When provision against shear is made by using small steel rods placed either vertical or inclined downward toward the center of the beam, as mentioned above, these rods may well be made in the form of inverted U-shaped stirrups, with their ends securely fastened to the reinforcing metal in tension.
In many cases all the provision necessary is given by the use of two longitudinal bars, parallel and close together near the center of the span, but one of them leading to a plane near the top of the beam at the supports. This system is very conveniently applied in concrete slabs supported by i-beams, one bar of the pair being hooked over the upper flanges of the i-beam and sagging toward the center. The Hennebique system (§571) is a combination of the inclined bar and U-shaped stirrups.
608. A modification of the single inclined bar is the Cum-mings system, wherein there are several pairs of bars of varying lengths; these are all horizontal and near the bottom along the center of the beam; a short distance from the center the shortest pair turns up at an angle of about forty-five degrees; a little farther toward the end a second pair of bars is turned up, and so on, leaving a single pair to go through straight to the support.
Another, and more radical modification, is the Kahn system (§573), in which the bar is square with wings of metal on opposite corners which are sheared and bent up at angles of forty-five degrees, so that the outline of the steel work in a beam resembles the tension members of a Pratt truss.