GIRDERS AND TRUSSES 131 



Some of the trusses shown have the joints lettered with a capital 

 U on the upper chord and a capital L on the lower chord. The 

 subscript figure represents the number of the joint from the left 

 end, the joint, or joints, at the abutments being 0. In the draw- 

 ings a joint is referred to by the U or L and the subscript indi- 

 cating the number of the joint. A member is identified by giving 

 the letter and subscript number of the joint at each end of the mem- 

 ber. This method of identifying joints and members is common. 



Architects and designers of buildings have to deal with the 

 simpler forms of trusses, but when it is desirable to introduce the 

 maximum economy into a design, that truss is most economical in 

 which the stresses in the chords are constant from end to end. This 

 points to a truss having the general outline of a bowstring girder. 

 The top chord should be straight and not curved between joints. 

 To obtain a curved outline for a roof it is easy to use fillers or vary 

 the depths of the purlins resting on the trusses. For an exposed 

 chord where the polygonal form would be unsightly the expedient 

 is sometimes adopted of curving the segments, thereby introducing 

 bent beams with arching action. This should never be done. It 

 is better to use a false curved chord in segments to hide the short 

 straight pieces. 



The stresses in the top and bottom chord of a bowstring truss 

 are found with sufficient accuracy by assuming the truss to be 

 uniformly loaded. The moment divided by the depth gives the 

 maximum stress at the center of the top chord and throughout the 

 lower chord, the formula being 



in which T = total tension, 



C = total compression, 

 I = length in feet, 

 w - uniform load per lineal foot, 

 d - depth in feet at center of span. 



The chord assumes some of the functions of braces as the ends 

 are approached, where the inch* nation of the chord increases, and 

 the compression is nearly uniform throughout the length. The 

 compression at any point distant y feet from the center is given 

 by the following formula, 



c . JK 



