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:

(9.77)

wherefcomputed 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 mmultiply the right-
hand side of Eq. (9.77) by
For deflection determinations, that is, in computing moment of inertia to be
used in deflection calculations or other calculations involving stiffness:

(9.78)

For a flat-width ratio w/tgreater than 10 but not over 25, the compressive
stress should not exceed:

(9.79)

wherefbbasic design stress, not to exceed 30,000 psi
wwidth of element
tthickness

Iffb20,000 psi, this formula reduces to

(9.80)

Figure 9.7 shows a number compression elements used in lightweight steel con-
struction.
Forw/tgreater than 25, not over 60, the compressive stress should not
exceedfcas given by Eqs. (9.79) and (9.80). For angle struts,

(9.81)

For other sections,

(9.82)

Stiffened Light-gage Elements Subject to Local Bucking Compute section
properties based on an effective widthof each stiffened compression element
(Fig. 9.8). Determine the effective width bby means of Eqs. (9.83) and
(9.84).

fc20,000 282

w

t

fc

8,090,000

(w/t)^2

fc24,700 470

w

t

fc

5

3

fb8,640

1

15

(fb12,950)

w

t



w

tlim



5,160

f

1.65/m.



w

tlim



4,020

f