PHYSICS, CHEMISTRY AND ENGINEERING 



169 



of the story, in the opposite direction, to prevent it from mov- 

 ing horizontally in response to the forces above. That is, 

 there is a shear in the story, and this shear is equal to the sum 

 of the horizontal forces acting upon the bent above the story 

 in question, including the force at the top of the story. 



Fig. 1 represents the tenth story of a three-span bent. The 

 force W applied at the eleventh floor is equal to the sum of 

 all the horizontal forces above, including the one at the elev- 

 enth floor. W tends to push the top of the tenth story to the 

 right. The force w is the wind load at the tenth floor. The 



H^O 



(JJ 



I W+UJ 



force W+w is the resistence which the ninth story offers to 

 prevent the bottom of the tenth story from moving to the 

 right. If the columns and girders were hinged at the corners 

 the frame would collapse as indicated by the dotted lines. 

 The bracing added to prevent the frame from collapsing is 

 called the wind bracing. 



The easiest way to prevent the frame from collapsing would 

 be to put in a diagonal tension member from A to B. This, 

 however, it is not practicable to do inasmuch as diagonal brac- 

 ing in the exterior walls would interfere with the windows, 

 and all interior walls must be made so that they can be re- 

 moved to meet the changing needs of the tenants. It is there- 

 fore necessary to make the frame capable of resisting shear 

 without interfering with the clear rectangular space between 

 the columns and girders. 



If the connections between the columns and girders are 

 made rigid, that is, if the columns can not turn relative to the 

 girders, the frame, when subjected to a shear, instead of col- 



