Disproportionate Collapse

The Regulations

It is a requirement under the UK building regulations that buildings comply to a minimum level of robustness. This is referred to as ‘disproportionate collapse’. The building regulations for Scotland and England & Wales in essence state, although worded slightly differently from each other, that: “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”.

Until a British code is available and referenced in the building regulations the route for timber designers is to adopt alternative methods. The STA (Structural Timber Association) have developed a method and forms the basis for the following information.

The Methods

Platform timber frame is a lightweight building process that under accidental damage is known to be robust and has significant capacity to span over gaps caused by accidental damage.

This was demonstrated by tests carried out on the BRE/TRADA TF2000 six storey building in 1998 which concluded that: “...timber frames designed and built correctly were robust against disproportionate collapse”.

Platform timber frame comprises wall and floor components mechanically fixed to each other. Unlike other structural concepts, buildings falling outside the scope of platform cellular layouts e.g. post & beam or portal frames, platform timber frame relies on the full diaphragm action of the floors to transfer horizontal forces to an evenly distributed layout of load bearing walls, which provide both vertical support and horizontal load resistance.

The building regulations have classified buildings into 4 classes according to building type and risk.