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Anchorage into a mat slab foundation
7

Anchorage into a mat slab foundation

Anchorage into a mat slab foundation

(OP)
This isn't related to work that I do directly. But, rather a technical issue that I was helping someone else with. And, I realized I didn't have a great solution. I'm curious what I'm missing or what other options are available.

They had a steel SMF frame coming down directly onto a reasonably thick mat slab foundation (36~48"). Some relatively shallow (4") pockets so that the slab surface would be undisturbed by the base plates or stiffeners or such. The pockets aren't particular important except they lead to question #1:

Question #1: Does the presence of these pockets reduce the pullout cone strength of the rods. My thought was no. Provided the pullout cone extends beyond the width of the pockets.

Now, because it was a moment connection there were huge tension forces that needed to get developed from the anchor rods into the concrete slab. Embedment depth of the headed anchors wasn't going to be enough. So, the project would need some kind of supplemental reinforcement to resist the anchorage forces.

Question #2: What type of supplemental reinforcement to add?

There is plenty of horizontal bars in the slab that would intersect the pullout cone. But, I don't know whether those bars can be considered to resist it or not. Maybe a stud rail like you would use to resist a punching shear failure?

I am a number of years removed from true design engineering. But, even so, I am disappointed that I didn't fully grasp the solution to what should be a pretty common engineering issue these days.

RE: Anchorage into a mat slab foundation

On question 1, I agree with you that as long as the pocket does not intercept the plane of the failure cone, it should not reduce the pullout capacity (but it may reduce the capacity of other failure modes). That said, even if it did intercept the failure cone and did not pass, I would want to re-think the connection because if that was the straw that broke the camel's back, the design was probably barely passing anyway.

On question 2, you can add supplemental steel as long as it is developed on both sides of the failure plane. That can be tricky because of limited space for bar development. Also, that steel should not serve double duty unless its designed to do so (i.e. flexural steel in the mat and supplemental steel to resist pullout)

RE: Anchorage into a mat slab foundation

For headed anchors, I also agree with you. I think epoxy anchors can also fail without creating a cone if those were being used.

I use the concept in this paper often for the second question:

http://files.engineering.com/download.aspx?folder=...

RE: Anchorage into a mat slab foundation

#1) I'm fairly certain that it would make little difference. That said, I would tend towards conservatism here. If you envision the situation in STM terms, it becomes a bit disconcerting to be counting on a bunch of concrete placed above your top steel. On the other hand, if the pocket is very small relative to your projected failure cone, then chances are that most of the useful top steel will be located at the "normal cover" elevation rather than the "below the pocket" elevation.

#2) If extending the anchor bolts to the bottom of the footing wasn't getting the job done, I'd do one or all of the following:

a) Thicken the footing locally below the anchors to get a larger failure frustum.

b) Install an embedded steel plate bolted to the anchors at the bottom of the footing such that I could call the tension failure mode upside down punching shear.

c) If upside down punching shear doesn't work, add "candy cane" rebar ties in a uniform pattern about the anchors extending out until punching shear does work (just like a column terminating at a transfer slab).



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

(OP)
So, let's say you add some supplemental reinforcement to the top slab above and beyond what is required in that slab. My thought is that the angle the top slab reinforcement makes with pullout cone would be too steep to really transfer the tension into the bar.

Therefore, we'd have to do one of the following:
1) Bend the bars down at approximately 45 degrees (truss bars) at approximately where they would intersect the pullout cone. Then lap them with bottom bars.

2) Add in some vertical shear ties in that region like there is a grade beam contained within that slab. The vertical nature of the ties will intersect the pullout cone and transfer that tension into this pseudo grade beam.

3) Same concept as #2, but using some type of stud rail system. Thinking that the concept of punching shear and this type of pull out cone are basically the same.

RE: Anchorage into a mat slab foundation

(OP)
Last post written before KootK's candy cane bar response. My same concern with these candy cane bars as I have for the supplemental bars in the top of the slab. The angle is perpendicular to the original tension force that we're trying to resist. Can we really rely on horizontal bars like this for the supplemental reinforcement.

I hadn't thought of an embed plate at the bottom. Not a bad idea. But, doesn't it seem odd that this would give me so much more capacity than just the anchors. The pullout cone is basically the same dimensions as punching shear would have been. I know it's true though because this slab worked fine for punching shear.

