Reinforced concrete columns
Reinforced concrete columns are designed by following specific procedures that require meeting certain rules and requirements. The conditions mainly relate to reinforcement ratio, rebar size, spacing of steel bars, the thickness of concrete cover, size, and spacing of lateral ties or spirals, and dimensions of the column. The requirements or specifications related to the RC column design are commonly provided by codes such as ACI 318-19, IS 456, etc. In this article, we will discuss the tips and rules for the design of reinforced concrete columns, which are crucial for their safety and stability.
Dimensions of Columns Cross-Section
According to ACI 318-19, there is no limitation on the minimum dimension for columns. This allows reinforced concrete columns with a small cross-section in lightly loaded structures, such as low-rise residential and light office buildings. However, a small cross-section requires careful workmanship. It is desirable to have the column's cross-section as a multiple of 5 cm for practical purposes.
Longitudinal reinforcement is the main bar in the RC column. They are arranged in a square, rectangular, or circular pattern.
2.1 Minimum and Maximum Reinforcement Ratio
According to ACI 318-19, the area of longitudinal reinforcement should not be less than (0.01Ag) and not greater than (0.08Ag). Where "Ag" is the gross cross-sectional area of the column. A reinforcement ratio higher than (0.08*Ag) is not economically and practically desirable as it leads to steel congestion that hinders proper placement and concrete consolidation. Steel congestion may lead to the formation of honeycomb in concrete.
The majority of columns are designed with a maximum reinforcement ratio of (0.04*Ag). This significantly reduces the chance of congestion. The use of large steel bars can reduce steel congestion. The largest commercially available steel bars are No. 43 and No. 57, mainly manufactured in columns.
2.2 Number of Longitudinal Bars
According to ACI 318-19, the minimum number of bars for concrete columns is as follows:
Four within rectangular or circular ties.
Six were enclosed by spirals or columns of special moment frames held by circular hoops.
Three within triangular ties.
For columns with large axial forces and small moments, longitudinal bars should be spaced more or less uniformly around the perimeter. If bending moments on the column is significant, much of the longitudinal steel rebar is concentrated at the highest compression or tension faces, i.e., at maximum distances from the axis of bending.
2.3 Thickness of Concrete Cover
The minimum thickness of the concrete cover is 40 cm. However, it may be required to be increased if in special circumstances or when the general building code needs greater concrete cover for fire protection. For columns that are cast and permanently in contact with the ground, the minimum cover is 7.5 cm. For columns exposed to weather or contact with the ground and embedded bars are No. 19 or greater, the minimum concrete cover is 5 cm.
2.4 Spacing Between Longitudinal Bars
The spacing between the longitudinal reinforcement of the column should be the greatest of the following:
· 4 cm
· 1.5 times the diameter of the longitudinal bar
· (4/3) times the diameter of the maximum aggregate size
2.5 Bundled Bars
Bundled bars are groups of parallel bars that are in contact with each other to act as unit bars. It is used where a heavy concentration of reinforcement is required. Bundled bars save space and reduce congestion for the placement and compaction of concrete. The maximum number of bundled bars is four. Bundled bars shall be
3. Size and Spacing of Lateral Ties or Spirals
The size and spacing of lateral ties or spirals are important factors in the design of reinforced concrete columns. Lateral ties or spirals are used to prevent the longitudinal bars from buckling and to provide lateral support to the column.
3.1 Size of Lateral Ties or Spirals
According to ACI 318-19, section 18.7.2, the size of lateral ties or spirals shall not be less than:
· 1/6 of the diameter of the longitudinal bars
· 6 mm for ties
· 1/5 of the diameter of the longitudinal bars
· 6 mm for spirals
6 mm for ties and spirals in columns where longitudinal bars are larger than No. 36.
The maximum spacing of the lateral ties or spirals shall not exceed
· 16 times the longitudinal bar diameter
· 48 times the tie or spiral diameter.
3.2 Spacing of Lateral Ties or Spirals
The spacing of lateral ties or spirals shall be as follows:
· Not more than 16 times the longitudinal bar diameter
· Not more than 48 times the tie or spiral diameter.
3.3 Number of Lateral Ties or Spirals
The minimum number of lateral ties or spirals in a reinforced concrete column shall be as follows:
· 4 for circular columns
· 6 for rectangular columns
· 8 for columns with a side dimension greater than 750 mm.
The number of lateral ties or spirals shall be increased if the longitudinal bars are closely spaced or if the column is subjected to high bending or axial loads.
4. Shear Reinforcement
Shear reinforcement is used in reinforced concrete columns to resist shear forces. The shear reinforcement can be in the form of stirrups, bent-up bars, or inclined bars.
Stirrups are the most commonly used form of shear reinforcement in reinforced concrete columns. The stirrups are placed around the longitudinal bars and spaced at regular intervals. The stirrup spacing is governed by the maximum size of the aggregate and the column diameter.
4.2 Bent-up Bars
Bent-up bars are used in reinforced concrete columns with high shear forces. The bent-up bars are used in conjunction with the longitudinal bars and the stirrups to provide additional shear resistance.
4.3 Inclined Bars
Inclined bars are used in reinforced concrete columns with high shear forces. The inclined bars are placed at an angle to the longitudinal bars and the stirrups to provide additional shear resistance.
In conclusion, the design of reinforced concrete columns requires careful consideration of several factors. The cross-section dimensions, longitudinal bars, lateral ties or spirals, shear reinforcement, and concrete cover all need to be carefully selected to ensure the safety and durability of the structure. The specifications and guidelines provided by codes such as ACI 318-19 and IS 456 should be followed to ensure that the design meets the requirements. A well-designed reinforced concrete column will be able to resist the loads imposed on it and provide long-lasting support to the structure.
The design of reinforced concrete (RC) columns according to the ACI Code involves the following steps:
Determine the axial load and moment at the critical section of the column.
3. Select a column section based on the load and moment, and check for adequate gross and net areas.
4. Check the column for slenderness effects, and determine the effective length factor based on the boundary conditions, and end restraints.
5. Determine the required longitudinal reinforcement based on the factored axial load and moment and calculate the spacing and amount of longitudinal bars.
6. Determine the required transverse reinforcement based on the column size, longitudinal reinforcement, and axial load, and calculate the spacing and amount of ties or spirals.
7. Check the column for strength and serviceability requirements, including the maximum allowable axial load, moment, and deflection.
8. The above steps involve many design equations and factors. A detailed understanding of the ACI Code and relevant design standards is essential for proper RC column design.