AD 297: Disproportionate collapse regulations
This AD is to advise engineers how they might account for the differences between the disproportionate collapse regulations in England and Wales, Scotland and Northern Ireland. The regulation for each country has slightly different limits defining when disproportionate collapse should be considered.
The requirements for the Building Regulations of England and Wales regarding disproportionate collapse are detailed in SCI publication P341. On page 2 of P341 there is a note that suggests in Scotland and Northern Ireland disproportionate collapse only needs to be considered for buildings of five or more storeys. This AD shows why this is an over simplification for Scotland.
In Scotland, the Building Standard 1.2 of the Building (Scotland) Regulations 2004 states:
‘Every building must be designed and constructed in such a way that in the event of damage occurring to any part of the structure of the building the extent of any resultant collapse will not be disproportionate to the original cause.’
Despite this, the Technical Handbooks for domestic and non-domestic buildings states that progressive collapse should be considered only if the building is five or more storeys.
In view of this uncertainty, it is recommended that agreement is reached with the relevant building control body when buildings are less than five storeys.
The obligation to limit disproportionate collapse currently applies only to buildings of five or more storeys.
England & Wales
Although no storey limit is given in Regulation A3, Approved Document A defines four classes of building and the structural provisions required for each class to comply with the regulations. Further guidance is provided in SCI-P341.
How to comply
It is advised that to comply with the regulations in Scotland and in Northern Ireland hot-rolled steel framed buildings with five or more storeys should comply with Clauses 220.127.116.11 and 18.104.22.168 of BS 5950-1: 2000. Buildings with less than five storeys should comply with Clause 22.214.171.124 of BS 5950-1: 2000.
The next amendments to BS 5950-1 (expected summer 2006) will include the building classification system from Approved Document A. Therefore, buildings designed to BS 5950-1 will follow the classification system even though they may not be covered by the England and Wales Building Regulations.