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Hazard

Partial or structural collapse: Fires in buildings under construction or demolition

Hazard Knowledge

When subjected to fire some types of buildings under construction or demolition may collapse suddenly, with little or no warning. It is important to understand the type of construction involved and the method and sequence of building work being undertaken. 

Fires in buildings under construction or demolition present an increased risk of partial or structural collapse. Factors that may affect the structural integrity include: 

  • Their derelict, deteriorated or incomplete condition 
  • The type of building or structure, and the method of construction 
  • Structural alterations that have weakened the building 
  • The type and extent of structural work being undertaken 
  • Temporary supports or propping 
  • Structural fire protection features being removed, damaged or not yet installed 

Any lack of fire compartmentation or fire stopping may result in rapid fire development. This will increase the speed at which the integrity of the building is affected, potentially resulting in collapse. 

Any localised collapse or demolition work could either leave slender sections of the structure vulnerable to wind or other loads, or could result in the weight of upper parts being redistributed, which in turn can lead to further overload and progressive collapse. 

Fire can spread rapidly through voids and cavities or to external parts of the building, affecting the cladding and glazing systems of buildings. Glass (glazing) or other flat panels or lightweight systems, such as rain screens used for cladding, may travel significant distances from the building when falling from height, particularly in windy conditions. 

Collapse may not be limited to the building or structure itself, as scaffolding or tower cranes, for example, may be affected by intense radiated heat, putting them at higher risk of collapse. Therefore, the footprint of the area affected may be significantly larger than that of the building or structure itself.  

Structural timber buildings under construction are particularly vulnerable to rapid firespread, which may lead to early collapse. 

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Fire in timber building under construction – photograph courtesy of London Fire Brigade

Older buildings may have no mechanical connection between floors, beams and walls so any separation causes collapse. 

Structural steel elements are vulnerable during the construction phase because fire protection elements such as intumescent coatings, sprays and boards may not yet have been installed. Inappropriate storage of combustible materials on-site, if not carefully managed, can increase the risk of firespread. 

The applied load of firefighting media used in the building may affect structural stability, particularly in incomplete buildings or structures.  

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Building construction showing hoarding as a site boundary – photograph courtesy of Fire Scotland
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Post-fire incident site showing debris outside of site boundary – photograph courtesy of London Fire Brigade