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Structural defect or further collapse

Hazard Knowledge

Most structural collapses occur because of a loss of stability: the basic shape and integrity of the structure is significantly changed through being subjected to a combination of forces. The newly altered structure/shape is then less capable of supporting the forces and loads imposed on it. The structure continues to change until it finds a new shape that is more stable. Structural collapse can follow a number of generic patterns, each with its own hazards and areas where survivors are likely to be found. Collapse patterns can be categorised as internal, external or total collapse.

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

Equally, 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.

Types of internal collapse

Pancake collapse

A failure in load-bearing walls or an upper floor fails and falls horizontally (or 'pancakes') onto the floor below. The added weight causes this floor, and subsequent floors, to fail and fall to a lower level (not always to ground level). Pancake collapse is sometimes referred to as progressive collapse and can be mistaken for total collapse.

Lean-to collapse

A supporting wall, column or beam fails at one end. Triangular voids are created beneath and can offer refuge for occupants.

V-shape collapse

Heavy loads from above cause a collapse at a given point of a floor level. The excess load causes the point to fail in the middle. This creates triangular voids that can act as safe havens.

Tent collapse

Structural supports fail near the outer walls but remain in situ on the interior load-bearing element.

External collapse

Types of collapse in this category include:

90° collapse

A wall falls outwards to a distance that is at least equal to its height. 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 the base of the wall.

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.