Building Design and Construction Handbook - Merritt - Ventech!

8.14 SECTION EIGHT
The Committee on Specifications of the American Iron and Steel Institute has
strived to put all formulas in the ‘‘Specification for the Design of Cold-Formed
Steel Structural Members’’ on nondimensional bases so that their use with English
or SI units is rigorous and convertible.
(AISI ‘‘Cold-Formed Steel Design Manual,’’ American Iron and Steel Institute,
1101 17th St., NW, Washington, DC 20036.)
8.5 STRUCTURAL BEHAVIOR OF FLAT
COMPRESSION ELEMENTS
In buckling of flat, thin compression elements in beams and columns, the flat-width
ratio w/t is an important factor. It is the ratio of width w of a single flat element,
exclusive of any edge fillets, to the thickness t of the element (Fig. 8.4). Local
buckling of elements with large w/t may be resisted with stiffeners or bracing.
FIGURE 8.4 Compression elements.
Flat compression elements of coldformed
structural members are accordingly
classified as stiffened or unstiffened.
Stiffened compression elements
have both edges of the element parallel
to the direction of stress stiffened by a
web, flange, or stiffening lip. If the sections
in Fig. 8.1a to n are used as compression
members, the webs are considered
as stiffened compression elements.
The wide, lipless flange elements and
the lips that stiffen the outer edges, however,
are unstiffened elements. Any section
can be broken down into a combination
of stiffened and unstiffened elements.
Only part of an element may be considered effective under compression in computation
of net section properties. The portion that may be treated as effective
depends on w/t for the element.
The cold-formed structural cross sections shown in Fig. 8.5 indicate that the
effective portions b of the width of a stiffened compression element are considered
to be divided into two parts, located next to the two edge stiffeners of that element.
(A stiffener may be a web, another stiffened element, or a lip in beams. Lips in
these examples are presumed to be fully effective.) In computation of net section
properties, only the effective portions of stiffened compression elements are used
and the ineffective portions are disregarded. For beams, because flange elements
subjected to uniform compression may not be fully effective, reduced section properties,
such as moments of inertia and section moduli, must be used. For computation
of the effective widths of webs, see Art. 8.7. Effective areas of column cross
sections are based on full cross-sectional areas less all ineffective portions for use
in the formula for axially loaded columns, Eq. (8.22), in Art. 8.13.
The critical load, Pcr, kips, for elastic flexural buckling of a bar of uniform cross
section, concentrically end loaded as a column, is given by the Euler formula:
2 2
P � � EI/L (8.1)
cr