Rebar Detailing (Offset Bars)
Rebar Detailing (Offset Bars)
(OP)
Many detailers use offset bars (slightly bent on a 1:6 slope) to facilitate placement of lap splices. My common sense tells me they should not be allowed in areas of maximum tensile stress, but unless I have missed something, ACI does not seem to clarify where they can or cannot be used. Can anyone advise on this issue?






RE: Rebar Detailing (Offset Bars)
RE: Rebar Detailing (Offset Bars)
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.
RE: Rebar Detailing (Offset Bars)
Dik
RE: Rebar Detailing (Offset Bars)
I see a detail # 4 in ACI 315-23 with the offset bars in the "upside down" position which provides full moment capacity and makes good sense as it places the "offset" further away from the column/slab or column/beam connections and closer to an inflection point (assuming you are dealing with a rigid frame structure). I assume the other details are for structures with shear walls (no lateral loads on columns).
However what about using offset bars in walls where there are no ties (see Dik's comment) or in beams to avoid rebar congestion.
RE: Rebar Detailing (Offset Bars)
RE: Rebar Detailing (Offset Bars)
RE: Rebar Detailing (Offset Bars)
With walls the lapping bars can be placed side by side in the longitudinal direction, unless they are at very close spacing.
RE: Rebar Detailing (Offset Bars)
The main problem lies in the column / beam junctions where rebar congestion is very heavy. As a consequence we have reduced our max. aggregate size to 1/2" in many pours. In fact, we are using 1/2" aggregate in all columns, beams and elevated slabs. The Eng. has also aurthorized the use of offset bars in the beam steel in order to feed the horizontal bars through the column cages, but I don't feel comfortable with having any offset bars near a column/beam connection, especially in a severe siesmic zone.