Page 296 - Civil Engineering Formulas

230 CHAPTER NINE

but not less than 4.8t. A simple lip should not be used as an edge stiffener for
any element having a flat-width ratio greater than 60.
For safe-load determination, that is, in computing effective area and section
modulus:
4,020 (9.77)
w
t lim f
where f computed unit stress in psi in the element based upon effective width.
Equation (9.77) is based on a safety factor of about 1.65 against yield stress
at the outer fiber of the section. For any other safety factor m multiply the right-
hand side of Eq. (9.77) by 1.65/m.
For deflection determinations, that is, in computing moment of inertia to be
used in deflection calculations or other calculations involving stiffness:
lim 5,160 (9.78)
w
f
t
For a flat-width ratio w/t greater than 10 but not over 25, the compressive
stress should not exceed:
5 1 w
f c f b 8,640 (f b 12,950) (9.79)
3 15 t
where f basic design stress, not to exceed 30,000 psi
b
w width of element
t thickness
If f 20,000 psi, this formula reduces to
b
w
f c 24,700 470 (9.80)
t
Figure 9.7 shows a number compression elements used in lightweight steel con-
struction.
For w/t greater than 25, not over 60, the compressive stress should not
exceed f as given by Eqs. (9.79) and (9.80). For angle struts,
c
8,090,000
f c (9.81)
(w/t) 2
For other sections,
w
f c 20,000 282 (9.82)
t
Stiffened Light-gage Elements Subject to Local Bucking Compute section
properties based on an effective width of each stiffened compression element
(Fig. 9.8). Determine the effective width b by means of Eqs. (9.83) and
(9.84).