Building Materials, Third Edition

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Long narrow beams must be restrained laterally and supported in such manner that the
wide sides are vertical. If the first condition is not fulfilled, the member will fail from column
action in the compression fibres. Unless the second condition is met, strength of the beam may
be very seriously reduced through the inclination of the neutral axis. These requirements are
satisfied by diagonal bracing known as bridging. The bending strength of some of the timbers
is given in Table 4.3.

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ƒ— ƒX Wood has low shearing strength of 6.5–14.5 N/mm^2 along the fibres.
Resistance of wood to cutting across the fibres is 3 to 4 times greater than that along the fibres,
but pure shear generally does not take place since the fibres are also subjected to crushing and
bending.

ƒX Stiffness in a timber structure is often of as much importance as strength, but it is
much more frequently neglected in designing. Floors must be sufficiently stiff so that they will
not deflect appreciably under working loads as else they give one the feeling of insecurity.
Likewise, the deflection of rafters should be limited, if it is desirable, to avoid the disagreeable
appearance of a sagged roof. In general, denser woods are more stiff. A green timber is less stiff
than when seasoned. The structural sizes of timber are about as stiff as the clear small sticks.

„ XA wood which has a large capacity to resist shock or blows is called tough. In
order to be tough a wood must have both strength and flexibility. Hardwood as a class excels
in toughness. Long leaf pine is the only one of the conifers possessing much toughness. In
general, green wood is tougher than seasoned wood. Toughness is best measured by the
energy of the blow required to rupture a beam in transverse impact.

g—&—˜)is the measure of the ease with which wood may split. Most hardwoods split
more easily along radial planes than along tangential surfaces. Since splitting is accomplished
by wedging apart the longitudinal elements, it is closely related to tension, across the grain.
Woods which must be fastened by nails and screws should have a high resistance to splitting.
Among the conifers, with an exception of longleaf pine, the difference in cleavage strength in
the two directions is not great.