What Are The Types Of Stirrups Used In Construction? . A stirrup is vital in reinforced concrete. It’s a closed loop of rebar that plays a crucial role. Stirrups hold together the main reinforcement, also known as tension (RFT) bars, which lie within a concrete part. And there are two types of stirrups used in reinforcement. They are closed stirrups and open stirrups. They are used to achieve specific design goals. The choice between these two types often hinges on the project’s unique demands. For instance, closed stirrups look like a rectangle or square. They must confine concrete. It is ideal for high-stress uses like skyscrapers or bridges.
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Open stirrups are different. They have a “U” or “C” shape. They offer more flexibility. They are often preferred in situations that need adaptability. This is the case in construction with complex shapes. The design’s requirements determine the choice. The designer may change or switch between these stirrup types to ensure good strength and performance. Discover more about various types of stirrups and their many uses in this in-depth study.
What Is Stirrup?
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).
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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.
Types of Stirrups in Construction
Single Legged Stirrups (Open Stirrups)
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The single-leg stirrups are reinforcing bar holders. They have seen limited use in construction projects due to their restrictions. This is mainly because they are made for binding only two rods. This is a fairly rare need in most buildings.
For instance, many rods must be secured in a typical concrete foundation. This is to ensure stability and strength. In such cases, the single-leg stirrups would not be enough. Other types, like the double-leg or four-leg ones, would be better.
Two-Legged or Double legged Stirrups (Closed)
The most widely used stirrup type is the two-legged stirrup. This construction staple requires at least four rods to ensure stability and proper operation. The two-legged stirrup has been crucial for centuries, dating. back to the early days of bridge construction in the 18th century. This two-legged stirrups are used in lintel construction
Imagine a bridge without these vital stirrups. It would be like a spider’s web without its threads. The two-legged stirrup’s design allows for flexibility and movement. It is perfect for structures that need to resist environmental stressors. These include strong winds or earthquakes.
Four-Legged Stirrups (Closed)
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The four-legged stirrup is a big advance, offering more security and stability than its predecessor. This design comprises two overlapping stirrups. They do not fully enclose the rods but provide a strong support system. Imagine a configuration with eight rods arranged in two rows of four. In this setup, the two stirrups will overlap each other. Each will circle six rods. Both stirrups encircle the four central rods tightly. This setup ensures the best hold. It minimizes the risk of rod slippage or misalignment. This clever design feature improves structural integrity. It also makes installation easier.
Six-Legged Stirrups (Closed)
Six-legged stirrups securely hold a beam reinforced with eight bars. In this closed setup, a single stirrup holds all eight rods. It provides a more secure foundation. Imagine a strong, sturdy bridge, its foundation unshakeable. The stirrups, which have six legs, are like a giant metal spider. They support a beam with eight reinforcement bars. Each bar strategically adds extra strength during placement. The bars are made of high-tensile steel. They are like the bridge’s skeletal system. They provide the internal framework needed to bear heavy traffic and harsh weather.
Circular Stirrups (Open Stirrup)
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When the strengthened column is round, use the circular stirrup. Ancient Greek and Roman architecture frequently used this. This stirrup is designed to hug the curved surface tightly. It even helps to stabilize and sustain the structure.
For example, the famous Colosseum in Rome dates back to the first century AD. It has round columns that are stirrup-reinforced. They have endured over time. Circular stirrups are useful. It guarantees the column can support the weight of the building. It accomplishes this without weakening the column.
Helical Stirrups (Open Stirrup)
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Helical Stirrups get their name from their striking similarity to the DNA double helix. This famous structure is a key part of science. This unique resemblance is not superficial; it holds a significant purpose.
Traditional stirrups wrap around rods in a perfect circle. Helical stirrups coil around them like a spring. They provide better support to primarily circular columns.
In essence, they are a type of open stirrups. They offer a clear advantage in structural design. They duplicate the twisted, double-coiled pattern of DNA. This makes helical stirrups grip the rods better for a more secure and stable connection.
This new design element is very useful. It is helpful in projects with circular columns. These columns need extra reinforcement.
How To Calculate The Length Of a Stirrup?
Measuring the length of a stirrup is a really easy process. Here is how you can:
Consider that you’re measuring the length of a stirrup used in 0.30m x 0.30m beam; now find the perimeter length of the beam, i.e., 0.30m x 4sides = 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)
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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
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Application Of Stirrups
Stirrups are widely used in reinforced concrete construction to reinforce columns, beams, and slabs. They are an essential component of the reinforcement scheme necessary for improving the strength and stability of the concrete structure. The use of stirrups can also help to improve the seismic resistance and crack resistance of the structure.
