Important Know-How on Progressive Collapse of Building Structures

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Progressive collapse or disproportionate collapse of a building occurs when the failure of a single structural element leads to the failure of other components in a progressive manner. The failure starts with the primary structural element, accompanied by the failure of adjoining elements, finally resulting in structural collapse.

Progressive collapse is associated with the collapse of structures, caused due to accidents, actions of overload, failure of the material, design deficiencies, or natural phenomena. These triggering effects result in a collapse that is out of proportion. Hence it is also termed as disproportionate collapse.

This article explains the important causes of progressive collapse and some of the notable cases around the world.

Causes of Progressive Collapse

Progressive collapse of structures can occur to varying degrees and is dependent on several factors. Based on years of studies and investigations conducted, the two main notable causes of progressive collapse of structures are:

1. Design or construction errors
2. Extreme loading conditions

1. Progressive Collapse of Structures due to Design or Construction Errors

The structures are designed and constructed efficiently to withstand different loads without undergoing excessive deformation. A bad design originates when a designer fails to consider the loads that a building is expected to resist. Some of the design errors that result in progressive collapse are:

1. Inaccurate design data
2. Erroneous design drawings
3. Invalid design assumptions

The construction errors that result in progressive collapse are:

1. Poor construction methodologies
2. Inadequate supervision of construction workers
3. Deficient assembly
4. Material deficiencies due to production errors
5. Non-conformance to the required design specifications

2. Progressive Collapse of Structures due to Extreme Loading Conditions

A structure is expected to experience extreme loading conditions during its lifetime. This load is more than the normal design assumptions and could result in abnormal loading hazards like impact, blast, or deformation-related loads.

Impact loads are high-intensity loads acting for a short duration, which results in unusual structural response compared to the conventional dynamic loads like wind and seismic loads. Some of the examples of impact loads are aircraft impact, vehicular collision, etc. Blast loads like bomb detonation and gas explosion also result in similar consequences.

Deformation-related loads include foundation subsidence and fire. Foundation subsidence is the phenomenon of downward displacement of supporting ground, whereas fire results in the softening of steel in structures, which results in progressive collapse.

The abnormal loading conditions on structures are not commonly observed and hence are not usually considered in the structural design due to economic issues and the inability to accurately predict the extreme scenarios.

Important Examples of Progressive Collapse of Structures

There have been numerous examples of progressive collapses in different countries. The severity of progressive collapse and the interest in studying the issue was initiated by the partial collapse of the Ronan Point apartment building in East London in 1968.

Progressive Collapse of Ronan Point Apartment East London

The Ronan Point was a 22-story building made out of precast concrete bearing walls. On 16^th May 1968, an explosion due to a gas leak on the 18^th floor of the building created an explosion that blew the exterior wall panel away from the building. This explosion triggered a partial collapse of the structure. The collapse resulted in the death of a few and caused injury to many.

It was later determined that the Ronan Point building was not designed for structural integrity. There was no redundancy assigned for load distribution to face any local failure within the building.

Progressive Collapse of the World Trade Centre Towers

The progressive collapse of the twin towers in the World Trade Center in New York is one of the most recognized cases around the world. Two planes crashed into the twin towers at a high speed on 11^th September 2001. The crash caused severe damage to the principal structural elements of both the towers like the core structural walls, the perimeter columns, and the floors.

The building withstood the impact for some time even after the initial damage. This is due to its structural integrity and a sound load distribution system. But the quickly spreading fire across the towers reduced the effectiveness of major structural elements and resulted in the collapse of the towers.

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