CIVIL ENGINEERING FORMULAS

210 CHAPTER NINE

sections. Fyis the minimum specified yield strengthof the steel, ksi (MPa).
Table 9.2 lists values of Fbfor two grades of steel.
The allowable extreme-fiber stress of 0.60Fyapplies to laterally supported,
unsymmetrical members, except channels, and to noncompact box sections.
Compression on outersurfaces of channels bent about their major axis should
not exceed 0.60Fyor the value given by Eq. (9.12).
The allowable stress of 0.66Fyfor compact members should be reduced to
0.60Fywhen the compression flange is unsupported for a length, in (mm),
exceeding the smaller of

(9.8)

(9.9)

wherebfwidth of compression flange, in (mm)
dbeam depth, in (mm)
Afarea of compression flange, in^2 (mm^2 )

The allowable stress should be reduced even more when l/rTexceeds certain
limits, where lis the unbraced length, in (mm), of the compression flange, and
rTis the radius of gyration, in (mm), of a portion of the beam consisting of the
compression flange and one-third of the part of the web in compression.

For , use

(9.10)

For , use

(9.11)

whereCbmodifier for moment gradient (Eq. 9.13).

Fb

170,000Cb

(l/rT)^2

l/rT 510,000Cb/Fy

Fb

2

3



Fy(l/rT)^2

1,530,000Cb

Fy

102,000Cb/Fy l/rT510,00Cb/Fy

lmax

20,000

Fyd/Af

lmax

76.0bf

Fy

TABLE 9.2 Allowable Bending Stresses in Braced

Beams for Buildings

Yield strength, Compact, Noncompact,

ksi (MPa) 0.66Fy(MPa) 0.60Fy(MPa)

36 (248.2) 24 (165.5) 22 (151.7)

50 (344.7) 33 (227.5) 30 (206.8)