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Last edited 20 Dec 2020
Disproportionate collapse (or progressive collapse) occurs in a building when the failure of one component leads to the progressive failure of a series of other components, often with catastrophic results. The effect is very similar to that seen in a game of Jenga, pictured.
A well-known example occurred in Newham, East London in May 1968 at a new 22-storey block of flats called Ronan Point. The reinforced concrete building featured prefabricated external wall panels. Unfortunately, a gas explosion resulted in the progressive collapse of an entire corner of the tower block, which killed four people and injured 17 other residents. The cause of the explosion was a resident striking a match to boil water which ignited the gas that was leaking from a joint in a newly-installed cooker.
The explosion blew out the load-bearing flank walls, which had been supporting the four flats directly above, lifting the ceiling and floor slab. This resulted in a house-of-cards progressive collapse as the floors above became unsupported. For further details see Ronan point.
Requirement A3 of the Building Regulations 2010 states:
In other words, had Ronan Point been constructed in line with requirement A3, the damage would have been restricted to the floor where the explosion took place. Reducing a building’s tendency to disproportionate collapse can be met by a range of constructions set out in Approved Document B of the Building Regulations.
In the US, the term ‘pancake collapse' is sometimes used to describe the phenomenon of progressive collapse. The term was first used to describe a structural failure in the Bronx, New York, in August 1980, where a fire had weakened a structure to the extent that the floors fell successively in on one another, i.e they ‘pancaked’ down.
A similar effect was seen at the destruction of the twin towers of the World Trade Center, New York in September 2001. The fire produced by the aircraft strikes resulted in the progressive failure of the floor system; although initiated by the attacks, this was later deduced to be the primary mode of structural failure in the steel frame buildings. A number of code changes were initiated as a result of the investigation, one of which was to increase by one hour the fire resistance of structural components.
- Practical guide to structural robustness and disproportionate collapse in buildings.
- Manual for the systematic risk assessment of high-risk structures against disproportionate collapse.
 Related articles on Designing Buildings Wiki
- Adam Curtis - The Great British Housing Disaster.
- Approved Document A.
- British post-war mass housing.
- De la Concorde overpass collapse.
- Defects in construction.
- Grenfell Tower fire.
- Modular buildings.
- Offsite manufacturing.
- Prefab bungalows.
- Ronan Point.
- Structural failures.
- The structural condition of Easiform cavity-walled dwellings (BR 130).
 External references
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