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National Advisory Committee for Aeronautics, Wartime Report - Charts for Calculation of the Critical Stress for Local Instability of Columns with I-, Z-, Channel, and Rectangular-Tube Section

Charts are presented for the calculation of the crit-
ical stress for local instability of columns with I~, 2—,
channel, and rectangular—tube section. These charts are
intended to replace the less complete charts published in
NACA Technical Note No. 743. The values used in extend—
ing the charts are computed by moment—distribution methods
that give sonewhat more accurate values than the energy
method previously used and also make it possible to deter—
mine theoretically which element of the cross section is
primarily responsible for instability.

An experimental curve is included for use in taking
into account the effect of stresses above the elastic
range on the modulus of elasticity of 34$~T aluminum alloy.

A determination of the dimensions of a thin—metal
column for maximum critical stress with certain given con—
ditions is presented.

One of the important requirements in the design of
thin—metal columns for aircraft is_ths determination of
the critical compressive stress at which local instability
occurs. Local instability of a column is defined as any
type of instability in which the cross sections are dis—
torted in their own planes but are not translated or re—
tated.

The critical stress for local instability can
usually be given in terms of the geometry of the seem
tion, the properties of the material, and a coefficient,
Reference 1 presented charts for the determination of
such coefficients for columns of 1—, 2—, channel, and
rectangular~tube sections. These charts, however, con-
tained relatively few.curves and in some cases required
interpolation over a wide range.

In order to make the charts of reference 1 more
nearly complete and to reduce the necessary range of
interpolation, each chart has been extended to include
eight intermediate curves. The values used in extend—
ing the charts are computed by moment—distribution
methods that give somewhat more accurate values than the
energy method previously used and also make it possible
to determine theoretically which element of the cross
‘section is primarily responsible for instability.