RECTANGULAR BEAMS 85 



8 in. in diameter and 16 in. long, under laboratory condi- 

 tions of manufacture and storage, using the same con- 

 sistency as is used in the field. In considering the factors 

 recommended with relation to this strength, it is to be 

 borne in mind that the strength at 28 days is by no means 

 the ultimate which will be developed at a longer period, and 

 therefore, they do not correspond with the real factors of 

 safety. On concrete in which the material of the aggregate 

 is inferior, all stresses should be proportionally reduced, and 

 similar reductions should be made when leaner mixtures are 

 to be employed. On the other hand, if, with the best 

 quality of aggregates, the richness is increased, an increase 

 may be made in all working stresses proportional to the 

 increase in compressive strength at 28 days, but this increase 

 shall not exceed 25 per cent. 



" Bearing. 1 When compression is applied to a surface of 

 concrete larger than the loaded area, a stress of 32.5 per cent 

 of the compressive strength at 28 days, or 650 Ib. per square 

 inch on the above-described concrete may be allowed. This 

 pressure is probably unnecessarily low when the ratio of 

 the stressed area to the whole area of the concrete is much 

 below unity, but is recommended for general use rather 

 than a variable unit based upon this ratio. 



"Axial Compression. (a) For concentric compression on 

 a plain concrete column or pier, the length of which does not 

 exceed 12 diameters, 22.5 per cent of the compressive 

 strength at 28 days, or 450 Ib. per square inch on 2000 Ib. 

 concrete, may be allowed. 



" (b) Columns with longitudinal reinforcement only, the 

 same unit stress as recommended for (a) . (It is recommended 

 that the ratio of the unsupported length of a column to its 

 least width should be limited to 15 in any type of reinforced 

 concrete column.) 



" (c) Columns with reinforcement of bands or hoops, 2 

 stresses 20 per cent higher than given for (b) , or 540 Ib. per 

 square inch on 2000 Ib. concrete. 



" (d) Columns reinforced with not less than 1 per cent 



1 For beams built into pockets in concrete walls, the lower compressive stress of 450 Ib. 

 per square inch should not be exceeded. 



2 Where bands or hoops are used, the total amount of such reinforcement shall not be 

 less than 1 per cent of the volume of the column enclosed. The clear spacing of such 

 bands or hoops shall not be greater than one- fourth the diameter of the enclosed column. 