RE: Anchorage into a mat slab foundation

The candy cane bars that I mentioned would be vertical. Basically, the construction friendly slab version of stirrups.

I like the truss bars in concept. I've seen some wood shear wall to slab tie down details published by Washington state that are very similar, just on a smaller scale. My only concern would be coming up with a numerical evaluation method that seemed "official" enough.

I like the mini grade beam idea too. That might in fact be the most constructible solution. Basically shear heads done in concrete.

I would think that the embed plate would produce the same failure cone. And one probably could just use plain headed anchors. I basically just go with the plate to produce a situation that I feel confident about with regard to its similarity to something accepted (punching shear).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

If it's a moment connection aren't you looking at roughly equal tension and compression force, or is there a lot of net uplift?

Plate on the anchor bolts is common for developing the anchors. Shear reinforcing would normally be vertical. Bent bars seem to make more sense but you don't see them often, presumably for constructability reasons. I think that aci 421 recommends additional reduction factor if using rebar instead of headed anchors for shear in mats.

RE: Anchorage into a mat slab foundation

(OP)
It is moment, that's true. There was some net uplift, but it wasn't huge compared to the tension caused by the moment.

I'd always looked at the anchorage requirements on the tension side as being independent of the compression on the other side. Are you suggesting that the embedded plate connected to the anchors would inherently transfer the moment without requiring that we design tension anchorage? Essentially doing what KootK said, and simplifying this down to a punching shear calculation.

If so, that actually makes me feel a lot better. These anchorage calculations seem so complex these days (compared to when I did them 15+ years ago at my previous employer). Relating it back to a a different failure method (punching shear) that I feel much more comfortable with brings me closer to first principles and expected behavior.... Rather than trying to shoe-horn my situation into matching code equations that were derived for single anchors acting independently.

RE: Anchorage into a mat slab foundation

I don't know about the embed plate transferring the moment without considering tension anchorage. I would feel comfortable with the plate (if very rigid) to resist the tension component in the form of punching shear as an alternate to App D calcs. Or develop the force into new rebar.

RE: Anchorage into a mat slab foundation

The common method for anchor bolts is to plate each bolt. This gives, potentially, 4 overlapping failure cones in the concrete....complicated to resist with rebar.

Consider the following....

A full plate that engages the bottom of all bolts, with a large hole in the center to allow continuity of the concrete flowing around and through the plate. This allows a very large failure cone that will have a large pullout resistance because of the larger shear area on the concrete and reduced shear stress as a result.

or....

A very stiff bottom plate of expanded metal that allows a lot of concrete paste continuity and interlock as well as promoting a larger shear failure cone as noted above.

RE: Anchorage into a mat slab foundation

These are those smaller scale details that I mentioned that are eerily similar to your ideas.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

Quote (Book)

If it's a moment connection aren't you looking at roughly equal tension and compression force, or is there a lot of net uplift?

I agree with what I think you're getting at. If the anchors are long enough I think that you get a situation as shown in the sketch below where the lion's share of the tension is countered by the applied compression from the moment couple. The moment largely gets resisted by horizontal force mechanisms in the footing which makes sense as base plate moment must ultimately be converted into footing moment.

Quote (JP)

I'd always looked at the anchorage requirements on the tension side as being independent of the compression on the other side.

I've done a lot of this too mostly just because you don't see much about this alternate path in the literature. From what I've seen most designers look at the tension anchorage independently.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

Correct, that's what I was getting at.

RE: Anchorage into a mat slab foundation

I wouldn't use the compression component method to justify a new anchorage to concrete design. I would stay with what is more conventional.

RE: Anchorage into a mat slab foundation

Hey KootK....great minds think alike!....rofl

RE: Anchorage into a mat slab foundation

With regard to question #2, I have often added vertical rebar, developed on both sides of the failure plane, adjacent to each anchor rod to avoid having to check concrete breakout strength for tension loading. This is explicitly permitted in ACI 318-11 D.5.2.9. A graphic of this condition is shown in Figure RD.5.2.9 in ACI 318-11. I typically use 90-degree or 180-degree hooks on the top and/or bottom if development length is a problem. The center-to-center spacing of the added rebar and the anchor rods must be less than or equal to 50% of the embedment depth (hef) per ACI.

