GIRDERS AND TRUSSES 



113 



plate girder is used, provided a rolled beam cannot be made to 

 serve, by attaching plates to the flanges. Tables of plate girders 

 are given in the steel handbooks, so the architect or builder finds 

 it as easy to select a plate girder for much of his work as it is to 

 select a rolled I-beam or channel for light loads on shorter spans. 

 When the load, or the span, either or both, make a plate girder 

 too heavy, a trussed girder is used. 



Fig. 72 shows a plate girder. The thin vertical plate is known 

 as the web and is made thick enough to carry the shear. It acts 

 also as a long slender column, so must be safe against crippling. 

 When proportioned to carry the shear and the thickness is greater 

 than ff V the depth between the rivets in the upper and lower flanges, 



Fig. 72 Plate Girder with Cover Plates and Stiffenere 



the plate is safe against crippling. When designed to carry the 

 shear and a thickness less than V the depth is obtained, it is neces- 

 sary to use stiffeners spaced regularly at intervals equal to the 

 depth of the girder. Additional stiffeners are placed under con- 

 centrated loads and at the ends. Intermediate stiffeners are some- 

 times crimped over the flange angles, but it is as common to have 

 fillers placed under them, the thickness of the fillers being equal 

 to the thickness of the flange angle, so the stiffeners will be straight 

 and have the ends resting on the outstanding leg of the flange 

 angles. No scientific rules seem to be commonly accepted for 

 designing intermediate stiffeners, the usual empirical method 

 being to have the outstanding leg equal in width to ^ the depth 

 plus 2 in. 



End stiffeners act as columns to carry the end shear, which is 

 delivered to them by the web plate and carried to the bearing 

 plate. Stiffeners under concentrated loads are designed as columns. 

 The ends of all stiffeners designed as columns should be milled 

 or ground to fit perfectly against the bottom flange angles. 



