive loading event, but these cannot be easily assessed. The idea that one might consider the immaculate removal of a column as a damage scenario, while leaving the rest of the building undamaged, is unrealistic. An explosive loading event near a building will cause extensive localized damage (e.g., terrorist attacks in London, Oklahoma City, etc.), affecting more than a single column. The remaining damaged structure is expected to behave very differently from the ideal situation. Therefore, it is critical to assess accurately the post attack behavior of structural elements that were not removed from the building by the blast loads in their corresponding damaged states. This requires one to perform first a fully-nonlinear blast-structure interaction analysis, determine the state of the structural system at the end of this transient phase, and then to proceed with a fullynonlinear dynamic analysis for the damaged structure subjected to only gravity loads. Such comprehensive analyses are very complicated, they are very time consuming and require extensive resources, and they are not suitable for design office environments.
Figure 2 Post-Incident View of Building Damage from the 1992 St. Mary’s Axe Bombing Incident in London Damaged structures may have insufficient reserve capacities to accommodate abnormal load conditions (Taylor 1975, and Gross 1983). So far, there are few numerical examples of computational schemes to analyze progressive collapse.
Typical finite element codes can only be used after complicated source level modification to simulate dynamic collapse problems that contain strong nonlinearities and discontinuities. Several approaches have been proposed for including progressive collapse resistance in building design. The alternative load path method is a known analytical approach that follows the definition of progressive collapse (Yokel et al. 1989). It refers to the removal of elements that failed the stress or strain limit criteria. In spite of its analytical characteristics, alternative load path methods are based on static considerations, and they may not been be adequate for simulating progressive collapse behavior. Choi and Krauthammer (2003) described an innovative approach to address such problems by using algorithms for external criteria screening (ECS) techniques applicable to these types of problems. As a part of such ECS, element elimination methods were classified into direct and indirect approaches, and compared with each other. A variable boundary condition (VBC) technique was also proposed to avoid computational instability that could occur while applying the developed procedure.