CONCRETE DESIGN 291 



Cinder concrete is not suitable for reinforced -concrete 

 structures. 



For reinforced -concrete work, a mixture based on the pro- 

 portion of 1-6 is generally used; that is, 1 part of cement 

 to a total of 6 parts of fine and coarse aggregates, measured 

 separately. The fine and coarse aggregates are often in the 

 proportion of 1-2, which makes a concrete that is commonly 

 called a 1-2-4 mixture. For columns, richer mixtures are often 

 required, and leaner mixtures can often be used in mass work. 



Moving live loads and suddenly applied loads require 

 special consideration. These loads can often be taken care 

 of by increasing the amount of live load used in the cal- 

 culation. The weight of the beam or the floor slab itself 

 should always be considered when estimating the dead load. 



The span length of beams should be taken as the length 

 from center to center of supports aad not of the clear span, 

 but it need not be considered longer than the clear span 

 plus the depth of the beam. Brackets are not considered 

 as reducing the clear span. 



In the formulas about to be given, rather than employ 

 ultimate stresses and divide the result by the factor of safety, 

 working, or safe, stresses should be used. 



Rectangular Beams. The following notation will be used 

 in the design of beams and columns: 



F s tensile stress in steel, in pounds per square inch; 



.F f = compressive stress in concrete, in pounds per 

 square inch; 



E s = modulus of elasticity of steel; 

 * E c = modulus of elasticity of concrete; 

 n-E f +E e ; 



M = moment of resistance of beam, in inch-pounds; 



A = area of steel, in square inches; 

 6 = breadth of beam, in inches; 

 d = depth of beam, in inches, from top to center of steel 



reinforcement ; 

 k = coefficient; 

 j coefficient; 



p = ratio of area of steel to bd = . 

 bd 



