Page 527 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.17

Deterioration
Decay, Insects, and Marine Borers. Where there is evidence of moisture, it is important
to consider the possibility of decay and insect damage. Remember timber doesn’t make an
immediate transition from solid material to mush. There is a condition called incipient
decay where the timber is weakened by the start of decay that is not totally obvious. Under
conditions favorable to their growth, fungi will attack wood, causing decay and seriously
reducing the structural integrity of timber members. These organisms require four essen-
tials to survive: food, air, moisture, and a favorable temperature. Wood decay cannot
progress when the organisms lack any of these four essentials. Wood is the food source.
Lacking air, continuously submerged wood will not decay from most wood-eating organ-
isms although in saltwater it may be attacked by marine borers.
The wood-eating organisms are dormant at temperatures below the middle 30°F range.
In normal use three of these essentials are usually present, requiring only moisture as the
catalyst to induce decay. Coring and stress-wave testing, as described under Properties,
Stiffness, are useful in situ methods to determine the extent and limits of decay in struc-
tural members.
Termite activity is a function of temperature, being more prevalent in warmer climates such
as Florida, with reduced activity as temperature decreases. Termites are virtually nonexistent
in the northern heartland states away from the tempering effect of oceans. Subterranean ter-
mites maintain ground colonies with mud tunnels to the wood structure above.
Carpenter ants and powder post beetles are other insects that can cause damage to tim-
ber. Pest control and epoxy injection for significant damage are a solution. The portion of
timber filled with appropriate epoxy (providing a similar “MOE,” the right viscosity for
injection, and adequate adhesion) is generally assumed to be fully effective in compression
and neglected as an improvement for tension. The debris lining the cavities interferes with
reliable bond.
Wood exposed to saltwater environments is subject to a family of wormlike marine bor-
ers which burrow below the wood surface. As with termites, their activity is a function of
temperature, and unprotected woods in tropical waters have become nonfunctional due to
marine borers within a period of a few months.

Chemical Deterioration. Wood is considered superior to many other materials of con-
struction in resistance to chemical action but is not totally free from chemical deterioration.
In the wet environment of the Aleutian Islands, untreated timber bridge members remained
in good condition where the steel bolts had disintegrated to rust traces due to corrosion
caused by the proximity to the ocean. The author was involved in the design of a timber
domed roof covering a chemical coking operation so corrosive that the previous steel roof
had collapsed after a reasonably short period of service.
Wood structures are used for bulk storage of chemicals such as urea fertilizer which cor-
rodes unprotected steel rapidly. The heartwood of many species is more resistant to chem-
ical action than sapwood. Water and some organic liquids such as alcohol cause wood to
soften and swell, causing a temporary strength reduction. Acids and acid salts cause per-
manent strength loss due to chemical changes in the cell structure. Alkalines cause perma-
nent strength loss due to destruction of lignins which bind wood fibers together.
Heat. Wood strength decreases with higher temperature and increases with lower tem-
perature. Tabular design values are set at approximately 20°C. Temporary strength reduc-
tion occurs at temperatures to approximately 150°F, and permanent strength reduction
occurs beyond that level. The effect of fire on timber members goes beyond the obvious
char layers due to the permanent heat damage. (See Case Study 5, Dept. of Social and
Health Services Building.)