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Unstable or collapsed structure

Hazard Knowledge

Structural collapses occur because of a loss of stability, where the basic shape and integrity of the structure is significantly changed through being subjected to a combination of forces. As the altered structure or shape is less capable of supporting the imposed forces and loads, it continues to change until it finds a new shape that is more stable.

Structures may become unstable or collapse due to:

  • Construction or demolition work
  • Derelict or deteriorated condition, including previous fire related damage
  • Involvement in a transport collision
  • Substandard or unregulated construction or modification
  • Exemption from, or non-conformity with, building regulations
  • Operational activity such as moving or cutting structural elements
  • Severe weather conditions, such as flooding, heavy snow or high winds
  • Shock due to severe impact or explosion

Inherent design defects can cause weaknesses to parts of a structure, which may subsequently fail if stresses are applied, such as severe weather conditions or abnormal loading by heavy machinery. A building under demolition or renovation may collapse if critical load-bearing walls or floors are removed without considering the effects on the other structural elements.

Substandard materials used in construction, or poor workmanship during the construction phase, can result in a building that is substantially weaker than intended. This increases the likelihood of collapse should the building be exposed to additional forces.

Some non-building or temporary structures can become unstable due to a combination of potential causes that render the environment hazardous to fire crews operating within or nearby.

Structures may fail for various reasons, such as insufficient strength to take the weight or force of a load or possibly through secondary collapse. People may be at risk if they are on, in under or attached to an unstable or collapsed structure. Loads or forces applied to the structure, directly or indirectly, may worsen the instability or progress a collapse. This could include rescue loads and the use of equipment.

Elements of structure, floors, walls, ceilings, ancillary items, fixtures and fittings can partially collapse. Partial collapse can follow on from the collapse of lightweight or decorative features.

If partial collapse is not controlled, it may increase the potential for falling debris and secondary or structural collapse.

Lightweight or fragile structural features may collapse, including non-structural elements; for example roof coverings, false chimneys and glazing.

False chimneys do not form part of the structural fabric of the building, can be a considerable weight and are only supported by roof timbers. If roofing timbers or lightweight trusses fail, they may collapse through the roof. False chimneys are not suitable as an anchor for working at height, as they may not be able to support any additional weight.

Collapse may not be limited to the structure itself, as scaffolding or cranes, for example, may be at risk of damage or collapse.

Moving or cutting structural elements during operational activity can have an impact on the stability of a structure.

The way and speed in which elements of construction distort or fail depend on the type of structure and how construction materials have been used or combined. There may be varying stages or severity of instability or collapse. A structural collapse may occur without warning, giving people little or no time to escape.

In a collapsed structure casualties may be located in voids or spaces, or be trapped under debris. The type of structure can provide some indication of the way it has collapsed, and the location of potential voids or spaces.

For more information on construction methods and materials see BRE building supplementary information.

Patterns of collapse

Collapse patterns can be categorised as internal, external or total collapse.

Internal collapse  
Pancake or progressive collapse Structural failure causes a floor to fall horizontally onto the floor below. The added weight may cause that floor, and subsequent floors, to fail and fall to a lower level, although not always to ground level. Pancake collapse can be mistaken for total collapse.
Lean-to collapse Where one supporting wall fails, resulting in the roof or floor hinging on the remaining wall creating a triangular void.
V-shape collapse Usually occurs when the centre support is compromised, and the floor or roof collapses and settles in the shape of a V. Triangular voids may be formed under the V-shape.
A-frame or tent collapse The floor is no longer supported at the outer edges, but remains supported on internal walls or structures, forming an A-shape.
External collapse  
90° collapse This is when the wall drops away from the building at a 90 degree angle. Debris will spread as the wall hits the ground.
Curtain fall collapse Much like a curtain cut loose at the top; walls collapse straight down and create a rubble pile near their base.
Inward/outward collapse Walls crack horizontally in the middle. The top half usually falls inwards and the lower half outwards.
Total collapse This is the most severe form of structural failure and occurs when all the floors have collapsed to the ground or basement level and all walls have collapsed onto the floors.


Partial or structural collapse may create other physical hazards such as

  • Unstable or fragile surfaces
  • Exposed structural members
  • Sharp edges
  • Heavy dust loads making atmospheres irrespirable
  • Damaged utilities
  • Unsafe cabling or wiring