1000

Note: b' is the distance measured from toe of flange to centerline of web, h' is the distance between centerline lines of flanges, h'' is the distance from centerline of flange to tip of stem, l1, l2 are the length of the legs of the angle, ti, t2 are the thickness of the legs of the angle, bf is the flange width, tf is the average thickness of flange, tw is the thickness of web, Ic is the moment of inertia of compression flange taken about the axis of the web, It is the moment of inertia of tension flange taken about the axis of the web, and Ix is the moment of inertia of the cross-section taken about the major principal axis.

Numerical values for ro and H are given for hot-rolled W, channel, tee, single-angle, and double-angle sections in the AISC-LRFD Manual (AISC 2001).

For unsymmetric sections: Fe is to be solved from the cubic equation

(Fe - Fex)(Fe - Fey)(Fe - Fez)- Fl(Fe - Fey) (-V - F^Fe - Fe*) (= 0

The definitions of the terms in the above equation are as in Equation 4.20.

Local Buckling (with width-thickness ratio > 1r): local buckling in the component element of the cross-section is accounted for in design by introducing a reduction factor Q in Equation 4.17 as follows:

r / 0.877\ . .r „ ffi-0.85[A^ —r) Fy], if VQ > 1.5

where l = 1c for flexural buckling and l = 1e for flexural-torsional buckling. The Q factor is given by

where Qs is the reduction factor for unstiffened compression elements of the cross-section (see Table 4.6) and Qa is the reduction factor for stiffened compression elements of the cross-section (see Table 4.7).

4.4.2 Built-Up Compression Members

Built-up members are members made by bolting and/or welding together two or more standard structural shapes. For a built-up member to be fully effective (i.e., if all component structural

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