Page 256 - Forensic Structural Engineering Handbook

7.40 CAUSES OF FAILURES

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beam area of 10.6 in (68.4 cm ). However, if the beams in this example were supported by
single-plate shear connectors to the walls of tube columns, at each of six locations (three
bays, one connection at each end of each beam), the wall of the tube column would need to
deform by only 1/32 in (0.8 mm) under the action of the axial load in the beam to relieve
the stress. In fact, if the column walls deform, the axial load that develops in the beams can-
not achieve the theoretical 76 kips (338 kN), and equilibrium is reached at a lower load.
To the extent that strain due to temperature changes occurs over a long time in materi-
als such as concrete that are subject to creep and shrinkage distortions, the stresses that
develop from self-strain in building materials may be mitigated by these other self-straining
phenomena.
In most enclosed and temperature-controlled buildings, thermal strains during service
conditions are not significant. Heated and air-conditioned buildings commonly experience
interior temperature excursions that rarely exceed 10 to 20°F (5.5°C to 11°C). Normally
this range of temperature change is insufficient to damage structural elements as long as
connections have sufficient capacity to support the resulting loads in restrained members.
Unusual building uses and structures that are not temperature-controlled can experience
large service temperature excursions. For example, open parking structures in some north-
ern regions of the United States experience ambient annual air temperature excursions of
100°F (56°C) or more. Restrained structures that respond fully to temperature ranges on
this order should be expected to experience damage.
Thermal gradients are important considerations for designs of structures. These gradi-
ents arise from transient conditions when structural elements are responding to changes in
ambient temperatures, in structural components with surfaces that are subjected to differ-
ent steady-state temperatures, and in structural components that are subjected to solar and
other thermal radiation. The top level of an unshaded parking garage in a southern climate
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easily could experience a gradient through the slab thickness of 35°F (19 C) or more.
Thermal stresses can build in structures when dissimilar materials or systems with dis-
similar exposures are locked together against thermal movement. Curtain walls experience
fluctuations in ambient temperatures and solar radiation, whereas the frames that support
them are insulated against this environment. If these two elements are connected rigidly,
the differences in thermal strains in the two elements will generate stresses in both.
Conversely, if connections are flexible, sufficient movement joints must be provided to
allow freedom from constraint.
Data on ambient thermal variations can be obtained from the National Climatic Data
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Center and local news agencies. In addition, local airports often maintain accurate records
of temperature. These data must be adjusted for the variations possible within a reporting
region. For instance, temperatures often are warmer in city centers than in nearby suburbs.
Large bodies of water often mitigate daily and seasonal temperature fluctuations. Also,
radiational cooling during the night is influenced by objects, such as trees, over or around
structures. Wind can affect the rate at which heated objects change temperature by accel-
erating heat loss (wind chill).

Loads Due to Moisture Changes
Perhaps the two most common types of structural damage due to moisture changes are (1)
movements and cracking due to shrinkage of moist construction materials that tend to con-
tract when drying and (2) spalling and distortions of masonry construction as it absorbs
moisture and tends to expand in service.
The phenomena of shrinkage cracking and movement in concrete are well docu-
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mented. In general, common types of damage include cracking of long and broad ele-
ments such as walls and slabs, cracking of beams, curling of slabs (elevated and on grade),