168 REINFORCED CONCRETE CONSTRUCTION 



strength of cylinders to crushing strength of cubes, see Art. 10). 

 Strength of plain concrete columns of an average 1:2:4 mixture 

 at 60 days may be taken at about 1800 Ib. per square inch al- 

 though tests made at the University of Wisconsin in 1908 

 indicate that, with careful workmanship and testing, an average 

 value of about 2000 Ib. per square inch can be obtained. Plain 

 concrete is entirely suitable for short columns up to lengths of 

 6 to 10 diameters, but for more slender proportions hooped or 

 banded columns are much to be preferred. The Joint Committee 

 recommends that the length of a plain concrete column be limited 

 to 12 diameters. 



The compressive strength of concrete is approximately pro- 

 portional to the amount of cement which it contains, so that 

 increasing the richness of the concrete in 

 either a plain or reinforced column is an effec- 

 tive means of strengthening the column to 

 permit smaller section. Tests show that for 

 a 1:2:4 plain concrete column there is no 

 excessive variation in individual tests. For 

 a weaker mixture than this, however, the 

 individual tests are much more at variance, 

 indicating greater unreliability. A rich mor- 

 tar for the above reasons may often prove to 

 be the more economical in column construc- 

 tion. 



Bending stresses in columns due to ec- 

 centric loads must be provided for by increas- 

 ing the section until the maximum stress does 

 not exceed the allowable. A formula for 

 homogeneous columns follows, and formulas 

 applicable to reinforced concrete will be given 

 under " Bending and Direct Stress." 

 The ordinary formula for the compressive fiber stress due to 

 eccentric loading upon solid rectangular columns of homogeneous 

 materials (Fig. 78), is as follows: 

 R = total load. 

 A = area of column. 

 x eccentricity. 



t = breadth of column. 



f c = total unit pressure on outer fiber nearest to line of vertical 

 pressure. 



..t 

 < - 



FIG. 78. 



