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Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
I have some high load interior shear walls I need to attach to a proposed PT slab on grade foundation. (550 PLF) Finishing concrete around placed anchor bolts on the interior of the slab will be a nightmare. I do not want anything nailed or shot into PT deeper than 1/2 in, which is basically nothing.

Can I use construction adhesive? I have looked and looked for data or testing on this scenario, and I can't find it. Does anyone have any references to testing data on this? ICC-ES hopefully? Thanks in advance.

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

No chance you can depend on glue for that purpose. Why are you against post-installed (drilled in) anchors? You just need to locate and miss the strands, and that should be simple enough.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
This is a long, skinny, four story wood structure with a lot of interior shear walls every 35'. Slab strands at 3-4' on center each direction, drilled anchors would be a nightmare. Most other similar plans I've seen use friction and PAF that penetrate concrete by 1.5". NO WAY I can specify that, without xraying the slab everywhere, and those are not very strong.

Why do you say "no chance"? We rely on epoxies for long term concrete bonds. We rely on glue in glue-laminated beams for long term bonds. Have you seen a code limitation for construction adhesive? Is there data that shows long-term delamination or relaxation? There is another argument for the glue increasing the friction coefficient.
-
I see 28-day shear strength of Liquid Nails multi-purpose wood adhesive listed at over 450 psi. Even trowel-applied polyurethane wood flooring adhesive lists shear strength at 150 psi. There is practical strength there. Anyone that has removed a glued bottom plate knows the wood shears, not the glue or the bond. Is there a code limitation that prevents using that strength? I'm not trying to be combative. Someone must have previously studied this one way or another. I'm looking for references. Thanks for responding.

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

I suppose I should have said "No chance I would depend on glue...", and now that I know it is a 4 storey structure, I am even more sure, notwithstanding what any code says. You may get different answers from others.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
Has anyone seen glued wood-to-concrete connections fail long-term? If so, do you have thoughts on why?

More stories means a higher the sustained dead load for friction resistance. The math says that 60% of the sustained dead load resists the base shear in friction, if you believe 0.6 friction coefficient. Would you also neglect friction resistance and size drilled anchor bolts alone for the base shear?

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

If you have problems with drilling in the sill plate anchors that are relatively flexible in location, you must be mortified at the thought of drilling in the holddowns with their inflexible locations. Can't glue those sucker.

These details should have been specified in the plans prior to construction, not after the slab is in place.

Mike McCann, PE, SE (WA)


RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

TopKnot:
The way I see it.... The codes don’t say you can’t use adhesives for an application like this, but they don’t explicitly say you can either. It’s for you to prove the sufficiency of what you want to do, and then acceptance is up to the local AHJ, who may not know adhesive from shinola. And, he has no incentive to stick his neck out, even though never held responsible as you could be. There are plenty of adhesives which seem to perform pretty darn well, but their makers and suppliers won’t guarantee them for 50 years, and we don’t know what various environmental situations and various material interactions might do to the bond joint over that time. So, the code writing people say it is an unproven system, and may not be able to be relied upon, long term. It should be an ideal system for distributing that shear load in a uniform, low unit stress fashion; but less certainty in tension visa-a-vi a concrete slab. That’s an ideal stress distrib. for that application. It shouldn’t creep because the loading is only occasional, but it might age poorly and embrittle.

GlueLams were developed at a time when code scrutiny was much less stringent. They had the backing of major players in the wood industry. I’ve been designing with them since the mid 60's and we had plenty of design info. at that time, and very few problems with them. They are manufactured in a controlled environment, by a trained crew of workers, doing the same thing over and over, with the same materials. They certainly went through plenty of early tribulations in the way of glue formulations, etc., and consistency of product, susceptibility to moisture, etc. Anchor bolts (A.Bs.) are pretty well tested and proven, but they are somewhat susceptible to creep, very much dependant on the quality and care taken during installation, etc. We have certainly had some real disasters with their misuse and poor installation or selection of the proper epoxy. Again, they have had strong developmental backing from some major players. You’ll have to get some adhesive makers interested in backing this use of their product. So there is someone who will supporting long term testing and research to prove the system. MikeMc is right, these walls, their A.Bs. vs. the PT’ing. should have been coordinated on the plans and specs. Do you have enough slab depth for A.B. embedment?

