20' Tall Pin-Pin Foundation Wall

[SarBear]

Fellow Engineers,

A discussion in our office lately has been about tall restrained pin-pin foundation walls. We've had quite a few residential projects come in lately where they have tall rooms underneath garages (15' tall theater room, 20' tall sports court, etc.). The discussion originally started because of a disagreement about how thick the footings for these type of walls should be. We've normally had these footings for super tall restrained walls be 12" thick, but the question of why 10" wouldn't work has been brought up. The loads coming down from the roof above, the weight of the concrete slabs in the garage, and the fnd wall itself are not such that there would be a shear issue with a 10" thick footing.

But what about rebar embedment from the wall above? These walls often have #6 or #7 bars on the tension face. For a #6 bar the ldh for hooked dowels into the footing should be around 12", so I'd think the footing should actually be around 15" thick for the ldh plus 3" cover. But that ldh is for hooked bars in tension. Since this is a pin-pin wall these bars aren't transferring a moment into the footing, correct? The biggest bending moment from soil lateral load will be at the center height of the wall and then will be 0 at the top restraint and the stem/footing junction, so do these bars need to extend into the footing at all? I'd think they should extend into the footing to help with shear loads at the stem/footing interface, but is that necessary?

And on the compression-side bars since those are just for crack control, they wouldn't need those deep ldh embedments since they're not in tension, correct? We recently had a project where we had 5' wide x 12" thick footings for a wall such as this, but the contractor only poured the footings 10" thick instead of 12". We told them that was ok, so I brought up what is the point of 12" thick in the first place if 10" is ok. Thus all the discussion about rebar embedment. I have noticed in RetainPro when you have a pin-pin foundation wall that the rebar on the tension face is only shown at the area of maximum moment plus a little extension above and below. Please see below (ignore dimensions, rebar, etc.). I know this is just a rudimentary diagram provided by EnerCalc but it does seem to imply that the rebar embedment into the footing is not a major concern of the design.

I'd be very interested to hear anyone's opinions on the question of footing thickness, rebar embedment, etc. for a pin-pin design. Thanks!

 

[BAretired]

Personally, I have no issue designing this wall as pin-pin, but if I were to have rebar hooks embedded in a "much wider than normal" footing on the outer face of the wall, I would prefer any "failure" to be through those bars yielding, not through a brittle concrete failure due to too small of an embedment length. Where as, if the foundation is narrow and I thought it likely the foundation itself would rotate before those bars fail, I wouldn't be too concerned about it.

I completely agree with douwerda. In a case such as this, it is important to choose your failure mode; avoid brittle failure. In that regard, an 'L' shaped dowel is difficult to analyze because of its eccentricity and radius of curvature at the bend. A concentric anchorage would be much preferred. In any case, the anchorage must be sufficient to develop the bar in tension so that failure will be ductile if the moment is larger than expected.

BA

 

[SarBear]

Really appreciate your comments, dauwerda.

Likely one of the bigger issues that hasn't been discussed here is the pinned assumption at the top. For a 20ft tall wall, it will take a substantial amount of anchorage and good detailing to ensure that shear can actually be transferred into the floor diaphragm (I'm assuming it's a traditional wood floor diaphragm).

Usually our diaphragm for this situation are 8" thick precast concrete panels in a garage above. The precast panels have a 4" thick topping slab poured on top and we provide hooked bars that go from this topping slab into the wall.

Now say, the footing does provide plenty of moment resistance, causing the wall to try to act like a propped cantilever, however the reinforcing embedment in the footing can't handle this load so it fails. What will happen? The load will redistribute to act like the original pinned connection you designed for. The question you need to ask yourself is after this "failure", will the wall still perform as intended? Will there still be a safe way to transfer the shear? Will this small movement cause any issues to waterproofing?

I guess this is what the very first drawing I posted is attempting. It's sort of fixed and sort of pinned at the base. Rebar on both faces is embedded into the footing which imparts moment to the footing. Here's hopefully an adequate summary of what you said:

1A. The rebar in both faces delivers a moment to the footing. The footing rotates into the soil. The soil is strong enough and we end up with a propped cantilever. If the rebar on the exterior face has been adequately designed for bending and proper anchorage then we're good. If not, then number 1B below happens.

1B. The rebar on the exterior face of the wall fails, causing our propped cantilever to become a pin, which puts bending into the rebar on the interior face of the wall. If the interior face has been adequately designed for bending then we're good. If not, goodbye wall.​

2. The rebar in both faces delivers a moment to the footing. The footing rotates into the soil. The soil fails and the footing rotates to match the rotation at the bottom of the wall and we end up with basically ends up the same as 1B.

If all that sounds accurate, seems like the exterior face bars should be designed for a propped cantilever case, and the interior face bars should be designed for a pinned case? Shouldn't the rebar in both faces have the hook pointing toward the toe? We usually detail the interior face bars to hook toward the toe, and the exterior face bars to hook toward the heel, but if the exterior face bars are to resist propped cantilever bending then I'd think they should point toward the toe like a regular cantilever wall.

 

[jayrod12]

Generally it's best to orient your hooks such that the compression strut confines them. So for the inside face bars they'd hook towards the outside, and vice versa.

 

[BAretired]

A plan layout of the garage would help. A mat foundation may well be the best solution.

BA

 

[OldDawgNewTricks]

SarBear, it sounds like you now have a much better understanding of the various concerns that have been raised throughout this thread. Now you can make a better-informed engineering judgment about how to design it and it seems like you are coming around to the envelope solution that I mentioned in the very first reply. I'm not saying that's the best or most correct solution, but in my opinion it's probably the simplest way to balance the various issues (code compliance, analysis complexity, economic concerns, etc.). Your supervisors may have considered all of these things and decided that they can live with some failure types like allowable soil bearing or rebar development as long as they prevent global collapse. I tend to be a little more conservative with foundation design because it's a lot more difficult to fix later if something goes wrong.