Page 298 - Forensic Structural Engineering Handbook

DEFECTS, DETERIORATION, AND DURABILITY 9.9

Examples: Wood
Defects. Wood is an organic material, so some defects are natural; those are usually
detectible during structural condition assessments. Natural wood defects generally result
from growth or drying. Knots develop from branch growth; shakes are grain separations
that develop between growth cycles; and checks are cracks resulting from the differential
shrinkage rates of wood parallel and transverse to the grain that result from improper cut-
ting and drying of dimension lumber. Splitting occurs in wood as a result of the same mech-
anism as checks. 2,10,11 Knots, shakes, checks, and splits create local stress concentrations in
structural lumber and may lead to failure.
Other defects affecting the structural integrity as well as the aesthetics of wood compo-
nents include deformations, such as bowing, twisting, crooking, and cupping. 12 These
deformations may result from improper drying in fabrication, improper material storage,
and one-sided coating applications.

Deterioration. Wood deterioration often can be attributed to deficiencies in maintenance,
such as exposure to weathering factors without proper protective coatings. Leaking build-
ing envelopes, peeling paint, and damp conditions often lead to the accelerated deteriora-
tion of structural wood components.
Wood is especially vulnerable to consumption—by fire, rot (fungus), and insects. Wood
rot results from a combination of conditions within a structure. A fungus breaks down the
cellular structure and leaves the wood with little to no structural integrity. Various species
of fungus result in different types of consumption—dry rot, wet rot, soft rot, and white
rot—but they each require a food source (cellulose), oxygen, water, a particular tempera-
ture range, light, and a particular pH range to thrive. Wood typically does not rot if it is con-
tinuously submerged in water or if it is continuously well protected from water; however,
the intermittent wetting and drying of wood, or constant medium water content, provides
ideal conditions for fungal attack and resulting deterioration.
Softened, damp wood also provides an ideal environment for burrowing insects.
Termites, ants, carpenter bees, marine borers, and various beetles consume wood fibers;
loss of structural integrity and premature failure may result.

Examples: Masonry

Defects. Masonry units may consist of brick, concrete (CMU), terracotta, natural stone,
or other materials. Each material has unique criteria for design, fabrication, and installation.
However, there are some characteristic defects common to all types of masonry systems.
Masonry structures are composite systems; many defects in masonry arise from improper
attention to the interface of the components. The most common structural applications for
masonry are wall systems composed of masonry units, mortar, and reinforcement. For an
exterior wall system to function, both structural (load bearing) and environmental (weather-
proofing) aspects must be addressed in the design.
Defects in the structural systems of a wall often result in increased stress factors. For
example, deficiencies in the masonry units themselves, such as under-fired brick, make the
units more susceptible to cracking, spalling, and crushing. Improperly selected mortar, for
either initial construction or repointing, may lead to crushing of the surrounding masonry
due to differences in stiffness or thermal expansion. If mortar selected for repointing is
more impervious to water than the masonry units, water does not evaporate as readily,
which may lead to accelerated weathering.
Deficient detailing of control and expansion joints may restrain normal movement of
masonry and result in elevated internal stresses within the wall. Similarly, the improper