Page 285 - Civil Engineering Formulas

BUILDING AND STRUCTURES FORMULAS 219

PLATE GIRDERS IN BUILDINGS

For greatest resistance to bending, as much of a plate girder cross section as
practicable should be concentrated in the flanges, at the greatest distance from
the neutral axis. This might require, however, a web so thin that the girder
would fail by web buckling before it reached its bending capacity. To preclude
this, the AISC specification limits h/t.
For an unstiffened web, this ratio should not exceed

h 14,000
(9.44)
t F y (F y 16.5)

where F yield strength of compression flange, ksi (MPa).
y
Larger values of h/t may be used, however, if the web is stiffened at appro-
priate intervals.
For this purpose, vertical angles may be fastened to the web or vertical plates
welded to it. These transverse stiffeners are not required, though, when h/t is less
than the value computed from the preceding equation or Table 9.4.
With transverse stiffeners spaced not more than 1.5 times the girder depth
apart, the web clear-depth/thickness ratio may be as large as

h 2000
(9.45)
t F y

If, however, the web depth/thickness ratio h/t exceeds 760/ F b , where F ,
b
ksi (MPa), is the allowable bending stress in the compression flange that would
ordinarily apply, this stress should be reduced to F b , given by the following
equations:

F b R PG R e F b (9.46)
A w h 760
R PG 1 0.0005 1.0 (9.47)
A f t F b
12 (A w /A f )(3 ) (9.48)
3
R e 12 2(A w /A f ) 1.0

TABLE 9.4 Critical h/t for Plate Girders in Buildings
14,000 2,000
F , ksi (MPa) 2 F y (F y 16.5) 2 F y
y
36 (248) 322 333
50 (345) 243 283