Page 257 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.41

and differential movements between concrete elements and elements of other materials,
such as steel, that are not subject to shrinkage.
The amount of shrinkage that a concrete element experiences is influenced by the con-
crete mix design, the method of placement, the amount and type of curing that are applied
during the days and weeks after placement, and the environment while in service. 40
Swelling by clay masonry units upon take-up of moisture can be a serious problem in
large masonry structures. Clay masonry units, such as brick, are manufactured by kiln dry-
ing. The moisture content of brick when it is first manufactured is significantly less than
the moisture content later when the brick is in service. Expansion can be on the order of
0.03 to 0.05 percent.
Large exterior applications of brick usually require careful detailing to allow the brick
to expand without causing distress. Midrise and high-rise buildings without horizontal
relief joints can develop severe distress in brick facades as the brick expands vertically
while the frame remains at constant height. Differential movements can damage connec-
tions to the backup structure, shelf angles, lintels, and other interconnected elements. If
adjacent segments of interlocked brick are supported at different levels (such as when a seg-
ment that rests on supported shelf angles is interlocked with a segment that extends to the
foundation), vertical and step cracks can form near the interface. It is possible for structures
with concrete frames to have more acute potential for damage than do structures with steel
frames because differential movements can be greater if the concrete frame shortens due to
creep and shrinkage.
Horizontal expansion of clay masonry will cause damage if the spacing and size of ver-
tical expansion joints are inadequate. Common forms of damage are cracking and outward
displacement at building corners, as perpendicular faces of brick move relative to each other.
Wood-frame structures also can experience damage due to moisture changes. New lum-
ber often is intentionally dried before being sold for construction. However, at a maximum
moisture content of 19 percent as specified by the National Forest Products Association 41
for wood at installation, further drying is possible over time. When wood dries, it contracts,
especially in directions perpendicular to the grain of the lumber. Construction details that
include stacking wood elements across the grain can lead to dimensional shortening.
Accumulations over multiple details with this potential can cause damaging distortions.

SETTLEMENT

Nature and Consequences of Settlement
Structural settlement normally occurs when the foundation is inadequate to support the
loads applied to a structure. Settlement can occur if a foundation is inadequately designed
or installed, or if conditions unforeseen during design affect the structure during its life.
Certainly, settlement can occur if, due to changes in use, loads on a foundation exceed those
envisioned by the designer. However, excavations near existing foundations can cause set-
tlement as well. Changes in groundwater elevation can cause consolidation that can affect
foundations. Bridge piers in rivers can be undermined by scour of foundation materials
below footings. Decay of organic materials below footings can cause volume changes in
the foundation materials. In most cases, the reasons for settlement are best determined by
a thorough geotechnical investigation and assessment.
Settlement causes distortions that affect the function of a structure, stresses that can
damage structural members and their connections, cracking of structural elements, over-
loads in structural elements that acquire extra load due to distortions of indeterminate
structural systems, and damage to architectural components. Usually when settlement is