PEMB Thrust - Shear Breakout 1

[bearjew]

Hi all,

Let's say we have a PEMB structure with large kickout forces at the base of each rigid frame. These rigid frames are supported on column pedestals and isolated footings (non-moment resisting). If this is the case, it seems we have (2) ways to resist the kickout forces, either hairpins into the slab or tension beams that tie together each end of the rigid frame. In either situation, it is very difficult to get the column pedestal to check OK with regards to shear breakout following Appendix D of ACI.

I understand that many people on this forum have differing opininons on the effectiveness of hairpins, but for the sake of argument lets assume we went with the tension beams. If we develop our tension beam reinforcing fully into the pedestal and 'hook' the pedestals vertical dowels, is shear breakout still a controlling failure mechanism? The way I am imagining this situation is that the thrust gets transferred thru the PEMB anchor bolts as shear into the vertical dowels. The vertical dowels act together due to the ties spaced vertically. If our tension beam is then developed into the pedestal, wrapping the pedestal cage, no shear breakout?

 

[KootK]

If we develop our tension beam reinforcing fully into the pedestal and 'hook' the pedestals vertical dowels, is shear breakout still a controlling failure mechanism?

Depends. If you're using the conventional breakout provisions of appendix D, then what you'd really need to do is develop the tension force required into the presumed breakout cone, not just into the pier.

The way I am imagining this situation is that the thrust gets transferred thru the PEMB anchor bolts as shear into the vertical dowels. The vertical dowels act together due to the ties spaced vertically.

This sounds quite a lot like the strut and tie method proposed by Widianto: Link. See figure four and the associated text.

The vertical dowels act together due to the ties spaced vertically. If our tension beam is then developed into the pedestal, wrapping the pedestal cage, no shear breakout?

I suspect that this will probably be the case. It's just demonstrating it that's a hassle. If you strut and tie your way from the anchor bolts to the beam tension reinforcing, dotting all your eyes and crossing all your tees as you go, I don't see how anyone could argue with that.

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.

 

[KootK]

If your firm does a lot of this kind of thing, you probably have Alexander Newman's book that deals with exactly this issue in a very practical and up to date fashion. I highly recommend it.

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.

 

[dik]

Can you use a plate cast into the concrete with holes for the anchor rods, flush with the base plate. The plate would have a couple of weldable rebar anchors welded to it bent into the slab mid depth so that the slab reinforcing can take care of the tension. If the tension is too great, then actual bars can be added to the slab to provide a tie across. Completely negates ACI requirements for Appendix D.

Dik

 

[KootK]

Another thing to keep in mind is that, with a tension tie, I do not believe that ACI allows lap splicing. Gotta go mechanical or welds.

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.

 

[T_Bat]

Good point KootK. Another item to keep in mind is axial elongation of your reinforcing (elong. = (PL)/(AE)) and it's potential effect on your frame. We had job where we used a tie beam to help with thrust and the elongation numbers were interesting to say the least. We increased our reinforcing to limit the behavior.

 

[bearjew]

Depends. If you're using the conventional breakout provisions of appendix D, then what you'd really need to do is develop the tension force required into the presumed breakout cone, not just into the pier.

If mechanical splices are used (per code since it is a tension beam, as others noted) then wouldn't this be as simple as providing a mech. splice in the breakout cone, that is between your tension beam steel and bars that wrap around and engage the pedestal cage? Or would this splice have to wrap and engage the embedded anchors themselves?

Dik,

I like your suggestion, however to me it seems this would involve many of the same problems that hairpins face, namely clients wanting to cut the slab in the future without consulting an engineer.

 

[KootK]

I'm not sure that either of your proposals will improve matters significantly bearjew. Might be time to post some sketches of what you've got in mind.

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.

 

[dik]

Bearjaw... reminds me of the old adage, "Evolution fails when stupidity is no longer fatal."

KootK:I wasn't aware that ACI had restrictions on tension tiea not spliceable. Will check into this. Usually make these long enough to develop the slab reinforcing.

Dik

 

[structSU10]

I'm going off on a tangent here, so I apologize to the OP...

to T bats point, I don't think the axial elongation issue disappears when you have hairpins either, its just harder to figure what it may be.

I was always curious about it, as it will affect the moment in the frame if you allow for a horizontal displacement at the bottom (I believe higher negative moments). I have used the same concept, however, with increasing reinforcing for the tie beam to help limit the elongation, I think I tried to size the rebar to when my moment would only be 3-5% greater in the frame.

 

[bearjew]

I'm not sure that either of your proposals will improve matters significantly bearjew. Might be time to post some sketches of what you've got in mind.

I thought more on your original comment about developing tension steel into the presumed breakout cone, and my repsonse wasn't exactly appropriate. I understand now, thank you.

In reponse to the others commenting on tie beam elongation, what is an acceptable amount to limit this to? I suppose it will depend on the clear span of the rigid frame you are tieing together. I have Newman's book, and for a clear span of 150' he uses an elongation limit of 0.5", which seems pretty conservative to me.

 

[T_Bat]

In reponse to the others commenting on tie beam elongation, what is an acceptable amount to limit this to? I suppose it will depend on the clear span of the rigid frame you are tieing together. I have Newman's book, and for a clear span of 150' he uses an elongation limit of 0.5", which seems pretty conservative to me.

I'll be honest, our condition was actually a retrofit for some under-designed frames and foundations. We were able to play with the elongation to limit their effect within the tolerances we needed (and we were being paid to do so). Often, there isn't much room in the fee to piddle around with springs and deflections.

Having said that, I would agree that the limit proposed by Newman does sound conservative. Newman seems to base this limit off of experience so I would be inclined to take a look at the difference in steel area this relates to. It may not be huge difference in the number of bars. Also consider that the total elongation corresponds to a deflection of half the elongation at each end of the frame. You could use this to justify a larger limit based on typical deflection limits for steel frames.

I would be interested in hearing another engineers thoughts on this.

 

[KootK]

KootK:I wasn't aware that ACI had restrictions on tension tiea not spliceable. Will check into this. Usually make these long enough to develop the slab reinforcing.

Not sure if the limit still applies when the reinforcing is in SOG rather than a discrete member. Quite similar logically but, obviously, nobody's doing that with SOG reinforcing. At least not that I've seen.

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.

 

[dik]

Sorry KootK, I've been using the SOG reinforcing for decades... No extraordinary cracking or ill effects, yet. just careful that you don't saw cut it. Also minimise strain, and likely why it works...

Dik

 

[KootK]

No need to be sorry on my account. I've been doing the same. From a reliability perspective, I do see a difference between using laps in a tie beam at 2% reinf and an SOG at 0.1% reinforcing.

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.