Page 211 - Handbook of Civil Engineering Calculations, Second Edition

1.194 STRUCTURAL STEEL ENGINEERING AND DESIGN

FIGURE 21. Bending-moment diagrams.

2
3
19.25(12) /[3(29)(10) ] 0.382 in. (9.7 mm). For dimensional homogeneity, the left
side of Eq. 5 must be multiplied by 1 kip (4.45 kN). The product represents the external
work performed by the unit loads.
REDUCTION IN WIND DRIFT BY USING
DIAGONAL BRACING

With reference to the previous calculation procedure, assume that the wind drift of the
bent is to be restricted to 0.20 in. (5.1 mm) by introducing diagonal bracing between lines
B and C. Design the bracing, using the gross area of the member.

Calculation Procedure:
1. State the change in length of the brace
The bent will be reinforced against lateral deflection by a pair of diagonal cross braces,
each brace being assumed to act solely as a tension member. Select the lightest single-
angle member that will satisfy the stiffness requirements; then compute the wind drift of
the reinforced bent.
Assume that the bent in Fig. 22 is deformed in such a manner that B is displaced a hor-
izontal distance A relative to D. Let A cross-sectional area of member CB; P axial
force in CB; P h horizontal component of P; L change in length of CB. From the
geometry of Fig. 22, L cos a /L approximately.
2. Express P h in terms of
Thus, P aAE /L ; P h P cos Pa/L;
2
then
2
a AE
P h (6)
L 3
3. Select a trial size for the
diagonal bracing; compute the
tensile capacity
A section of the AISC Specification limits
the slenderness ratio for bracing members in
FIGURE 22 tension to 300, and another section provides