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
It's under design. Finishing a slab around 700 placed anchor bolts in the interior is tough. Also tough to get them in the right place on 20,000 SF pours. X-raying a 44,000 SF slab for 700 drilled anchors is a pain. And the likelihood that an anchor is hit is too high for me to consider. I'd like to use some strength from adhesive and friction to reduce the bolt placement. I'm not advocating using adhesive alone.

Does anyone use sustained dead load to account for friction resistance under the bottom plate? I have enough load to take the shear in friction alone.

Maybe the best thing is to lose the PT and drill anchors after. That increase cost by $1.20-$1.40/SF. What have you seen done elsewhere?

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

What is so hard about locating tendons? You know where the ends are, and they should be in straight lines. If you can't control that, you have no chance of controlling the quality of field applied glue.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

TopKnot,

I have easily been able to relatively accurately locate post tension cables in up to 8" thick slabs using a covermeter (rebar locator). Over the years I have located and specified work around 1000's of post tension tendons. I never used X-ray for 'large' situations, it was too impractical and not cost effective for widespread location. With tendons located 3' to 4' apart, and given a reasonable tolerance around the locations where tendons were identified, there should be ample room to drill and install anchors. The Contractor needs full time supervision by technical personnel who understand tendon layouts.

Pretty much any product adhered to concrete eventually debonds. Toppings, waterproofing membranes, coatings, etc... I could see using an adhesive in conjunction with post installed anchors as a 'belt and suspenders' type situation. Also, perhaps some limited percentage of adhesive use could be justified to bridge areas where tendon locations prevent the installation of the specified anchors.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
Thanks for the responses. Tendons are not hard to locate. Just tedious with 1800 interior anchor bolts. That is a lot of tendons to miss when drilling past. I may end up coming full circle and shooting down plates with PAF. Those will be less likely to damage tendons. Contractor can choose to locate tendons or rely the concrete guy got the tendon deep enough.

No one commented on my question about using a portion of the available friction from sustained dead loads. 0.6*0.6*DL of sustained dead load seems like a real resistive force that would reduce required mechanical attachment. Has anyone used this? In a four story building with 3/4" gypcrete on all floors and perpendicular joists, that is a lot of available load for friction. If anything, the adhesive will help improve the coefficient of friction.

I see stories about 7,8 & 9-story wood framed structures. I imagine they are not relying on shear anchorage alone to resist base shear. But maybe those all have concrete stair/elevator cores?

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

OK...

Friction, to me is very unreliable in this case, particularly when the load to generate it can be very problematical and in a high seismic event with a lot of vertical motion, as well as horizontal. During the vertical motion downswing, the gravity forces that would generate the friction would be much less to zero to resist the lateral forces. I say no. If you want to provide the glue as an unquantifiable extra, OK, but do the primary with bolts.

Mike McCann, PE, SE (WA)


RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
Lateral is not controlled by seismic. Wind only.

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
Do you ever consider vertical movement for seismic design? If horizontal accelerations are 0.3g-0.6g, vertical movement is 1/3rd of that. Which is why vertical movement is generally ignored. It is not enough to impact a structure's vertical design. But I am no seismic expert. It's an interesting consideration though.

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

Different fault types generate different loads. Lateral slip produces lateral forces, and strike/slip faults produce uplift as well as lateral. Just depends where you are and what you have.

Mike McCann, PE, SE (WA)


RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

Back to locating the tendons...the simplest way should be to mark the tendon straight lines on the slab surface. Then just miss the lines when you drill. No need to x-ray or use a covermeter.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

hokie66, for a suspended slab, marking the forms at the tendon locations before concrete placement works well, but the OP has a PT slab on grade. Form marks nor x-ray will work to pre-locate the tendons in a SOG.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

Maybe you can explain that for me. Aren't the tendons straight?

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

(OP)
Some tendons will have minor deflections to work around conflicts, slab depressions, etc. But the slab is huge. There would likely be too much variance in location to risk marking a tendon from the end. They are criss-crossing everywhere. 2'-10" on center each way.

