Last edited 20 Dec 2020

Disproportionate collapse

1 Introduction
2 Ronan Point
3 Building regulations
4 Pancake collapse
5 Related articles on Designing Buildings Wiki
6 External references

[edit] Introduction

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.

[edit] Ronan Point

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.

Progressive collapse can be exploited deliberately as a demolition method to ensure a relatively quick and economic collapse if it is known the construction will respond appropriately.

[edit] Building regulations

Requirement A3 of the Building Regulations 2010 states:

‘The building shall be constructed so that in the event of an accident the building will not suffer collapse to an extent disproportionate to the cause.’

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.

[edit] Pancake collapse

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.

The Institution of Structural Engineers (IStructE) has published a two-volume guide to structural robustness and disproportionate collapse in buildings:

Practical guide to structural robustness and disproportionate collapse in buildings.
Manual for the systematic risk assessment of high-risk structures against disproportionate collapse.