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National Advisory Committee for Aeronautics, Technical Notes - Notes on the Design of Latticed Columns Subject to Lateral Loads

The increasing interest in the use of metal for the con-
struction of airCraft makes timely a discussion of the problems I
and difficulties to be met in the design of efficient compression
members. No rational column formula has yet been developed
which gives results which are sufficiently precise for the de—
sign of airplane members, and consequently it is necessary to
fall back upon experimental testing. In order to derive the max--
imum benefit from experiments, however, it is necessary that the
eXperiments be guided by theory, and it is the object of this
paper to suggest a method of procedure by means of which the data
needed to modify existing formulae may be obtained with a minimum
of tests.

Although it is common in wing construction to find wing beams
continuous over several supports, for the sake of simplicity this
discussion will be limited to that of a simple column supported
at both ends and subjected to uniformly distributed loads perpen—
dicular to its axis and to end loads either axially or eccentri~
sally applied.

It should be kept in mind that the critical load calculated
from the above formula is the end load required to buckle the
strut and that for loads smaller than this the ideal column re—
mains perfectly straight. It is apparent also that the column- I
will fail elastically as soon as the stress at the ends reaches
the elastic limit of the material. ConseQuently the curve of ul—
timate stress vs. L/K for an Euler column has the form of the
right hand curve of Fig. l:‘

If now instead of being axially applied, the end load has an
eccentricity, h, bending stresses are introduced which increase
the stresses in the fibers of the column and decrease the magni—
tude of the load which will cause failure. In the case of a prac—
tical strut, variations in the shape and.thickness of the section,
initial curvature and other imperfections have the effect of giving
an eccentricity to the end load.