Quantcast Chapter 3. Inspection of Wood Trusses

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Elements of a typical heavy-timber truss are shown in Figure 3-1. Any of
the following signs of distress (Figs. 3-2 through 3-6) indicate a need for more
detailed investigation as described under PROCEDURES in Chapter 2. Since
trusses are often supported by heavy-timber columns, these columns should be
evaluated at the same time. The same principles of deterioration apply. Specific
application to columns is discussed under various signs of distress.
3.1.1 DEFLECTION. Most trusses will deflect some, so that unusual
deflection is the main concern. Many trusses are built with camber (upward deflec-
tion) such that deflection under dead load results in the bottom chord being
horizontal. Generally, the downward deflection (sag) should not exceed the total
span divided by 180. This calculates to 2 inches to 30 feet (360 in.), 4 inches
in 60 feet (720 in.), or 6 inches in 90 feet (1,080 in.). Deflections less than these
amounts may be considered slight, and given a rating of 60 or above.
Greater deflections should be easily visible (Fig. 3-5), and these should be
measured and the amount recorded. If repairs are not made after inspection,
the amount of deflection should be compared during successive inspections to
see if the truss is stable or is continuing to deflect more. Excessive deflection
is always a signal to look for causes such as loose connectors, decay in wood
members, or a fracture in the lower chord.
3.1.2 CHECKS AND SPLITS. The importance of checks and splits
depends partly on the type of stress in the member. In single-span trusses, lower
chords of trusses are stressed in tension; top chords are stressed in compres-
sion. Diagonals may be stressed in either tension or compression depending on
the type of truss.
Normally, checks are of relatively little importance unless they become water
traps. If they are extensive and deep, their seriousness may be evaluated from
the following guidelines on splits, on a basis of their relative importance com-
pared with splits.
At the ends of the members stressed in compression parallel to the grain,
such as top chords, checks and splits may be disregarded, provided there is no
evidence of slip from the wedging action of connectors and bolts.
For members stressed in tension parallel to the grain, such as bottom chords,
splits outside the connector area that are approximately parallel to the grain (Fig.
3-2B) may be disregarded. However, splits with a slope greater than 1 in 8 should
be noted.


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