Handbook of Civil Engineering Calculations

FINDING THE LIGHTEST SECTION

TO SUPPORTA SPECIFIED LOAD

Find the lightest W8 in A36 steel to support a factored load of 100 kips (444.8 kN) in ten-

sion with an eccentricity of 6 in (15.2 cm). The member is 6 ft (1.8 m) long and is lateral-

ly braced only at the supports; Cb = 1.0. Try the orientations (a) to (c) shown in Fig. 41.

Calculation Procedure:

1. Try the first orientation, (a). Fig. 41
   Given

Pn = 100 kips (44.8 kN); M 14 = Pue = 100klPs*6m = 5Q kip_ft ^

12 in/it

For orientation (a) in Fig. 41

P 14 = 100 kips, M^ = 50kip-ft, Muy = Q

Try a W8 x 28: the design tensile strength (for a cross section with no holes)

<$>tPn = ^tFyA 8 = 0.90 x 36 ksi x 8.25 in^2 = 267 kips (1188 kN)

For (Lb = 6.0 ft) < (Lp = 6.8 ft), the design flexural strength for jc-axis bending

FIGURE 41