What is the role of bundle bars in column?

Bundling of bars becomes necessary when large number of bars are required to be accommodated in a structural member. Concrete codes dictate minimum criteria for spacing of the reinforcing bars to ensure that during construction fresh concrete can be placed easily in between and around the bars. Therefore, when there are large number of bars required to be provided based on design, it may not be possible to place the bars separately with necessary clearance. In such cases, there are two options:

1. Increase the size of the member (columns, beams).
2. Bundle the bars in groups of two, three or four bars.

However, option 1 means unnecessary cost implications due to the increase in the volume of concrete. Hence, engineers mostly resort to bundling of bars.

Image credit: http://civcal.media.hku.hk/

It is important to note that bundled bars have low contact area with the surrounding concrete when compared to the bars placed separately. This affects the bonding between concrete and the rebars which is of concern especially in beams. To compensate this, the development length is increased suitably and curtailment, wherever prescribed, is done at different points for each bar. 

The development length of each bar of bundled bars shall be that for the individual bar, increased by 10 percent for two bars in contact, 20 percent for three bars in contact and 33 percent for four bars in contact.

-- Clause 26.2.1.2, Code 456-2000, Bureau of Indian Standards.