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National Advisory Committee for Aeronautics, Technical Notes - Comparison of Measured and Calculated Stresses in Built Up Beams

Web stresses and flange stresses were measured
in three built-up beams: one of constant-depth
v.ith flanges of constant cross section, one linearly
tapered in depth with flanges of constant cross section,
and one linearly tapered in depth with tapered flanges.
The measured stresses were compared with the calculated
stresses obtained by the methods outlined in order to
determine the degree of accuracy that may be expected
from the stress— —analysis formulas. These comparisons
indicated that the average measured stresses for all
points in the central section of the beams did not exceed
the average calculated stresses by more than 5 percent.
It was also indicated that the difference between average
measured flange stresses and average calculated flange
stresses based on the net area and a fully effective web
did not exceed 6.1 percent.

In an effort to improve the accuracy and consistency
of strength predictions of aircraft structures, an '
increasing tendency has been evident in structural
engineering to supplement static tests with strain
readings. The advent_ of the electrical strain gage has
accelerated this tendency. Because the aircraft struc-
ture is quite complicated and the-location of the
failure in a well-designed structure cannot be easily
determined, even a relatively large number of gages
(several hundred) may be just sufficient to place a few_
gages on each spot where failure is likely to occur.
The situation is further complicated because Structures

built up from sheet are not so uniform nor so consistent
in their behavior as, for example, beams of solid cross
section. The simple formulas for beams of solid cross
section consequently are not applicab_le to built— —up
structures. The successful interpretation of strain
readings on airplane structures re.quires, therefore,
basic information on the conSistency of the behavior of
built- -up structures. This information may be obtained
by multigage tests of structural elements simple enough
to permit very complete coverage. The test data thus
obtained may be compared with the results obtained by
stress-analysis formulas, such as those presented in
references 1, 2, and 5, to determine the accuracy with
which these formulas may predict the stress of built-
up structures.