Four-Legged Stirrups in Construction – Advantages & Tips

A layman would wonder if they got to know the fact that steel bars wrapped around the columns and beams can reinforce frameworks. This may sound simple. However, this simplistic of responsibilities holds an important role in the construction phase. The reinforcing component named stirrups prevents the concrete from cracking under pressure. They also offer strength and stability that contradicts their appearance. People will have this inherent question about stirrups when we mention them as components that reinforce the framework of the structure. What can these stirrups, a small component, do for a colossal structure? Many know these stirrups give the structure better lateral support, improving longevity and structural strength. Thus, this article provides a detailed account of stirrups and their types. Following this, the article boils down to the importance of four-legged stirrups in construction and their advantages and disadvantages.

What are Four-Legged Stirrups?

Before we understand what a four-legged stirrup is, we need to have a basic idea of what a stirrup is. As mentioned in the introduction, the stirrups are structural elements that reinforce the strength and stability of construction structures. These stirrups are made of steel which resists the tensile forces occurring in the structure. In contrast, four-legged stirrups are designed to comprise two overlapping stirrups. Unlike other stirrups, four-legged stirrups do not enclose the rod.

Types of Stirrups Used in Construction

Single-Legged Stirrups

The light nature of single-legged stirrups is preferred in low-intensity construction works. The design comprises a bar bent in a U-shape that is tied to the primary reinforcement bar. This type of stirrup could be found in buildings with less shear stress. However, their minimal strength and limited use made them obsolete.

Double-Legged Stirrups

This type of stirrup is the most commonly used in construction. They have a bar with two bends providing better strength and stability to the structures. Excessive shear strength in column construction and beam construction makes this the highly preferable among constructors.

Four-Legged Stirrups

As the name suggests, this type of stirrup consists of four bends. The four bends provide additional strength and resistance to shear forces. High-rise buildings and bridges exert more loads, so four-legged stirrups are used to resist this factor.

Circular Stirrups

These types are commonly used in foundations, bridges, and cylindrical structures. As the name suggests, these stirrups are designed for cylindrical or spiral columns. The circular cross-section distributes the load uniformly, thereby providing resistance to both axial and shear forces.

Helical Stirrups

These stirrups are similar to circular stirrups but do not take the same form. They are also called spiral stirrups. Unlike circular stirrups, helical stirrups are in continuous form. The ultimate strength is added and allows better confinement of the concrete is achieved by the stirrups encircling the TMT bars. This feature provides flexibility and load-carrying capacity to the columns.

Torsion Stirrups

The main purpose of torsion stirrups is to resist the torsional forces in the structures. Beams and slabs are the structures that make use of torsion stirrups as they hold heavy torsional loads.

Combined Stirrups

These are hybrid versions used in structures that have multiple forces like shear, torsion and other axial forces occurring simultaneously. They exert maximum resistance and reinforcement concerning respective design needs.

Applications of Four-Legged Stirrups

Beams and Columns: In tall structures and industrial buildings, beams and columns are subjected to substantial axial loads and bending moments. The use of four-legged stirrups offers exceptional shear resistance and structural integrity.

Foundations: Mat foundations and raft foundations in construction use four-legged stirrups to confine the concrete and prevent the emergence of shear failure in the foundation.

Retaining walls: These walls rely on their weight to resist lateral earth pressure and in some cases are subjected to bending moments and shear forces. The use of four-legged stirrups negates all these factors, providing the structure with better stability.

Advantages of Four-Legged Stirrups in Construction

• Use of four-legged stirrups in beams can bear the shear forces exerted near supports and can distribute these forces preventing shear failure.
• The four legs of the stirrup can distribute the load evenly across the beam. This reduces the risk of localised failure.
• The high ductility of four-legged stirrups can provide us with a warning before the structure completely collapses. Before falling, the ductility causes deformation in the structure.
• The configuration provides better anchorage for the reinforcement bars. This increases the bonding between the concrete and steel. Overall, the integrity of the concrete is improved.

Disadvantages of Four-Legged Stirrups in Construction

• The need for more steel reinforcement compared to other stirrups increases the material costs.
• The fabrication process where the four-legged stirrups are used involves a complex and time-consuming process.
• Improper usage of four-legged stirrups leads to over-reinforcement which can be unnecessary and inefficient.
• In congested areas, the usage of four-legged stirrups may minimise flexibility.

How to Install Four-Legged Stirrups

• The main reinforcement bars are arranged according to the design drawings. Ensure the space between the bars is intact.
• Four stirrups are positioned around the longitudinal bars. Ensure the space between the stirrups is even.
• Tie wires are used to secure the stirrup with the longitudinal bars. They must be tight to prevent any displacement during concrete pouring.
• Stirrups overlapping mount ranges between 150mm to 200mm.
• The above steps are repeated along the length of the reinforcement cage.

Difference Between One-legged, Two-Legged Stirrups and Four-legged stirrups

Conclusion

The use of four-legged stirrups is prevalent in contemporary construction practices. Most notably for stiff structural elements that experience substantial forces and shear. The four-legged stirrups are advantageous over the conventional type due to their ability to provide high shear resistance and confine the concrete. This contributes to increasing the strength of the structure. Even though they have some notable benefits, their advantages should be tempered by their higher volume of material usage and higher complexity of fabrication. Understanding both the benefits and limitations of four-legged stirrups allows engineers and contractors to make informed decisions on their use in various construction projects, ultimately ensuring safe, stable, and long-lasting structures.