COLUMNS AND STRUCTURES 257 



Stress in AB - 1 

 Stress in CD - 1 

 Stress in diagonals = =*= H ( + 7 ) 

 Stress in columns = J 



A/6 on columns = H x T- 



Diagonal bracing is the cheapest in tall buildings, but it is not 

 possible always to use it on account of window or other openings. 

 It is, therefore, necessary in most 

 cases to use girders at floor levels, " >N yv ^V /% %-a: 



xxxx 



A 



as illustrated in Fig. 166, or ^ S\/\/\/\ 9 

 trusses, as illustrated in Fig. 167. V x c / Dl~T 



The figures show but one frame y' 



and apply to single story build- . ^ ? 



ings. It is easy to say that a 



high building may be considered 



to ho a series of such frames 



superimposed and side by side. 



Difficulties arise when it is neces- 2 z 



sary to apportion the wind load Fi - 167 Case II of Portal 



on each story and on each column. 



It is assumed that at each floor level the stiff floors will distribute 



the load to the columns and it is only in the vertical frames that 



attention must be paid to proportioning of members to resist wind. 



Every building having a height twice the width must be pro- 

 portioned to resist wind and the pressure per square foot of the 

 wind is fixed by the ordinance followed by the designer. If, 

 there is no building ordinance to be followed then the usual require- 

 ment is 30 Ibs. per sq. ft. of exposed surface. 



The designer has a choice of lines through which the wind 

 force may be carried to the foundations. Look up the specifica- 

 tions and see just how the stresses are fixed when wind is included. 

 Then test each line of columns to see if they can carry wind as 

 well as the gravity loads. If they can do this the building is 

 stable against wind. If they will not do it the designer must fix 

 on some lines of columns, with their connecting floor girders, 



