Plate Girders

48.10 Plate Girders

Plate girders are built-up beams. They are used as flexural members to carry extremely large lateral loads.

A flexural member is considered as a plate girder if the width–thickness ratio of the web, h c /t w , exceeds 760/ ÷F b (F b is the allowable flexural stress) according to ASD or l r (see Table 48.8 ) according to LRFD. Because of the large web slenderness, plate girders are often designed with transverse stiffeners to reinforce the web and to allow for postbuckling (shear) strength (i.e., tension field action) to develop. Table 48.9

TABLE 48.9 Web Stiffeners Requirements Range of Web Slenderness

Stiffener Requirements h £ 260

Plate girder can be designed without web stiffeners t w

260 < h £ . 0 48 E Plate girder must be designed with web stiffeners; the spacing of stiffeners, a, t w

F yf ( F yf + . 16 5 )

can exceed 1.5h; the actual spacing is determined by the shear criterion . 0 48 E < h £ . 11 7 E Plate girder must be designed with web stiffeners; the spacing of stiffeners, a,

F yf ( F yf + . 16 5 t ) w

F yf

cannot exceed 1.5h

Note: a = clear distance between stiffeners, h = clear distance between flanges when welds are used or the distance between adjacent lines of fasteners when bolts are used, t w = web thickness, F yf = compression flange yield stress (ksi).

summarizes the requirements for transverse stiffeners for plate girders based on the web slenderness ratio h/t w . Two types of transverse stiffeners are used for plate girders: bearing stiffeners and intermediate stiffeners. Bearing stiffeners are used at unframed girder ends and at concentrated load points where the web yielding or web crippling criterion is violated. Bearing stiffeners extend the full depth of the web from the bottom of the top flange to the top of the bottom flange. Intermediate stiffeners are used when the width–thickness ratio of the web, h/t w , exceeds 260, when the shear criterion is violated, or when tension field action is considered in the design. Intermediate stiffeners need not extend the full depth of the web, but they must be in contact with the compression flange of the girder.

Normally, the depths of plate girder sections are so large that the simple beam theory, postulating that plane sections before bending remain plane after bending, does not apply. As a result, a different set of design formulas for plate girders is required.

Plate Girder Design Allowable Stress Design Allowable Bending Stress

The maximum bending stress in the compression flange of the girder computed using the flexure formula

shall not exceed the allowable value, F ¢ b , given by

(48.87) where

F b ¢= b PG R e F R

F b = the applicable allowable bending stress, as discussed in Section 48.5 (ksi)

R PG = the plate girder stress reduction factor, 1 – 0.0005(A w /A f )(h/t w – 760/ ÷F b ) £ 1.0 R e = the hybrid girder factor, [12 + (A w /A f )(3 a – a 3 )]/[12 + 2(A w /A f )] £ 1.0 (R e = 1 for nonhybrid girders)

A w = the area of web

A f = the area of compression flange

a = 0.60F yw /F b £ 1.0

F yw = the yield stress of web