Page 255 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.39

strain gradients in constrained elements, and temperature or moisture changes in elements
of dissimilar materials that act compositely.
While damage is possible in almost any structure, usually movements induce damage in
rigid elements that in some way anchor the movement of relatively long portions of struc-
tures that are subjected to volume changes. An example is a structure with rigid masonry
walls that infill opposite ends of a multibay steel frame that is erected in a northern climate
in the winter. If the frame is cold when it is erected and then is allowed to heat when the
building is enclosed, the frame will expand. If insufficient relief is provided for this expan-
sion, masonry walls built into the frame will receive very large forces as they tend to
restrain the growth of the frame. Rigid nonstructural elements, such as precast curtain wall
panels, also are susceptible to damage if not properly isolated from the movements due to
volume changes of the supporting structures. These elements, which usually are designed
for self-weight and tributary pressure loads, often are of insufficient strength to withstand
the very large forces induced on them by the movement of their supporting frame elements.
Damage due to expansion or contraction of a frame is most likely to occur at the lowest
levels of buildings. At these locations, the lateral restraint created by the foundation system
can inhibit movement of the frame, thereby creating stresses that are less likely to occur at
higher levels where the frame is not restrained by the foundation system.
The most common forms of the damage due to self-straining forces are cracking and
spalling of structural and nonstructural components. Cracking occurs due to tensile stresses
usually caused by direct tension, shear, or bending. Spalling results from high compressive
stresses. Often, spalling occurs at joints and connections where high local contact stresses
can exceed by severalfold the average stress in an element.

Loads Due to Temperature Changes
One source of thermal loads in a structure is the dimensional changes that occur between
the time a structure is erected and the time that it is put into service. In northern regions
of the country, structural frames sometimes are erected in subfreezing weather. Assuming
that the framing elements of a steel structure are bolted together at a temperature of 32°F
(0°C), and the structure is then raised to an operating temperature of 70°F (21°C), the com-
pressive strain in elements that are fully locked against movement (if this could be
achieved) would be 38°F × 0.0000065 in/in/°F = 0.00025 in/in, or approximately 15 percent
of the yield strain of ASTM A992 steel. Similarly, over a 60-ft (18.3-m) length of expan-
sion, this temperature change would cause free expansion (if this could be achieved) of
approximately 3/16 in (4.8 mm). In practice, full restraint and free expansion both are
improbable. Typically, the actual stresses and the amounts of movement in the frame will
be less than these limits.
Structural analyses are necessary to assess stresses and the amounts of movements.
Accurate estimates must be made to model relative flexibilities of the elements assumed to
change temperature and the elements restraining movement. Consideration needs to be given
to any slippage of connections, local flexibilities in connections or members, and deformed
shapes attributable to other loads on a structure at the time of a temperature change.
All these conditions, which normally are neglected in design for loads that are not self-
straining, can have significant impact on stresses due to self-straining loads. For instance,
for the example of temperature change from construction to service given above, cumula-
tive deformations and connection slippage of 3/16 in (4.8 mm) over the length of 60 ft
(18.3 m) will entirely relieve thermal stresses. Consider that this condition of temperature
change might exist in a frame with three bays of W16 × 36 ASTM A992 steel beams, each
spanning 20 ft (6.1 m). Fully restrained, these beams would generate compressive stress of
approximately 7.2 ksi (0.05 MPa), or approximately 76 kips (338 kN) axial load for the