170 ILLINOIS ACADEMY OF SCIENCE 



lapsing will take the form shown in Fig. 2. The columns 

 of this latter frame tend to fall over to the right, the same as 

 the columns of the frame in Fig. 1, but the girders at the tops 

 of the columns hold the top ends in a nearly vertical position 

 and likewise the girders at the bottom hold the bottom ends 

 in a vertical position. That is, a column as a whole is not 

 free to fall over, and the top can only move to the right by 

 moving relative to the bottom when both the top and bottom 

 remain in a nearly vertical position. Under these conditions 

 a column is capable of resisting a horizontal shear. 



VS/&u> 



F/G. 2L 



The columns naturally take the position shown by the dot- 

 ted lines in Fig. 1, that is, they tend to rotate and they are 

 prevented from rotating only because the girders hold the ends 

 in a nearly vertical position. In other words, and this is a 

 fact that should be kept clearly in mind, the columns are the 

 overturning members and the girders are the resisting mem- 

 bers. 



As stated above, the columns represented in Fig. 2 are sub- 

 jected to shear. Since it is considered that no horizontal 

 forces act upon the steel frame between the floors, the shear 

 on a column is uniform for a story height. This shear pro- 

 duces a bending moment in the column, that is, the stress in 

 the column due to the wind is tension on one side and com- 

 pression on the other. From the form which the column 

 takes when strained, it is apparent that for a particular side of 

 any column if the stress at one end is compression that at the 

 other end is tension, or the reverse. That is, the bending 

 moment and also the bending stress changes sign, or passes 

 through zero, some place between the two ends of the column. 



