Disproportionate collapse – what does it mean?

Ronan Point was a 22 storey block of flats in Newham London. In 1968 it suffered a partial collapse due to a gas explosion in a kitchen on the 18th floor . The tower block was built using a technique called ‘Large Panel System Building’ or ‘Large Panel Construction’. The system consisted of bolting together floor and wall panels in a systematic way. The system relied on the bolted connection to maintain structural integrity. The effect of the gas explosion was to cause a ‘progressive collapse’ of the whole south east corner of the building. (See inset picture) To overcome the problem buildings are constructed so that the failure of a beam, column or wall does not cause a ‘disproportionate’ failure of other elements in the building.

Requirement A3 states that ‘A 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’. The requirement does not apply to all buildings. It usually applies to large and/or tall buildings including dwellings. In order to understand the guidance it is usual to classify the buildings. Class 1 buildings are generally small buildings that do not require any special robust detailing. Class 2A buildings such as five storey dwellings or offices not exceeding four storeys attain robustness by the provision of horizontal ties. Class 2B buildings such as hospitals not exceeding three storeys attain their robustness through horizontal and vertical ties. Class 3 buildings are buildings that that are defined above Class 1, 2a or 2B and are generally large or tall buildings, grand stands or buildings containing hazardous materials. In these buildings the construction requires a systematic structural risk assessment. The risk assessment takes into account all normal and abnormal hazards ranging from failure due to accidental impact to explosion. The relevant structural codes and standards provide further guidance.
The requirements can often impact on change of use, refurbishment work where structural alterations take place or where a building is extended especially by the addition of a floor.