258 PRACTICAL STRUCTURAL DESIGN 



which must be designed as a frame to carry the wind loads. If, 

 by slightly deepening the girders on all lines of columns the 

 matter can be accomplished, then all the lines of columns may be 

 called into service. Usually, however, the necessity for making 

 openings through interior walls requires that a minimum of depth 

 be used in girders and that the sizes of columns must be a mini- 

 mum. When it is decided to keep the area between columns as 

 free as possible from obstructing beams, ties, and struts, it will 

 be necessary to select a few lines of columns parallel with the 

 wind, which, with their connecting girders, trusses, or ties, will be 

 designed as frames to resist the force of the wind. Wall columns 

 are usually chosen and the spandrel beams are deepened, be- 

 cause, being in the walls, they cannot be in the way of partitions 

 or alterations in the interior of the building. When necessary to 

 strengthen framework across the interior of a building it is usual 

 to do it on the wall lines of light wells. 



For wind alone the columns of a building may be considered 

 to have two fixed ends, except the columns supporting the roof. 

 If the footings are not designed to resist the additional force of 

 the wind the lower columns are not considered fixed. It is hardly 

 probable that the foundations will not be so designed. Assum- 

 ing the columns to be fixed and the trusses or girders connecting 

 the columns to be strongly attached to them, there will be a point 

 of contraflexure in each column and in each girder. For con- 

 venience in designing this point of contraflexure is taken to be 

 in the middle. It is not an accurate assumption, but it is safe 

 and lessens the time required for computation and simplifies the 

 work. There is moment only at the ends, and, so far as the wind 

 force is concerned, the columns and girders can be hinged at the 

 points of contraflexure. This really means that each column 

 consists of two cantilevers extending upward and downward 

 from the floor beam with a length equal to half the story height; 

 and each girder consists of two cantilevers extending to the right 

 and left of the column and with a length equal to one-half the 

 span. 



For each story the total amount of wind on the story is assumed 

 to be concentrated at the middle of the column on. This force 

 is horizontal shear. Assuming the tall building to be a vertical 

 cantliever beam the wind loads are the loads (horizontal shear) 

 in each story. Adding these loads from the top down the total 



