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.

Structural collapse can be caused in many ways including:

• Naturally, such as:
  • Natural deterioration
  • Earthquake
  • Subsidence
  • Flood water
• Accidentally, such as:
  • Explosion
  • Impact
  • Fire
• Deliberate actions, such as:
  • Terrorist attack
  • Explosive device

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
• Geological effects
• Shock due to severe impact or explosion

Poor design or 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.

Low quality materials, or poor building techniques used 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.

Geological weakness may cause buildings to collapse through movement of the strata on which the foundations are laid, for example, subsidence. This movement can place excessive stress on a structure, overloading it and causing collapse. Alternatively, the ground on which the building is constructed may weaken to such an extent that it is unable to support the weight of the building. Liquefaction, where the water content in the soil increases to such an extent that the soil loses all cohesiveness and strength, resulting in the building sinking into the ground, is the most common form of failure.

There may be occasions when the unstable or collapsed structure may be affected by some form of geological movement, such as earthquakes. This is rare in the UK, but similar types of ground movement within the substrata in and around the area of an unstable structure could occur.

The reasons for this movement may not be known and may not be noticeable, but the history of the initial cause of the structure's instability or collapse may indicate a need to be aware of potential ground movement. For example, in areas of known mining, the cause may be assigned to the collapse of old workings and therefore the possibility of further movement will need to be considered.

Some non-building or temporary structures can become unstable due to a combination of potential causes that render the environment hazardous to personnel 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
• 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

People may be at risk if they are in the vicinity of an unstable or collapsed structure.

Structural elements: 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:

• Roofs
• False chimneys
• Glazing

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 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 refer to 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 or 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.

In the event of a partial or structural collapse, the presence of other hazards should be considered, including:

• Fire, heat and smoke
• Damaged utilities
• Heavy dust loads and airborne particulates

Fire, heat and smoke

Fire, heat and smoke are always a hazard to operations, but in unstable or collapsed structures the effect of even a small fire will be amplified. There are many reasons for this, which include:

• Restricted access and egress
• Reduced ability to get firefighting media or ventilation equipment to the scene
• The possibility that fire could weaken the structure further

Incident commanders should consider not committing personnel to the hazard area if there is evidence or a risk of fire. They should also consider withdrawing personnel from the hazard area if a fire is suspected.

For further information refer to:

• Fires and firefighting
• Fires in buildings

Damaged utilities

Gas, electricity and water services may be affected or damaged as a consequence of any structural collapse. This can create the potential for leaking gas, localised flooding or exposed electrical services. Isolating services should be considered in the early stages, along with liaising with the appropriate agencies to provide advice and assistance.

Equal consideration should be given to the possible need to keep certain services functioning to aid in resolving the incident. The list of utilities will depend on the building; a hospital, for example, may have piped steam or oxygen. Unusual services such as these need to be identified in the Site-Specific Risk Information (SSRI) and this should inform the incident commander's decision.

For further information refer to Utilities and fuel.

Heavy dust loads and airborne particulates

Collapsed structure incidents will generate large quantities of dust as a result of the collapse or as a consequence of operations undertaken and the associated equipment used.

Such dust may be inherently carcinogenic or hazardous and have the potential to travel off-site, particularly in some weather conditions. The type and amount of such contaminants will inform the use of appropriate personal protective equipment (PPE) and respiratory protective equipment (RPE). Consideration may also need to be given to people away from the incident, who may be affected by dust plumes.

For more information on specific substances such as asbestos and silica refer to Hazardous materials - Health hazards.

A partial or structural collapse may create other physical hazards such as:

• Unstable or fragile surfaces
• Exposed structural members
• Sharp edges
• Unsafe cabling or wiring, including hanging cables