COLUMNS 181 



tageous to use in combination with a rich concrete more than 

 2 or 3 per cent of longitudinal steel. 



"4. The amount of data presented on tests of columns sub- 

 jected to repeated or time loadings is far too small to warrant 

 drawing definite conclusions as to the limiting stress for repeated 

 loadings which will hold true for all kinds of columns and for an 

 infinite number of repetitions, or for a prolonged loading. 

 However, it does appear from the results presented that there is 

 practically no increase in set or deformation after a few repetitions 

 of loads equal to 40 to 50 per cent of the yield points of the 

 columns tested. The results of the repeated load tests also 

 plainly indicate that there is considerable additional strength 

 and toughness afforded by the spiral after the yield point of the 

 longitudinal steej has been passed. That all of this additional 

 strength may not be permanent is suggested by the slopes of the 

 deformation-time curves for two of the columns. 



11 5. The close agreement between theoretical values and values 

 derived from test data shows that the formula commonly used in 

 designing short homogeneous columns eccentrically loaded, may 

 be applied to reinforced concrete columns, provided suitable 

 allowance be made for the steel. 



"6. The strength of a column resting upon a footing will be 

 about as great as when bedded on a metal plate, provided con- 

 siderable lengths of the longitudinal reinforcing bars are bent 

 outward into the slab or footing. The results given show that 

 the use of metal base plates and longitudinal steel milled to their 

 required length leads to greater uniformity and strength. 



"7. Although only a few test pieces were reinforced with corru- 

 gated bars of high carbon steel, the results were so uniform and 

 the strength so high that attention should be called to this type 

 of reinforcement. It is quite evident that, with certain ratios of 

 unit prices, the use of deformed bars with high elastic limits will 

 be more economical than the use of plain round bars of mild steel. 



"8. Briefly summarizing the foregoing, it seems economical to 

 use for reinforced concrete columns a very rich mixture, and 

 advantageous to employ about 1 per cent of closely spaced high 

 carbon steel lateral reinforcement combined with two or three per 

 cent of longitudinal reinforcement. From the test data pre- 

 sented it seems apparent that such columns, centrally loaded, 

 may be subjected to a static working stress equal to one-third of 

 the stress at yield point." 



