Civil Engineering Reference
In-Depth Information
The detailed provisions of the Code relating to shear design for deep beams, together
with the applicable ACI Section numbers, are as follows.
1.
Deep beams are to be designed using the procedure described in Appendix A of the
Code (Appendix C in this textbook) or by using a nonlinear analysis. (
AC
I 11.8.2)
2.
The nominal shear strength
V
n
for deep beams may not exceed
c
b
w
d
10
f
. (ACI
11.8.3)
3.
The area of shear reinforcing parallel to the span must not be less than 0.0015
b
w
s
2
and
s
2
may not be greater than
d
/5 or 12 in. (ACI 11.8.5)
s
2
is the spacing of
shear reinforcing measured in a direction perpendicular to the beams longitudinal
reinforcement.
4.
The area of shear reinforcing
A
v
perpendicular to the span must at least equal
0.0025
b
w
s
and
s
may not be greater than
d
/5 or 12 in. (ACI 11.8.4)
s
is the spac-
ing of the shear or torsion reinforcing measured in a direction parallel to the logi-
tudinal reinforcing.
You will note that more vertical than horizontal shear reinforcing is required because
vertical reinforcing has been shown to be more effective than horizontal reinforcing. The
subject of deep beams is continued in Appendix C of this textbook.
8.15
INTRODUCTORY COMMENTS ON TORSION
Until recent years, the safety factors required by design codes for proportioning members
for bending and shear were very large, and the resulting large members could almost al-
ways be depended upon to resist all but the very largest torsional moments. Today, how-
ever, with the smaller members selected using the strength design procedure, this is no
longer true and torsion needs to be considered much more frequently.
Torsion may be very significant for curved beams, spiral staircases, beams that have
large loads applied laterally off center, and even in spandrel beams running between exte-
rior building columns. These latter beams support the edges of floor slabs, floor beams,
curtain walls, and façades, and they are loaded laterally on one side. Several situations
where torsion can be a problem are shown in Figure 8.23.
When plain concrete members are subjected to pure torsion, they will crack along 45
spiral lines when the resulting diagonal tension exceeds the design strength of the con-
crete. Although these diagonal tension stresses produced by twisting are very similar to
those caused by shear, they will occur on all faces of a member. As a result, they add to
the stresses caused by shear on one side of the beam and subtract from them on the other.
Reinforced concrete members subjected to large torsional forces may fail quite sud-
denly if they are not provided with torsional reinforcing. The addition of torsional reinforc-
ing does not change the magnitude of the torsion that will cause diagonal cracks, but it does
prevent the members from tearing apart. As a result, they will be able to resist substantial
torsional moments without failure. Tests have shown that both longitudinal bars and closed
stirrups or spirals are necessary to intercept the numerous diagonal tension cracks that
occur on all surfaces of beams subject to appreciable torsional forces. There must be a lon-
gitudinal bar in each corner of the stirrups to resist the horizontal components of the diago-
nal tension caused by torsion. Chapter 15 of this text is completely devoted to torsion.
