Short Answer:
The importance of longitudinal reinforcement in columns lies in its role to resist axial loads and bending moments. These vertical steel bars help the column carry the weight of the structure and maintain its strength and stability, especially during compression and slight bending caused by eccentric loading or lateral forces.
Longitudinal reinforcement also prevents buckling of the column, controls cracks, and ensures ductile failure instead of sudden collapse. It is essential for enhancing load-carrying capacity, durability, and overall safety of reinforced concrete columns.
Detailed Explanation:
Importance of longitudinal reinforcement in columns
In reinforced cement concrete (RCC) design, columns are compression members that carry vertical loads from beams and slabs down to the foundation. Concrete is strong in compression but weak in tension. To improve the strength, stability, and performance of columns, steel reinforcement is provided. This reinforcement is of two types: longitudinal bars and lateral ties. Among these, longitudinal reinforcement plays a vital role.
Longitudinal reinforcement refers to the vertical bars placed along the height of the column. These bars are designed to take care of both axial loads and bending moments that the column may experience due to unequal loading, wind, seismic forces, or any eccentric load. The placement, number, size, and arrangement of these bars are based on structural calculations and codal guidelines.
Functions and Importance
- Resisting Axial Loads
The primary function of longitudinal reinforcement is to support axial compression in combination with concrete. As the building’s weight is transferred through the column, the steel bars share the load with concrete, especially when loads are high.
In heavily loaded columns, the concrete alone may not be enough, and the steel reinforcement carries a significant portion of the load, increasing the load-carrying capacity of the column.
- Handling Bending Moments
Columns may not always be centrally loaded; often, loads act with eccentricity, leading to bending. Longitudinal reinforcement resists the tensile forces generated due to bending moments.
Especially in corner or edge columns, or those subjected to wind or seismic forces, the bending can be significant, and vertical bars help maintain stability.
- Enhancing Ductility
Concrete is a brittle material and can fail suddenly. Steel reinforcement makes the column more ductile, meaning it can deform before failure and give warning signs like cracks or deflection.
This property is crucial during earthquakes or sudden overloads, helping prevent sudden collapse and saving lives.
- Preventing Buckling of Column Core
Longitudinal bars, when tied properly with lateral ties or spirals, help prevent the buckling of the core concrete. The steel bars maintain the alignment and hold the concrete in shape during vertical loading.
This prevents structural instability, especially in slender columns.
- Crack Control and Durability
With proper placement, longitudinal reinforcement helps in controlling cracks, particularly in columns subjected to environmental changes or temperature variations. Cracks reduce the lifespan of structures, so reinforcement helps maintain the durability of columns.
Design and Placement Guidelines
- IS 456:2000 specifies a minimum of 0.8% to 6% of the cross-sectional area to be provided as longitudinal reinforcement.
- A minimum of 4 bars is required in a rectangular column and 6 bars in a circular column.
- Bars are placed uniformly around the column to balance the load distribution.
- Proper clear cover and anchorage are maintained for safety and durability.
Conclusion:
Longitudinal reinforcement in columns is crucial for resisting axial loads and bending moments, increasing strength and ductility, and preventing sudden failure. It supports the concrete core, controls cracking, and ensures the column performs safely under different loading conditions. Properly designed longitudinal bars are key to strong and stable RCC structures.