Page 246 - Forensic Structural Engineering Handbook

7.30 CAUSES OF FAILURES

Large explosions inside closed structures, such as when natural gas leaks into an
enclosed building, can literally blow the structure apart. Simply put, the energy released
during the explosion often far exceeds the energy-absorbing capability of the building
elements, and there is insufficient venting to allow the energy to be dissipated into the
atmosphere.
The forces of a blast often remove nearby partitions and exterior walls that are not
specifically designed for the effects of blast. Severe blasts also can remove important struc-
tural elements, such as columns, leading to progressive collapse. It is unusual for blasts out-
side buildings to impart enough energy to a building to fail structural elements (i.e.,
columns and beams) other than those directly receiving the shock and pressure loads.
Progressive failure is the most common reason why buildings collapse when there is an
exterior blast.
Explosions sometimes cause fires, particularly when the fuel causing the explosion is a
gas that has a continuing source. More commonly, though, for high explosives, the deto-
nation consumes the available oxygen and post-explosion fires do not ignite. Typically,
when there are fires they start at the periphery of the area of blast damage, where the heat
generated by the explosion is substantial, but beyond the region where the available oxygen
is consumed by the blast itself.
The collateral damage caused by fire can exceed the damage of the blast itself. The most
common damage from blasts is the devastation of weak and nonductile architectural fea-
tures, such as windows, nonductile curtain walls (Fig. 7.21), partitions, and unreinforced
masonry walls. These elements often have insufficient ability to absorb energy to resist the
momentum imparted by the explosion.

FIGURE 7.21 Curtain wall blast damage, Hong Kong and Shanghi Office Tower, London.
(Credit: R. Massa, Lorron Corporation.)