Progressive Collapse: Emerging Challenges for the Design Professional

Robert Smilowitz is a principal in the Applied Sciences Division of Weidlinger Associates. He serves as adjunct professor of engineering at the Cooper Union. Dr. Smilowitz has more than 29 years of experience participating in protective design and vulnerability studies of numerous federal courthouses, federal office buildings, embassy structures, airline terminals, and commercial properties. He analyzed the World Trade Center’s underground parking garage slabs following the 1993 bombing and the Khobar Towers following the terrorist vehicle bomb attack. He was a member of the ASCE/FEMA World Trade Center Building Performance Study after the terrorist attack of September 11, 2001, and developed protective design retrofits for the Pentagon façade before, and after the aircraft impact. Dr. Smilowitz participated in the explosive testing of full-scale curtainwall systems and is a principal developer of analysis software for evaluating curtainwall response to explosive terrorist threats. He is a GSA National Peer Professional, a National Associate of the National Academies and a Registered Engineer in the states of New York and California. He holds a Ph.D. from the University of Illinois at Champaign–Urbana.Robert Smilowitz

The practice of structural design is very different from what it was decades ago. Sophisticated computer software provides design engineers with considerable analytical capabilities that often result in more efficient and effective designs that satisfy building-code requirements and address both expected and mandated loading conditions. Some of these mandated loading conditions relate to low-probability high-consequence events, such as earthquakes and hurricanes. Modern codes adopt a performance-based approach to these extreme loading conditions, and the trend is to require reduced levels of performance in response to successively greater intensities of loading. Risk management can be quantified, and rational design decisions can be tailored to specific conditions. As a result, performance-based design allows the project team to weigh the risk-reduction measures against the consequences of their design decisions. However, the drive to greater efficiency and economy in design often comes at the expense of robustness and fault tolerance.

Extreme loading (whether accidental or intentional)—and the resulting damaged state—often impose unforeseen demands and unanticipated reductions in capacity. Older structures were able to accommodate some of these unforeseen situations as a result of less efficient construction techniques and imprecision, and therefore conservatism, in the design process. Modern design eliminates many of these undocumented factors of safety while accommodating ever-greater challenges of architectural expression; as a result, modern building design may be more vulnerable to unforeseen conditions during the life of a building. U.S. design professionals addressed some of these concerns through changes in detailing practice; however, the general structural-integrity requirements of ASCE 7-05 are nonspecific in an attempt to “limit the effects of local collapse and to prevent or minimize progressive collapse.” Although debate exists among design professionals about the need for more specificity given the limited number of historic events that resulted in structural collapse, U.S. government construction practice is demanding greater attention to damage-state design requirements, and developers of commercial property are demanding greater robustness of their major construction projects. The American Society of Civil Engineers (ASCE) and the National Institute of Standards and Technology (NIST) are leading the profession toward standards that may some day become design requirements.

The following papers address different technical challenges and observations associated with increased structural robustness. Some of these papers help define the issues, whereas others relate the practices mandated by different criteria. Other papers present observations that are based on bomb-damage data and controlled demolition practice. Risk mitigation for a range of hazardous conditions is compared, and the means to describe the consequences of low-probability events are identified. Many of these papers address the different analytical approaches that may effectively predict the performance of structures as a result of damage-state conditions. As you will note, many conflicting opinions exist among design professionals, and there are calls for additional research to resolve some of these differences of opinion. Although several papers seek the holy grail of simplified analytical methods whereas others incorporate sophisticated nonlinear, dynamic finite-element software, they all serve to stimulate the industry and prompt design professionals to explore new approaches. However, these approaches must be faithful to both the physics and the numerical methods that are required to produce accurate results. The reader is urged to challenge each paper’s methods and assumptions.

Furthermore, all approaches must demonstrate global stability even if local collapse appears to be arrested; for example, catenary behavior can be developed only if the undamaged portion of the structure can resist the catenary forces. Aggressively innovative approaches must be vetted by the design community to determine whether they live up to their claims and representations. Physically meaningful testing must be conducted to validate design concepts and analytical methods; it is important that these tests correctly represent the boundary conditions to accurately capture all possible failure mechanisms.

Fundamentally, however, there needs to be greater attention to the subject of disproportionate collapse in the academic curriculum so that graduating engineers have a background in and theoretical understanding of the subject matter. It is hoped that these papers will serve as a valuable resource in the classroom to prompt discussion and improve on the state of the practice. Ever more powerful hardware is likely to permit advanced analytical techniques to become ubiquitous; the challenge is to train students to use the suite of analytical techniques to accurately represent the problem.

In short, design to prevent or minimize progressive collapse is an evolving discipline, and we can expect considerable growth pains and awkwardness associated with its ongoing development. As with any new area of engineering, a couple of generations of engineers will probably be required to achieve true maturity and the stability of practice and governing concepts that come with it.

As a profession, we need to adopt a standard of safety that protects society from a broader range of abnormal loading conditions than are currently addressed in our building codes. Construction failures, fire-weakened structures, accidental impact, and deliberate acts of terrorism are unlikely to cease any time soon. We are therefore ethically bound to provide reasonable and effective measures to protect the public against disproportionate levels of damage resulting from unforeseen events. There are likely to be events that cannot be addressed by a reasonable and prudent level of improved structural robustness; however, if a collapse does progress despite a professional’s best efforts to arrest it, that should not happen because of lack of forethought.