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National Advisory Committee for Aeronautics, Report - Form Factors of Beams Subjected to Transverse Loading Only

This publication is one of a series of three reports prepared by the Forest Products Labo-
ratory of the Department of Agriculture for publication by the National Advisory Committee
for Aeronautics. The purpose of these papers is to make known the results of tests to determine
the properties of wing beams of standard and proposed sections, conducted by the Forest
Products Laboratory and financed by the Army and the Navy.

Nearly all of the mechanical properties of wood, especially those affecting its fiexural
strength, have been determined from tests on rectangular specimens and, of all of these proper-
ties, the modulus of rupture is the one most used in design. The term modulus of rupture does
not correspond to any of the fundamental properties of wood, but it is that value obtained by
substituting maximum bending moment in the ordinary beam formula which gives stresses in
the extreme fiber for moments within the elastic limit. When confined to rectangular sections,
however, the term modulus of rupture in this restricted sense may well be applied to wooden
beams. However, when applied to beams of I and box sections we obtain results which are not
comparable with those obtained for rectangular beams. The computed values for such sect-ions
may, in extreme cases, be 50 per cent less than corresponding values computed for rectangular
beams made of material from the same plank.

If the properties of wood as based on tests of rectangular sections are to be used as a basis of
design for any other section, a factor whose value is dependent upon the shape of the section
must needs be applied to the usual beam formula. For convenience in this discussion this factor,
which is the ratio of either the fiber stress at elastic limit or the modulus of rupture of the section to the similar property of a rectangular beam 2 by 2 inches in section made of the same material, will be called a “Form Factor.”