PHYSICS, CHEMISTRY AND ENGINEERING 171 



The point where the moment changes sign is called the point 

 of contra-flexure. The conditions at this point are the same, 

 as far as stresses are concerned, as if the column were cut and 

 the two parts were connected with a frictionless hinge. There 

 is a shear but there is no moment at the point of contra- 

 flexure. 



Since a girder is subjected to forces resulting from the 

 wind load only at points where the girder is connected to the 

 columns, the shear on a girder is uniform between columns. 

 The shear in the girder produces a bending moment and this 

 bending moment changes sign in the girder the same as in a 

 column. 



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F/G. 3. 



III. APPROXIMATE METHODS OF CALCULATING WIND STRESSES 



In Fig. 2 the action of the portion of the bent above the tenth 

 floor, upon the tenth floor is represented as a single force W; 

 and the action of the tenth story upon the story below is like- 

 wise represented as a single force W+w. In reality the hor- 

 izontal shear is transmitted from story to story through the 

 columns. 



If a column is divided at the point of contra-flexure, repre- 

 sented in Fig. 3 by a small circle, the upper portion of the 

 column will exert upon the lower portion a horizontal shear, 

 but no moment. (There is also a vertical force, but it is 

 neglected in this discussion.) The total shear above the tenth 

 floor is represented by W. It is equal to the sum of the 

 shears, P lf P 2 , P 3 and P 4 , in the columns. The shear is con- 



