By that I mean
that the cross-sectional dimensions must be correct,
as otherwise there will be bulging places on the
outside, with the result that the stress will not be
evenly disposed about the centre of strength, and
a bending stress may be produced.
3. Struts, spars, etc., must be undamaged. Remember
that, from what I have already explained about
bending stresses, the outside fibres of the wood are
doing by far the most work. If these get bruised
or scored, then the strut or spar suffers in strength
much more than one might think at first sight;
and, if it ever gets a tendency to bend, it is likely
to collapse at that point.
4. The wood must have a good, clear grain with no cross-
grain, knots, or shakes. Such blemishes produce
weak places and, if a tendency to bend appears,
then it may collapse at such a point.
5. The struts, spars, etc., must be properly bedded into
their sockets or fittings. To begin with, they must
be of good pushing or gentle tapping fit. They
must never be driven in with a heavy hammer.
Then again, a strut must bed well down all over its
cross-sectional area as illustrated above; otherwise
the stress of compression will not be evenly disposed
about the centre of strength, and that may
produce a bending stress. The bottom of the strut
or spar should be covered with some sort of
paint, bedded into the socket or fitting, and then
withdrawn to see if the paint has stuck all over the
bed.
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