Design of column according ACI codes
- 1. DESIGN CONSIDERATION OF COLUMN ACCORDINGTO ACI CODE
- 2. According to ACI Code 2.2, a structural element with a ratio of height-to-least lateral dimension >3 COLUMN: support vertical loads transfer these loads to the footings. usually support compressive loads with or without bending
- 3. Types of Columns:divided into three types according to reinforcement. Tied Columns The longitudinal reinforcement bars are tied together with separate smaller diameter ties spaced at some interval along the column height. These ties help to hold the longitudinal reinforcement bars in place during construction and ensure stability of these bars against local buckling.
- 4. The cross sections of such columns are usually square, rectangular, or circular in shape. A minimum of four bars is used in rectangular and circular cross sections.
- 5. Spirally-Reinforced Columns : The longitudinal bars are arranged in a circle surrounded by a closely spaced continuous spiral. These columns are usually circular or square in shape. A minimum of six bars is used for longitudinal reinforcement.
- 6. Composite Columns: A column made of structural steel shapes or pipes surrounded by or filled by concrete with or without longitudinal reinforcement.
- 7. FAILURE OF TIED AND SPIRAL COLUMN: Axial loading on tied and spirally reinforced columns having the same cross-sectional areas of concrete and steel reinforcement behave in the same manner up to the ultimate load. At that load tied columns fail suddenly due to excessive cracking in the concrete section followed by buckling of the longitudinal reinforcement between ties within the failure region.
- 8. For spirally reinforced columns, when ultimate load is reached, the concrete shell covering the spiral starts to peel off. Only then, the spiral comes to action by providing a confining force to the concrete core, thus enabling the column to sustain large deformations before final collapse occurs.
- 9. ACI Code Requirements forColumns: ForTiesColumn: No. 3 ties for longitudinal reinforcement no. 10 bars or less, no. 4 ties for no. 11 bars or larger and bundled bars. Tie spacing shall not exceed 16 diameter of longitudinal bars, 48 diameters of tie bars, nor the least dimension of column. Every corner bar and alternate bars shall have lateral tie provide the angle shall not exceed 135 degree.
- 10. No longitudinal bar shall be spacing more than 6 inches without a lateral tie. Forspiral Columns: At least six longitudinal bars must be used within spiral ties. Spirals may not have diameters less than 3/8 in. The clear spacing between spirals may not be less than 1 in. or greater than 3 in.
- 11. Design Considerations: MaximumandMinimumReinforcementRatios ACI Code 10.9.1 specifies that a 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.
- 12. MinimumNumberofReinforcingBars: ACI Code 10.9.2 specifies a minimum of 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
- 13. Clear Distance between Reinforcing Bars: ACI Code 7.6.3 and 7.6.4 specify that for tied or spirally reinforced columns, clear distance between bars, is not to be less than the larger of 1.50 times bar diameter or 4 cm
- 14. Concrete Protection Cover: ACI Code 7.7.1 specifies that for reinforced columns, the clear concrete cover is not to be taken 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.
- 15. Minimum Cross Sectional Dimensions: With the 1971 Code, For practical considerations, column dimensions are taken as multiples of 5 cm.
- 16. 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.
- 17. Ties: According to ACI Code 7.10.5.1, for longitudinal bars 32 mm or smaller, lateral ties 10 mm in diameter are used. In our country and in some neighboring countries, ties 8 mm in diameter are used in column construction.