Everyone has different opinions, but in the absence of publish data or unknown code issues, my opinion is as follows:
1. Drilling 1800 anchors 2.75" into a PT slab on grade is way too much risk. I would never require that in good conscience. I'd rather make the concrete guy finish around placed anchor bolts.
2. I prefer the PAF attachment, with tendon location by a covermeter. The fastener will theoretically not hit the tendon even if placed directly above it. 1/4" is too close for comfort, but there's a lot less risk in that than drilling an anchor past a live tendon.
3. The only reason given to ignore friction effects from sustained dead loads is seismic, and this is not a seismic area. Even if it was, the vertical effects of gravity will act to reduce the sustained column dead load by 10-30%. Which is still within the 0.6 of sustained dead load. If the ground dropped away from the plate, you had better keep your column unity check under 0.4, because your static gravity loads just went dynamic. I wouldn't want to rely on friction for more than 50% of the base shear. But using friction effects to reduce attachment makes sense to me. I was hoping someone could point me to a published article addressing this one way or another.
4. Construction adhesive should only be considered as a belt and suspenders approach. If anything, it ensures that you could at least obtain a friction resistance of 0.6*0.6*Sustained DL.

For whatever it is worth, I'm going RC for this one. The cost benefit is not worth PT in this case. But I could see other squishier sites having a big savings with PT. I will check back to this over time to see if any other experienced PT slab on grade guys can comment on the issue. Thanks for the responses.

_________________________
TKE

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

You are probably choosing the right course to use conventional reinforcement. You would also largely avoid the issue of drilling if you used bonded PT, but I suppose you are in the US.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

Just trying to think a bit outside the box. If you were going to continue with PT, would there be any merit to increasing the concrete cover by 1" or 1-1/2" (or more if necessary) to give additional cover above the tendons for the post installed anchors. There would be additional cost for the concrete, however, no covermeter work, no finishing around pre-placed anchors, etc...

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

It would still not cover the shear wall hold downs, only the intermediate anchor bolts.

Mike McCann, PE, SE (WA)


RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

I realize that the thread is some what old but I thought i might point you to AISC Design guide 01 "Column Base Plates" (I know its steel and we are talking wood) within this design guide AISC discuses friction resistance in resisting shear forces. I believe that they also site some reference articles that have provided a friction coefficient (steel on concrete) therefore allowing you to calculate the 'conservative' available capacity due to friction. All that being said, I am envisioning you slathering glue to the bottom plate of all of your wood shear walls and I do not understand why you are not concerned about the T & C forces and how to mount your tie downs. Are all your walls in net tension? If they are, I would look at FRP (FIFE web site is good example) to glue down anchors to the concrete.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

Pour a stem wall with vertical dowels and embed into that if the slab is proposed and not poured.Them you finish up to a surface.

Inspector Jeff

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

For what its worth, TopKnot, I always use the allowed 0.6 DL on a shear wall to help with my hold down forces. As far as friction for shear anchors, I have not done that. Honestly, I'm not sure if the code speaks to whether or not that is allowed. It seems reasonable (only because you are asking it to be supplemental to require fewer not zero bolts), but I would think your concrete under the sill plate would need to be roughened to provide a predictable coefficient of friction. However, shallow PAF's are probably the quickest route to achieve your shear. I'm not a huge fan of them knowing how they tend to break out the concrete. However, if you have significant dead loads, they could basically be viewed as creating that friction you are wanting even if they do cause break out.

I have looked into using structural adhesive several times and have yet to find a single source with published load values I can use. There are simply too many field variables to make it predictable. Glulams only work because of the tight shop control and regulation. Maybe talk to the FRP (carbon fiber) folks about their epoxy. They rely on it for a predictable adhesion of their FRP to concrete, but they require the concrete to be ground prior to install. HJ3 is a good resource company.

I'm sure you are past all this already, but good luck.

RE: Attaching Interior Shear Wall Bottom Plate to Post-Tensioned Slab on Grade

You should be able to specify that the tendons need to be moved away from the walls. Temp steel can be located easily and marked on the slab or on drawings during construction. P-t is pretty forgiving. You can also cast sleeves through the slabs at the hold downs for through bolts and other high loads.

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