ICF Wall - footing to wall rebar schedule

[Mellimac]

Hi All - Newb - just building myself my own little bunker (20'x36'), as it were. Consulted several ICF manuals regarding the placement of the vertical rebar in footings (9"x22" w/ 3L of #5). With an above ground 6"/8" wide ICF wall, vertical rebar is centred. Makes sense (no static load on either side). Usually, they recommend at least 8" of vertical (wall) rebar into footings. In my case, I'll be doing a 90 degree bend at bottom of vertical rebar. This way, I tie into 2 longitudinal footing rebars (3 in total, 3" off floor). I'll also have a (2x4) keyway for cold joint.
It perplexes me as several ICF manufacturers have rebar sitting on longitudinal rebar. In a seismic event, wouldn't one get better results if vertical 'tail' of rebar was curled underneath footing rebars? I have plenty of room insofar as to maintaining a healthy floor clearance (2-1/2"/62mm min).
I'll be alternating sides of vertical tail @ 16" oc.

I'm trying to visual the seismic strain on rebar and concrete. If Vrebar was underneath footing rebar, I'd have to think there would be less chance of cold joint cracking or worse, wall becoming detached from footing.

Your thoughts appreciated. Attached a pic for visual reference. Vrebar in pic has too short of tail...not attached.

 

[Mellimac]

I see I got blown away with the trash....
After a few hours of reading footing to stem wall theory, strut and tie modeling, studies of hooks in concrete, lapping etc., I feel I can proceed. Have to thank the folks who contributed to this thread:

http://www.eng-tips.com/viewthread.cfm?qid=401855

Kudos to KootK for hammering home his view...kept the thread alive and then some.
Seriously, thanks! I may be deficient, but I know a good read when I see one...

Figure 13-28 resonates with me (minimizing or clamping the crack (plastic hinge) with a diagonal/strut/incline reinforcement). What is that called?!
I really got the sense ICF (Insulated concrete forms) manuals were not terribly concerned with how they were 'attached' to footings (an afterthought - "min 8" vertical dowel into footing" /end of story). A little disconcerting since some engineer signed off on them (I called a manufacturer to inquire - he sent me technical drawings with sig - supposedly, to save me the trouble).

The problem with skinny walls such as a 6" ICF wall, is one cannot really pack it with sufficient rebar to mitigate tension/shear at the joint. Albeit, the forms themselves, are in essence, a collection of plastic stirrups. The wall is not the issue.
The joint, is the issue. I alternate hook direction every 16" c/c, and add opposing diagonal to keep hinge in check or I go with double hooks (both directions), and throw in a diagonal strut in footing for good measure (or not).

BTW - I will also be strapping the interior wall with full length horizontal 2x4 @ 3' intervals (anchored into wall). Plus an interior 2x4 standard framed 'dummy wall' for services (well attached to strapping). A tad old school method to dampen and contain wall (inspiration - Nepal). Although my walls are not very long, a seismic event will likely induce resonance, and the wood will dampen that. Not sure if anyone cares, but some of Nepal's older buildings fared better than the newer ones in last big earthquake because they had timber moment frames embedded in them. And we are talking about brick buildings!

 

[KootK]

Kudos to you if you actually managed to read all of that other thread. And thank you very much for the kind words.

I do, however, disagree with your approach here. You are correct that these joints, as typically detailed, would be poor joints for transferring bending moments from the wall into the footing. Most engineers will design these joints as though they are pinned, however, so the conventional detailing really is fine.

You get full marks for studiousness, that's for sure.



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.

 

[Mellimac]

Most engineers will design these joints as though they are pinned, however, so the conventional detailing really is fine.

Are you saying because the wall is 'weak', pinning is sufficient?
Maybe I am misreading what you said...

My concern lies at that joint. As far as I am concerned, it is the weakest link in the whole building, yet it has to be the stiffest (never will be, but it never hurts to try). I'm in a high seismic region 0.70 < Sa(0.2) < 1.0 (or another way on a scale of 1-6, with 6 being highest, I'm in Zone 4, bordering zone 5). Personally, I am on a solid bedrock extrusion, on a hill, so I am not terribly concerned (footings are fixed for the sake of discussion).
There are two ways to approach seismic considerations, build it like a rock, or be flexible, like many wood framed exterior shear wall buildings are (with good anchoring). Since, I am dealing with concrete, I want to avoid the flexibility, especially the footing to wall shear zone. The moment-arm load on that zone will be huge in an earthquake. Yet, I need it to be like a tree trunk...pinning will not suffice.


BTW - that was an amazing thread. You guys jumped around a bit, but hey, it is all good. I know we look for analogues in everything...

 

[TehMightyEngineer]

Also gonna give you some props for reading that. It's quite a read but has a lot of great info.

Are you saying because the wall is 'weak', pinning is sufficient?

What KootK is saying (at least my interpretation) is that such wall to footing joints are rarely detailed to get the fully efficient/idealized flexural strength of the joint. But, because the wall is not cantilevered most designers idealize it as a pin connection and thus conservatively any additional flexural strength at the joint is a bonus.

For seismic, without fully knowing your design, I would say that there's nothing wrong with the joint being flexible as long as your building has ways to dissipate the seismic forces elsewhere and the building has the deformation compatibility for that joint rotation, then it should be fine. Remember that while concrete is not as flexible as steel it's all about relative flexibility. Regardless though, if you can achieve a rigid joint then I suspect you're correct that that would be the easiest way to go from a design standpoint.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Mellimac]

TME - I see. Thanks for interpretation.
Yep, a 20x36 rectangular ICF building with 10' walls is relatively small and both horizontal x-y axis will have strong moment frames to resist rotation.

I look at the end results of seismic events around the world, for instance Northridge/Chile...lots of compression/shear failures, but also lack of strong lateral moment resisting frames, which really contributed to the former. IIRC - in Chile they actually went overboard on rebar in some cases, resulting in poor compression. So, they retrofitted in strong lateral moment frames. If you get no or limited rotation at footing-wall connection, your all good for the most part. As I'm sure your aware, concrete can take compression all day long (within reason), provided you take care of the lateral axis. New building codes in earthquake prone areas reflect that (IBC - braced wall line is what they call it).

Realistically, I won't be able to throw in a foundation damping and isolation system like LA's City Hall or throw a water based damping system on roof, so I look at stiffening the footing-wall joint. ICF's prescription for openings, it to add some peripheral rebar to limit shear cracking. I've also limited openings, which is the best solution (although, everybody wants huge windows - esthetics trumps silly seismic issues).

I love analogues, and a tree is a perfect analogue for what I am trying to do. The roots and trunk are analogues to footing and base of wall, however, the connection between the two in concrete buildings is an issue. You, KootK and others did a wonderful job exploring this problem (heck, that thread should be collated, written up and submitted as a discussion paper on the subject - you know when your googling for info, and you never get exactly what you need...in this case, I felt like I hit the jackpot).

All I'm trying to do, within reason, is stiffen that joint without compromising compression or shear. Dissipation will take place higher up (roof mostly, as it will be wood framed). I believe the ideal seismic dissipation should be linear as one goes up (tall buildings typically don't behave that way - more of a wave function). If I get too much flexibility at joint, it will dissipate seismic forces, but then I'm looking at a rebuild. Even a small seismic event will cause issues, as noted in your discussion (cracking at footing-wall joint is a recipe for water intrusion/corrosion). Plus, one never gets a clean break, and things will get messy (imagine some of the wall rotating, and portions of wall still fixed).