It is often due to shake or season checks, common in large timbers, which reduce the actual area resisting the shearing action considerably below the calculated area used in the formulae for horizontal shear.

(See page 98 for this formulae.) For this reason it is unsafe, in designing large timber beams, to use shearing stresses higher than those calculated for beams that failed in horizontal shear.

The effect of a failure in horizontal shear is to divide the beam into two or more beams the combined strength of which is much less than that of the original beam.Fig.18 shows a large beam in which two failures in horizontal shear occurred at the same end.

That the parts behave independently is shown by the compression failure below the original location of the neutral plane.
[Illustration: FIG.

18 .-- Failure of a large beam by horizontal shear.