20 Jun 06 16:07
For years, the typical ACI detailing manual, and most engineer's, specified the 1:6 taper in columns as starting just beneath the floor and tapering up through the floor thickness such that as the bars came out of the top surface of the floor structure, they would be fully offset to lap with the bars for the column above, which were set in the corners.
ACI includes in their detailing manual an alternative method where the offset occurs in an upside-down position.
Here's an attempt to describe it:
1. The lower column vertical bars are in the corners, extending straight up, and through the floor above.
2. These bars then extend from the upper floor a distance of the column lap length.
3. The upper column bars are offset to lay just inside the straight bars from below.
4. Once the upper bars lap past the straight bars, they begin to taper 1:6 outward to the four corners.
5. At the end of the taper, they then extend straight up through the floor above and the process repeats
This creates the greatest "d" value in the column reinforcing where its needed most - at the column-floor interface.
The old way created a diminished "d" at the floor, where usually the moment is greatest. Also with this "old" way, the taper occurs within the column/beam intersection and can cause a lot of interference and congestion while the alternative method only has straight, vertical bars.
I've used this detail on a lot of projects and it works well. You just have to be sure to sort of red flag this for the contractor as most will try to still put them in the "old" / wrong way.