Analysis of the Ultimate Strength and Optimum Proportions of Multiweb Wing Structures Page: 16 of 35
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HACA TN 3633
in the webs must be considered. For deep wings, the webs called for by
this analysis offer a large area to resist shear loads. For the shallow
wings with low hits values, however, additional shear area may be
required.
Effect of Differences Between Actual and Ideal Structures
Attachment members.- Equations (5) and (6) express the maximum moment
carried by the skin and flat-sheet portion of the web of a fabricated beam
if the web-skin connection is a strongly riveted one. This requires small
rivet-offset distances. (See ref. 5.) An attachment member, either the
flange of a channel web or an angle cap or T-cap, must usually be consid-
ered. Such an attachment member adds to the solidity of a beam but it
also adds to the moment-carrying capacity. The stress carried by small
attachment members will be approximately equal to the edge stress a2.
By a development similar to that of -equation (17), it can easily be shown
that, when attachment members are added to an optimum design which is
selected from figure 4, the deviation of the actual beam from the optimum
integral beam (when both are plotted in fig. 4) is approximately along a
line of the same slope as the maximum-efficiency curve.
On the basis of this development, the range over which attachment
members increase beam efficiency can be determined. When the slope of
any hftS curve equals 1/ l2, the slope of the maximum-efficiency curve,
it becomes more efficient to move to the right in figure 4 by adding
T-caps or angle caps than to decrease web spacing and increase web
thickness.
Neutral-axis location. - When because of cover-skin buckling, unequal
thickness of cover skins, or other lack of beam symmetry, the neutral-axis
location is not at the web midheight, the maximum-strength equations are
affected. The change in neutral-axis location results in unequal edge
stresses at the two covers and a resultant axial force as well as a
bending moment acting on the webs. In general, the effect of this change
on the maximum-strength equation (eq. (7)) will be small. For example,
consider a web stress distribution with the distance of the neutral axis
from each cover changed by 20 percent, that is, 40 percent of the web
under tensile stress. If the stresses are again approximated by a rectan-
gular stress distribution of magnitude ce, the plate will have an axial
compression force equal to 0.2aebWtW and a moment equal to 0.24aebw2tW.
The moment is essentially that given in equation (6), and it is seen that
the effect is a small one. For beams with an adequate tension cover, the
effect of a neutral-axis shift on equation (5) will also be negligible.15
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Rosen, B. Walter. Analysis of the Ultimate Strength and Optimum Proportions of Multiweb Wing Structures, report, March 1956; (https://digital.library.unt.edu/ark:/67531/metadc57902/m1/16/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.