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National Advisory Committee for Aeronautics, Technical Notes - An Analysis of the Stability and Ultimate Bending Strength of Multiweb Beams with Formed-Channel Webs

Design curves and procedures are presented for calculating the
stresses for instability and failure of multiweb beams with formed-
channel webs. The ultimate bending strength of this type of construc-
tion is shown to depend upon the deflectional stiffness of the web
attachment flanges. A simple criterion is also given for predicting
whether a multiweb beam with a given attachment—flange design will be
susceptible to a wrinkling instability or will buckle as if the webs
were integrally Joined to the cover skins.

The criteria for predicting buckling and failure stresses are com-
pared with experimental data. These criteria are sensitive to the off-
set, pitch, and diameter of the rivets used on the web attachment
flanges, and the riveting specification is, therefore, emphasized as an
important design consideration.

Experiments have shown the existence of two distinct modes of insta-
bility and failure for multiweb beams subjected to bending loads.’ One
mode, referred to herein as local buckling, is characterized by longi—
tudinal nodes along the Joints between webs and cover skins. The other
mode is evidenced by a buckle pattern with troughs and crests extending
across the entire width of the beam and without formation of longitudi—
nal nodes. This mode is referred to herein as wrinkling, by analogy to
a form of instability which may occur in the face plates of compressed
sandwich panels.

In reference 1, design information for multiweb beams is presented,
which is based on the assumption that the beam.webs possess enough
deflectional stiffness to enforce longitudinal nodes in the cempressive
cover skin when it buckles. In the tests of multiweb beams reported in
reference 2, it was observed that, when local buckling occurred, the
buckling stresses correlated with the theory of reference 1. However,
for beams of this series with fonmed—channel webs, wrinkling instability
and failure were obtained. These tests, as well as the extensive experi-
mental investigation of multiweb beams with formed—channel webs reported
in reference 5, showed that, for a range of beam proportions, wrinkling
instability and failure can occur at stresses far below those predicted
by the local-buckling theory of reference 1.