Page 250 - Forensic Structural Engineering Handbook

7.34 CAUSES OF FAILURES

first compressive wave to travel though the object, but for significant explosions it is pos-
sible that the reflected tension wave will have high enough stress to progressively and
explosively shed spalls from the far face of the object. In effect, the concrete can be stripped
away from the back side of structural elements.
Comprehensive references 30,31 provide means to calculate blast loads and their effects
on structural elements.

FIRE LOADS

Nature and Consequences of Fire Loads
In general, fires affect structures directly in two ways: High temperatures reduce the
strength and stiffness of structural materials, and thermally induced expansion damages
structural and nonstructural components. However, collateral damage from falling debris
and other causes can be high as well.

Effects of Fire on Materials
Extensive research and testing have been performed to determine the effects of heat on
materials of construction. These studies have shown that all common building materials are
affected by fire temperatures, which often exceed 1700°F (927°C) in the fire area where
flashover has occurred.
Structural steel is, perhaps, the building material that is most susceptible to damage dur-
ing fires. When steel is exposed to heat, its temperature increases rapidly. At approximately
o
800°F (427°C), steel begins to lose strength and at approximately 400 F (204°C) steel begins
to lose stiffness. 32 Above this temperature, permanent deformations can occur, stability
begins to be compromised, and factors of safety diminish. When a simply supported steel
beam reaches 950°F (510°C), its strength can be reduced by approximately 30 percent, 32
which places it at risk of failure if it is at the same time loaded to its design capacity. This,
of course, is why external fire protection is often applied to structural steel members.
The strength of concrete is first affected at approximately 575°F (302°C). 33,34 However,
most reinforced-concrete members can be exposed to temperatures far in excess of this
level without failure. Compared to steel, concrete is relatively slow to increase in temper-
ature when heated. The strength of most reinforced-concrete members is governed by the
35
strength of the reinforcing steel, which is insulated from heat by the concrete. In general,
reinforced-concrete members do not fail due to direct exposure to heat until the concrete
cover over the steel has spalled, exposing the reinforcing steel to heat.
Pre-stressed concrete is, in general, more prone to failure than reinforced concrete when
exposed to heat. Tendons lose strength rapidly above approximately 575°F (302°C). 36
Members with unbonded tendons can be particularly susceptible to damage, depending on
the tendon patterns and the amount of mild steel, because they can lose strength over their
entire length (both positive and negative moment regions) when any one section of the
member has been heated enough to soften the tendons.

Thermal Expansion from Fires
The coefficient of expansion of many building materials is temperature-dependent. For
−6
2
instance, the coefficient of expansion for structural steel is = (6.1 + 0.0019t) 10 /°F