Page 254 - Forensic Structural Engineering Handbook

7.38 CAUSES OF FAILURES

during a fire and witness accounts of the fire intensity and duration, can be used to estimate
maximum fire temperatures. The estimated fire temperatures, in turn, can be used to
estimate the effects that a fire had on material strength, material stiffness, and thermal
deformations and associated stresses.
Useful information about fire temperatures can be acquired by assessing the condition
of building materials found in the fire debris. Different materials burn or melt at different
temperatures. When samples of various materials found in the fire debris can be related to
locations of fire damage, the conditions of those materials can be indicators of the maxi-
mum fire temperatures nearby. Reference 37 contains a useful list of the melting points of
materials commonly found in buildings.
Information about fire temperatures also can be revealed through metallurgical analysis
of metallic materials and petrographic analysis of cementitious materials. Petrographic
analyses can reveal changes in material characteristics, and the depth that heat penetrated
members.
Once ambient fire temperatures are known, analyses must be performed to estimate the
temperatures reached in building components. These calculations require assessment of the
exposure of the component to ambient temperature and direct radiation, the influence of
insulation that remained on the component during the fire, and the characteristics of heat
flow into the component. For calculation of the overall effect of a fire on a structure, it is
important as well to understand and account for the progress of the fire through the build-
ing. It is possible that the first area to burn began to cool as another area of the building
became fully involved in a conflagration.
There are sophisticated software programs for the analysis of fire growth and progres-
sion, and for analyzing the temperatures of structural elements subjected to a fire. Users of
these programs should have thorough understanding of fire science phenomena and heat
flow mechanisms.

SELF-STRAINING LOADS

Nature and Consequences of Self-Straining Loads
Self-straining loads arise primarily from volumetric changes in building materials. These
volumetric changes, which are due to environmental changes or in some cases aging
effects, cause strains that lead to structural movements. If these strains and movements are
constrained in any way, then stresses build in structural components. It is these stresses due
to constrained movements that can cause damage in structures. Since these stresses arise
not from any externally applied loads, but instead from changes within the building mate-
rials themselves, loads from these causes usually are classified as self-straining loads.
The most common environmental conditions that cause volumetric changes and self-
straining loads in building materials are changes in temperature and moisture. All common
building materials expand and contract with changes in temperature. The amount of expan-
sion and contraction depends on the temperature change and the coefficient of expansion
for the building material. In addition, some building materials such as concrete and
masonry shrink or swell with changes in moisture.
In most cases, if the temperature or moisture content of an isotropic object changes uni-
formly, the object experiences no stress change when it comes to equilibrium if it is not oth-
erwise constrained externally. Uniform growth or contraction of a body that has no external
restraints creates no internal stresses of consequence to the performance of normal build-
ing elements. In structures, conditions that can cause detrimental stresses are external
restraint of building movement, internal restraint of movements of portions of structures,