Progressive Collapse of structures

Words by BSBG Media Team, Thursday 19 July 2018

The latest BSBG blog is written by Structural Engineer Mark Juinio, who provides a study of progressive collapse in structures, which begins with a minor failure, and results in widespread collapse. BSBG's Engineering division has delivered some of Dubai's most prestigious developments, and is currently working on a variety of projects under construction, including Bluewaters Island Residential, Jebel Ali Park Hotel, E15, Festival Plaza, Banyan Tree Residences - Hillside Dubai and BLVD Crescent.

Do you find falling dominoes satisfying to watch? When you line up a series of rectangular blocks in any form you want – a simple line, a curve, a circle - once everything is arranged, it only takes one gentle push on the first block to start the thrilling chain reaction.

In structural engineering, there is a similar phenomenon, except that it is never a satisfying thing to watch. We call it Progressive Collapse.

What is Progressive Collapse?

Progressive Collapse, as defined by ASCE 7, is the spread of an initial local failure from a structural element resulting, eventually, in the collapse of an entire structure or a large part of it (just like how a single domino can knock down an entire series of blocks). This phenomenon is also called disproportionate collapse, for the reason that the collapse is out of proportion to the triggering event.

Figure 1. Partial collapse of Ronan Point in East London caused by a gas explosion, triggered by a single matchstick (1968)

What triggers Progressive Collapse?

The susceptibility of structures to progressive collapse varies to a certain degree and is dependent on many factors. Over the years different investigations and studies have been conducted to further examine its nature. Among the many triggering events considered, the most notable are design or construction errors and extreme loading conditions.

Design or construction errors

All structures are expected to withstand many kinds of loads without deforming excessively. Most of the time, a bad design does not come as a result of computational errors, but by failure to consider loads that a structure is expected to resist. In addition to this, invalid design assumptions, inaccurate data, and erroneous design drawings are amongst the other causes of design error.

Other forms of human error take place during the construction stage. Construction failures can be categorised in terms of performance and materials. Performance errors are associated with poor construction methodologies, deficient assembly, inadequate supervision of construction workers, and non-conformance to the design intent. On the other hand, material deficiencies are caused by production errors and non-conformance to design specifications.

Figure 2. Remains of L’Ambiance Plaza after collapse during construction (1987)

Extreme loading conditions

During its lifetime, a structure may be subjected to extreme loading conditions that far exceed normal design assumptions, leading eventually to catastrophic consequences. These abnormal loading hazards may be classified as blast, impact, or deformation-related loads. Blast (e.g. gas explosion, bomb detonation) and impact (e.g. vehicular collision, aircraft impact) loads are short in duration and but high in intensity, resulting in unusual structural responses when compared to more conventional dynamic loads such as earthquakes and wind. Furthermore, deformation-related loads include fire (which causes softening of steel) and foundation subsidence (i.e. the process of downward displacement of the supporting ground).

Since these potential abnormal loading conditions have a low probability of occurrence, they are not usually considered in the structural design due to economic reasons and inability to accurately predict extreme scenarios.

Figure 3. Alfred P. Murrah Building after a truck explosion (1995)

Some notable historic events

Over the past decades, there have been numerous cases of progressive collapse in different countries. The interest in the issue of progressive collapse was initiated after the famous 1968 partial collapse of the Ronan Point apartment building in East London (See Figure 1.)

Figure 4. A closer look at Ronan Point’s exterior walls blown out by a gas explosion in 1968

Ronan Point (1968)

The structure was a 22-storey precast concrete, bearing wall building. On the morning of 16th May 1968, a tenant on the 18^th floor lit a match for her stove, unaware of a gas leak which caused an explosion that blew out an exterior wall panel of her apartment. This eventually triggered a partial collapse of the structure, killing a few and injuring many (See Figure 4.).

The collapse of the Ronan Point building illustrated a lack of structural integrity. Its design had no redundancy for load redistribution in cases of local failure and did not have any fail-safe mechanisms. It was, quite literally, a house of cards.

World Trade Center Towers (2001)

Undoubtedly, one of the most recognised accounts of progressive collapse is the case of the twin towers of the World Trade Center in New York City. On the morning of 11th September 2001, two planes crashed into the twin towers at high speed. According to an investigation published by the National Institute of Standards and Technology (NIST), the aircraft impact caused severe damage to the principal structural components of both towers, such as core walls, perimeter columns, and floors.

Figure 5. The Twin Towers of the World Trade Center prior to an aircraft impact (2001)

Despite all the immediate damage, the building stood for some time. The structural integrity of the buildings helped to withstand the initial impact, allowing load redistribution. Ultimately, the timing of the eventual collapse was largely dependent on the rate it took for the fire to diminish the effectiveness of the major structural elements. Although it may seem that the aircraft impact was the primary cause of collapse, the NIST report concluded that it was the ensuing fire that played a major part in the collapse of the towers.

Where are we now?

The Ronan Point and World Trade Center incidents are just two of the classic examples of local failures that eventually led to catastrophic collapse. These historical events generated widespread concerns over the progressive collapse of structures. New design approaches and code provisions have been developed to ensure and enhance the structural integrity of new and existing structures. Furthermore, new procedures and practices have been set up to ensure fire resistant design. Read Design Confidence’s Fire Safety Masterclass to learn more.

The mitigation of progressive collapse calls for action by all parties involved in a certain project regarding standards, codes, regulations, and practices. Each entity should be able to coordinate with all disciplines to ensure convergence and minimise the gap between design and construction.

Additional references:

ASCE 7: Minimum Design Loads for Buildings and Other Structures

NISTIR 7396: Best Practices for Reducing the Potential for Progressive Collapse in Buildings

NIST NCSTAR 1: Final Report on the Collapse of the World Trade Center Towers