43$ PROBLEMS 



PROBLEM 13. WIND LOAD STRESSES IN A FINK TRUSS ROLLERS ON 

 THE WINDWARD SIDE BY GRAPHIC RESOLUTION. 



(a) Problem. Given the same truss and wind load as in Prob- 

 lem 12, and with rollers under the windward side. Calculate the reac- 

 tions by means of force and equilibrium polygons. Calculate the 

 stresses by graphic resolution. Scale of truss, i" = lo'-o". Scale of 

 loads, i" = 3ooo Ibs. 



(b) Methods. Start stress diagram at (4.6", 6.7"). Reaction 

 R! will be vertical, while the direction of R 2 will be unknown, the only 

 known point in its line of action being at the right end of the truss. 

 Use the method for finding the reactions described on page 52 and in 

 Fig. 34. In this solution the equilibrium polygon is started at the 

 right reaction, the only known point in R 2 , and the polygon is drawn. 

 The intersection of ^ in the force polygon, and a line through 0, 

 parallel to the closing line is at Y, and R 2 is then determined in magni- 

 tude and direction. The stress diagram is carried through and checked 

 at R 2 . 



(c) Results. The load P must be considered as it produces 

 stresses in the truss. If R 2 coincides with the top chord there will 

 be no stresses in the other members of the truss on the leeward side; 

 if line of action of R 2 passes outside and above the truss the lower 

 chord will be in tension; while .if the line of action is below the upper 

 chord the lower chord will be in compression. These statements may 

 be checked by taking moments about the upper peak of the truss. It 

 will be seen in Problems n, 12, and 13 that there will be no reversal 

 of stress when the dead load and wind load stresses are combined. 

 This is commonly true for simple Fink trusses resting on walls; but 

 is not true for Fink trusses supported on columns, nor is it always 

 true for Fink trusses simply supported. 



PROBLEM i3a. WIND LOAD STRESSES IN A FINK TRUSS ROLLERS ON 



THE WINDWARD SIDE BY GRAPHIC RESOLUTION. 

 (a) Problem. Given the same truss and wind load as in Prob- 

 lem I2a, and with rollers under the windward side. Calculate the reac- 

 tions by means of force and equilibrium polygons. Calculate the 

 stresses by graphic resolution. Scale of truss, i" = lo'-o". Scale of 

 loads, i" = 3ooo Ibs. 



