130 PRACTICAL STRUCTURAL DESIGN 



with the parallelogram of forces completed by drawing horizontal 

 lines to intersect the triangles. Then a = stress on U\ Uz - stress 

 on LiZ/2. It is twice 6 which represents the stress on UQ U\ and 

 L Li. The thrust of the brace C/iL = the pull of the tie U Li. 

 It is resolved at the point of support on the abutment into a hori- 

 zontal component along the chord, and a downward vertical com- 

 ponent, which latter is resisted by the upward reaction of the 

 abutment. 



A usual ratio of depth to span in trusses is one- tenth, but 

 circumstances may alter this. It may be used in the absence of 

 computations to ascertain the economic depth and economic 



Fig. 84. Multiple System Warren Truss. (Lattice Truss.) 



ratio of depth to span. For Howe trusses the best angle for the 

 diagonals is 45 degrees. When any different angle which indicates 

 a panel length greater or less than the depth is adopted, the 

 Pratt truss is better. For trusses of the Warren type the angle 

 should be 60 degrees. 



Deflection is usually taken care of by making the horizontal 

 panel length at the upper end inch longer than the horizontal 

 panel length at the lower end, in every ten feet of span. This 

 does not alter the lengths of the verticals but does alter the lengths 

 of diagonals and when the truss is in place the bottom chord will 

 be cambered upward. Were it perfectly straight it would appear to 

 the eye to sag. The amount of camber in inches is found as follows: 



d = depth of truss in inches. 



s = span of chord in inches. 



c = camber in inches. 



Sd 

 c = 



s 



In some of the figures of trusses the spaces are lettered. This 

 is the system introduced by Mr. Bow for the graphical analysis 

 of frames. The member is indicated by the letters between which 

 it lies. In addition to this system of lettering the spaces the 

 joints are sometimes numbered. The spaces are lettered to identify 

 the member in the graphical analysis and the joints are numbered 

 only when the detail drawing of the joint is to be referred to. 



