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PROGRESSIVE STRUCTURAL COLLAPSE

This is a form of "domino effect" failure that can occur in a reinforced concrete structure whereby a failure starting in a particular component rapidly propagates to other components precipitating a major or even a total collapse. The three most common occurrences of this type of collapse are as follows:

  1. High rise concrete flat-plate structures (during construction or earthquake).
  2. Formwork for concrete structures.
  3. High rise structures constructed with precast concrete elements.

 

Recently, two more types of construction have joined this group: lift-slab and hung ceiling systems.

The most well known failures in these categories are the Commonwealth Avenue collapse in Boston, the Bailey's Crossroads collapse in Virginia, and the Ronan Point explosion-collapse in London.

The Ronan Point failure occurred in May 1968. A natural gas explosion in a kitchen located in one of the four corners of the 23-storey precast building triggered a progressive collapse of all the corner units above and below that unit. The spectacular nature of the collapse created an enormous impact on the philosophy of structural design and resulted in important revisions of design codes. The Ronan Point report of the Court of Inquiry stated:

"It is the common aim of structural engineers so to design their structures that if one or two component parts or members fail due to any cause, the remaining structure shall be able to provide alternative paths to resist the loads previously borne by the failed parts."

The Ronan Point failure resulted in the addition of an amendment to the British Building Regulations of 1970, later developing into BS Cp 110-1972, which made it mandatory in Britain for buildings of five or more stories to be designed for the possibility of progressive collapse. This amendment applies to all structures of more than five stories and is not limited to designs using precast panels. It follows the alternate path theory and requires, in essence, that every building be designed using either of the following alternatives:

•The designer shall ensure that the removal of any of the structural components essential to the stability of the building does not produce the total collapse of the structure and that any resulting "local" damage or collapse be restricted to the stories above and below the one at which the removal of the component was made.

•Structural members shall not collapse if subjected to the combined dead and imposed loads acting simultaneously with a pressure of 5 psi (34.48 kPa) in any direction and any extra loads transmitted from adjacent parts of the structure subjected to this 5 psi pressure.

The need to safeguard against progressive collapse was beginning to be recognized, though not formalized in design codes, even prior to the Ronan Point collapse. The Comite European du Beton produced and published in March 1967 a comprehensive code covering the design and construction of systems buildings. This Code drew attention to the danger of progressive collapse in the following words:

"One can hardly overemphasize the absolute necessity of effectively joining the various components of the structure together in order to obviate any possible tendency for it to behave like a "house of cards"…"

In North America the first provisions to deal with progressive collapse were those in the BOCA (Building Officials and Code Administrators) 1981 Code. However, it was not until 1989 that the ACI - 318 Code made a first mention of the problem and not until the issuance of the 1995 Code that provisions comparable to those of BS-CP 110: 1972 were adopted.

The Canadian Standard CSA - A23.3-94 for the design of concrete structures now recognizes structural integrity as a separate limit state. The Standard includes several provisions to enhance structural integrity especially in precast and tilt-up structures, mixed or unusual structural systems and structures subjected to severe loads such as vehicle impact or chemical explosions.

The Ronan Point case stands as one of the few landmark failures that have had a sustained impact on structural thinking, an impact that affected even institutions that traditionally tend to resist change such as the ACI.

 
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Information contact: engineering@waltersforensic.com