72 



In order that the compressive stress per linear foot re- 

 sulting from a chosen value for I) shall not exceed the safe 

 compressive strength of the concrete there must be 16 

 sq. in. of concrete above the bars for each ton stress or 

 16 x stress equals 12 x D; from which stressrz%I>. Sub- 

 stituting this value' of stress in the above formula and 

 reducing, we have D=s square root of 

 4XLXS 

 21 



Having obtained D, the total stress in tons % D. The 

 total thickness of the floor must be */ 2 In. greater than D 

 If 14 in. bars are used and % in. greater if l / z in. bars are 

 used. 



Assume a flat floor of 10 ft. span loaded with 100 Ib. 

 per sq. ft. 



It will be necessary to assume the dead weight of the 

 floor guided by experience. We will suppose it to be GO 

 Ib., making the total load 160 Ib. per sq. ft. The total load 

 L on a strip of floor 1 ft. wide would be 



10X160 



~2000~ = ' 8 ton 

 and D would equal the square root of 



4X-8X10X12 =4 



The total stress in the bars would equal % x 4.28 3.21 

 tons, requiring % in. bars 2*4 in. on centers. The total 

 thickness would equal 4% In. 



Ribbed Floors: In calculating the dimensions and ten- 

 sion bar for ribbed floors we use the general formula 



LXS 



In order that the concrete in the top member of the floor 

 may not be strained beyond its safe compressive strength, 

 we make the condition that the upper third of the beam 

 including the flat slab connecting the beams shall contain 

 at least 5 sq. in. of concrete for each ton stress given by 

 the formula. 



Assume a ribbed floor of 20 ft. span loaded with 150 Ib. 

 per sq. ft., to find the dimensions of floor and size of 

 tension bar. 



