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United States of America – In the United States, buildings are constructed to withstand the usual occurrence of rain, wind, snow, and earthquakes. The abnormal events of gas explosions, vehicle impacts, and uncontrolled building fires are not usually considered by architectures and engineers. Now, a new building standard may aid engineers in preventing and preparing for the worst as a string of recent collapses have jumped in recent news .
The American Society of Civil Engineers (ASCE) has released the ASCE/SEI 76023 Standard for Mitigation of Disproportionate Collapse Potential in Building and Other Structures. This national building standard has been in development for over a decade and has garnered research from the National Institute of Standards and Technology (NIST). The new standard provides design requirements and guidance in keeping small, isolated mishaps in a structure from spreading and resulting in the partial or complete collapse of a building.
This is a phenomenon which the standard has defined as “disproportionate collapse.”
NIST research engineer Dr. Joseph Main, who is a member of the ASCE committee which developed the standard, has stated that if there is, “an unanticipated load that you didn’t explicitly design for, it shouldn’t cause the whole building to collapse.”
Although disproportionate collapse has always remained a risk for large buildings, the event of an actual collapse is rare. The need for a standard aimed at disproportionate collapse served to be a controversial topic among architectural and engineering experts in the United States. It was not until the 1990s and early 2000s, following the fall and failure of several distinguished buildings, in which a consensus began to form.
The Alfred P. Murraah Federal Building collapsed due to the Oklahoma City bombing in 1995. Initially, three columns of the building were destroyed but the damage led to the eventual crumbling of nearly half the building. During the 2001 attack on the World Trade Center (WTC), the heat of the unmitigated fires caused deformations in WTC7 which severed the connection between a girder and a column, resulting in a collection of failures that led to the building’s collapse. Following these instances, the NIST emphasized the need for a standard directed at mitigating such collapses and constructing guidelines and tools for building designs.
The ASCE formed a new standards committee of building experts from the realms of industry, academia, and the federal government. The committee set performance goals for structures and gave engineers autonomy in creating their own design solutions. Donald O. Dusenberry, consulting engineer and chair of the ASCE committee said, “The standard points toward solutions but doesn’t prescribe them…It offers proven methods to analyze and design structures for disproportionate collapse but allows for cost-effectiveness and creativity.”
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Throughout the past decade, researchers have created multiple computer simulations of building components wavering and giving out. In some of the simulations, researchers removed the middle of three columns from a structural frame and applied force above the missing column to showcase how the connections fractured.
The performance and level of hazard differs depending on the building’s size, occupancy, and utility. NIST research structural engineer and member of the ASCE disproportionate collapse committee Dr. Fahim Sadek, said that individuals “designing government buildings or any buildings that are critical for communities, I think, would be the first groups looking carefully at this standard.”
NIST researchers have made a tremendous effort to illuminate the mechanics as to what happens when sections of a building structure are removed. Joseph Main states that the loading scenario for the connections is “quite different from what you would typically get under wind, earthquake or gravity loads that people are used to designing for…It’s a much more complex condition.”
Since there is a lack of data regarding these types of events, researchers replicated such scenarios physically. The committee would build and destroy fractions of or an entire building to produce data which would ensure that the simulations were realistic. These experiments helped identify the degree that beams and connections are able to rotate and bend before a structure crumbles.
Researchers additionally used the findings to created simplified modeling approaches which would evaluate the susceptibility of a building’s disproportionate collapse potential.
Another ASCE technical committee, led by Main, is working to distribute guidelines which will help designers use computer modeling tools for disproportionate collapse mitigation. Various modeling approaches are presently available ranging from advance models which provide detailed analysis of individual building components to simplified models which provide a general evaluation of entire buildings. Engineers could utilize these resources to examine and test the integrity of their designs under several hazard scenarios.
For now, the standard serves as a voluntary resource. The ASCE remains hopeful that in the future, a portion, or the entire standard, will be incorporated into the model building code and adopted by state and local jurisdictions.
