Use or Purpose of Shear reinforcement
Reinforced Concrete(RC) beams have two sets of steel reinforcement, namely: Long straight bars ( Longitudinal Bars) placed along its length & Closed loop of small dia bars( Stirrups) at regular intervals along its length.
The two types of failures occur mainly in a beam are:
1. Flexural(Bending ) failure
2. Shear failure
The beam sags under increase loading which results in flexural failure. Therefore, Longitudinal bars are provided to resist tension due to flexure.
A Shear crack may develop which is inclined at 45° to the horizontal and it develops at mid-depth near the support grows towards the top and bottom. Shear failure is brittle and, therefore must be avoided.
So Stirrups are provided to resist such shearing action and also prevent the longitudinal bars from bending outward.
These are the main purpose or use of shear reinforcement.
More about shear reinforcement
The term stirrups are usually applied to lateral reinforcement in flexural members and the term ties to lateral reinforcement in vertical compression members.
- In uniaxial compression test of concrete, upon reaching the ultimate load failure of concrete occurs where major cracks line up in the vertical direction and the concrete cube would be split up. The development of vertical cracks involves the expansion of concrete in lateral directions. In case the concrete is confined in lateral directions, it was observed that the formation of vertical cracks would be hindered as indicated in past experiments. As a result, the concrete strength is increased with also a rise in failure strain.
- The above theory is often used in the design of bridge columns. Steel stirrups are installed at around the vertical main reinforcement. Other than the function of shear reinforcement, it helps to avoid the lateral deformation of the interior concrete core so that the strength of a concrete column is increased.
- Also, Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.
- Concrete beams vary in depth. The deeper the beam, the more shear capacity. When the depth is not adequate, steel stirrups must be added to increase the shear capacity of the beam. These stirrups are usually one piece of steel that is bent into a rectangular shape. Often small diameter steel is used, such as #3 and #4 rebar. The stirrup typically wraps around the bottom and top bars of the beams.
- Too often the stirrup is not prefabricated and the installer tries to make the stirrup in the field after the horizontal bars are already in place. This is usually obvious because the stirrup is constructed from two pieces with inadequate lap splice. It is much easier and efficient to install a stirrup at the same time the horizontal reinforcement is being installed.