Page 251 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.35

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[ = (6.16 + 0.0034t)10 /°C], where t is the temperature. At fire temperatures, the rate of
expansion is at least 30 percent higher than at room temperature.
At fire temperatures, steel and concrete can expand by 1 to 1.5 percent. For a 26-ft (8-m)
span, this expansion corresponds to approximately 3 in (8 cm) to 5 in (13 cm) of movement.
Few building systems can accommodate this movement even for one bay, much less the
cumulative effects caused by expansion over several bays.
Normally in floor systems, this full expansion in the horizontal plane never occurs.
Beams often are protected against heat, fire durations are short enough that members do not
achieve the fire temperature, or the supporting members sag as strength and stiffness
reduce. Sometimes, if restraint of expansion is sufficient, the floor system buckles before
the full horizontal movement can occur.
However, structural slabs and certain other structural elements can undergo horizontal
expansions on the order of 1 percent when conditions permit. For example, when a struc-
tural slab is supported on beams and subjected to a fire above, it can heat without failure
out of plane because the supporting members are shielded from the fire and retain their
strength and stiffness. The resulting movement of the slab can cause severe damage in
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structural elements that are outside the fire zone (Fig. 7.22). Another common form of

FIGURE 7.22 Thermal Expansion Damage Due to Fire on Floor Above, Amherst College,
Massachusetts. (Credit: D. Dusenberry, Simpson Gumpertz & Heger Inc.)

damage is cracks that radiate from the center of the fire zone in regions of slabs adjacent to,
but outside, the fire zone.
In fires, expanding floor systems often cause severe damage to rigid bearing walls and
infill walls and to curtain walls and cladding. Rarely are these elements designed with suf-
ficient strength to withstand the very high expansion forces, which can be orders of mag-
nitude greater than lateral forces due to normal environmental loads.
The forces due to thermal expansion can have damaging effects on structural systems,
even in areas relatively remote to the fire location. If a structural steel member could be
fully restrained axially, heating it from room temperature to a mere 250°F (121°C) would