High demand 3-5/8" jamb stud connection cantilevering 2" past rough slab edge.

[KootK]

This thread introduction may well be too long for busy folks to read. If so, you can probably get a pretty good jist of it just by scrolling through the graphics. I won't be lambasting anybody because they bring up something that I have already mentioned in the text.

I've presented some possible solutions below that I would like to have vetted by the gang. Naturally, I would also be utterly thrilled to entertain alternate, better solutions. In my heart of hearts, I'd like to rip out the walls and replace them with the 6" studs that should have been there to begin with. That would not be good client care in this instance however.

THE SITUATION

1) existing 3-5/8" jamb stud for a rather large wall opening in a highrise condo.

2) jamb stud will deliver about 1500 lbs ULS wind shear to its slab connection.

3) jamb stud is already being reinforced with a nested, 3" steel channel.

4) original connection design was some aggressively designed clips and concrete screws.

5) site work has revealed that, in some locations, the slab edge was originally saw cut as shown in the the photo below. Some kind of field error presumably.

6) my hardworking jamb stud connection now has a stud cantilever of up to 2".

7) expensive folks are standing around on site seeking direction.

THE CONSTRAINTS (AT LEAST FOR NOW)

8) Existing wall system is to remain in place during repair. Exterior sheathing will come off.

9) Need a solution that works for stud cantilevers ranging from 1/2" min to 2" max.

10) No field welding.

11) Need to be a little sensitive to building envelope concerns. I can, locally, have a bit of steel that would project into the exterior sheathing/insulation space.

THE OPTIONS CURRENTLY TABLED

OPTION 1. For a short length of slab, feather out the slab edge with repair concrete somehow. This would, ostensibly, be pretty great. Can that be done in such a way that we'd trust that repair concrete to be able to handle the fastener shear loads coming in from the jamb studs though? It gives me the heebee jeebess. I also worry that it's hard to do without removing the existing stud work. I would be interested to hear if this solution is viable and, if it is, recommendations for appropriate materials and procedures.

OPTION 2. This is a clever solution suggested by the client. My main concern with this is the viability of the grouting which would need to work at thicknesses ranging from 1/2" to 2". If the grout is installed before the bolt, will it just fall off during drill & epoxy'ing of the bolt? Would it be feasible to install the bolt first, grout around it, and then attach the angle? Could the bolt and angle be installed first, with the angle kept still somehow, and then grout be stuffed in behind the angle as a final operation?

OPTION 3. Similar to option two but no longer reliant on grouting feasibility. It's just... kinda silly looking. I would like to do something similar with a single back side nut, much like traffic sign structure bases are handled sometimes. Given the access situation, however, is there any way that I could use a single, backside not and be confident that it would be locked in place for the long haul? This would be easier at the 2" dimension and harder at the 1/2" dimension I reckon.

 

[SwinnyGG]

For option 3- is the assumption here that you're filling that cavity with concrete?

Have you considered an option similar to option 3- except with a full-thread adhesive anchor instead of your bolt/nut/washer stack; which is then filled with concrete (or grout or whatever) and then anchors are preloaded once the concrete is cured? This would give your concrete 'patch' a lot of confinement as well as place it in compression, potentially enough to cancel out all possible shear load being applied by screws coming in for the overhung jamb stud.

 

[DTGT2002]

I would think bolting a stiffened channel or even just a 2x6 to the relatively flat concrete face would be more straight forward.

Is the slab mild steel reinforcement only? Any concerns with drilling the face? Evidence of spalling or other concrete loss? It appears to have two mats of reinforcement.

 

[KootK]

For option 3- is the assumption here that you're filling that cavity with concrete?

Not necessarily. I could ask for grouting but the intention is that the lateral load capability of the connection would no longer be dependent on the feasibility nor quality of that grouting. For what it's worth, vertical load carrying capacity here isn't much of an issue.

Have you considered an option similar to option 3- except with a full-thread adhesive anchor instead of your bolt/nut/washer stack; which is then filled with concrete (or grout or whatever) and then anchors are preloaded once the concrete is cured?

Interesting. With options two and three, I'd been considering pre-load as a way to at least keep the grout in place. I'd not made the leap to then switching back to the original, top side fasteners under that scenario. Under that condition, I take it that you would be comfortable considering the outer fasteners as being as good as they would have been in monolithically cast concrete correctly placed? Specifying the preload strikes me as a bit tricky. Without meaningful stretch length etc, I doubt that I could reasonably expect much more than "torque wrench tight"

 

[WARose]

I lean more towards Option #2. I'd use a epoxy grout to fill that space (after roughening the existing concrete).....then install the bolt after it sets. Some pre-load in the bolt will help get it across via friction.

