Art. 127. 



STRESSES IN SIMPLE TRUSSES. 



235 



loads are generally taken in strict accordance with some stand- 

 ard specification. 



Uniform loads are taken of indefinite length, and as ad- 

 vancing from either end of the bridge until the whole bridge 

 is covered. As in the case of the dead load, the construction 

 converts the uniform load into panel loads concentrated at the 

 panel points of the loaded chord. 



127. General Considerations Regarding Stresses in 

 Simple Trusses. The methods of Chapters IV and V are 

 adequate for the determination of stresses in any statically 

 determinate truss (42), but it will be advisable to consider the 

 common forms of trusses more in detail, and particularly with 

 regard to live load and maximum stresses. No new methods 

 are introduced, although the simplicity of some of the trusses 

 may make old methods seem new. 



The object is to find the maximum stress in each member and 

 if a reversal of stress is possible, both the maximum tensile and 

 maximum compressive stresses are to be found. 1 (28). 



It is as simple a matter to find live load stresses as to find 

 dead load stresses, but first the proper position of the live load 

 must be known, which will produce the maximum stress in each 

 member, and for those members which are subject to a reversal 

 of stress, two positions of the live load must be determined. 



With uniform loads, the maximum panel loads will be equal 

 for a symmetrical truss having equal panel lengths, and the 

 maximum stress in any member will be equal to that in the 

 member which is symmetrically located with it, with regard to 

 the center line of the truss ; in other words, the maximum stresses 

 will be the same on both sides of the center line. It follows that 

 only half the stresses need be calculated. It is customary and 

 convenient to assume that the live load advances from right to 

 left and to find (in those members subject to a reversal of stress) 

 the maximum stress of one kind to the left of the center line of 

 the truss, and the maximum stresses of the opposite kind (counter 

 stresses) to the right of the center. This procedure avoids turn- 

 ing the live load around so as to advance from left to right. 



*A few specifications also require minimum stresses, and, in this case, 

 it must be remembered that the dead load stress is not always the minimum 

 stress; the stress in a main tie, for example, may be zero. 



