228 PRACTICAL STRUCTURAL DESIGN 



and the maximum angle of slope will be 50 degrees. Professor 

 Ricker a few years ago proposed a similar formula, using 30 Ibs. 

 pressure and 30 degrees maximum slope, to accord with modern 

 practice. 



The horizontal wind pressure is fixed in specifications. It is 

 usually taken as 30 Ibs. per square foot against vertical flat sur- 

 faces. The following modifications are made for surfaces not 

 flat: 



Cylindrical chimney, 67 per cent of horizontal pressure. 



Octagonal chimney, 71 per cent. 



Rectangular building of large size, 80 per cent. 



Concave side of shallow cylinders, channels and cups 115 to 

 130 per cent. For deep cups and concave side of spheres, 

 130 to 170 per cent. (Ketchum.) 



When the wind pressure against a roof is reduced to the normal 

 pressure a stress diagram may be drawn after finding the reac- 

 tions. The pressure normal to the surface acts at each joint the 

 same as any load for which the roof may be designed. The effect 

 the wind force on the roof will have on the walls or columns 

 determines the stresses in the roof members. 



There are three general cases: 



1. The roof may be fastened to the support at both ends. 



2. The roof may be attached to one end support and the other 



end may rest on a plate and be free to move. 



3. The roof may be attached to one end support and the other 



end may rest on rollers. 

 With Case 2 and Case 3 



(a) The wind may come from the attached end. 



(b) The wind may come from the free end. 



With Case 1 the reactions cannot be vertical and the horizontal 

 thrust causes bending in the roof support, whether it be a wall 

 or a column. The reactions are parallel to the resultant wind 

 pressure. 



With Case 2 the reaction at the fast end is parallel to the re- 

 sultant wind pressure and the reaction at the "free " end makes 

 an angle with the vertical equal to the coefficient of friction 

 between steel and steel, about 18 degrees. 



With case 3 the reaction at the free end is vertical. 



The cases selected for illustration are very simple. The matter 

 is simple. To give a number of force diagrams showing the effect 



