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

(9.44)

whereFyyield strength of compression flange, ksi (MPa).
Larger values of h/tmay 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 stiffenersare not required, though, when h/tis 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

(9.45)

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

(9.46)

(9.47)

Re (9.48)



12 (Aw/Af)(3^3 )

12 2(Aw/Af) 

1.0

RPG 1 0.0005

Aw

Af 

h

t



760

Fb

1.0

FbRPGReFb

Fb

760 / Fb

h

t



2000

Fy

h

t



14,000

Fy(Fy16.5)

TABLE 9.4 Criticalh/tfor Plate Girders in Buildings

Fy, ksi (MPa)

36 (248) 322 333

50 (345) 243 283

2,000

2 Fy

14,000

2 Fy(Fy16.5)