12.4. Progressive Collapse

12.4.1. What is progressive collapse?

Progressive collapse is the propagation, by a chain-reaction, of a local structural failure into the failure of a substantial portion of the building, disproportionate in magnitude to the original failure.

last updated 5 January 2004

12.4.2. What events caused progressive collapse incidents in the past?

The 1968 failure of one corner of a 23-story residential precast concrete building in London (Ronan Point) was caused by poor connection detailing and was triggered by an explosion from a gas deflagration. In the aftermath, the UK introduced building regulations addressing progressive collapse. In North America, some examples of progressive collapse include the 12-story steel-framed Union Carbide office building, in Toronto, 1958; a 16-story cast-in-place reinforced concrete apartment building, Boston, 1970; and a 16-story post-tensioned concrete lift-slab building in Bridgeport, CT, 1987.

The Alfred P. Murrah Federal Building in Oklahoma City was a dramatic example of progressive collapse of a weakly redundant reinforced-concrete building, with collapse triggered by the vehicle bomb near the front of the building. As mentioned earlier, the building had minimal resistance to upward loads generated by the blast at street level.

last updated 5 January 2004

12.4.3. What is robustness? How can one add robustness to a building?

When referring to a building, the term robustness implies the strength and sturdiness to resist excessive loads. A highly redundant steel-framed building can be considered robust.

It is more difficult and expensive to add strength to an existing building than to consider this aspect in a new design, especially for high-rise buildings. One notable building that has undergone strengthening is the Citicorp Building in New York City. After the building was built, it was discovered that it would not likely survive a particular wind condition. The building was strengthened effectively, but at a significant expense.

last updated 1 January 2006

12.4.4. What official design specifications exist for reducing the risk of progressive collapse?

The sector of our economy that researches the protection of government buildings from terrorist attack and mitigates progressive collapse of these buildings is the General Services Administration (GSA), Department of Defense (DoD) and their contractors. GSA and DoD have developed guidelines for the protection of buildings against blast effects. Civilian-sector engineering firms that work for GSA on Federal Buildings receive these guidelines as dictated by a particular project. Some of these are available to the general public.

The GSA’s “Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects,” is available for free download at Also, “DoD Minimum Antiterrorism Standards for Buildings,” is available for free download at

last updated 5 January 2004