A stirrup is a closed loop of rebars in a reinforced concrete component that keeps the main reinforcement (RFT) bars together. Stirrups may be of various types depending on the design and shape of the structural members.
Stirrups are made of steel rebars wrapped around the top and bottom bars of beams (rectangular shape) or columns (rectangular or circular shape).
Stirrups are usually perpendicular to the longitudinal direction of RFT, although they can also be diagonally positioned.
It is done to avoid shear failure, which occurs in the case of beam cracks and is generally diagonal. The designer should preferably specify the stirrup spacing along the beam.
Why Are Stirrups Used In Construction?
• Stirrups are mainly used to hold the main reinforcement rebars together in RCC Structure.
• Stirrups are required to prevent the columns and beams from buckling and aid in the resistance to lateral stresses (e.g., earthquake loads).
• Stirrups are used to provide stability to the reinforcement of steel bars.
• Stirrups are used to control shear capacity more effectively.
Types Of Stirrups Used In Construction
Different types of stirrups based on different conditions are described below.
1. Types Of Stirrup Based On Shape:
i) Rectangluar Stirrup
ii) Square Stirrup
iii) Circular or Round Stirrup
iv) Triangular Stirrup
iv) Stirrups On a Spiral
2. Types Of Stirrup Based On the Nature Of Construction
i) Single-legged Stirrups (Open Stirrups)
Single-legged stirrups aren’t commonly used, although they can come in handy when just two rods need to be tied. These single-legged stirrups have a flattened U-shaped form rather than a complete loop.
ii) Two-legged or Double-legged Stirrups (Closed Stirrups)
Stirrups with two legs are the most used type of stirrups in construction. This type of stirrup requires at least four rods to be completed.
iii) Four-legged Stirrups (Closed Stirrups)
The four-legged stirrup is the next level higher, and it’s made up of two overlapping stirrups that don’t surround all of the rods. If you have eight rods in two rows of four, the two overlapping stirrups will encircle six rods each, with the four in the middle wrapped by both.
iv) Six-legged Stirrups (Closed Stirrups)
Instead of eight rods, a six-legged stirrup can be used to support the same eight-rod configuration. A single stirrup encircles all eight rods in this arrangement, and extra legs provide additional support. These additional legs are generally positioned between two rods that are nearby.
Purpose Of Stirrups
• It improves the column’s compressive strength.
• It can increase the column’s ductility.
• Stirrups keep a column’s main reinforcement from buckling.
• It inhibits the movement of the primary reinforcement during the concreting process.
• Stirrups are employed in beams for the same purpose as in columns for lump-sum calculations.
• To produce longitudinal bars with lateral confinement.
• Its resistance to shear and torsion is excellent.
• During construction, to keep longitudinal bars in place.
• It keeps the concrete in the core, making it more ductile and strong.
• It prevents individual bars from buckling prematurely.
Some Design Consideration
How To Calculate The Length Of a Stirrup?
Measuring the length of a stirrup is a really easy process. Here is what you can do:
Consider that you’re measuring the length of a stirrup used in a 0.30m x 0.30m beam; now find the perimeter length of the beam, i.e., 0.30m x 4 sides = 1.20m.
Now you have to consider clear cover (which is 1 inch for all sides, both in vertical and horizontal directions) spacing for concrete.
Deduce 8 inches (an inch is 2.54cm), which is approximately 0.20m from the beam’s perimeter length, which gives us 1.00m.
Lastly, you need to add 2 inches to each end of the stirrup rod while bending to a square shape which gives 1.10m (1.00m + .10m)
How Far Apart Do Stirrups Have To Be?
Throughout the lapping length, the vertical stirrup spacing should be 100 mm c/c. Stirrups should have a minimum diameter of 8 mm.
Stirrup Spacing In Beam Formula
• The maximum stirrup spacing, according to IS 456, is 0.75 d.
• Vertical stirrups should be spaced no more than 0.75d or 300mm apart.
IS: 456 specifies the requirements for fixing the vertical stirrup spacing in a structural member, which must not be greater than the least of the following:
- least lateral dimension
- The diameter of the smallest longitudinal bar is multiplied by sixteen.
- 300 millimeters
Conclusion
If you don’t provide proper specifications of stirrups, spacing in the construction building may collapse, so the designer should preferably specify the spacing of the stirrups along with the structure.
So in this article, we learned about different types of stirrups used in construction. Hope you now have enough knowledge about it. If you have any questions, let me know in the comments.