Types of structural collapse

1 What is a collapse?
2 Classifications of collapse
3 Related articles on Designing Buildings

[edit] What is a collapse?

Collapse means to fall down. It can be used to refer to a person, potentially a site worker, who falls down, often as a result of illness or injury, but more specifically in construction it relates to the mechanisms of structural failure that can lead to the partial or complete collapse of a building or other structure.

Collapse may be initiated intentionally during demolition.

[edit] Classifications of collapse

There are different descriptions and classifications of structural collapse, culturally but also depending the specific information needed for different professions. For example in terms of fire and safety classification may relate to understanding how best to deal with rescue operations in a collapsing building, whilst in engineering terms it may relate to predicted failure mechanisms and developing design solutions. In earthquake zones it may be about maximising possibilities for rescue.

Below are just some examples of how collapse can be classified, both in process and severity and how terms in different countries and different professions relate to one another, along with examples.

[edit] Partial and complete collapse

Partial collapse is when elements or whole areas of a building or other structure fail for a variety or reasons and through a variety of processes whilst the majority of the structure stays standing.
Complete collapse is when a structure in its entirety, either though a series of staged collapses or in a singular failure, collapses as a whole.

[edit] Disproportionate collapse

Disproportionate collapse may also be called progressive collapse or cascade failure and is a general term that refers to a failure of one component that leads to the progressive failure of another or a series of other components. It can result in both partial collapse or complete collapse of a structure. One form of such collapse is described in the US as pancake collapse, describing a stacking effect of failure.

[edit] Pancake collapse

The term pancake collapse, a form of progressive collapse, was first coined to describe a building failure that occurred in the Bronx, New York, in August 1980. A fire weakened structure caused floors to fall successively onto one another, pancaking down. In this case the entire building was destroyed.

In a more infamous and catastrophic collapse this effect was seen to its extreme during the destruction of the twin towers of the World Trade Centre, New York in September 2001. Again fire, on this occasion initiated by terrorist aircraft strikes, resulted in progressive failure of the floor system, which it was later deduced was a primary catalyst in complete structural failure of the steel frame and total collapse of the building.

[edit] Total progressive collapse

Total progressive collapse describes disproportionate collapse that occurs through stages and ends with complete destruction.

[edit] Partial progressive or pancake collapse

Partial progressive collapse describes disproportionate collapse that occurs in stages, the stages themselves in their nature may differ, though elements of the structure may still remain standing or lead to total collapse.

One infamous example occurred in 1968 at Ronan Point in Newham, East London as the result of a gas explosion where a new 22-storey reinforced concrete block of flats constructed by what was known as as Large Panel System (LPS) with prefabricated external wall panels and flank walls supporting the floors. The explosion blew out the load-bearing flank walls, lifting the ceiling and floor slab, resulting in a house-of-cards effect of progressive collapse as the floors above became unsupported. It might be classified as partial collapse because the majority of the building remained standing but a complete collapse of the wing of the building where the gas explosion occurred because all floors on that part of the plan collapsed.

For more information see: Ronan Point.

An example that had no fatalities and that perhaps more suitably describes partial progressive or pancake collapse occurred in 2005, when the upper 11 floors of the 28 story Windsor Tower in Madrid, Spain collapsed. The design of the tower included a seven-foot thick reinforced concrete transfer floor acting as a bulkhead and supporting the steel framework of the top 11 stories along with a reinforced concrete inner core surrounding the steel-frame. After a number of hours of fire burning on the 21st floor the core failed, so no longer supporting the framework. The upper 11 stories collapsed to street level with the upper three floors collapsing to the transfer floor.

[edit] Symmetrical collapse

Symmetrical collapse describes partial collapse, often in smaller buildings, which occurs occurs at the centre or at the edges of structural members, where some elements remain standing. It is less likely in taller buildings as the significant weight of floors above may result in total collapse. The end result of this type of collapse in simple terms creates a V or A shape in the debris that is left and these types of collapse are notable from a fire and rescue perspective because of the voids that remain (potentially only temporarily) from which survivors can potentially be rescued.

[edit] A-frame collapse

A-frame collapse might also be described as a tent collapse, and is where floors become separated from the exterior supporting walls but the central interior supporting walls remain intact and support the part of the floor directly above. It is likely to be caused by weakness and shear forces at the junction between the external supporting walls and the floors, as the edges fall and the centre remains supported the shape created from the debris resembles an A-frame. This type of collapse might be described as progressive collapse when the failure of the floors then causes the central supporting wall to collapse as a result of the extra weight.

[edit] V-frame collapse

V-frame collapse is where beams fail at their centre point due to overloading or damage, through weathering or rotting and where there are few or no central structural supporting walls. The failure occurs in the centre of the structural member, whilst side connections and their supporting elements such as external walls may remain intact, this forms a V- shape from the debris left.

[edit] Asymmetrical collapse

[edit] Lean to collapse

Lean-to collapse is where one or other load-bearing wall fails, one half of the structure is effectively removed and connection points remain and bending moments cause the remaining elements to lean to one side. If the remaining connection points give way to shear forces the floors will fall, stack or pancake to the ground. The remaining wall may or may not remain standing. These types of collapse are notable from a fire and rescue perspective because of the voids that remain (potentially only temporarily) from which survivors can potentially be rescued.

[edit] Cantilever collapse

Cantilever collapse refers to failure where a beam is supported only at one end, with load carried over the overhang - in contrast to a simply-supported beam, which is supported at both ends. Collapse may be progressive starting with failure of one or two elements of this single sided structure, such as counterweights, props, foundations or reinforcements.

[edit] Other types of collapse

There are numerous scenarios such as subsidence, earthquakes, flash floods and others in which the grounding of a structure is compromised. In these scenarios similar forms of failure and collapse can occur but others do not fit in these simple categories such as inclined total collapse (a building falling as a whole to one side), outspread multi layer collapse (a building spreading flat to one side) or sinking collapse (where the ground of a building gives way and it effectively sinks in whole or in part) as well as other potential scenarios.