180 REINFORCED CONCRETE CONSTRUCTION 



Prof. Withey draws the following general conclusions from the 

 results of these tests: 



"1. If materials can be obtained at average unit prices, rich 

 mixtures are more economical than lean ones. Considering 

 materials similar to those employed herein, the more economical 

 mixtures will be produced if the proportion of cement to aggre- 

 gate, by weight lies between 0.2 and 0.7. 



"2. Although the yield point of a reinforced concrete column 

 is practically independent of the percentage of spiral reinforce- 

 ment, the ultimate strength and the toughness are directly 

 affected by it. On account of the excessive deformations accom- 

 panying loads beyond the yield point, on account of the prob- 

 ability that both yield point and ultimate strength are less in 

 repeated or long time load tests than in the progressive load 

 tests ordinarily made in the testing machine, and on account of 

 the uncertainties which always surround the hypothesis adopted 

 in designing, good practice demands that only a portion of the 

 stress producing disintegration of the outside shell be used as a 

 working stress. Consequently, only enough lateral reinforce- 

 ment is needed to prevent the longitudinal rods from bulging 

 outward, and to provide an additional factor of safety against 

 an overload by increasing the toughness and raising the ultimate 

 strength somewhat above the yield point. From these tests 

 1 per cent of a closely spaced spiral of high carbon steel seems 

 to be sufficient for this purpose. 



"3. By the addition of longitudinal steel the yield point, 

 ultimate strength, and stiffness of a spirally reinforced column 

 can be considerably increased. If maximum economy in floor 

 space is desired, if a column is so long or is so eccentrically loaded 

 that tension exists on a portion of the cross-section, or if a large 

 dead load must be sustained by the column while the concrete is 

 green, a high percentage of longitudinal reinforcement may often 

 be advantageously employed. Such reinforcement is also a 

 valuable safeguard against failure due to flaws in the concrete. 

 If the cost of cement is extremely high, it may be economical to 

 use a leaner mixture than suggested in (1) and considerable 

 longitudinal steel to increase the stiffness and strength; columns 

 like those of Series 1 may profitably be used. In general, how- 

 ever, cement is a more economical reinforcement than steel. 

 Therefore, for ordinary constructions it does not seem advan- 



