Retaining Wall Design...Pinned Pinned? 1

[bigmig]

I was brought up under an engineer who designed retaining walls for residential wood framed homes with a 'propped cantilever' method. In other words, the walls are not designed as pinned pinned, but pinned top and fixed at the base. This is accomplished by providing a footing big enough to provide so-called base fixity. The end result is this: the soil loads do make it into the diaphragm, are minor and completely manageable without the use of shear walls and hardware.

After going out on my own, I'm beginning to see many engineers designing the same wall as pinned pinned. These walls are characterized by extremely narrow footings. The obvious load path is 2/3 the load goes to the slab on grade below, and 1/3 goes to the floor diaphragm.

After a half hour of searching the Engtips posts, it seems many of you design like this.

A couple of questions for this crowd:

1. Where does this rather significant soil load go once it gets into the diaphragm?
2. If the answer to question 1 is 'it cancels out from loads on the opposite side of the building' what happens in the case of a walkout basement?
3. If you have sloping grade, the load into the diaphragm will be varying based on your location at the perimeter wall. Do you actually calculate the summation of this 'diaphragm' load from all 4 (or 3) sides and come up with a resultant that you then.....resist? This seems like a lot of calculation and hope that the load will actually make it to the location you want it to go.
4. In cases where you have a wood framed floor that is stacked on top of your wall, your concrete wall has to transfer force through a relatively weak element....a 1. 1/2" thick mud plate with a perp to grain anchor bolt located only 2. 3/4" from the edge of the mud plate. For a simple 8' wall, that is 333 # per foot. Your anchor bolts have to be spaced pretty tight to get that to work. How do you transfer loads to your diaphragm in this scenario?

The reason I ask is that many contractors immediately point out that it takes a lot more concrete to pour a pinned-fixed wall than a pinned-pinned wall. Resisting full time lateral loads (as opposed to seismic and wind) with things like specially detailed wood shear walls and sheet rock makes me cringe. Any input would be helpful.

 

[CELinOttawa]

I have, and continue to, design basement retaining sections both ways. I typically use the pin-fixed designs for situations where it makes my life easier, and for nearly all cases of irregular footprints.

I think you need to go through the exercise of designing an example house as a pin-pin foundation wall condition. Chase the loads as you've presented the problem above (b/c you are correct) and then compare this to the strength of the diaphragm action available in your timber floor. I think you'll be pleasantly surprised by how low the demand/capacity ratio turns out to be...

As a small aside, it is less the diaphragm fastening of the Plywood/osb floor panels which is of concern and much more so the detail at the top of the wall where the load must be transferred into the floor. This is particularly important to get right, and can be finicky at the parallel joist to wall connection. You also need to watch that the contractor doesn't blow through a whole lot of blocking with ducts or other services.

Both have their place, and once you've worked through a full pin-pin I'm confident you'll see it as a useful tool. If it reassures you at all, most prescriptive pre-engineered residential codes have pin-pin as the standard foundation wall.... If it is robust enough to allow without specific engineering, it is pretty hard to have be a problem when a competent engineer is involved.

 

[XR250]

Those are excellent questions. I usually only do pin-pin or horizontally spanning if there is enough jogs. Getting the load into the floor diaphragm can be a challenge if there is a lot a of backfill. Our code only requires bolts at 6 ft. O.C. Usually, the math requires a much closer spacing and then lots of nails to get it into the joists. When the joists are parallel, I usually stop them 4 ft. back and then ladder frame the rest. Still not foolproof, as there is an uplift at the end of the ladder due to the load coming in at the bottom and being transferred upto the diaphragm (shear times joist depth). Once the load is in the diaphragm, it has to be transferred to the side walls, so those connections must be addressed as well as the shearwalls - if there are pony walls on top of the side foundation wall. I have witnessed numerous instances of entire houses racking when there is a lot of openings in the side wall or a garage door. I worked on a job recently (designed by another engineer) that had 14 ft of backfill. The wall carrying the backfill began leaning and slid the entire floor system off the remainder of the foundation.

It has been my experience, that basement walls always seem to perform better than the numbers say they will.

 

[dcarr82775]

If the wall allows it I look at it as pinned on the bottom and the 2 sides and free at the top (2 way action). I try to avoid pinned/pinned top and bottom as the connections to the floor are too much to ever have the contractor get it right. I have often used fixed bottom and pinned top. I think where the load goes at the top is often ignored by a designer.

But like Excel said, basement walls seem to work a whole lot better than they should for a typical instance.

 

[XR250]

@dcarr;

What is your limit for length of wall before the 2 way action becomes ineffective?

 

[CELinOttawa]

The seemingly automatic conservatism in the actual performance of foundation walls is due, in my opinion, to the conservation of necessary safety factory re against the worst cases in soil loading.... The geotech end is simply much less predictable, and as a result has more built in safety that is often not actually required, but better be there if you want to keep practising for a long time!

 

[XR250]

I agree with CEL. Also, un-intended 2-way action seems to play a part - even in really long walls. I see plenty of cases weekly where the top of the basement wall is essentially un-braced as the joists are running parallel and there is no blocking. Most basements around here are CMU. I push concrete when I can as it is much more forgiving to the contractors mistakes such as poor attachment to the floor system or backfilling before the floor is even constructed.

