Art. 152. MAXiMt M FLOOR BEAM CONCENTRATION. 



297 



That is, the loads in the two panels must be equal for a max- 

 imum load on the floor beam between them. With concentrated 

 loads this will, in general, require a load at the floor beam so that 

 part of it may be considered in one panel and part in the other. 



The criterion of equation (90) is the same as for maximum 

 moment at B for a span of length Pi+p 2 ? as ma y be seen if (90) 



... 

 is taken by composition. 



^ TT .,, , I <2 



We will have = = 



Pi P* 



, that is, the average load per foot on either panel must 



equal the average load per foot on the two panels. This is the 

 same as the criterion of equation (84) in. which G-^-\-G z corres- 

 ponds to G! and a to p x . 



There may be several different loadings which will satisfy 

 the criterion for maximum floor beam concentration. Having de- 

 termined the positions it is a simple matter to calculate the con- 

 centrations. It is sometimes convenient to calculate the floor 

 beam concentration from the maximum moment at the middle of 

 a span equal to 2p, since the position of the loads is the same in 

 both cases. If the panels are equal, 6r 1 =6r 2 and 



R 2 = 



B =E 1 pG 1 (c-\-xpa 1 ) 



For span=2p R L = -^-(c-\-x a^+a^+x) . Substituting we have 



c+oj. 



4 



It is evident that if the maximum moment at B as given by 

 the last equation is multiplied by - - we get the maximum floor 



beam concentration R^. 1 



153. Stresses in a Pratt Truss from Wheel Loads. 



Fig. 207 shows a single track Pratt truss railroad bridge of seven 

 panels. We will assume the following loads : 



Weight of trusses and bracing = 1650 Ibs. per ft. of bridge. 



Weight of floor system =1000 Ibs. per ft. of bridge. 



Total dead load 



= 2650 Ibs. per ft. of bridge. 



1 A method of calculating bridge stresses from wheel loads, embodying 

 this principle, together with a complete table for Cooper's E50 loading, is 

 given in Eng. News, Vol. 55, page 695. 



