Reinforced Cement Concrete (RCC) columns play a pivotal role in the structural integrity of buildings and various other construction projects. These vertical members are designed to bear the weight of beams, slabs, and other structural elements, transferring the load down to the foundation and ultimately to the soil. The design, shape, and construction methods of RCC columns are crucial to ensuring that these elements function properly under varying loads and conditions. This article explores the different types of RCC columns, their functions, and the methods used to construct them.
Types of RCC Columns
RCC columns can be classified based on various factors, including shape, length, and the nature of forces acting on them. Each classification serves a specific function in the overall design of a structure.
1. Classification Based on Shape
- Circular Columns: These columns are typically used for aesthetic purposes and are often placed in areas where they will be exposed. Their circular cross-section provides a smooth, uniform look and allows them to resist lateral forces like wind or earthquake-induced moments efficiently. This shape is especially useful for architectural designs where visibility and elegance are priorities.
- Square or Rectangular Columns: These are the most common types of columns used in construction. They are often preferred for structural purposes due to their simplicity, ease of formwork, and cost-effectiveness. Square or rectangular columns are typically found in buildings with conventional architectural designs and are capable of bearing large axial loads. Their uniform shape also makes them easier to reinforce and form during construction.
2. Classification Based on Length
- Short Columns: When the ratio of the column’s height (L) to its width (B) is less than or equal to 12 (L/B ≤ 12), the column is considered short. Short columns are typically used in structures with standard floor heights, typically around 3 meters (10 feet). Due to their relatively low height, short columns are less prone to instability and can be designed to handle typical loads found in most buildings.
- Long Columns: If the height-to-width ratio (L/B) exceeds 12 (L/B > 12), the column is considered long. These columns are more susceptible to buckling and other forms of failure due to the increased height. Consequently, they must be designed with greater attention to load distribution and stability. Long columns are often found in high-rise buildings, where floor heights exceed 3 meters, and the columns are required to bear higher loads.
3. Classification Based on Moments
- Biaxial Columns: These columns are designed to carry axial loads as well as moments in two directions. Typically found in building corners, biaxial columns must withstand bending in both directions due to wind, seismic forces, or accidental loads. These columns are critical for ensuring the structural stability of buildings in areas where forces act in multiple directions.
- Uniaxial Columns: Unlike biaxial columns, uniaxial columns are designed to bear axial loads and bending moments in only one direction. These columns are often located along the sides of buildings or in the interior, where the forces acting on them are more predictable and less varied.
Functions of RCC Columns
The primary function of RCC columns is to transfer loads from the beams and slabs above them down to the foundation and, ultimately, to the soil. Columns act as compression members, meaning they are designed to bear vertical loads that push down on them. The load-carrying capacity of a column depends on several factors, including the material used, the column’s dimensions, and the type and quantity of reinforcement.
It is crucial to design columns in such a way that they avoid the development of tensile stresses in the cross-section. This can be achieved by carefully planning the placement of the column, ensuring that it is properly aligned and reinforced. Additionally, the location of columns should be chosen so that they are either fully or partially hidden within walls, preventing them from interfering with the aesthetic and functional aspects of the building’s design.
Construction Methods of RCC Columns
The construction of RCC columns involves a series of well-defined steps, each crucial for ensuring the strength and durability of the column. These steps are as follows:
1. Column Layout
The first step in constructing an RCC column is the layout. This involves marking the precise locations for the columns on the ground. Typically, a grid system is used to position the columns according to the structural design. A rope is laid along the grid lines to mark the exact points where the columns will be placed, ensuring that they align correctly with the overall building plan.
2. Column Reinforcement Work
Column reinforcement is a critical part of the construction process. The reinforcement ensures that the column can withstand the forces it is subjected to without failing. The following checks must be made during reinforcement:
- Number and Diameter of Bars: Ensure that the correct number and diameter of vertical reinforcement bars are used as per the structural design.
- Spacing of Vertical Bars: The spacing between the vertical bars must be as specified in the design to ensure adequate load-bearing capacity.
- Development Length: The development length of the bars, which depends on their diameter, must be checked to ensure proper bonding between the reinforcement and the concrete.
- Lapping of Bars: Lapping (overlapping) of reinforcement bars should occur at the specified heights (typically at 1/3 or 2/3 of the column’s length). The laps should never fall within the beam or slab areas.
- Stirrups and Binding: The spacing between stirrups and the proper positioning of vertical bars must be checked. Stirrups should be bent at right angles, and the vertical bars should be placed correctly at the corners of the stirrups, bound securely with wire.
3. Column Formwork
Formwork or shuttering is used to shape the column and hold the wet concrete in place during curing. The following guidelines must be followed for effective formwork:
- Starter and Alignment: A concrete pad called a “starter” is cast before fixing the shuttering. This starter ensures proper alignment and provides a base for the formwork.
- Shuttering Materials: The formwork is typically made of plywood or steel sheets with adequate stiffeners to hold the shape under the pressure of wet concrete.
- Oil or Grease: A thin film of oil or grease should be applied to the inside surface of the shuttering to prevent the concrete from sticking and to make removal easier after curing.
- Verticality and Support: The formwork should be aligned properly to ensure the column is vertical. Diagonal measurements must be checked for accuracy, and the formwork should be adequately supported with props to prevent any movement during pouring.
4. Pouring Concrete
Once the formwork and reinforcement are in place, concrete is poured into the column formwork. The pouring can be done manually or with a machine pump, depending on the scale of the project. The following guidelines must be followed during concrete pouring:
- Concrete Mixing: For smaller quantities, manual mixing of concrete is acceptable, but for larger quantities, ready-mix concrete (RMC) is preferred.
- Concrete Placement: Concrete should be poured up to the slab bottom. The remaining portion of the column will be filled during the pouring of the slab and beams above.
- Vibration: A mechanical vibrator should be used to compact the concrete and remove air pockets. Excessive vibration should be avoided to prevent segregation of the mix.
- Slump and Temperature: The target slump of the concrete should be 160 mm, and the pouring temperature should remain below 30°C to ensure proper setting and curing.
- Curing: Once the concrete has been poured and compacted, it must be allowed to cure for the necessary time to achieve its full strength.
Conclusion
RCC columns are fundamental components in any construction project, playing a crucial role in load-bearing and maintaining the structural integrity of buildings. Understanding the types of columns, their functions, and the steps involved in their construction is essential for engineers, architects, and construction professionals. By following the correct design principles and construction methods, RCC columns can ensure that structures remain stable and safe throughout their lifespan.