 

[SteelPE]

What were you standing over my shoulder a few days ago.

I had these same questions. I was designing a mixed use wood structure and I was wondering if the pinned-pinned assumptions I learned was correct. In this instance had a decision to make with regards to the attachment to the footing. I had some L shaped bars that turned up into the wall. I contemplated turning them up to the outside face of the wall essentially creating a partial fixity at the base. In the end, after some discussion with the design engineer (I am only designing the foundation) we decided to uses the pinned-pinned condition for now.

 

[bigmig]

I can tell you right now that a majority of pinned-pinned designers never check global stability for diaphragm connections and strength, let alone the deflection in the diaphragm that results from a full time soil load.

I know this because you never see any diaphragm chord connections. Butt joints, jogs, and step in elevations, all without any load transfer strap or fasteners. Floor irregularities, stair openings, elevators etc., which are always present in the homes I design, would make any accurate diaphragm analysis and 1 day finite modeling exercise. The text book rectangle home with continuous diaphragm chords is pretty rare, at least in this neck of the woods.

The problem with running soil loads in your diaphragm is that you are guessing. One side of the house is in the shade. The soil is damp and at full design pressure. The other side is dry as a bone. The soil is cohesive and never places a portion of the design load into the floor. You would have to pattern load umpteen load cases to get the worst load on your floor system. Again, I'm going to go out on a limb and say that I don't think most engineers go to this amount of examination.

I always model my stuff in Risa, to milk out all the 2 way action I can get. My impression is that once you get a horizontal run of over 10-12', the 2 way action is not a very helpful factor (8-10' tall walls).

'Well, I have never seen a problem with this in the years I have been doing it' is the exact same answer I get from the contractor who is always telling me I'm the idiot engineer making him follow the over designed code. It is just a poor answer.

 

[XR250]

Yup, I never check diaphragm chords etc. The things just seem to work.
When i get over 10 ft of backfill, I consider using concrete walls with external buttresses and take the floor system out of the equation.
Or I try to get it to span horizontally if the jogs are big enough and not too far apart.

 

[SteelPE]

bigmig, you model all of your walls in RISA? That must take a while to accomplish correctly. Designing a wall as pinned-pinned fixed-pinned fixed-free is pretty easy once you make the decision on which way you would like to go.

 

[bigmig]

SteelPE, It takes a day to do a 4000 sq. foot walkout basement house. I draw the model in cad, import it, and run analysis on a model built of 1'x1' plates.
For simple projects, or a project with no budget, I definitely do the 1' strip, hand method, as it is quick, albeit conservative generally speaking.

 

[JAE]

[blue]1. Where does this rather significant soil load go once it gets into the diaphragm?[/blue]
It extends into the diaphragm which must span to orthogonal walls of some kind if there is not an opposite soil force from the other side (see your question 2)

[blue]2. If the answer to question 1 is 'it cancels out from loads on the opposite side of the building' what happens in the case of a walkout basement?[/blue]
In the case of a walk-out basement, this load extends into the diaphragm and on to the building side walls - which act as shear walls to transmit the force down to the earth - via friction or passive pressure, depending on the nature of the building. The entire load path should be checked - i.e. the connections from loaded wall to diaphragm, the diaphragm shear nailing, diaphragm chords, and shear transfer to the orthogonal walls.

[blue]3. If you have sloping grade, the load into the diaphragm will be varying based on your location at the perimeter wall. Do you actually calculate the summation of this 'diaphragm' load from all 4 (or 3) sides and come up with a resultant that you then.....resist? This seems like a lot of calculation and hope that the load will actually make it to the location you want it to go.[/blue]
Usually approximate (on the conservative side) the loadings.

[blue]4. In cases where you have a wood framed floor that is stacked on top of your wall, your concrete wall has to transfer force through a relatively weak element....a 1. 1/2" thick mud plate with a perp to grain anchor bolt located only 2. 3/4" from the edge of the mud plate. For a simple 8' wall, that is 333 # per foot. Your anchor bolts have to be spaced pretty tight to get that to work. How do you transfer loads to your diaphragm in this scenario?[/blue]
Agree with your comments above - the anchor bolts must transfer through the sill plate into perpendicular joists or blocking in the case of parallel joists. The more recent IBC / IRC codes do show significant bolt requirements for the loads so these would confirm you comments more. In our local city code, they've amended the anchor bolt requirements and relaxed them quite a bit - also they use 35 pcf for equiv. soil pressure. This works generally in practice but we do get cases where walls have leaned inward under bolt slip and cause wall cracks.


I might point out that your fixed-pin option isn't totally correct and actually leans to the unconservative side because no footing is truly fixed but will rotate somewhat under load - this would then cause higher stresses in your wall and also larger resultant forces at the top of the wall.

 

[dcarr82775]

Excel,

I will use the PCA tank tables to estimate moments and deflections for 2 way action. I don't have the L/D ratio in my head right now. Then I decide what deflection I can live with. If it is a little messier I will use RISA. Like BigMig said, it doesn't take as long to do as you might think. If you start cracking sections and re-distributing moments in RISA you can get values that begin to explain why walls can work when at first glance they should not.