COLUMNS AND STRUCTURES 251 



rately, but usually it can be determined in advance just what 

 maximum size is permissible. In the figure the meaning of the 

 letters used is plain, but attention must be called to 19 in which 

 "6" is the distance back to back of the channels and it must be 

 not less than 63 per cent of the nominal size of the channel. The 

 procedure is to assume the form of section and the extreme 

 dimensions. Then apply the rules given in Fig. 163 and thus 

 get an approximate value for the radius of gyration. Proceed as 

 before and when the allowable fiber stress is found proceed to 

 get the area and then select the plates and shapes to make the 

 selected section. When it has been designed find the exact 

 radius of gyration and test for the fiber stress. 



Wrought iron columns are seldom used, for the material is 

 hard to obtain and steel is stronger pound for pound. Wrought 

 iron and steel columns are usually two or three stories long. 

 Column splices should be so arranged that not more than one- 

 half the total number of columns splice at any one floor level. 

 All connections between columns, girders, and beams should be 

 rivrtted. Theoretically it is best to vary the sizes of columns 

 from story to story, but it is less expensive with steel and wrought 

 iron columns to have them not less than two stories long, of the 

 same size, for the extra amount of material often costs less than 

 the labor required to change sizes at each floor. Cast iron columns 

 and wooden columns are never more than one story in length 

 and the practical impossibility of making rigid connections at 

 floor levels limits the use of cast iron and wood for columns to 

 low buildings, for they offer poor resistance to wind. 



No column is free to turn as though the end were round or 

 as if it bore against a pin. Such conditions do not arise in build- 

 ing construction, although they may be nearly attained in bridges. 

 The columns in massive buildings are sometimes considered as 

 fixed at the ends, but mass implies positive rigidity. In sheds and 

 low mill and shop structures columns are not considered as fixed 

 at the ends unless specially massive foundations are used for 

 the purpose of assuring such a condition, something seldom 

 done. The majority of engineers advocate the assumption 

 of two rounded ends for all cases short of positive fixety, as 

 there are so many secondary stresses, experiments showing 

 that columns tested to destruction fail in detail rather than as 

 a whole. 



