Page 276 - Civil Engineering Formulas

210 CHAPTER NINE

TABLE 9.2 Allowable Bending Stresses in Braced
Beams for Buildings
Yield strength, Compact, Noncompact,
ksi (MPa) 0.66F (MPa) 0.60F (MPa)
y
y
36 (248.2) 24 (165.5) 22 (151.7)
50 (344.7) 33 (227.5) 30 (206.8)

sections. F is the minimum specified yield strength of the steel, ksi (MPa).
y
Table 9.2 lists values of F for two grades of steel.
b
The allowable extreme-fiber stress of 0.60F applies to laterally supported,
y
unsymmetrical members, except channels, and to noncompact box sections.
Compression on outer surfaces of channels bent about their major axis should
not exceed 0.60F or the value given by Eq. (9.12).
y
The allowable stress of 0.66F for compact members should be reduced to
y
0.60F when the compression flange is unsupported for a length, in (mm),
y
exceeding the smaller of
76.0b f
l max (9.8)
F y
20,000
l max (9.9)
F y d/A f
where b width of compression flange, in (mm)
f
d beam depth, in (mm)
2
2
A area of compression flange, in (mm )
f
The allowable stress should be reduced even more when l/r exceeds certain
T
limits, where l is the unbraced length, in (mm), of the compression flange, and
r is the radius of gyration, in (mm), of a portion of the beam consisting of the
T
compression flange and one-third of the part of the web in compression.
, use
For 102,000C b /F y l/r T 510,00C b /F y
2 F y (l/r T ) 2
F b 3 1,530,000C b F y (9.10)
, use
For l/r T 510,000C b /F y
170,000C b
F b (9.11)
(l/r T ) 2
where C modifier for moment gradient (Eq. 9.13).
b