When buildings collapse during earthquakes, the failure often starts in the columns. What goes wrong? In many cases, it happens due to missing or poorly placed lateral ties. These small steel rings don’t carry the load but perform other important functions in columns and buildings as a whole. This blog explains the critical role of lateral ties in column design, their types, how they’re used, and why they’re different from stirrups in beams.
What are Lateral Ties in Columns?
Reinforcement bars in columns carry and transfer applied loads in buildings. Lateral ties are small metal binding elements that help hold these bars in their correct position and prevent them from moving during concrete compaction. These ties do not bear or share the column load but are uniformly spaced throughout the full height of the column to ensure that reinforcement bars are positioned correctly and maintain stability.
Importance of Lateral Ties in Column Design
Lateral ties are small but essential elements used in reinforced concrete columns. Some of their important functions are:
Hold Longitudinal Bars in Position
Lateral ties wrap around the main vertical bars (longitudinal reinforcement) in a column to maintain proper spacing during and after the concrete pouring process. According to the Indian Standards Code IS 456:2000, this ensures a properly designed geometry of the reinforcement for efficient load transfer.
Prevent Buckling Under Axial Load
Columns support and transfer heavy loads. Under such loads, the vertical bars can buckle. Closely spaced lateral ties prevent this by bracing the bars at intervals. This is particularly necessary for slender columns and in case of seismic action.
Enhance Shear Strength and Ductility
Lateral reinforcement enhances a column’s resistance to shear forces. It also enhances ductility and enables the building to deform safely under loads such as earthquakes or blasts. Ductility prevents sudden collapse and provides individuals with additional time to evacuate in the event of an emergency.
Improve Seismic Performance
In seismic areas, spacing and anchorage of ties should be uniform. The Indian Standards Code IS 13920 (for ductile detailing) specifies that reduced spacing of ties is necessary at column ends where bending and shear are highest. Properly detailed ties enable the column to remain unharmed during extreme ground motion.
Avoid Concrete Spalling and Core Crushing
Inadequate lateral restraint will lead to the spalling or failure of the concrete core under stress. Spalling leads to surface breakdown or peeling and weakens the column. Ties are placed in the correct position to help prevent these issues and resist brittle failure.
Structural Code Compliance
All major building codes of different countries, such as Indian Standard IS 456:2000, ACI (American Concrete Institute) 318, and Eurocode 2, mandate minimum tie spacing, diameters, and hooks to achieve structural integrity. Apart from ensuring building safety, adhering to these codes ensures legal compliance in a project.
Types of Lateral Ties Used in Columns
Rectangular or Square Ties
These are the most common types of lateral ties and are used for columns of a rectangular or square cross-section. They secure the longitudinal bars in position and provide lateral support to prevent buckling. They are easy to execute, as steel can easily be bent to the required square or rectangular shape.
Circular Ties
These are used in columns with round cross-sections. These ties function like rectangular ties, and offer lateral support to longitudinal bars and assist with the structural stability of circular columns. They are often employed in spiral columns, such as silos or chimneys.
Helical (Spiral) Reinforcement
This is an unbroken spiral around the reinforcement bars across the column. It offers equal confinement to the concrete core and promotes ductility and energy absorption, particularly for seismic loads. These are used in seismic zones since they perform better when it comes to energy dissipation and ductility.
Diamond and Triangular Ties
These are unconventional forms and are applied in certain design conditions where regular tie shapes will not work or where appearance is a factor. They are used occasionally in architectural columns with irregular shapes or where there are special structural requirements.
Multi-Legged Ties (Two, Three, Four-Legged)
These ties have two or more legs to give columns additional support. They provide columns with extra lateral support through the use of ties that overlap each other.
Accurate detailing and setting of the lateral ties are extremely important to maintain the structural stability of reinforced concrete columns, particularly against seismic and axial loads.
Requirements for Lateral Ties in Columns
Lateral reinforcement is necessary, whether it is in the form of ties or spirals, because they hold the columns together, provide confinement to the concrete, and protect the vertical bars. To ensure safety and structural performance, specific standards as per the Indian Standards Code IS 456:2000 are followed, and these include:
1. Diameter of Lateral Ties
The diameter of lateral ties must be at least the greater of the following.
- One-fourth (¼) of the diameter of the largest longitudinal bar, or simply 6 mm. This makes sure that the ties are strong enough to hold the vertical bars in place and resist forces while loading.
2. Spacing of Lateral Ties
The spacing between lateral ties should be carefully maintained and must not exceed:
- The smallest lateral dimension of the column.
- 16 times the diameter of the smallest longitudinal bar or 300 mm.
The least value should be used for tight confinement and effective support.
3. Diameter of Spiral Reinforcement
For circular columns with spiral reinforcement, the bar diameter used for the spiral must be at least the highest of,
- One-fourth (¼) of the diameter of the largest longitudinal bar, or 6 mm.
4. Pitch of Spiral Reinforcement
The pitch, which means the spacing between spiral turns, should be the smaller of the following.
- One-sixth (⅙) of the diameter of the concrete core (measured to the outside of the spiral), or 75 mm.
Lateral Ties and Stirrups: How Do They Differ?
At first glance, lateral ties and stirrups may appear similar. Yes, both of them are transverse reinforcement elements. However, their purpose and placement differ.
Lateral Ties
These ties are commonly used in concrete columns, where the main force experienced is compression. Their primary job is to keep the main vertical bars, i.e., longitudinal reinforcement, in position and secure them even under heavy loads. Lateral ties usually wrap around the core bars and hold everything together during concrete pouring and after the structure is loaded. They also help in confining the concrete core and prevent buckling of individual rebars, especially in seismic zones.
Stirrups
These are used in beams, where the element mainly faces bending and shear forces. Their main purpose is to resist shear stress and prevent diagonal cracking. Moreover, they wrap around the longitudinal bars in a beam and are carefully designed and spaced to provide extra shear capacity where needed.
In simple terms, lateral ties tie bars together in columns, whereas stirrups in beams provide shear strength in beams.
Conclusion
Lateral ties may look like simple elements, but they play a big role in keeping a column strong and safe. They prevent buckling, improve earthquake resistance, and help the column hold its shape. Different types of ties are used depending on the column design. By following building codes and placing ties correctly, you can ensure your structures last longer and remain safe. Understanding the difference between ties and stirrups is also crucial for proper construction and design work.
Apoorva H P Achar