234 



PRACTICAL STRUCTURAL DESIGN 



roofs, plus the recommended snow load; for a high wind may 

 succeed a heavy sleet. Not all engineers use the snow load in 

 addition to the wind load, arguing that a high wind will blow 

 away the snow. The possibility, however, of a high wind follow- 

 ing sleet, which cannot be blown away, must be considered. A 

 minimum of 10 Ibs. per sq. ft. should be used except in localities 

 mentioned on Fig. 155. A sleet storm may follow a heavy snow- 

 storm, and, in its turn, be followed by a heavy wind. 



Wind on a Curved Roof 



In Fig. 156 is shown the graphical method to follow in obtain- 

 ing the reactions for a roof having curved chords. First find 



the inclination of each panel 

 of the top chord in degrees 

 and find the normal compo- 

 nent of the wind on each 

 slope. Multiply the area of 

 each panel by the normal 

 force of the wind on the panel 

 and set this off at each end of 

 the panel (i.e., at each joint) 

 and complete each parallelo- 

 gram of forces. Draw the dot- 

 ted lines, representing the re- 

 sultant for each parallelogram. 

 At (6) draw the polar diagram. The line ed is parallel with the 

 resultant at the joint DEP. The line dc is parallel with the re- 

 sultant at the joint CDR, etc. The length of each line is equal 

 to the amount of the resultant wind force at the joint through 

 which the resultant passes. Connect the points ae and the direc- 

 tion of the resultant wind force on one side of the roof is found, 

 and its amount. 



At (c) draw the equilibrium polygon. The line o2 is parallel 

 with the line ob of the polar diagram. Similarly the line 23 is 

 parallel with the line oc and the line 3u is parallel with the line 

 od, of the polar diagram. The line ou is transferred to the polar 

 diagram as the resultant closing the equilibrium polygon. The 

 line oe on the polar diagram is transferred to the equilibrium 

 polygon as the line u4 and the line oa is transferred as the line 



Fig. 156 Reactions for Wind on 

 Curved Roof 



