Building Design and Construction Handbook - Merritt - Ventech!

8.22 SECTION EIGHT
8.9 MAXIMUM FLAT-WIDTH RATIOS OF
COLD-FORMED SHAPES
When the flat-width-thickness ratio (w/t) exceeds about 30 for an unstiffened element
and about 250 for a stiffened element, noticeable buckling of the element
may develop at relatively low stresses. Present practice is to permit buckles to
develop in the sheet and to take advantage of what is known as post-buckling
strength of the section. The effective-width formulas, Eqs. (8.5) to (8.7), are based
on this practice. To avoid intolerable deformations, however, w/t, disregarding intermediate
stiffeners and based on the actual thickness t of the element, should not
exceed the following:
Stiffened compression element having one longitudinal edge connected to 60
a web or flange, the other to a simple lip
Stiffened compression element with both longitudinal edges connected to 500
a web or flange element, such as in a hat, U, or box-type section
Unstiffened compression element 60
8.10 UNIT STRESSES FOR
COLD-FORMED STEEL
For sheet and strip of A611, Grade C steel with a specified minimum yield strength
F y � 33 ksi, use a basic allowable stress ƒ � 20 ksi in tension and bending. For
other strengths of steels, ƒ is determined by taking 60% of the specified minimum
yield strength F y. (This procedure implies a safety factor of 1.67.) However, an
increase of 33 1 ⁄3% in allowable stress is customary for combined wind or earthquake
forces with other loads. It should be noted that the 1996 AISI specification uses
‘‘strength’’ (moment, force, etc.) rather than unit stress.
8.11 LATERALLY UNSUPPORTED
COLD-FORMED BEAMS
If cold-formed steel sections are not laterally supported at frequent intervals, the
allowable unit stress must be reduced to avoid failure from lateral instability. The
amount of reduction depends on the shape and proportions of the section and the
spacing of lateral supports. (See AISI ‘‘Specification for the Design of Cold-Formed
Steel Structural Members.’’)
Because of the torsional flexibility of lightweight channel and Z sections, their
use as beams without close lateral support is not recommended. When a compression
flange is fully connected to a deck or sheathing material, the flange is considered
braced for its full length and bracing of the other flange may not be needed
to prevent buckling of the beam. This depends on the collateral material and its
connections, dimensions of the member, and the span.
When laterally unsupported beams must be used, or where lateral buckling of a
flexural member is likely to occur, consideration should be given to the use of
relatively bulky sections that have two webs, such as hat or box sections (Fig. 8.1o,
p, and q).