200 



PRACTICAL STRUCTURAL DESIGN 



ROOF 



-? ^ 3 k e ^ u in 



plat each moment on opposite sides of the members and draw a 

 straight line connecting the end lines. The shear is constant, as 

 it is a concentrated load. 



The author has here presented the method he uses in designing 



frames of buildings to 

 resist wind. There are 

 several other methods 

 use, and not all 

 engineers will agree 

 with the method here 

 given. It is, however, 

 simple and agrees well 

 with such meager 

 knowledge as we now 

 possess of the actual 

 force of the wind on 

 tall buildings. It is 

 probably in more 

 common use than any 



Moments in Girders. 



Shear on Girders. 

 Fig. 168 Graphic Representation of Moments 



and Shears in Frame of a Building 



other method. Another method is to assume the building frame 

 as a vertical cantilever beam loaded at the mid-point of each 

 story with the wind as a concentrated load. The total moment 

 is found for the beam at each floor level 

 and this is divided among the lines of 

 columns proportionately to their dis- 

 tance from the neutral axis, which is 

 assumed to be midway between the ex- 

 terior columns. The bending moments 

 in columns and girders are equal at the 

 respective floor levels. 



In Fig. 169 is shown the method of 

 bracing frames by means of diagonal 

 ties, known as sway bracing. It is as- 

 sumed that the bracing is provided only 

 between the exterior columns and the 

 first line of interior columns. The wind 

 load is applied at each floor level. The 



Fig. 169 Diagonal Wind 

 Bracing in Tall Buildings 



two lines of columns become, respectively, the upper and lower 

 chord of a cantilever truss, the floor beams or girders are the 

 verticals, and the diagonal ties carry shear. Such a method 



