Shear and Diagonal Tension - Design of Reinforced Concrete - page 223

Secondary crack Flexure-shear crack

Initiating or
flexural cracks

Figure 8.1 Flexure-shear crack.

tinuous beams or near simple supports. At such locations small moments and high
shear often occur. These types of cracks will form near the mid-depth of sections and
will move on a diagonal path to the tension surface. Web-shear cracks are illustrated in
Figure 8.2.
As a crack moves up to the neutral axis, the result will be a reduced amount of concrete
left to resist shear—meaning that shear stresses will increase on the concrete above the crack.
It will be remembered that at the neutral axis the bending stresses are zero and the
shear stresses are at their maximum values. The shear stresses will therefore determine
what happens to the crack there.
After a crack has developed, the member will fail unless the cracked concrete sec-
tion can resist the applied forces. If web reinforcing is not present, the items that are
available to transfer the shear are as follows: (1) the shear resistance of the uncracked
section above the crack (estimated to be 20% to 40% of the total resistance), (2) the
aggregate interlock, that is, the friction developed due to the interlocking of the aggre-
gate on the concrete surfaces on opposite sides of the crack (estimated to be 33% to
50% of the total), (3) the resistance of the longitudinal reinforcing to a frictional force,
often called dowel action (estimated to be 15% to 25%), and (4) a tied-arch type of be-
havior that exists in rather deep beams produced by the longitudinal bars acting as the
tie and by the uncracked concrete above and to the sides of the crack acting as the arch
above. 2

web-shear cracks

Figure 8.2 Web-shear cracks.

2 Taylor, H. P. J., 1974, “The Fundamental Behavior of Reinforced Concrete Beams in Bending and Shear,”
Shear in Reinforced Concrete , Vol. 1, SP-42 (Detroit: American Concrete Institute), pp. 43-47.