Art. 156. 



EQUIVALENT UNIFORM LOADS. 



311 



from the moment table by means of the following equa- 

 tion. 1 



(93) 



in which P n = concentration at the nth panel point. 

 p = panel length. 



of the moments about the nth panel point of 

 all the loads to the left of this point. 



f the moments about the (n + l)th panel 

 point of all the loads to the left of this point. 

 M n _ 1 =sum of the moments about the (n l)th panel 

 point of all the loads to the left of this point. 



156. Equivalent Uniform Loads. For stringers and 

 deck plate girders, the determination of an " equivalent uni- 

 form load " practically amounts to determining by the preceding 

 methods the maximum moment and shear for each span and 

 then calculating the uniform loads per foot which will produce 

 equal maximum moments and shears. 



The equivalent uniform load for trusses is usually based 

 upon the maximum moment at the center of the span, or upon 

 the maximum shear in the end panel of the truss. The former 

 gives chord stresses that are a few per cent too large for trusses 

 with an odd number of panels, and in all trusses gives shears 

 which are a little too small. The end shear loading gives chord 

 stresses which are too large, and in the case of long spans the 

 error may be considerable. 



For multiple intersection trusses, draw spans, arches and 

 cantilever trusses, the criteria for position of wheel loads to 

 produce maximum stresses are complicated. Methods for ob- 

 taining equivalent uniform loads for these are usually more or 

 less approximate. 



Panel lengths for multiple intersection trusses should be so 

 chosen that there will be a reasonable distribution of the loads 

 to the several systems of web bracing. For example, if there 

 are two systems of webbing, and the distance center to center 

 of driving wheel bases is 56 ft., the panel length should not be 

 28 ft. but preferably about 19 ft. 



1 For demonstration see Johnson's 

 Art. 101, page 87. (5th Edition.) 



Modern Framed Structures," 



