248 STRESSES IN PRATT TRUSS BY COEFFICIENTS. Art. 130. 



L. L. P. sec 0=14,400 ^1=19000 Ibs. 

 D. L. P. tan 6= 5,400 ^-=4630 Ibs. 



L. L. P. tan 0=14,400 7^12340 Ibs. 



Fig. 182 shows the left-hand reactions for the various cases 

 which it is necessary to consider; having these the shears in the 

 different panels are easily gotten. The coefficients on the web 

 members are these shears for unit panel loads. In Fig. 182 (/) 

 R l =3, and the shear in panel 4-5 is 3 2 = 1, which, by the 

 method of sections, determines D 3 . At joint 7, L t must be 

 equal to the horizontal component of D v hence its coefficient 

 is 3. At joint 6, L 2 =L 1 , hence they have the same coefficient. 

 At joint 5, the horizontal component of D 2 is added to L 2 to 

 make L 3 , hence the coefficient of L 3 =3+2=5. At joint 8, 

 the horizontal components of D 1 and D 2 both act toward the 

 right and are resisted by U 2J hence the coefficient of U 2 is 3 +2 =5. 

 At joint 9, the horizontal component of D 3 is added, making 

 the coefficient of U 3 , 5 + 1 =6. 



It will be noted that all the coefficients for interior web 

 members are smaller for full load than those for partial load. 

 Likewise would the coefficients for the chord members be smaller 

 for any partial load than for full load. Since the dead load 

 is a full load, the coefficients of Fig. 182 (/) may be used for 

 dead load coefficients, with the exception of the vertical posts 

 as noted later. 



The diagonal members (except the end posts), the suspend- 

 ers, and the lower chord are always in tension; the posts and 

 top chord are always in compression. The dead load shear in 

 the middle panel is zero and therefore there can be only live 

 load stress in Z> 4 . The dead load shear in panel 4-5 is +1 X5400 

 = +5400 Ibs.; the maximum live load negative shear is, from 

 Fig. 182 (6), -fX 14,400 = -6170 Ibs.; the resultant shear 

 is 6170-1-5400= -770 Ibs., and therefore a counter-tie will 

 be required in this panel. The dead load shear in panel 5-6 

 is +2X5400= +10,800 Ibs.; the maximum negative live load 

 shear is, from Fig. 182 (a), -|X 14,400 = -2060 Ibs.; the 

 resultant is positive and therefore no counter-tie will be required 

 in this panel. 



