Building Design and Construction Handbook - Merritt - Ventech!

COLD-FORMED STEEL CONSTRUCTION 8.19
FIGURE 8.7 Curves relate effective-width ratio b/t to flat-width ratio w /t at various stresses ƒ
for (a) unstiffened elements and (b) stiffened elements.
stiffened element in a cold-formed steel member loaded as a beam. However, in
beam strength determinations requiring use of the section modulus of the cross
section, 1.67ƒ is the stress to be used in Eq. (8.9) to calculate the effective width
of the stiffened element and provide a margin of safety.
In determination of the safe loads for a cold-formed steel section used as a
column, effective width for a stiffened element must be determined for a nominal
buckling stress, Fn, to ensure an adequate margin of safety.
Since effective widths are proportional to �k,
the effective width of a stiffened
element is �4.00/0.43 � 3.05 times as large as that of an unstiffened element at
applicable combinations of ƒ and w/t. Thus, stiffened elements offer greater
strength and economy.
Single Intermediate Stiffener. For uniformly compressed stiffened elements with
a single intermediate stiffener, as shown in Fig. 8.6c, calculations for required moment
of inertia I a of the stiffener are based on a parameter S.
S � 1.28�E/ƒ (8.10)
For Case I, S � b o/t, where b o � flat width, in, including the stiffener. I a � 0
and no stiffener is required.
For Case II, S � b o/t � 3S. The required moment of inertia is determined from
4 I a/t� [50(b o/t)/S]
� 50 (8.11a)
For Case III, b o/t � 3S. The required moment of inertia is determined from
4
I /t � [128(b /t)/S] � 285 (8.11b)
a o