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.
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 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
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
#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
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
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
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
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
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
RE: Anchorage into a mat slab foundation
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
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 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.
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
RE: Anchorage into a mat slab foundation
RE: Anchorage into a mat slab foundation
RE: Anchorage into a mat slab foundation
RE: Anchorage into a mat slab foundation
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
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
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
RE: Anchorage into a mat slab foundation
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
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
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
@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 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
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
RE: Anchorage into a mat slab foundation
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
RE: Anchorage into a mat slab foundation
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
RE: Anchorage into a mat slab foundation
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
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
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 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
http://www.ancon.co.uk/downloads/s1/l1/ksn%20ancho...
RE: Anchorage into a mat slab foundation
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
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
RE: Anchorage into a mat slab foundation
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.