Design Of Columns As Per ACI:
1. Maximum and Minimum Reinforcement Ratio:
The minimum reinforcement ratio of 1 % is to be used in tied or spirally reinforced columns. This minimum reinforcement is needed to safeguard against any bending, reduce the effect of shrinkage and creep and enhance ductility of columns.
2. Minimum Number of Reinforcing Bars:
Minimum four bars within rectangular or circular sections; or one bar in each corner of the cross section for other shapes and a minimum of six bars in spirally reinforced columns should be used.
3. Clear Distance between Reinforcing Bars:
For tied or spirally reinforced columns, clear distance between bars should not be less than the larger of 150 times bar diameter or 4 cm.
4. Concrete Protection Cover:
The clear concrete cover should not be less than 4 cm for columns not exposed to weather or in contact with ground. It is essential for protecting the reinforcement from corrosion or fire hazards.
5. Minimum Cross-Sectional Dimensions:
For practical considerations, column dimensions can be taken as multiples of 5 cm.
6. Lateral Reinforcement:
Ties are effective in restraining the longitudinal bars from buckling out through the surface of the column, holding the reinforcement cage together during the construction process, confining the concrete core and when columns are subjected to horizontal forces, they serve as shear reinforcement. Spirals, on the other hand, serve in addition to these benefits in compensating for the strength loss due to spalling of the outside concrete shell at ultimate column strength.
For longitudinal bars, 32 mm or smaller, lateral ties 10 mm in diameter should be used. In our country and in some neighboring countries, ties of 8 mm dia are used for column construction.