232 



PRACTICAL STRUCTURAL DESIGN 



end is inclined at an angle of 18 degrees from the vertical away 

 from the wind, this being practically the angle of friction of steel 

 on steel. Trusses are of course designed for the maximum stresses, 

 and with the majority of trusses the maximum stresses occur 



with the wind from 

 the fast side. Ana- 

 lyze the truss with 

 wind from either side 

 and then proportion 

 each member for the 

 greatest force it is 

 THIS end expected to resist, 



free on 



rollers the two sides of the 

 R 2 truss being alike. 



Concentrated Loads 



Sometimes a roof 

 truss must be de- 

 signed to carry a 

 trolley at some joint. 

 The designer does 



not always know in 

 Fig. 154.- Wind Pressure on Roof-Wind on Free advance on which 

 bide. 



panel the trolley will 



be carried, the owner of the building wishing to be free to change 

 such things at pleasure. Instead of a trolley it may be a shaft 

 for machinery, or a heavy pipe. 



The method to pursue in such a case is to design the truss for 

 the dead load, which will include the allowance for snow if any, 

 then design for wind, then make diagrams for the concentrated 

 loading at each joint where it is liable to come. This brings up 

 the question of maximum and minimum stress and reversal of 

 stress. 



Maximum and Minimum and Reversed Stresses 



Specifications usually state the safe allowable unit stress for 

 all materials, but seldom give the stresses to use for members 

 subjected to changing stresses and reversal of stress. It is cus- 

 tomary in such cases to use a "Range Formula." 



