296 



MAXIMUM SHEAR IN A GIRDER. 



Art. 151. 



creased by P x . Now as the loads move to the left the shear is 

 again increased (because 72 is increased and the shear==K 1 P x ) 

 until P 2 reaches 6', when it is again a maximum. This may be a 

 larger or smaller maximum than the first, depending upon the 

 relation between P : and the other loads, and upon c^. When all 



the loads move a distance a , R x increases SP-^ 1 . If this increase 



is greater than P t , P., at C gives a greater maximum shear at C 

 than Pj at the same point. This is the criterion for finding which 

 of two consecutive wheels at a point will give the greater shear. 

 It will be found that the first or second wheel of the first engine 

 at C will usually give the maximum shear at C. It is sometimes 

 a very simple matter to calculate the shear for several possible 

 positions without testing by the criterion. It should be remem- 

 bered that for maximum shear at C, we should have as much 

 weight near C and to the right of it as possible and no load to the 

 Jeft of it, unless such load will decrease the shear less than the 

 movement to this position will increase R . If when P 2 is at C, 

 P! has passed off the span, there has been a sudden increase of 

 shear at C, which may have been still further increased by loads 

 coming on at the right. 



The maximum shear in the beam occurs of course at the end. 



152. Maximum Floor Beam Concentration and Stress in the 

 H i p Vertical. 

 Let Fig. 206, re- 

 present loads in , 

 two panels. It 

 is required to /P, 

 find the criter- 

 ion for maxi- 

 mum load on the Fig. 206. 

 floor beam at #, or maximum R 2 . 



ABC is not a continuous beam, but two consecutive panels 

 of stringers. 



for maximum R 2 , - - 



when the panel lengths are equal G l =G. 



Pi 



= *_! or ft_,_ 



lh />i P\ Vi 



(91) 