 

[BridgeSmith]

I think I like option 2, possibly substituting a couple of wedge anchors for the epoxy anchor.

 

[KootK]

I would think bolting a stiffened channel or even just a 2x6 to the relatively flat concrete face would be more straight forward.

You consider that relatively flat? Three issues that I have with a ledger option:

1) ledger will want to twist under load. that'll be a tensile force in the anchor and compression against the slab edge. I don't feel great about the compression given how rough the slab edge is.

2) amount of cantilever varies all over which would be a nightmare for coordinating the dimensions of the ledger piece.

3) it tends to detail out such that the top side fasteners either land pretty much at the gap between concrete and ledger or, in the case of a steel piece, at the heel which would be tough to fasten through.

Is the slab mild steel reinforcement only? Any concerns with drilling the face? Evidence of spalling or other concrete loss? It appears to have two mats of reinforcement.

We believe this to be a mild slab but, yeah, some concern. We'll have to check it out. No slab edge live ends encountered yet. No evidence of spalling or other concrete loss. It's normal here to see two layers of slab reinforcement at slab edges. You'll have the chord bars running the perimeter. and some short hook bars up to the slab edge.

 

[KootK]

I lean more towards Option #2. I'd use a epoxy grout to fill that space (after roughening the existing concrete).....then install the bolt after it sets. Some pre-load in the bolt will help get it across via friction.

I do like the epoxy grout idea. Thanks.

I think I like option 2, possibly substituting a couple of wedge anchors for the epoxy anchor.

Is the wedge anchor preference just an economy thing?

 

[Celt83]

Think I would agree with DTGT2002's approach.

My concern is when you go to PAF or screw down into your new 2" of grout/concrete patch there is a pretty good chance of splitting making that anchor useless again. with a channel or even a set of angles top and bottom you can PAF to the steel and handle the tensile loading from suction with an epoxy anchor into the slab with better cover and windward pressure gets picked up with bearing between the steel and edge of slab. Maybe in this approach to a skim coat of repair mix to just level the surface.

What is your intention with the angle you show on top of the plate, seems installing this would require you to cut the jamb studs and weld them on to get resistance for the windward pressure?

Edit: you replied to DTG as I was posting with some reasons to avoid that approach

Open Source Structural Applications:

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

 

[Celt83]

A track + nested stud ledger will get you some more flexibility on the depth.

Open Source Structural Applications:

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

 

[BridgeSmith]

"Is the wedge anchor preference just an economy thing?"

There is that, but also, there have been failures of epoxy anchors due to creep under sustained tension, the most famous being some of the ceiling panels that came crashing down in Boston's "Big Dig" project several years ago.

 

[BridgeSmith]

Apparently, I glanced too quickly through the sketches and picture in the OP, and didn't realize there was a support surface below the slab. Knowing that, I like Celt83's second detail the best.

A possible improvement on it if the length of this repair is limited and access permits - use top and bottom pieces for the ledger that are slightly trapezoid shaped, and install them as overlapping wedges to provide some vertical precompression. Post-install the anchors after the ledger pieces are tightly wedged between the support surface below and the sill plate above.

 

[bones206]

Depending on the degree of exposure to the slab reinforcement, I think some grouting/skim coat would be prudent simply to ensure long term durability and corrosion protection. Aside from the connection strength considerations.

 

[KootK]

What is your intention with the angle you show on top of the plate, seems installing this would require you to cut the jamb studs and weld them on to get resistance for the windward pressure?

That is a whole other ball of wax. As I mentioned, the jamb studs are getting reinforced with a tiny, hot rolled channel. So I'll ultimately need to reach over from the angle bracket things to get that channel. Complexities include:

1) Tolerances in both horizontal directions.
2) Need to deal with top of slab and bottom of slab connections simultaneously at many location.
3) Consideration of vertical slip at top which is made extra awkward by the fact that there is no allowance for that now.

I left that bit out because a) hoping I can handle it myself and b) didn't want to make a long post even longer.

A track + nested stud ledger will get you some more flexibility on the depth.

Better. Thanks for the sketch. My concern is still the range of possible stud cantilevers that I need to deal with. Even as you've shown it, I imagine that any cantilever less than about 1-1/2" is going to require another solution. The nature of this is that there are a few towers with, presumably, this problem all over. It'll onerous for everybody if we're having to come up with a custom solution every other week for the next six months. Although, perhaps I could manage with two solutions: yours + small can't relying on back edge PAF. I'll look into it.

 

[KootK]

here is that, but also, there have been failures of epoxy anchors due to creep under sustained tension, the most famous being some of the ceiling panels that came crashing down in Boston's "Big Dig" project several years ago.

