COLUMNS 177 



was obscured. The results of tests of these two columns, 

 together with those of columns B and B 4} indicate that the 

 protective shell does not carry its share of the load in this type 

 of column, especially when there is relatively little direct bond 



between it and the concrete core A comparison of 



the stresses sustained by the column before and after the shell 

 was removed shows that the shell sustained only about 40 per 

 cent of the average unit stress carried by the entire cross-section. 

 However tests of these two columns revealed that the protective 

 shell remained intact until the yield point of the steel was passed. 

 The point on the stress-deformation curve at which this outside 

 coating began to crack is indicated by a cross in Fig. 83. These 

 tests demonstrate that this type of column possesses considerable 

 toughness and a high load-carrying capacity; that the steel and 

 concrete act together in carrying the load and in resisting 

 deformation; and that the protective coating, although it remains 

 intact until the yield point of the steel is passed, should not 

 be counted upon to resist deformations or stresses. 



" Columns E l to E 3 exhibited considerable stiffness when 

 tested. No cracks of any consequence appeared until the 

 deformations indicated that the yield point of the steel had been 

 reached. At this point, however, owing to insufficient lateral 

 support offered by the widely separated 1/4-in. ties, they failed 

 very suddenly. The increase in strength for each per cent of 

 longitudinal reinforcement for these columns, over the strength of 

 the plain columns A l to A 4 , is approximately 156 Ib. per square 

 inch. These columns were lacking in toughness and did not 

 have a high ultimate strength. 



"The preliminary tests of the spiral columns C 1 to D 4 demon- 

 strated that this type of column has great toughness and a high 

 ultimate strength, which is accompanied by large deformations 

 and deflections. By comparing the test records and the stress- 

 deformation curves for columns D to D 4 , which had vertical 

 reinforcement, it was observed that the outside shell of concrete 

 cracked at about the same time that the stress in the steel 

 reached the yield point. As may be seen in Fig. 83, the stress- 

 deformation curves for columns C 1 to C 4 are practically the same 

 up to a stress of 1600 Ib. per square inch. As the shell concrete in 

 columns C lf C 2 , and (7 4 cracked at about this stress, it seems 

 evident that the shell and core act in unison up to this point 

 in this type of column. However as the shell on a column in 



