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National Advisory Committee for Aeronautics, Report - Stresses in Wood Members Subjected to Combined Column and Beam Action

This publication is one of a series of three reports prepared by the Forest Products Labora—
tory 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.

Often in airplane construction a member resisting fiexure from transverse loads has a
further direct stress brought upon it due to an end thrust or pull. The resultant intensity of
stress at any point of the member will then be the algebraic sum of the bending stresses and the
direct stress of tension or compression. Our analysis of the stresses in a wooden member sub-
jected to axial and lateral forces will, however, be limited to the condition of combined beam
and column action.

It is universally conceded that for this particular condition maximum stress is intermediate
between the ultimate compressive strength of the wood and its modulus of rupture and that
its value depends in some way on the ratio between the bending unit stress and the total unit
stress due to both bending and direct compression.

The Army and Navy aeronautical bureaus have assumed a lineal variation of maximum
stress in preparing specifications. This. scheme of representing maximum stress variation
by a straight line from ultimate compressive strength to modulus bf rupture was adopted
becauseof its simplicity and in the absence of any data which would show the true form of the
curve. It is recognized that this maximum stress curve, even it correct, does not solve the
problem of design for combined loading because maximum load does not occur simultaneously
with maximum stress, but at a stress ,below the maximum. For example, an Euler column
with a very slight side load reaches its 'maximum load at a stress but slightly above the fiber
stress at elastic limit.