The two different ways you can joggle the bars in the columns are as shown below.
As you can observe, the bottom bar of the column is joggled in the left side drawing & the top bar is shown joggled on the right side. Technically speaking both are correct as it does not affect the calculated design strength and load transferring ability.
But from the practical point of view, is it possible to joggle the bottom bars in columns? If not, what are the difficulties that we have to look into?
On the construction site, always the top bar of the column is joggled for the lapping purpose.
Now, let us go through some of the reasons why we should joggle the top bar while lapping work.
1. After concrete work if you joggle the bottom bar, it may produce cracks or break the portion of concrete, due to the force applied for joggle work.
2. It is practically easy to joggle the top bar with greater accuracy as it is not embedded in concrete. You can use bending machines to joggle them and then you can tie them to the bottom bar.
3. Once the column is concreted, it is practically tough to joggle the bottom bar, due to in-situ congestion and difficulty in handling the bending equipment.
4. Maintaining the required clear cover is possible when you use joggled top bar for the lapping purpose.
Now, let us look into the correct lapping procedure in the columns.
Lapping should be done in such a way that the clear cover should be maintained at the lapped portion of the bar.
The joggled part of the top bar should be placed on the inner surface of the bottom bar ( as shown above ) so that the clear cover for the rebar is equal as specified, throughout the column.
2. The bars in the lapping zone should be staggered, and in any case not more than 50% of the lapped column bars should fall at the same section.
3. The spacing of the stirrups in the lapping zone should be minimum or in other words, you have to provide more stirrups (lateral ties ) in the lapping zone of the column.
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