RE: Anchorage into a mat slab foundation

Quote (Hokie)

With regard to question #2, I have often added vertical rebar, developed on both sides of the failure plane, adjacent to each anchor rod to avoid having to check concrete breakout strength for tension loading.

I've two, related concerns with this:

1) My impression is that app D, and the supplementary reinforcement business shown in RD5.2.9 in particular, is intended for small scale stuff. For the implied case of something like a curtain wall anchor, it's reasonable to assume that the supplementary reinforcing might double or triple your breakout capacity and can therefore just be deemed to be sufficient once the supplementary reinforcement has been provided. I think that a tension connection between a primary frame column and a foundation warrants more rigor.

2) With the supplementary reinforcement, all that you're effectively doing is lengthening the anchor bolts. There's still a breakout frustum to be checked, it's just a larger, more difficult to define frustum. Its a bit worse that lengthening the anchor bolts really because your anchorage occurs faster with heads/plates than it does via rebar knurl engagement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

If App D wants to be the way and the truth and the light, I really feel that it needs a version of the breakout mode shown below. I've tinkered with it for sport and the results have turned out ver6y promising numerically. I've yet to implement it in practice, however, because I feel as though it's a stretch to be making this stuff up myself given all of the research effort it took to get us appD.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

(OP)
I just want to thank everyone who commented. The issue is a lot more clear and rational in my mind now.

In my case, I'd probably go with something akin to KootK's "Sample Detail 2". But, where the bar then laps with the bottom bars for some significant length. Seems like the simplest way to detail it when the mat isn't too congested and you have room to develop those bars.

And, for what it's worth, the discussion of moment and Strut-Tie method really helped as well. Just as a means of bringing the discussion back to first principles. I think that's really where I was having issues with the anchorage calculations. The code requirements are not unclear if you're talking about a single bolt or a group of bolts in pure shear or pure tension. But, moment and anchorage reinforcement (to resist that moment) were the aspects that weren't making sense to me.

Thanks again everyone!



RE: Anchorage into a mat slab foundation

If you are trying to get fixity at the base and they are large moment frames. Embed the column into the mat and provide reinforcing in the slab. AISC Seismic Manual has a sample of both designs, embed is on 4-119.

RE: Anchorage into a mat slab foundation

I've got 10 scratch scratch sheets on this just trying to get my head around it -

I think this arrangement makes more sense at the anchor head due to symmetry. I've also kept the theoretical ties orthogonal although I imagine the ties would be perpendicular to the splitting plane - a direction that makes the STM even more confusing. With this arrangement I can better visualize the moment compatibility through the depth of the footing (moment at the bottom of column = moment at the bottom of the anchorage.


RE: Anchorage into a mat slab foundation

(OP)
Excellent idea Sandman.... I actually thought of that originally. But, I didn't take it very far. The owner / developer didn't like the idea. And, it was unconventional enough that I wasn't going to push for it.

But, now I'm going to read through that example some more to make sure I understand it as well.

RE: Anchorage into a mat slab foundation

In a mat its even easier to do than in grade beams, as you have more space to move bars. It will require less reinforcing than anchor bolts as you are using material that already needs to be there. The additional reinforcing is minor compared to the trying to get App. D to work. The connection will get a little messy close to the edge of the mat.

We only use base plates when we have small OMF's or assuming pinned base. Larger moment frames the base plate is significantly more expensive, 2" 50ksi plate, stiffeners, 10 - 2" 105ksi bolts, to build and install. The add benefit is that you are also closer to a fixed connection and have less drift.

RE: Anchorage into a mat slab foundation

@sandman: usually the objection that I get regarding embedding in a raft is difficulty in accurately setting and plumbing the columns. In that respect, a grade beam spanning over pad footings or piles seems to be much more palatable. How do you deal with this for the raft scenario?

@Teguci: I had to stare at your strut and tie model for a good twenty minutes before I fully grasped the subtlety of what you did there. It's brilliant. Creating a strut and tie model patterned after an Appendix D breakout failure really elucidates things. After mulling it over for a few days, I took the liberty of attempting to refine and improve your model. My thinking was as follows:

1) Your STM suggested a breakout frustum slightly different from the one that I proposed earlier. I adjusted the model to reflect my assumptions regarding the breakout frustum. I believe that it would be inherently asymmetrical about the tension anchor.

