Strengthening New Opening in 2-Way Slab w/ Externally Bonded/Bolted Steel Plate

[csways_]

Hi,

I am working working on the strengthening of a new opening in a 2-way RC slab slab. We had originally designed the strengthening using FRP, but the Contractor balked at the cost, cut the opening, shored the slab, and now we have to come up with a cheaper solution. We are looking into externally bonded and/or bolted steel plate, but I am having a hard time finding a good design reference. We know we need to replace the area of reinforcement lost, but what about development, bond length, through bolting so close to the slab edge, etc.

If anyone can provide any guidance or recommend a good design reference, I would sincerely appreciate it. Thanks!

 

[BridgeSmith]

If the opening is just slightly bigger than required, perhaps angles with a vertical leg extending up along the faces of the opening, with anchors through the vertical legs into the faces of the opening, so you get the section modulus without a beam underneath, and the concrete anchors are in shear, instead of tension.

 

[jayrod12]

I'm thinking he's talking about providing external tensile reinforcement from support to support on either side of the opening. So just bolting an angle on the opening likely isn't enough.

Do you need to carry it opening to opening? or just engage more of the slab beside the opening?

 

[oldestguy]

Just what portion of the slab is cut? If not out in the middle, maybe nothing needed, but accept some local increase in stress of remaining bars.

 

[Celt83]

Have used and not an overall fan of the anchored plate reinforcement for a few reasons:
[ul]
[li]Slab soffit will not be a flush/flat surface so the anchors will experience some bending along with tension[/li]
[li]# of anchors required to develop the plate need to be on both sides from the location where the additional steel is needed, to satisfy spacing criteria of the anchors and give enough play to avoid the in slab steel you may have 15 ft anchorage zones[/li]
[li]the plate method is really only going to get you reinforcement in one direction, can't criss-cross the plates[/li]
[li]overhead anchor installation which will be in sustained shear/bending[/li]
[/ul]

Check out the Book "Structural Renovation of Buildings" by Alexander Newman

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[jike]

A couple of considerations/comments:

1) If the slab has a fire rating then the steel plates may need to have sprayed on fireproofing.
2) Make sure that the holes are NOT oversized so that the bolts can bear against the steel so the load can get transferred without slippage. Alternates may be a) to fill the gap between the bolts and the oversized holes so bearing can be accomplished. b) Use washers with standard holes that slip over the bolt heads and get welded to the steel plate.
3) Could the intersecting plates be butt welded together to transfer load? Check tri-axial stress in the intersecting plates using Mohr's Circle or a similar method.

 

[jike]

Another consideration is if you are bonding the plate directly to the concrete (and using the bolts only to hold the plate in place until the epoxy cures) then you better find out how the epoxy may be affected by moisture and/or fire. If you have a fire rating, you may not be able to use a bonded plate.

 

[KootK]

1) I've got some decent details for two way reinforcing as jike has proposed it. They are all based on anchors rather than adhesive though.

2) Slip in bolt holes has always bothered me a bit. Even in holes that are not over-sized, you've got some play that would cause some anchors to engage before others. And it doesn't take much slack takeup before our common assumption of composite behavior gets dubious. A detail that I like mechanically is to use plate with nelson studs on top inserted into cored slab holes from the underside. The holes are cored from the top and then grouted once the plate is placed. It mitigates the slip issue but creates a situation where it's a pretty safe bet that you'll lose some reinforcing as a result of the coring. It's also a fair bit of work.

3) I don't know of a good design guide for designing bonded plates. There are a fair number of research papers touching on this subject and you may be forced to resort to a combination of those and first principle design. Here's one paper, for beams, by one of my former professors: Link

4) If at all possible, I'd be inclined to develop the reinforcing quickly at the ends, similar to what we do with bar joist reinforcing.

5) I this case, I think that one actually has to do more than merely develop the plates on either side of the maximum moment. That strategy is really suitable for linear elements like beams where there is no need to transfer the tension laterally. In many slab opening situations, what you're really doing is creating an offset lap splice between the reinforcing and the cut rebar. Viewing things this way will lead to a longer "development" length of course. This effect can be ignored if you carry the reinforcing all the way back to the assumed slab panel support lines such that the interrupted reinforcing at the opening can be considered to have been abandoned entirely from a structural perspective. This approach will often lead to reinforcing lengths that look a little nuts to contractors though.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[Ingenuity]

ACI has (had?) a Special Publication SP-165 entitled "Repair and Strengthening of Concrete Members with Adhesive Bonded Plates"

Author(s): Editors: R. Narayan Swami and Robert Gaul

Link

I purchased a hard copy back in the late 90's, so I somewhat doubt ACI till publish it, but you may be able to find individual papers that make up the SP.