Materials and Methods of Construction in Light Structures Page: 23 of 38
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1 .A.C.A. Technical memorandum o, 515 21
and made the condition tht for equal weights the steel
must have a strength of 46 (7.8 2,8) = about 110 i:g/-m2
This condition, however, is not fully applicable, be-
cause the choice of a material can never be based on its
tensile strength. alone. At best this could be the case
for purely tensile parts, and then only when the latter
r-e not combined with other parts which could be affected
by their deformation. An example will illustrate this.
A cuanticever girder is supported by a rod which is a ouro-
ly tension memboner. On the assumption that the rod is a
stooeel one with a tensile strength of 80 kg/mm2 (113,788
lb./sq.in.) requiring the area F, a corresponding rod of
dural'u.iin would require an area of 2F. On this assump-
tion and on the basis of the tensile strengths alone, we
obtain for, the steel rod a weight excess of 7.8 (2 x
2,3) = 1.39 as compared with the duralumin rod. In order,
however, for the girder to receive the same stresses in
both cases, the deformation of both rods must be the same.
2e;.ce we must have FD = (220 70) FS 3.15 rS (the ex-
refsion 220 70 representing the ratio of the moduli
of olasticity of the materials compared).
Since te ratio of the specific weights is only 7.8
2,3 = 2.8, the aluminum rod, on the above assumptions,
will be hoavier to the amount of 3.15. 2.8 = 1.12. If,
terofore, the cu.lraluuin rod should be given only the same
,weiht as the stooeel rod, then under certain conditions, a
corresponding excess weight would have to be given the
The above statement is likewise applicable to compros-
sion struts. Such a strut, .f it satisfies tho Euler
formula, can be made lighter of stool than of duralumin.
Even a girder subjected to bonding stresses, in which some
bonding deformation is to be expected, can be made lighter
of steel than of dura.lumin.
In lattice girders the lattices can well be duralumin,
since the o-fect of their deformation on the total defor-
mation of the girder is extremely small (See Schwengler,
"Elastizitattheorie im isenbau"). In an actual girder
test, the deformation of the lattices was found to be only
56 of t:,e total deformation.
A: important factor in the choice of the materials is
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Rohrbach, Adolf. Materials and Methods of Construction in Light Structures, report, May 1929; (https://digital.library.unt.edu/ark:/67531/metadc65364/m1/23/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.