Fungus Damage. Wood decay fungi are primitive plants that obtain their
food from wood. If wood is kept in a damp condition (30 pct or higher moisture
content) for any length of time, it becomes infected with wood-decay fungi which
bring about its decomposition more or less rapidly, depending on the species
of fungus and the kind of wood.
The growing stage of fungi consists of microscopic threads called hyphae
which in mass form cottony or felt-like growth/ It is hyphae that penetrate and
"branch out" within wood and cause decay. When conditions are suitable, fungi
produce spore-bearing structures called fruiting bodies (Fig. 1-6). The fruiting
bodies of most decay fungi are mushroom or bracket shaped.
Insect Damage. Various types of termites, beetles, carpenter ants. and
bees utilize wood as a food source or for nesting purposes. Damage by wood-
boring insects can usually be recognized without difficulty (Fig. 1-7). Evidence
of their presence may be entrance or departure holes, the size of which depends
upon the insect involved. In many instances the accumulation of powdery material
in the vicinity of the hole indicates insect activity. Extensive insect activity can
result in structural failure of members.
The damage caused by termites is different from that caused by most other
insects. Termites work inside pieces of wood, excavating large tunnels and
galleries, sometimes hollowing out the wood completely. They never form exit
holes on the surface of wood which they most carefully avoid damaging. However,
their presence may be evidenced by earthen tunnels which are used for passage
from the ground to wood members. Severe damage by termites in the United
States is usually limited to warm, temperate areas.
1.2.3-B PHYSICAL. Physical damage may take the form of excessive
deflection of structural members, failure of wood members, loosening or failure
of connectors, or development of splits. When wood members are loaded to or
beyond their design level for a long period, the fibers become permanently
elongated or shortened in a process referred to as creep. High or changing
moisture contents while the member is loaded has a similar effect. While this
creep may not indicate a loss of strength, the deflection or sag that results is
usually permanent. In the case of multiple wood members connected together,
such as in trusses, the deflection may be due to loose connectors. Joints are
loosened by moisture cycling that causes the wood to shrink and swell in thickness,
or by the initial drying of wood that causes a gradual shrinking over a long period.
Potential connector problems or joint failures may also be caused by splits in
the immediate area of the connector, (illustrations are shown under section 3.1.2.).
Splits outside the connector area in a tension member are generally not critical
if they are parallel to the member axis. Splits in areas with sloping grain are
potential failure points. Complete failure of the wood member may be in the