Torsion - Design of Reinforced Concrete - page 478

Figure 15.3 Closed stirrups (these types frequently
impractical).

one-piece stirrups such as these are used, the entire beam cage may have to be prefabri-
cated and placed as a unit (and that may not be feasible if the longitudinal bars have to be
passed between column bars) or the longitudinal bars will have to be threaded one by one
through the closed stirrups and perhaps the column bars. It is easy to see that some other
arrangement is usually desirable.
In the recent past it was rather common to use two overlapping stirrups arranged as
shown in Figure 15.4. Although this arrangement simplifies the placement of longitudinal
bars, it has not proved very satisfactory. As described in the ACI Commentary
(R11.6.4.1), members primarily subjected to torsion lose their concrete side cover by
spalling at high torques. Should this happen, the lapped spliced stirrups of Figure 15.4

will prove ineffective, and premature torsion failure may occur.
A much better type of torsion reinforcement consists of stirrups, each with a prop-
erly anchored top bar, such as the ones shown in Figure 15.5. It has been proved by test-
ing that the use of torsional stirrups with 90 hooks results in spalling of the concrete

outside the hooks. The use of 135 hooks for both the stirrups and the top bars is very

helpful in reducing this spalling.
Should lateral confinement of the stirrups be provided as shown in parts (b) and (c) of
Figure 15.5, the 90 hooks will be satisfactory for the top bars.

Should beams with wide webs (say, 2 ft or more) be used, it is common to use multiple-
leg stirrups. For such situations the outer stirrup legs will be proportioned for both shear
and torsion, while the interior legs will be designed to take only vertical shear.
The strength of closed stirrups cannot be developed unless additional longitudinal re-
inforcing is supplied. Longitudinal bars should be spaced uniformly around the insides of
the stirrups, not more than 12 in. apart. There must be at least one bar in each corner of
the stirrups to provide anchorage for the stirrup legs (Code 11.6.6.2); otherwise, if the
concrete inside the corners were to crush, the stirrups would slip and the result would be

Figure 15.4 Overlaping stirrups used as torsion
reinforcing but not desirable .