Correct Answer - Option 3 : 0.8 and 6
(i) CI. 26.5.3 of IS 456:2000, specifies that the total area of longitudinal bars in a column section must NOT be less than 0.8% of the gross column area. This limit on minimum reinforcement is imposed because of the following reasons:
- In order to ensure that a minimum flexural resistance of the column exists due to unexpected eccentricities in the column loading.
- In compression members, creep under sustained loading is very predominant, especially at low percentages of steel. Thus, the resulting creep stress (due to creep strain) tries to yield the bars.
(ii) Maximum Reinforcement: The maximum area of cross-section of longitudinal bars must NOT exceed 6% of gross column area. However, in practice, a maximum of 4% is recommended.
Diameter and Number of bar:
(i) The diameter of longitudinal bars in column NOT be less than 12 mm. These bars must NOT be spaced more than 300 mm apart on the column perimeter.
(ii) For rectangular columns. a minimum of 4 bars is provided.
(iii) For Circular columns, a minimum of 6 bars be provided.
Cover = 40 mm or bar diameter