Addition of new floor in between existing floors 3

[struggle66]

Hi all,

In one of my company previous project, someone who is no longer in this company designed a flat slab. The floor is added half-way in between existing floors. They drilled the rebars into the existing columns. Enlarge the column below the new floor by 200 mm on each side and become 1400x1400 (originally 1000x1000). What should be the stiffness column or is it pin support? He used only 5% stiffness.

Thanks

 

[racookpe1978]

Add a drawing. Then ask.

 

[KootK]

For the sake of the slab flexural design, I'd call it pinned. For punching shear and column design, I'd go 100% fixed based on the 1400 dimension stiffness. Much depends on the actual detailing however. It would be great if you could share that with us.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[manstrom]

Pinned would be conservative for the slab design.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[struggle66]

Hi all,
Attached rebar detail.
Thanks

 

[KootK]

Thanks for the details. They look great. With the the top bars passing through the column, you could make an argument for full fixity with regard to the slab flexural design. Your ability to transfer moment from the slabs to the columns may be limited to that which you can transfer via flexure at the slab-column interface. I wouldn't claim any moment transfer via eccentric punching shear. In reality, you'll no doubt you'll get some through friction and dowel action of course.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jike]

I am a little surprised that there is no reinforcing in the 200 mm additions to the column (unless it is shown in other details).

For details where it is difficult to determine how much fixity the column provides, I will try to bound the problem and run it both ways (full fixity and pinned).

 

[KootK]

Yeah, I was assuming verts and ties in the jacket and probably dowels into the column as well. How hard is it to drill holes through a 1000 column at 200 o/c ea way? I'm not being snarky. I'm genuinely curious. I would have tried to avoid that but sometimes I underestimate what construction folks are capable of. The holes through the column will remain ungrouted, right? No rebar bond through the column.

From a pure code compliance standpoint, the one thing missing is integrity steel.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[struggle66]

Hi jike,

The column design and detailing was done by structural consultant. We are PT specialist.

Hi KootK,

According to my site engineer, they didn't drill through. They drilled up to some distance and used some kind of paste like Hilti for the bonding inside the hole. Sorry for wrong info earlier.

Thanks

 

[CELinOttawa]

Lol... I was so sure that this would have turned out to be an epoxy job... And low and behold "some kind of paste"; You mean epoxy my friend.

Likely Hilti RE-500 if this was recent.

As to the drilling of the long bored through the columns - This is easy to the point of being trivial. NOT CUTTING any existing vertical steel, that is hard to the point of being impossible.

Where I have seen this done, in a multi-storey structure in NZ, the bars were cut and either rationalised as being redundant in the location, or new bars were lapped on and jacketed in with a concrete patch.

I am not a fan of concrete patching critical areas of cover like this without lab testing. Too likely to create incompatibility issues and not behave as you expect. See the excellent CAN/CSA 448; and here I used to be a BS fan for all field and existing structure repair works...

 

[KootK]

According to my site engineer, they didn't drill through. They drilled up to some distance and used some kind of paste like Hilti for the bonding inside the hole. Sorry for wrong info earlier.

This changes things for me, assuming that the epoxied rebar on either side of the column isn't effectively lapped somehow. Full mobilization of the epoxied rebar may result in cross column tension splitting. For that reason, I would consider the epoxied rebar ineffective.

All this is likely the EOR's issue rather than your own. However, it means that you can't reliably transfer slab moments to the columns via flexure or punching shear. And that leaves you with nothing. Consequently, I'm back to recommending zero fixity for use in slab flexural design.

I am not a fan of concrete patching critical areas of cover like this without lab testing. Too likely to create incompatibility issues and not behave as you expect

@CEL: can you elaborate on your concern and the type of lab testing that would alleviate it? Is it just general bond concern? I'm always skeptical when it comes to patching already stressed concrete. Feel free to reference S448. I have that.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Ingenuity]

Horizontal through-drilling within RC columns/beams/girders can be challenging, as CELinOttawa identified.

Percussion drilling is preferred, as it creates a superior bond-line for epoxy, will result typically in less damage to rebar, but is awfully difficult to maintain good hole alignment through the hole length.

Commonly, concrete coring (wet diamond cutting) is used using small diameter bits (1" dia say) with long shafts or extensions. A diamond bit will 'eat' through rebar and other embedded items. The bond line needs to the thoroughly cleaned of cutting slurry before rebar and epoxy is installed.

A technique we recently used on a EXISTING RC beam to NEW RC girder was:

1) percussion drill horizontal through holes through the existing beam;
2) drill small 1/4" dia inlet/outlet holes angled at 45°, at each end of beam face, intersecting the horizontal hole from the top;
3) clean holes with compressed air;
4) install longitudinal rebar;
5) install surface-mount epoxy injection ports to the inlet/outlet holes;
6) seal up the annular space at rebar/beam ends with paste epoxy;
7) let epoxy paste cure - several minutes for fast set;
8) using a handheld or pneumatic operated cartridge gun, inject 2-component low-viscosity epoxy into inlet ports;
9) continue injection until epoxy exits from outlet port;
10) seal ports.

The attached photo shows the injection process in operation. The black 'nipples' are the surface mounted inlet/outlet injection ports.

This method ensures complete bond/encapsulation of the rebar to concrete.