Journal of Performance of Constructed Facilities

“Disproportionate collapse” is structural collapse disproportionate to the cause; it is often, though not always, progressive, where “progressive collapse” is the collapse of all or a large part of a structure precipitated by damage or failure of a relatively small part of it. There have been many attempts to develop design guidelines and criteria that would reduce or eliminate the susceptibility of buildings to this form of failure. In recent years, the particular focus has been on the prevention of progressive collapse due to deliberate attack. The present study suggests, however, that these guidelines and criteria may be of limited value. Arguably the most important deficiency in the state of the art of design to prevent disproportionate or progressive collapse is uncertainty about the design event: We have the technology now to design for almost anything, but most recent building failures due to explosions and terrorist attacks have involved insults to the building not anticipated in design guidelines and criteria.

A progressive collapse initiates as a result of local structural damage and develops, in a chain reaction mechanism, into a failure that is disproportionate to the initiating local damage. Such collapses can be initiated by many causes. Changes in building practices to address low probability/high consequence events and to lessen building vulnerability to progressive collapse currently are receiving considerable attention in the professional engineering community and in standard-writing groups in the United States, Canada, and Western Europe. Procedures for identifying and screening specific threat scenarios, for assessing the capability of a building to withstand local damage without a general structural collapse developing, and for assessing and mitigating the risk of progressive collapse can be developed using concepts of probabilistic risk assessment. This paper provides a framework for addressing issues related to low probability/high consequence events in building practice, summarizes strategies for progressive collapse risk mitigation, and identifies challenges for implementing general provisions in national standards such as ASCE Standard 7, Minimum design loads for buildings and other structures.

The behavior of nonmilitary buildings subjected to blast is considered. Case studies from World War II are described, as well as more recent events from the detonation of large vehicle borne devices in the Middle East, North America, and Europe. Conventional methods for nonseismic design are shown to lead to frames with overstrong beams connected together by relatively weak connections. This may explain much of the evidence from bomb damaged buildings in which building connections have been observed to fracture in a brittle manner when subjected to blast. The risk of progressive collapse may be minimized by strengthening beam to column connections located at close proximity to potential vehicle borne devices and a capacity design method for such strengthening is advocated.

On April 19, 1995, a truck loaded with an ammonium nitrate and fuel oil bomb caused collapse of fully half of the total floor area of the nine-story, reinforced concrete Murrah Federal Building in Oklahoma City. The extent of the collapse, which extended well beyond the zone of direct structural blast damage, prompted studies of progressive/disproportionate collapse and development of new design guidelines for important buildings. While there is no question that the collapse was the result of the loss of only four columns, there is a common belief that direct blast effects destroyed three of those columns. Firsthand observation of debris, collapse patterns, damage patterns, and thousands of photographs taken during search and rescue activities at the building suggest the possibility that only one column was destroyed by direct blast effects, while the other three buckled due to loss of lateral support provided by beams and floor diaphragms that were destroyed by the blast. While the distinction may be subtle, it has significant implications for the design of tougher buildings. Specific lessons include ductile detailing, the necessity of maintaining the integrity of a three-dimensional frame, and explicit consideration of structural fuses to protect critical elements.

For several decades, the engineering profession has considered techniques to analyze the potential that structures could experience disproportionate collapse and to design them for greater resistance to such collapse. First interest in such design followed the partial collapse in 1968 of the Ronan Point building in London, a high rise residential structure that experienced full height collapse of a portion of the building following a relatively small kitchen-related gas explosion. Interest in collapse phenomena continued to build following the attack on the Alfred P. Murrah building in 1995 and has been at an apex since the collapses of the twin towers at the World Trade Center and the nearby World Trade Center 7 building in 2001. Presently researchers and engineers are studying structural performance during extreme deformations, systems to resist disproportionate collapse, and methods to analyze collapse potential. The goal is to develop techniques to accurately and cost efficiently assess collapse potential and to enhance robustness at appropriate cost. Analysis methods in common use include sophisticated dynamic, nonlinear modeling of structural systems with high-fidelity structural analysis computer software, and simplified approaches that are intended to capture the essential behaviors during collapse scenarios. Unfortunately, the sophisticated approaches require software not normally owned by design engineers, substantial experience in the modeling of collapse phenomena, and time and cost implications that cannot be supported by the present design fees and, indeed, are not warranted for many situations. Simplified analysis methods in common use are generally empirically based. Hence, they do not capture the essential behaviors of collapse mechanisms, and are of uncertain applicability for all but structural systems for which they have been calibrated. This paper presents two energy-based methods that capture the essential physics of collapse phenomena, and have potential to be developed into simplified procedures for collapse potential assessment.

The existing General Services Administration (GSA) and Department of Defense Unified Facilities Criteria (UFC) guidelines make use of the alternate path approach for evaluation of a structural system to determine susceptibility to progressive collapse. The alternate path approach presumes that one critical or key member, typically a column, is damaged and rendered incapable of supporting load. The remaining structure must be able to span across this lost member. The existing procedures incorporate material nonlinearity through allowable plastic deformations or through the use of a modified static capacity to incorporate plasticity. The procedures also permit an analyst to evaluate the response of a structure either statically or dynamically. Dynamic inertial effects can be considered directly through the equations of motion inherent in a dynamic analysis or considered indirectly through the modification of dead and live loads in a static analysis. Both the GSA and UFC procedures recommend a static “multiplier” of 2.0 to account for these inertial effects. The analysis presented in this paper illustrates that this multiplier may be conservative, resulting in structural designs less efficient than may be otherwise achievable. A dynamic multiplier of 1.5 better captures the dynamic effects when a static analysis is performed, and will result in more economical designs.

