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| Types of Stirrups: Quick Overview After reading our article, you will be able to: i. Understand what stirrups are and why they are required in RCC structures. ii. Identify different types of stirrups with their uses. iii. Get knowledge about stirrup spacing, diameter, and cutting length. iv. Calculate the basic cutting length of stirrups and understand their objective in reinforcement. |
1. What are stirrups?
Stirrups are closed-loop bars tied at regular intervals in beam reinforcement to hold the bars in position.1
The types of stirrups depend upon various factors such as the shape of the beam, reinforcement arrangement, and structural requirements, which we will discuss below.
Generally, we use Lateral ties to control the position of the reinforcement in a column without bothering the concrete space, i.e., preventing it from buckling.
If the reinforcement is set in a column, it is known as lateral ties, and if reinforcement is set in a beam, it is known as stirrups. The operations of both vary in beams and columns.
Mainly, the stirrups are developed in a rectangular or square shape, but in a few circumstances, they may be equipped in a circular or diagonal shape.
2. Uses of Stirrups
The uses of Stirrups are as follows:
a. Stirrups hold the main reinforcement bars together in an RCC Structure.
b. Stirrups are needed to hold the columns and beams safely from buckling and help in the resistance to lateral stresses (e.g., earthquake loads).
c. Stirrups help to maintain the proper position of steel reinforcement bars.
d. Stirrups help to resist shear and torsion where required by structural design.
e. Stirrups help to increase the shear capacity of the beam.
f. Properly detailed lateral ties help to improve the column’s compressive strength.
g. Properly detailed lateral ties help to improve the column’s ductility.
h. Lateral ties help prevent the main reinforcement from buckling.
i. Stirrups help prevent the movement of the primary reinforcement during the concreting process.
j. Lateral ties provide lateral support to longitudinal bars in columns.
k. During construction, stirrups help keep the longitudinal bars in place.
l. Properly detailed lateral ties keep the concrete in the core, making it more ductile and strong under compression.
m. Lateral ties help prevent individual bars from buckling prematurely.2
3. Types of stirrups
i. According to the Shape of Stirrups.
a. Square stirrups
Square stirrups are the square-shaped steel rebars generally used in RCC beams and columns to withstand shear forces and to keep the main bars in position.
b. Rectangular Stirrups
Rectangular stirrups are the rectangular-shaped steel rebars generally used in RCC beams and columns to withstand shear forces and to keep the main bars properly arranged.
c. Triangular Stirrups
Triangular stirrups are the triangle-shaped steel bars used in a special rebar arrangement to hold the main bars and provide lateral support.
d. Circular stirrups
Circular stirrups are the circular-shaped steel bars used in an RCC column to hold the main bars and provide lateral support.
e. Helical stirrups
Helical stirrups are the continuous spiral-shaped reinforcement that is used in columns and piles to confine concrete, improve ductility, and prevent the buckling of longitudinal bars.
f. Diamond stirrups
Diamond-shaped steel links used in a special reinforcement arrangement to hold the main bar in position and provide lateral support.
ii. According to the Nature of the Stirrups
a. Single-legged stirrups
The stirrups that exist on a single leg are known as single-legged stirrups.
b. Two-legged stirrups (Closed Stirrups)
Stirrups with two legs are commonly employed in the field of construction. This type of stirrup needs a minimum of four rods to be met.
c. Three-legged stirrups
The stirrups exist on three legs known as three-legged stirrups.
d. Four-legged stirrups
The four-legged stirrup is made up of two overlapping stirrups that don’t wrap all of the rods.
If your reinforcement has eight rods in two rows of four, the two overlapping stirrups will surround six rods each, with the four in the centre surrounded by both.
e. Five-legged stirrups
The stirrups exist on five legs, known as five-legged stirrups.
f. Six-legged stirrups (Closed Stirrups)
Instead of eight rods, a six-legged stirrup can be used to support the same eight-rod configuration.
In this arrangement, a single stirrup encircles all eight rods, and extra legs give extra support. These extra legs are normally placed between two rods that are close.
g. Seven-legged stirrups
The stirrups exist on seven legs and are known as seven-legged stirrups.
h. Eight-legged stirrups
The stirrups standing on eight legs are known as eight-legged stirrups.
4. Some Design Considerations
i. How To Calculate The Length Of a Stirrup?
The length of a stirrup depends upon the beam dimension, concrete cover, bar diameter, bends and hooks.
For example, assume that you’re calculating the length of a stirrup employed in a 0.40 m x 0.40 m beam. First, calculate the perimeter of the beam.
First, the perimeter of the beam:
Perimeter = 0.40 m x 4 sides = 1.60 m
Now, you have to take clear cover (which is 1 inch for all sides, both in vertical and horizontal directions) gapping for concrete.
Minus 8 inches (an inch is 2.54cm), which is precisely 0.20m from the beam’s perimeter length.
Length inside the clear cover = 1.60 – 0.20 = 1.40 m.
