14 >TES ON PLATE-GIRDER DESIGN 



ART. 4. LENGTHS OF COVER PLATES 



The plate girder, Ix-ing a composite structure, may easily 

 be constructed so that i section may vary approxi- 



|y :i> the moments and shears require. The full flange 

 section Ix'in^ required only where the moment is a maxi- 

 mum, a method of determining when- the parts may be 

 omitted when no longer required is necessary to economi- 

 cally design the girder. In general the cover plates are 

 the only parts of the flange which do not extend tin full 

 length. Two methods will be developed for finding where 

 cover plates (or any other part of the flange) may be 

 omitted. 



The first: For girders which carry a uniform load, 

 or a load which may be closely represented by a uniform 

 load. Deck plate girders for railway bridges may be 

 included in this classification. 



In Art. 2 the approximate moment of the internal 



stresses at any section =S/ix( A +- { ^J in which [A+-jrJ 



is the net flange area, designating this area by a for 

 simplicity, the moment of the internal stresses becomes 



9*0. 



l.'-t >r the uniform load per foot of girder, or for 



locomotive- loading the uniform load which would pro- 



the same end shear, then the bending moment at 



any point distant x from the end=-2~ "~o~ a d this 

 must equal the moment of the internal stresses. 



