This section is from the book "Cement And Concrete", by Louis Carlton Sabin. Also available from Amazon: Cement and Concrete.

The same total stress of 3,600 lbs. must be transferred through the concrete immediately above the bar. If the reinforcement is so distributed that the entire width of the beam has practically the same stress, and we consider, as before, that two-thirds of the length of the end foot is operative, we have mean shear = 3,600/(12x8) = 37.5 lbs. per sq. in. The value of stress in shear should not exceed one-tenth the safe value in compression, and there is a general tendency to use not more than one-twentieth.

If the same form of beam had a span of but five feet with same bending moment, the value of the shearing stress by this method becomes 75 lbs. per sq. in., and it will be necessary to provide against this stress coming upon the concrete.

Another approximate method is the ordinary one for rectangular beams, viz. to consider the shear in horizontal plane just above the steel reinforcement to be 3/2 times the total shear at any section, divided by the area of vertical section of the beam.

606. Provision is sometimes made for relieving the concrete of all shearing stresses. In this case the beam is divided into imaginary panels of length equal, say, to the depth of the beam, and the diagram of maximum shear is drawn. The shear in each imaginary panel is then provided for by a vertical or inclined bar of the proper dimensions. Or, what is usually better, the shear bars are all of one size and the proper number of them are distributed throughout each panel length; the spacing of the shear bars thus becomes wider near the center of the beam.

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