Disproportionate collapse

07-02-2024 | Posted by Joaquín Martí

Colapso desproporcionado

Buildings and other structures are generally designed to withstand the normal loads as well as conservative estimates of those arising from rain, wind, snow, and earthquakes. Abnormal events such as explosions, vehicle impacts, or uncontrolled fires are not usually considered. Disasters can happen when a vulnerable structure faces unanticipated events.

The American Society of Civil Engineers (ASCE) has released the ASCE/SEI 76-23 Standard for Mitigation of Disproportionate Collapse Potential in Buildings and Other Structures, the first national building standard of its kind. Developed over the course of a decade and informed by research led by the National Institute of Standards and Technology (NIST), the standard provides design requirements and guidance to keep small, isolated failures in a structure from propagating and bringing down the entire structure or a major part of it, which the standard defines as disproportionate collapse.

Disproportionate collapse has always been a risk for large buildings, but the events are rare. Discussions had been held for years about the need for a standard aimed specifically at that topic and several notable failures in the ‘90s and early 2000s finally led to a consensus.

Luchando contra la amenaza terroristaIn 1995, the Alfred P. Murrah Federal Building collapsed as a result of the Oklahoma City bombing, which initially destroyed three columns, damage that led to the subsequent crumbling of nearly half of the building. During the World Trade Center (WTC) disaster in 2001, the heat of unmitigated fires in the building called WTC7 caused deformations that severed a connection between a girder and column, triggering a progression of failures that brought the building down. At the conclusion of its WTC investigation, NIST highlighted the need for a standard and, based on their proposal to ASCE, a committee was formed with experts from industry, academia, and the federal government.

The standard uses a performance-based approach, which allows engineers maximum flexibility when finding solutions. Guidance is given about risk assessments and avoidance, as well as characteristics that enhance collapse resistance including the strength, ductility, deformation capacity, and robustness necessary to resist collapse without exceeding relevant limit states. It includes threat-specific and non-threat-specific methodologies and identifies direct design and indirect design approaches.

It is intended to address the same structures covered by ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. The required performance and level of hazard (for example, the extent of initial damage that the structure should withstand) varies depending on the size, occupancy, and utility of a building. 

Engineers commonly use computer models to confirm that their designs will hold up to the loads considered. A separate ASCE technical committee is working on guidelines to help designers harness computer modelling tools for disproportionate collapse mitigation.

Although the new standard Is certainly welcome, it is not as If the problems of progressive and disproportionate collapse were being completely disregarded before. Indeed, Principia has frequently carried out analyses of buildings, bridges, and other structures to minimise the risk of such catastrophes taking place. The triggering events in our studies were often impacts, explosions and fires, whether accidental or caused by terrorist action, but also less dramatic events such as the loss of a cable in a cable-stayed bridge.