Art. 59. STRESSES IN A TRUSS WITH PARALLEL CHORDS. 79 



In a truss of this kind, the calculation of the lever arms is 

 tedious; they may be taken by scale if a proper scale drawing of 

 the truss is made, but the stresses are usually gotten graphically 

 from a stress diagram (56). The above method is quite con- 

 venient for some members NM, for example. 



This method of finding the stress in the lower chord member 

 5-12 of Fig. 53 is sometimes used in connection with the graphic 

 solution, the computed value being scaled off on the stress 

 diagram, thus locating the point R directly without the auxiliary 

 diagram (42). 



59. Stresses in a Truss With Parallel Chords. Fig. 

 58 shows a truss of five equal panels carrying a load at each 

 upper joint. The diagonal length is Vl8 2 +20 2 =26.9 ft. Sec = 



26.9 m a 20 



. Tan 6=. 



18 18 



5 panels at ZOff- * /OO ft 



Stresses and loads /n thoi/sand-pound units 



/? = 37.5 



Fig. 58. 



Section pq. S vert. comps. = or shear in panel 7-8 = vert, 

 comp. ^ = ^^^=^==30000 Ibs. 



Di = 30000 sec 6 = 30000 -^ = 44800 Ibs. (-) 

 M 6 = or (Ri$P) 20 = 18 Ui = 30000x20 

 TT 30000X20 Qoonn iVia ( _i} 



(J\ = OOOVJU 1 Do. I -TT I 



18 



M 7 = or 18L = 0. That L is zero is also appar- 

 ent at joint 6 from 2 horiz. comp. =0. 



rs. S vert. cpmps.=0 or R l -$P = V 2 =30000 Ibs. 



tu. Shear in panel 8-9 = vert. comp. D 2 =Ri~ 1J P. 



D 2 = 15000 sec ^ = 15000-- = 22400 Ibs. (-) 

 18 



Section 

 Section 



