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National Advisory Committee for Aeronautics, Wartime Report - Tests of Beams having Webs with Large Circular Lightening Holes

Strength tests were made on two sets of beams
having webs with large circular lightening holes. The
main conclusion drawn from the tests is that allowable
web stresses derived from pure shear tests and allow-
able flange stresses derived from compression tests
cannot be applied in the design of beams without making
corrections for interaction. The test data are in-
sufficient to establish a method for making such a
correction.

Published design information on shear webs with
flanged, circular lightening holes is confined to
empirical formulas. Most of the tests on which these
formulas are based were made with fixtures producing
pure shear, or approximately pure shear, in the web,
and the structural members bounding the webs were very
heavy in order to distribute the shear as uniformly as
possible along the edges of the test specimen. In the
two most extensive investigations published (references 1
and 2), failure was always observed to be precipitated
by buckling of the sheet in the neighborhood of the line
joining the centers of the lightening holes even when
the holes were so large that their reinforcing flanges
almost touched the edge members of the webs; obviously,
the heavy edge members bridged over the dangerous region
where the transverse net section is very small.

In actual structures, the shear webs would be bounded
by angles or flanges of relatively much smaller cross-
sectional area than the edge members used in the tests
of references 1 and 2. These angles might not be
capable of bridging over the dangerous sections in webs
with large holes, particularly when subjected to large
normal stresses caused by bending of the structure. An
empirical solution of the problem would require an inves-
tigation several times larger than that of reference 2
and does not appear feasible at the present time. In.
order to obtain some preliminary information, however,
a few exploratory tests were made on two series of webs
furnished by the Curtiss—Wright Corporation Airplane '
Division (Buffalo, E. Y.). The results of these tests
and of some related tests are presented herein.