Example

Design an I-shaped cross-beam 12 ft (3.7 m) in length to be used as lateral braces to brace a 30-ft (9.1 m) long simply supported W30 x 90 girder at every third point. The girder was designed to carry a moment of 8000 kip in. (904 kN m). A992 steel is used.

Because a brace is provided at every third point, Lbr = 10 ft = 120 in., Mu = 8000 kip in. as stated. Cd = 1 for single curvature bending. ho = d - tf = 29.53 - 0.610 = 28.92 in. for the W30 x 90 section. Substituting these values into Equations 4.63 and 4.64 for nodal bracing, we obtain bbr = 30.7 kip/in., and Pbr = 5.53 kip.

As the cross-beam will be subject to compression, its slenderness ratio, l/r, should not exceed 200. Let us try the smallest size W-section, a W4 x 13 section, with A = 3.83 in.2, ry = 1.00 in., fcPn = 25 kip.

Stiffness: —— = —---- = 771 kip/in. > 30.7 kip/in.

Since all criteria are satisfied, the W4 x 13 section is adequate. Use W4 x 13 as cross-beams to brace the girder.

0 0

Post a comment