Civil Engineering Reference
In-Depth Information
Then the design shear strength of the member must at least equal the shear, to be
taken as
V
n
V
u
A
v
f
f
y
And the shear friction reinforcement across or perpendicular to the shear crack may
be obtained by
V
u
A
vf
f
y
This reinforcing should be appropriately placed along the shear plane. If there is no
calculated bending moment, the bars will be uniformly spaced. If there is a calculated mo-
ment, it will be necessary to distribute the bars in the flexural tension area of the shear
plane. The bars must be anchored sufficiently on both sides of the crack to develop their
yield strength by means of embedment, hooks, or other methods. Since space is often lim-
ited in these situations, it is often necessary to weld the bars to special devices such as
crossbars or steel angles. The bars should be anchored in confined concrete (that is, col-
umn ties or external concrete or other reinforcing shall be used).
When beams are supported on brackets or corbels, there may be a problem with
shrinkage and expansion of the beams, producing horizontal forces on the bracket or cor-
bel. When such forces are present, the bearing plate under the concentrated load should be
welded down to the tensile steel. Based on various tests, the ACI Code (11.9.3.4) says that
the horizontal force used must be at least equal to 0.2
V
u
unless special provisions are
made to avoid tensile forces.
The presence of direct tension across a cracked surface obviously reduces the shear-
transfer strength. Thus direct compression will increase its strength. As a result, Section
11.7.7 of the Code permits the use of a permanent compressive load to increase the shear
friction clamping force. A typical corbel design and its reinforcing are shown in Figure 8.20.
Figure 8.20
