202 



RESISTANCE OF MATERIALS 



snow load will be a similar figure, and the stresses in the two cases will be propor- 

 tional to the corresponding loads. Hence the snow-load stresses may be obtained 

 by multiplying each dead-load stress by a constant factor equal to the ratio of 

 the loads, 



Graphical analysis of French truss 



FIG. 161 



In drawing the diagram start at one abutment, say the left, and take the joints 

 in order, thus determining the stresses in OA, AM, AB, PB, BC, and CM. At the 

 middle of the rafter, where the load PQ is applied, there will be three unknowns, 

 and since these cannot all be determined simultaneously, one of the three must be ob- 

 tained by some other means before the construction can proceed. For this purpose, 



