170 REINFORCED CONCRETE CONSTRUCTION 



tion (1). Also the value of 15 for n (see Art. 25) as recommended 

 by the Joint Committee, is found to give very conservative 

 results when employed in the above formulas. In this form of 

 column the concrete fails suddenly and in a manner similar to 

 the diagonal shearing failure of a plain concrete column. 



The density and rigidity of the concrete when steel is employed 

 is apt to be less than in the plain column, so that for small 

 percentages of longitudinal reinforcement the gain in strength is 

 small. On the other hand, it is undesirable to use high per- 

 centages of reinforcement as the strength of such columns is not 

 well determined. If great strength is desired, an increase in the 

 proportion of cement is preferable to a high percentage of steel. 

 The riveted column unit has advantages over the separate rod 

 reinforcement if large amounts of steel are required. 



Where the concrete is depended upon to fireproof the steel in 

 a column, a certain thickness should be deducted in calculating, 

 strength. As already mentioned, the necessary thickness for 

 fireproofing is about 2 in., but if 1 1/2 in. be deducted all around 

 in calculating strength this will amply provide for the weakening 

 effect of fires. A less thickness than this should be sufficient 

 where the contents of a building are not especially inflammable. 



In the construction of columns the reinforcing bars should be 

 straight, and great care should be exercised in keeping them in 

 place during the pouring of the concrete. Assuming a load 

 uniformly distributed over the cross-section of the column, the 

 reinforcing rods should be arranged symmetrically over this 

 area. Ultimate failure is sometimes due to the buckling of the 

 reinforcement causing the outside concrete to scale and, for this 

 reason, at least 1 1/2 in. of concrete should cover the rods on the 

 outside. The rods should also be held securely in place by wiring 

 or banding the rods together at intervals of about a foot, but 

 such banding cannot be considered as hooping in the sense usually 

 employed. As will be seen later, hooping to be effective must 

 be spaced relatively close. Vertical reinforcement held in place 

 by hoops is shown in Fig. 79. 



In splicing columns, large rods (or structural shapes) should 

 have their ends planed true and well spliced. The splicing of 

 the large rods may be effected by placing a pipe-sleeve over the 

 upper end of the lower bar and projecting above it, and then 

 setting the lower end of the upper bar within the pipe and resting 

 upon the lower bar. The rods should either be grouted in place 