2) With #5 or smaller ties wrapped around longitudinal bars, I believe that it's fair to move your point "A" up to the top of the raft. This simplifies the model some.

3) I moved your point "C" up to the top of the raft and adjusted the last strut coming in from the right to hit that same point. This is consistent with my expectations regarding the compression stress trajectories in these areas.

4) I've got a compression field arching from the column/base plate compression zone all the way down to the bottom of the raft. Most of the compression would be absorbed into the tension anchor as the strut passes by.

Also, I finally figured out what you've been doing with that "v" shaped symbol that you draw over your ties that are represented by Vc. It represents SQRT(f'c), right? I'm adopting that from this point forward.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

I have done it a couple ways: provided construction pad footings with base plates to set the columns on below the mat, designed grade beams above the mat foundation and have a base plate sit on the top of the mat foundation.

I have seen others weld on members (Cx) to the sides of the columns they then provide reinforcing to support the column.

RE: Anchorage into a mat slab foundation

Thanks for the clarification sandman. Could you elaborate a bit more on the last option? Two channels welded to the sides of the column and then...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

To set the height of the Wx and check the column plumb they will weld a two or three foot piece of Cx to the flanges going in both directions near the bottom and top or as requried. They then set the Cx longitudinal bars while also tie to shear reinforcing, you can add as many Cx as need to keep the Wx secure. The provide I saw it on use the mat reinforcing and the contractor added some additional reinforcing to help. They only had the first splice at the first floor to limit the weight of the column.

RE: Anchorage into a mat slab foundation

Like this? I must have something wrong as I can't imagine one would leave any of the channels sticking up out of the footing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

That's the basic idea but without the Cx sticking out, they will add a layer of reinforcing, below the top mat reinforcing for the top Cx to sit on. I have seen this in deeper mat foundations, 8'+

RE: Anchorage into a mat slab foundation

Neat. I still find it surprising that one could install the embed that way, pour a bunch of concrete around it, and still be confident that the final position would be within tolerance. This shall be a new tool in my kit. Thanks for walking me through it sandman.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

I make no warranties or guarantees as I have only seen the drawings for it. It makes sense to me and like I said it was on a large mat foundation with several layers of reinforcing for bending and shear, so it had three rows if I remember correctly.

RE: Anchorage into a mat slab foundation

Oh no, I'm doing this now and referencing sandman21 (interweb weirdo) in my specifications.

Honestly, I'm amazed that anyone can actually build anything. In an effort to "keep it real", I joined a local trade school masonry lab once. I built a 4' lintel over two courses of wall pier. It took me a whole afternoon and wasn't even remotely within spec. In fact, had there been a job site supervisor, he or she would have been remiss to not have asked me for blood testing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

Only fair since you are referenced in my calculations. ;)

I just dont know if the contractor made them change the design or what. So your work sounds about right for what I have been seeing lately.

RE: Anchorage into a mat slab foundation

2
Did a little reading up on related issues last night: Link. The benefit of a moment induced compression force on the breakout frustum has been studied. I thought it prudent to share for the sake of others who might be interested.

In summary:

1) Work done by Zhao in 1993. I couldn't get my hands on the original stuff.
2) As expected, for the right combinations of lever arm versus embedment depth, there are marked improvements.
3) There's a proposed evaluation method but it requires determining a parameter (z) based on an elastic analysis. Not sure what to do with that in practice.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

I just can't get enough of this thread for some reason. Some crane base shop drawings crossed my desk this morning with a familiarly looking pullout detail. I'd have to think that such a setup is probably getting taxed quite heavily, and quite frequently, compared to a building's lateral system. Footing depth = 1400 mm. The stirrups that you see are cal lout out as "bar support". I'm not sure if they're intended to pull double duty.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

Australian structural engineering never ceases to amaze... Their modified failure cone is actually more aggressive than the one that I proposed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Anchorage into a mat slab foundation

Quote (KootK)

Australian structural engineering never ceases to amaze.

The Ancon brochure referenced by bookowski is actually from the UK, but Ancon are very active in AU.

Us Aussies are just a country of convicts smile

RE: Anchorage into a mat slab foundation

My bad. Let's reassign those cudos to Scotland then. Nice work Scotland.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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