152 PRACTICAL STRUCTURAL DESIGN 



10 ft. = 8 ins., so this width should be maintained. The depth 

 should not be less than this amount, which fixes the size of the 

 top chord at 8 ins. X 8 ins. and as the piece will carry nothing but 

 its own weight the deflection will not create anxiety, for by thus 



increasing the size of the piece the fiber stress becomes -^ ^-=834 



0x8 



Ibs. per square inch. 



In the truss being designed the loads are small and the members 

 are small, so there should be no difficulty in getting pieces of the 

 dimensions here given. There will be then no additional areas 

 to subtract for bolt holes. In trusses, however, which carry heavy 

 loads in which several pieces must be used to form the members 

 allowance must always be made for bolt holes. In this truss it is 

 assumed that all the loads are concentrated at the panel points. 

 If rafters rest on the top or bottom chords they must act as beams 

 to carry such loads and must be designed for the stress thus caused 

 in addition to the direct stress caused by tension in the lower 

 chord, if the rafters rest on it, or to the stress caused by com- 

 pression in the upper chord if the rafters rest on it. 



For a piece acting as a combined tie and beam or acting as a 

 combined strut and a beam use the following formula to obtain 

 the breadth when the depth is assumed. 



1 /6M 



in which 6 = the breadth of the piece, 



/ = the maximum fiber stress (compression for the upper 



chord, tension for the lower chord), 

 h = the depth of the piece, 

 M = the bending moment in inch pounds, 

 D = the total direct load (compression or tension). 



In practical work, in calculating a rectangular piece, the depth 

 may be assumed and the breadth computed to take care of M. 



Add enough breadth to carry the direct load. Or, assume a 

 breadth and design for a depth sufficient to take care of M , and 

 add enough breadth to take care of the direct load. 



Fig. 91 shows a bolted fish-plate splice used in connecting sec- 

 tions of a solid piece in tension. Formerly this was done on the 

 assumption that the bolts bent and they were designed to resist 

 the bending moment. The moment arm was equal in length 



