Best Reinforcement Detail by stepped column?

[BauTomTom]

Hi Struct.Engs.

I would like to ask you for your advice, which reinforcement arrangement did you make best experience?

The case is: A column changes the Dimensions from 450x800mm to 450x450mm
what is the best way to stop the reinforcement now?

i prepared some sketches for you, it would be nice if you could have a look and report back.

Regards

BauTomTom

 

[ron9876]

1---check to see if hooks will fit in slab

 

[BauTomTom]

hooks? Yes why not. The column bars are 20 mm diameter and the slab is 250mm, the beam is additional 250.

BauTomTom

 

[JAE]

Definitely NOT number 6.

 

[Teguci]

1 makes the most sense as long as the longitudunal bars are within 6" of each other and the lap is longer than 30". Otherwise, something like number 6 except:
- the 90 deg bends should be 30 deg
- additional confinement/shear reinf should be provided
- I would keep the tapered laps out of the beam column joint to minimize congestion

 

[csd72]

1 is the best way, the bars need to extend down past the top of the lower bars by their compression or tension development length as appropriate.

 

[BAretired]

I don't like any of them, but No. 1 is the best. It would be better to extend the vertical bars within the upper column to provide sufficient lap, then add two vertical bars each side of the lower column, hooked at the top.

BA

 

[JAE]

Teguci - ACI here in the US limits sloped bars to 1:6 slope - so a 30 degree bend wouldn't meet that criteria. That said, the 1:6 limit is for splicing bars, not stepping bars.

Usually I would simply terminate the lower, larger column bars, just under the floor. I'd then add dowel bars that matched the upper column bars. These dowel bars would lap splice with the upper column bars and extend down from the top of floor into the lower column to develop/splice down there.

If the floor is too narrow and the dowel bars would have to be extended below the bottom of the floor, then I might either extend them further to help with placement or go to hooks.

 

[TXStructural]

Number 1. In the 450 mm dimension, continue available straight bars for a typical splice above the slab. Discontinue bars which would be outside the upper column (or in required cover.) Where bars are bent/offset to continue into the upper column, the limit is still 1:6 per ACI.

As a general rule, if a location of a column face changes more than 3 inches (75 mm), do not try to offset the bars thru the slab/beam region. Discontinue bars and dowel the reinforcement into the lower column as dowels with one development length above and below the cold joint/top of slab (easy, essentially as shown in sketch #1.)

Where the reinforcement in the lower column must be developed at the top of column, bars should be hooked or anchored with headed reinforcing bars, as appropriate. Similar to your #5. Where you can get enough moment using the dowels or continuing certain bars into the upper column, that is preferred due to construction and rebar fabrication issues.

#2 is very complicated to construct, #3 even more so.
#6 does not work with such a sharp offset.

 

[dik]

A combination of 2 and 1. 1 for the dwls and bars to the column above and 2 because the compression on the hooked bars increases the bond.

Dik

 

[Teguci]

JAE - What is the reference for the 1 to 6 slope. Just briefed through chapters 10 through 12 and didn't pick it up.

As for the splicing into the base column where the upper to lower column dimension difference is more than 6", doweling in to the lower column will not meet ACI 12.14.2.3.

 

[slickdeals]

I think the 1 to 6 slope is in Chapter 7.

 

[JAE]

7.8.1.1 outlines the 1:6 slope limit. (318-05)