Manual, the allowable stress corresponding
to this ratio is fc = \.661fb - 8640 - \(fb -
12,950)g/f]/15 = 1.667(20,000) - 8640 - (20,000- 12,950)11.4/15 = 19,340 lb/in^2 (133.3 MPa).
3. Compute the allowable unit live
load If flexure is the sole criterion
Thus M =fcSx= 19,340(3.54)712 = 5700 ft-lb
(7729.2 N-m); w = 8M/Z^2 = 8(5700)716^2 = 178
Ib/lin ft (2.6 kN/m); WLL = 178 - 50 = 128 Ib/lin
ft(1.87kN/m).
4. Investigate the deflection under
the computed live load
Using E = 29,500,000 lb/in^2 (203,373 MPa) as
given in the AISI Manual, we have ALL .=
5wLLL^4 /(384EIx) = 5(128)(16)^4 (12)^3 /[384(29.5)
(10)^6 12.4] = 0.516 in (13.1 mm); ALLallow = FIGURE 23
16(12)7360 = 0.533 in (13.5 mm), which is satis-
factory.
5. Investigate the shearing stress
under the computed total load
Refer to the AISI Specification. For the individual channel, h = D - 2t = 6.79 in (172.5
mm); hit = 64.7; 64,000,000/64.7^2 >^2 Afb; therefore, vMow = 13,330 lb/in^2 (91.9 MPa); the
web area = 0.105(6.79) = 0.713 in^2 (4.6 cm^2 ); V=^1 X 4 (178)16 = 712 Ib (3.2 kN); v =
712/0.713 < #aiiow» which is satisfactory. The allowable unit live load is therefore 128
Ib/lin ft (1.87 kN/m).
LIGHT-GAGE STEEL BEAM WITH STIFFENED
COMPRESSION FLANGEA beam of light-gage cold-formed steel has a hat cross section 8 12 in (203.2 304.8
mm) of no. 12 gage, as shown in Fig. 24. The beam is simply supported on a span of 13 ft
FIGURE 24