Lastly, you need to provide 2 inches at each end of the stirrup rod while bending to a square shape.
Final perimeter of the stirrup = 1.40 m + 0.10 m = 1.50 m.
ii. How Far Apart Do Stirrups Have To Be?
Throughout the lapping length, the vertical stirrup spacing should be 100 mm c/c. The minimum diameter of stirrups must be selected according to the applicable design, code and structural requirements.
iii. Stirrup Spacing in Beam Formula
a. According to IS 456, the maximum spacing of the vertical stirrups must be kept no more than 0.75d or 300mm, whichever is less, where ‘d’ is the effective depth of the beam.3
IS 456 specifies the requirements for spacing reinforcement in structural members.
For column lateral ties, IS 456:2000 specifies that the spacing must not exceed the least of the following:
a. The least lateral dimension of the member.
b. Sixteen times the diameter of the smallest longitudinal bar.
c. 300 mm.
Note: Beam stirrups and column lateral ties have different spacing requirements under IS 456:2000.
5. Objective of Stirrups
The objective of stirrups is as follows:
a. It maintains the rebars in place and control deformation.
b. It places the rods in the exact alignment.
c. To prevent the beam and column from buckling and shear failure.
d. The stirrups withstand the rods under compression or tensile force.
e. The spacing of stirrups varies based on structural design requirements.
f. The lowest spacing is 100 mm to 250 mm, depending upon the diameter and number of bars used.
6. Difference between Stirrups and Ties
The role of both stirrups and ties in the RCC structure is identical and serves the same aim, but the difference is that stirrups are employed in horizontal member-like columns and ties are employed in vertical member-like columns.
7. Suitable Diameter of Stirrups
The diameter of the stirrups to be employed in the RCC structure relies upon the shear stress involved in the structure. There is no specified diameter for the stirrups; it purely depends upon construction.
However, for residential buildings, the right size of stirrups ranges from 6 mm to 10 mm.4
Commercial and vital buildings like hospitals and shopping malls should employ stirrups of more than 8 mm. The size of such stirrups may range from 9 mm to 10 mm.
Frequently Asked Questions (FAQs)
1. What is the purpose of stirrups in a concrete beam?
Ans: Stirrups hold the main reinforcement bars together in an RCC Structure, and help to resist shear and torsion where required by structural design.
2. What size stirrups are used in concrete beams?
Ans: The size of stirrups typically ranges from 6 mm to 12 mm, but the suitable stirrup diameter depends upon the structural design, shear requirement, and applicable building code.
3. What is the standard spacing of stirrups in beams?
Ans: The standard spacing of stirrups in beams is generally 100 mm to 300 mm, but the stirrup spacing depends upon the beam design, shear forces, and applicable building codes.
4. What is the difference between stirrups and lateral ties?
Ans: Stirrups are used in beams to resist shear forces, whereas lateral ties are used in columns to prevent buckling.
5. Why are stirrups placed closer near beam supports?
Ans: Due to high shear forces in these regions, stirrups are placed closer near beam supports.
6. How do stirrups prevent shear failure in a beam?
Ans: Stirrups prevent shear failure in a beam by holding the concrete together across diagonal cracks and preventing sudden shear failure.
7. How do you calculate the cutting length of a stirrup?
Ans: The stirrup cutting length is calculated by using the centerline dimension, bar diameter, hook lengths, and applicable bend deduction or allowance.
8. What happens if stirrup spacing is too wide?
Ans: If the stirrup spacing is too wide, it can reduce the shear resistance and increase the risk of diagonal cracking or shear failure.
9. What is the difference between two-legged and four-legged stirrups?
Ans: Two-legged stirrups have two vertical legs; four-legged stirrups have four vertical legs that provdes the addition shear resistance and support to the longitudinal reinforcement across wider beam sections.
10. Are stirrups required in every concrete beam?
Ans: No, stirrups are not required in every concrete beam, because the need for the stirrups depends upon the structural design and applicable building code, which determines the required shear reinforcement.
| Written By | Madhu Krishna Poudel |
| Role | Construction Site Supervisor | Founder, DreamCivil |
| Experience | 8+ years of practical construction experience |
| Technically reviewed by | Er. Bikash Kafle |
References:
ACI 318-14: Building Code Requirements for Structural Concrete & Commentary
https://studylib.net/doc/27705619/aci318-14-english [↩]
Ductile Design and Detailing of Reinforced Concrete Structures Subjected to Seismic Forces
https://archive.org/details/gov.in.is.13920.2016/IS13920%3A2016/ [↩]
Plain and Reinforced Concrete – Code of Practice
https://law.resource.org/pub/in/bis/S03/is.456.2000.pdf [↩]
Recommendations for detailing of reinforcement in reinforced concrete works
https://law.resource.org/pub/in/bis/S03/is.5525.1969.pdf#:~:text=6.12%20Edge%20Beams%20%2D%20Where,the%20size%20of%20this%20bar [↩]
