Residential Foundation Wall

[DKPE]

Recently, in our office, there has been disagreement regarding the design of residential foundation walls. Does one consider these walls cantilever retaining walls or are the ends pin-pin? When floor joists are perpendicular to and bear on the wall and are attached to a wood sill plate that is attached to the foundation wall by anchor bolts (48" o.c. max) do you need to check the stability of the floor system (joists, sill, anch bolts and plywood subfloor) to see if it will resist lateral movement of the top of the foundation wall?
What about found. walls parallel to floor joists? I know some have prescribed blocking the joists in the last few joist spaces but I do not believe this is standard construction practice in our area (midwest).
Typically, residential foundations are given in tables in IRC, however, we have a 12' tall foundation wall with heavy stone veneer and we will design from scratch. Just wondering if any of you deal with the cantilever vs. pin-pin issue much?
Thanks

 

[DaveAtkins]

I try to make a basement wall pin-pin. Sometimes (like wtih a "walkout" type basement) the diaphragm cannot resist the soil pressure; that is when I use a cantilever retaining wall.

DaveAtkins

 

[oldestguy]

More important than how you design it is how will it be built on and backfilled.

I've see all sorts of non-reinforced concrete walls used and work. Also, I've seen reinforced walls lean in, bulge in and even collapse due to what was then done to them.

One very important point is get some load on it. At least the deck. Your use of scabs inder the joists is good also.

Most important is how is it backfilled and when? I always advise never backfill until at least the deck is on, preferably also with more structure weight. In this situation you are looking at the wall being like a stack of kids building blocks. Put substantial compressive load on that stack and it can resist a lot of lateral pressure. So this wall needs no reinforcing, maybe? Well, it ought to have some to be sure. I have notseen that structural engineers go so far as to keeping the resultant in the middle third, but you could.

Of course you don't want any machine pushing on the wall, but you have to control the work so settlement later is not severe and collects water that then can weaken any shear strength that the backfill had. This saturated fill then results in it being a heavy liquid of sorts.

For fancy jobs, not a house, I spec that compaction of backfill requires that compactors stay at least 18, preferably 24 inches from the wall, to prevent building up a wedging action.

Drainage of the backfill is another topic I have preached on elsewhere in these threads.

 

[avogadro602]

Generally, (except in the instance cited where the walkout side may not be adequately braced) the assumption is that the basement floor slab and the main floor diaphragm provide adequate resistance to design the wall using pinned end conditions. If you can get your hands on one of the ICF manuals (e.g. BlueMax) a detailed procedure is provided for the reinforcement design. I incorporated these formulae into a spreadsheet that allows me to readily alter a few parameters as wall height, soils conditions and vertical loads change.

 

[gotwood]

I would go buy PCA rectangular concrete tanks book. It is very helpful. I use it a lot for residential design. Thi is what I do but you need the book I mentioned above.

When I design for the reinforcement, I consider it as fixed at the bottom and free at top. But when I use the table from the book above, I used fixed fixed (sides), fixed bottom and pinned at the top and figure out the shear at the middle of the top wall. If the top of the wall you have wood flooring, I dont want the shear at the top of the foundation to be more than 500 lbs. I think somewhere on IBC it will give you how much load the plywood can handle (350ish). I inreased that number a little bit since the shear at the top of the wall gets smaller toward the side of wall. If it is more than 500lbs then I will put a counter fort or buttress to make the span of the wall shorter. Since you assume the bottom is fixed, you need to make sure your foundation has enough resistive moment at the bottom so you will back up your assumption.


Now, if you assume the bottom is pinned, this can be done also. Just design the foundation big enough to take the axial load. But when you design the wall, it will act almost like a one way slab. So you will need a lot more horizontal reinforcement. Base on the cost of rebars vs concrete, I think it will be a lot cheaper to build the footings big enough to assume fixed at the bottom. Buy Javeed Munshi PCA rectangular concrete tanks fifth edition. It is very helpful.

 

[gotwood]

forgot to mention, my designs are mostly w/ basement so the walls are more than 10' high. So I design it like a tank.