I don't consider this a sustained tension case really. Granted, there's always some wind. I'd expect the connection to spend 95% of it's life at load levels too low to worry about creep though. I'd have to investigate a little more but I had some concern for wedge anchors possibly putting wedge stresses into the patch material. That may well be avoidable with smart geometric detailing though.

Another concern that I have here is the projection of the bolt into the envelope. You'll have a plate and nut which are okay but what about the bolt projection? How short can one rely on that being without having to ask the contractor to cut them off? I'm not sure how big of a deal cutting them off would be. For this reason, I've been given some thought to using an internally threaded epoxy anchor. That, to hopefully facilitate a flush nut condition.

Apparently, I glanced too quickly through the sketches and picture in the OP, and didn't realize there was a support surface below the slab. Knowing that, I like Celt83's second detail the best

.

Noted. Those aren't bearing studs though right? Or, at least, they're not supposed to be. So technically no support in the vertical sense.

A possible improvement...

I might need a sketch to understand that. It sounds like a lot of parts on pieces for cant dimensions down to 1/2".

Depending on the degree of exposure to the slab reinforcement, I think some grouting/skim coat would be prudent simply to ensure long term durability and corrosion protection. Aside from the connection strength considerations.

Yeah, that becomes a tricky ethical question. Everything is inside the envelope and has been doing fine for forty years it seems. And my scope has technically been limited to just the work around these large wall penetrations. It has crossed my mind that, with a possible track-slab overlap as low as 1.625", there may be wall segments elsewhere in the structure that may not be fastened robustly even if demand is much lower. It would be pretty terrifying to be on the 30th floor on a windy day and have your living room wall tear off into the ether while you're watching football. Although, if you're a Packers fan, it's hard to imagine things getting much more dire than they already are.

 

[CANPRO]

What's the access/framing look like below? Easiest solution I see is abandon the anchor closest (or past) the slab edge, and through-bolt the inner fastener to the underside of the slab. If you have another jamb lining up below with the same connection, your through-bolt does double duty (might need generous slots to make that work), if not, provide a washer plate on the underside of the slab.

 

[KootK]

What's the access/framing look like below?

Same as above.

Easiest solution I see is abandon the anchor closest (or past) the slab edge, and through-bolt the inner fastener to the underside of the slab.

Your overlap between the track and the slab is as little as 1.625" in spots. I don't see getting a through bolt in there and not having edge distance problems with the concrete. And possibly the track. Additionally, considering that the holes would need to be drilled near the interior face of a 3.625" stud cavity, I could see it being tough to get in there with a drill to get it done, particularly working at height.

 

[SwinnyGG]

Interesting. With options two and three, I'd been considering pre-load as a way to at least keep the grout in place. I'd not made the leap to then switching back to the original, top side fasteners under that scenario. Under that condition, I take it that you would be comfortable considering the outer fasteners as being as good as they would have been in monolithically cast concrete correctly placed? Specifying the preload strikes me as a bit tricky. Without meaningful stretch length etc, I doubt that I could reasonably expect much more than "torque wrench tight"

Yes, my idea would be to go back to the original fasteners. You'd be basically filling that cavity, assuming that your jamb stud screws are transferring all their load into the new concrete. Any shear load at the interface between new/old concrete is aided by the rough interface and the confinement provided by the plate/anchor arrangement; any tension load at that interface is handled by the anchors, which should have enough preload to handle it. 1500lb is a very light load for a group of anchors even if they are relatively small; I would think you could hit that reliably, with a LOT of excess margin, without having to do any crazy field work.

Your face looks relatively rough already, and it would be easy to roughen further to get some increased engagement between old and new concrete if you feel it necessary.. The benefit I see with this solution is that it doesn't have to change at all based on the depth of the cantilever. You could even size the anchors for the longest possible cantilever, and just wind up with more anchor depth for any smaller cantilever; this means the guys doing the word can do the exact same thing every time, without having to worry about correctly placing different part numbers.

If you go to wedge anchors instead of adhesive anchors, your installation gets easier. You would form and place concrete, remove your formwork and drill for wedge anchors, install your backing plate and wedge anchors, preload your anchors, then install your jamb screws.

 

[bones206]

I imagine this idea could be rejected for a multitude of reasons, but hey it's an idea...

You could install a new wall on the interior and sister it to the existing wall framing. You can rest easy knowing the anchorage of the new wall will be dependable and the edge can be left alone as it has been for 40 years. Most, if not all of the work can be done from the interior and it would be standard, fast, relatively cheap construction. The tenants would lose that 4" of space though.

 

[Celt83]

If this is just for the horizontal forces is something like these a non-starter:
Top of Slab:

https://www.steelnetwork.com/Product/VertiClipSLF

Soffit of Slab:

https://www.steelnetwork.com/Product/VertiClipSLS

Open Source Structural Applications:

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