What are Stirrups?
A Stirrup is a loop which is embedded in any concrete structure to hold the main rebars together. The stirrups prevent the columns and beams from collapsing under severe forces like one experienced under earthquakes.
Basic Types of Stirrups
These are the steel bars vertically placed around the tensile reinforcement at suitable spacing along the length of the beam. Their diameter varies from 6 mm to 16 mm. The free ends of the stirrups are anchored in the compression zone of the beam to the anchor bars (hanger bar) or the compressive reinforcement. Depending upon the magnitude of the shear force to be resisted the vertical stirrups may be one-legged, two-legged, four-legged and so on as shown in the below figure. It is desirable to use closely spaced stirrups for better prevention of the diagonal cracks. The spacing of stirrups near the supports is less as compared to spacing near the mid-span because the shear force is maximum at the supports.
What is the main purpose of using stirrups during construction?
1- The stirrups used are usually made out of a rectangular steel piece which is wrapped around top and bottom bars of the beams. Stirrups help to hold in place the primary reinforcement bars. The use of stirrups is needed to prevent the columns and beams from buckling.
2- Stirrups are sometimes placed diagonally and often vertically as well. This is done to prevent shear failure which is usually diagonal in case of cracks in beams. The primary reason for the diagonal shear is due to compression and tension caused by transverse and vertical tension. Eventually, diagonal tension occurs since concrete is stronger in compression as compared to tension. This tension is bound by steel stirrup which holds the cracked surface together.
3- The spacing of the stirrup along the beam is important and should ideally be specified by the designer. This will help the stirrups to be manufactured accordingly. They are usually placed in places where there are high chances of shearing such as beneath large load and bearing points.
4- Concrete is made strong by running bars of steel through them. However, stirrups are used to keep everything in a straight line. Stirrups help keep order and also add strength to the structure at critical points of probable vulnerability from use over time.
5- Steel rebars are stronger than stirrups. However, often it is seen that stirrups are used along with rebar. While rebar acts as the bones of the concrete, the stirrups aid the rebar to remain straight and provide enhanced backing to the column of concrete inside which it is placed.
6- Stirrups help secure much-needed resistance. When the pressure from above comes down on the column, the stirrups act like tendons. They help the rebar and concrete provide sustainable support for the extreme amount of load.