Wood has relatively little strength perpendicular to the grain in comparison to its strength along the axis of its grain.
For example, a sample of Douglas-fir might have parallel to-
grain tensile and compressive strengths of 15,600 and 3,470 PSI respectively. but perpendicular-to-grain tensile and compressive strengths of only 360 and 340 PSI, respectively.
The strength of a wooden beam in a spanning application derives from the fact that the forces experienced by the beam when loaded are primarily oriented along the grain (tension, compression and shear) with essentially no cross-grain tension.
This assumes, however, that the beam is supported underneath its ends and that the beam is of essentially uniform moss section without cuts or notches. This latter assumption may not always be true in practice. Beams may be cut or notched in various places to run utilities or to fit against other structural members. Notches that extend a significant distance into the beam may be an unavoidable part of the building’s design or may occur from poor construction techniques.
Generally, a notch in a beam causes some of the loading of the beam to be manifest as cross grain tension, a mode in which wood is relatively weak. Additionally, the stress concentration at the notch re-entrant corner produces stresses to initiate and propagate a crack. As result, if a spanning beam is to be notched, it is necessary to use reduced loading figures for that beam resulting in the need for larger or more beams than would otherwise be necessary. In renovation projects, where beam number and size is fixed, notching of the beams may not be allowable.
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