We compare four methods for progressive collapse analysis by analyzing a nine-story steel moment-resistant frame building, employing increasingly complex analytical procedures: linear-elastic static, nonlinear static, linear-elastic dynamic, and nonlinear dynamic methodologies. Each procedure is thoroughly investigated and common shortcomings are identified, along with advantages and disadvantages, using side-by-side comparison, including approximate time spent on modeling and computation. The evaluation uses current General Services Administration progressive collapse guidelines. Our objective is to provide clear conceptual step-by-step descriptions of various procedures for progressive collapse analysis by performing example analyses using commercially available structural analysis software, such as SAP2000, with the aim that the explanations in this paper will be clear enough that they will be readily understandable and will be used by practicing engineers. We demonstrate that dynamic analysis procedures not only yield more accurate results, but are also easy to perform for progressive collapse determination. Additionally, we show that current GSA performance limits for linear analysis procedures are unconservative, meaning that a structure designed with acceptable linear evaluation criteria may exceed allowable ductility and rotation limits when nonlinear dynamic analysis is performed on the same structure. Finally, our recommendations for the analysis procedures take into account accuracy as well as ease of use.

During the past decade, increasing attention has been focused on the design of buildings to resist progressive collapse. Previously, the authors presented a nonlinear solution procedure for progressive collapse analysis of planar frame structures. In the current study, a modeling strategy to account for the impact of failed members against other structural components is developed to extend the capabilities of the initial models. Assumptions made in approximating the effects of impact on the overall behavior of frame structures are discussed. An example illustrating the importance of accounting for the effects of impact on predicting progressive collapse is also given. Results indicate that the impact velocity plays the most significant role in causing failure of intact beam elements.

This paper describes progressive collapse assessment of an existing reinforced concrete commercial building. Prescriptive guidelines available to date are evaluated in light of alternate load path predictions. The building, exceedingly meeting ACI integrity requirements and the recent United Facilities Criteria tie force provisions, is still significantly vulnerable to progressive collapse triggered by the loss of an interior column.

When designing a building intended to be resistant to progressive collapse, it is instructive to consider this problem from the point of view of an implosion contractor who regularly demolishes buildings through explosives-induced progressive failure. All buildings want to fall down, but are prevented from doing so through their structural columns, walls and transfer girders. Innumerable ergs of potential energy are just waiting to be released. The implosion contractor creates a progressive collapse by releasing this energy through the sequential explosive removal of key structural supports, allowing gravity to do the remaining work, simultaneously using the minimum amount of explosives, creating the maximum amount of fragmentation, and minimizing the potential fly of debris. In this paper, we will explore several building structural systems and how their implosion has historically been achieved, comparing the amount of effort required in each system to affect an implosion as related to the susceptibility of that type of building to progressive collapse and identifying those types that lend themselves to it. The building structural systems described represent actual case studies. By comparison of different systems from the implosion contractor’s perspective, the design engineer will gain unique knowledge about systems that are inherently resistant to progressive collapse.

One of the most frequently used approaches for minimizing the potential for progressive collapse in buildings is the alternate path method. The appeal of this method is primarily due to its relative simplicity and threat independent specification. Applications of the alternate path method typically employ a component design strategy in which the adequacy of the system is based on individual structural components successfully satisfying the acceptance criteria. This design philosophy is also used in evaluating other extreme loading conditions such as seismic loads and direct blast loads. However, the adequacy of the global structural system is not usually investigated during this component design process. This paper details the importance of investigating global effects when evaluating the potential for progressive collapse in buildings. There are two types of frames that will be evaluated: moment-resisting (sway) frames and nonsway frames that include lateral-force resisting elements, such as shear walls. The necessity for considering the global response of a damaged structure becomes apparent following the evaluation of the overall stability of these systems. In addition, the conclusions concerning progressive collapse investigations will be generalized for application to seismic and direct blast hazards. A simple design and analysis method will be introduced, along with the associated acceptance criteria.

Progressive collapse is one of the most under-researched areas in structural engineering due to the relative scarcity of the circumstances leading to progressive collapse. The current design standards and building codes provide limited prescriptive or performance-based guidance on analysis or design to guard against progressive collapse. The collapse of the World Trade Center on September 11, 2001, led to demands by the public to amend current building codes and provide protection against collapse caused by extreme events. Following September 11, the literature on progressive collapse mitigation has expanded significantly. Important issues examined by investigators include events leading to progressive collapse, assessment of loads, analysis methods, and design philosophy. This paper seeks to explore aspects of the current state of knowledge on progressive collapse in the technical literature. The paper also discusses loads, structural analysis requirements, and design approaches in United States standards and guides as well as selected international building codes and standards.