Using stirrups to reinforce columns: In reinforced concrete columns, stirrups are usually installed around the longitudinal bars to prevent them from buckling because of the compressive forces. In a typical column, the stirrups are arranged in a vertical grid pattern with a spacing of typically 100 mm or more. The number of stirrups and their spacing depends upon the size of the column and the expected loads.
Using stirrups to reinforce beams: Stirrups are also used in reinforced concrete beams to transfer shear forces and provide lateral support for the longitudinal bars. A typical beam consists of two layers of stirrups arranged in a vertical grid pattern with a spacing of typically 100 mm or more. The number of stirrups and their spacing depends upon the size of the beam and the expected loads.
Using stirrups to reinforce slabs: In reinforced concrete slabs, stirrups are usually installed around the longitudinal bars to provide lateral support to the bars and reduce the risk of cracking. The number of stirrups and their spacing depends upon the size of the slab and the expected loads.
In all applications of stirrups, the stirrups must be securely attached to the longitudinal bars to ensure proper functioning. This is usually done by welding the stirrups to the longitudinal bars. It is also important to ensure that the stirrups are spaced evenly throughout the structure and placed at the same elevation as the longitudinal bars. The proper design and installation of stirrups are essential to ensure the strength and stability of the structure.
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Advantages Of Stirrups
Using stirrups in reinforced concrete construction has numerous advantages. The most important of these advantages are increased strength, improved stability, increased seismic resistance, and improved crack resistance.
Increased Strength: Reinforced concrete structures can withstand larger loads than un-reinforced structures. Even when subjected to heavy loads, reinforced concrete structures remain stable and strong due to the presence of stirrups. In addition, stirrups provide additional strength by preventing cracking and fragmentation of the structure.
Improved Stability: Stirrups help to keep the reinforced concrete structure in its intended shape. Stirrups act as a skeleton for the structure, keeping it in its intended shape without drastically changing it. This improved stability makes it possible for the structure to be more resistant to external forces.
Increased Seismic Resistance: Stirrups can also increase the seismic resistance of reinforced concrete structures. By providing additional strength and stability, stirrups can help prevent cracks and structural fragmentation during an earthquake.
Improved Crack Resistance: Reinforced concrete structures that use stirrups are more resistant to cracks than those that do not. This is because the stirrups help to distribute stress equally throughout the structure, thus preventing concentrated areas of stress, which can lead to cracking. Stirrups also help to keep the concrete in its intended shape, which makes it less likely to crack.
Using stirrups in reinforced concrete construction can greatly increase the strength and stability of the structure. This, in turn, can improve the structure’s seismic and crack resistance. Using stirrups makes it possible to create a reinforced concrete structure that is more stable, durable, and resistant to external forces.
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FAQs
How are column ties different from beam stirrups?
Though both appear to be the same but differ in their projected function, ties are used as transverse reinforcement in columns where the prime mode of load transport is compression. Primarily ties are installed to prevent premature buckling of bars and to restrict concrete in the core. While stirrups are transverse reinforcement placed on beams and the mode of load transfer is via bending and shear. The main objective of this reinforcement is to avert shear cracks in the beams which could lead to failure. Stirrups provide additional shear strength to concede and are crafted accordingly. Both steel and concrete act jointly in resisting shear force.
How do stirrups prevent earthquake damage?
In RC structures, vertical, horizontal structures (columns and beams) are integrally attached. The load is transferred from one to another, as the members act as a frame. Beams are installed with two reinforcements; one is long straight bars (longitudinal bars) placed horizontally, other is a closed loop or ring (stirrups) placed vertically at regular intervals. To prevent damage from moderate to severe earthquakes, the installed stirrups must be 6mm, and for beams more than 5 meters, it should be 8mm. Both ends of the vertical stirrups must be inserted into a 135-degree hook and extended adequately to prevent opening up in seismic activity. If the length of the beam is double its respective depth, the placing of stirrups must be more stringent.
Why are stirrups in beams and columns?
Stirrups are placed at stipulated intervals in beans to prevent buckle and shear failure. Steel rebar is tensile than stirrups, often installed with rebar. Rebar acts as bones to the structure. The stirrups reinforce them upholding their strength and straightness. The stirrups inside the concrete columns enhance their durability. Stirrups are installed vertically and diagonally, and it is done to prevent shear failure as cracks in beams mostly occur diagonally. Transverse steel tie bars are used to hold the main bars in position. The stirrups provide protection to the RCC structure during seismic activities. Stirrups hold the longitudinal bars in position and prevent the busting of lateral reinforcement into concrete.
