Page 295 - Civil Engineering Formulas

BUILDING AND STRUCTURES FORMULAS 229

w w

t t
w t w t

Stiffening
lip
Unstiffened Stiffened
element element
(a) (b)
FIGURE 9.6 Typical lightweight steel compression elements.
(Merritt–Building Construction Handbook, McGraw-Hill.)

For structural design computations, flat compression elements of cold-
formed structural members can be divided into two kinds—stiffened elements
and unstiffened elements. Stiffened compression elements are flat compression
elements; that is, plane compression flanges of flexural members and plane
webs and flanges of compression members, of which both edges parallel to the
direction of stress are stiffened by a web, flange, stiffening lip, or the like (AISI
Specification for the Design of Light Gage Steel Structural Members). A flat
element stiffened at only one edge parallel to the direction of stress is called an
unstiffened element. If the sections shown in Fig. 9.6 are used as compression
members, the webs are considered stiffened compression elements.
In order that a compression element may qualify as a stiffened compression
element, its edge stiffeners should comply with the following:

w
I min 1.83t 4 2 144 (9.75)
B t
4
but not less than 9.2t .
where w/t flat-width ratio of stiffened element
I min minimum allowable moment of inertia of stiffener (of any shape)
about its own centroidal axis parallel to stiffened element

Where the stiffener consists of a simple lip bent at right angles to the stiff-
ened element, the required overall depth d of such a lip may be determined with
satisfactory accuracy from the following formula:

6 w 2
d 2.8t 144 (9.76)
B t