20' high basement wall and diaphragm

[bylar]

Basement walls of 20’ in height
Residential
IBC 2006 North Carolina

Paragraph 1610.1 “Basement walls and other wall in which horizontal movement is restricted at the top shall be designed for at rest pressure”

Exception basement nor more than 8’ below grade with flexible floor

Similar wording in previous code

Table 1601.1 for most sandy types of backfill has an at rest pressure of 60 psf

For reinforced masonry with 12” CMU that puts a limit at about 12’ with 11’ of backfill.

If you go to concrete then the wall is possible but the resultant at the top is app 2100#/ft.

With a typical ¾” plywood floor as a diaphragm the plywood is overloaded.
Any ideas on how you have solved such a situation

 

[Lion06]

Design the wall as cantilevered, use the active pressure and make sure the wall is backfilled before the diaphragm is put in place.
Given that, I would still design the diaphragm to take some load from the wall for some possible surcharge or soil saturation after the diaphragm is in place.

 

[COEngineeer]

design the wall as if it were fixed at the bottom and both sides and free at the top (thicker wall and bigger footing). Almost like designing cantilevered retaining wall but not as strong since you have geometry of the house helping you a lot. I use PCA concrete tank design table to design the wall. It is mostly depend on the length of the wall. BTW, are you building a basketball court in the basement or something?

 

[COEngineeer]

structuralEIT, I must disagree with you. I think you must use at rest pressure. If you design it as cantilever, it is going to be HUGE. I think you have to design it like a concrete tank.

 

[bylar]

There is going to be another floor at the 10' level but if you count on that as a support you still have too high of a stress for a typical residential diaphragm.

I have suggested a concrete floor at either the 10' level or the 20' level but none of my clients ever want to pay for that.

apparently they find someone that comes up with a design because I never hear from them again and I never check out what someone else may have designed. I have seen CMU wall that High and are standing but I wouln't do it.

 

[271828]

What's the wall length? You might be able to design it for two-way bending like COEngineer is suggesting.

 

[COEngineeer]

Bylar, is it going to be a walk out basement also or there is going to be a 10' basement wall on the other side of the house? Watch out for slope stability. You might design the house correctly but the house wont stand if the whole hill slides. If you have a walk out basement you definately need to work very close w/ the geotech. eng.

Just design it like a tank with counterforts at long span and your footing and wall thickness wont be too crazy.

 

[Lion06]

I will concur with COEngineer on the at-rest pressure and not designing as cantilever to cut down on sizes, but I would still backfill before diaphragm is in place to avoid taking that load into the diaphragm.

bylar-
is the basement 20' high or are you using CMU for the entire height of the house? The OP seemed like the basement had a 20' high ceiling, but your last post seems like it is a 10' (or less for framing) basement ceiling and the CMU continues to the second floor. Which one is correct?

 

[JAE]

This topic was just discussed at length on another post.

Go here for the thread:

thread507-184078

 

[bylar]

As I say in the post I believe masonry wall is out of the question.

Yes it is a walk out basement with both side wall masnry or concrete.

I have already considered the sliding effect.

To me the only solution is have a concrete floor at either the 10' or the 20' elevation.

I don't believe a retaining wall is practical.

I feel somewhat confident in I analysis of the situation but I have seen it done with simple reinforced masonry and even though the walls are in tact and still standing I do not believe it complys with the chapter 16 that I originaly stated.

I had looked at the sited thread before I even posted mine but the conditons are not th same.

 

[271828]

OK, venturing into the dangerous territory of the other thread, LOL...

If you consider it to be a cantilevered wall, the big downsides are:

1. The wall (cip concrete for sure) and footing will be huge. Answer: too bad. It is what it is. They don't like the other option of installing a concrete diaphragm.

2. The wall will deflect later on and possibly damage your diaphragm or other LFRS elements.

Why not bite the bullet and cantilever the wall? Deal with Item 2 with detailing.

Depending on your situation, the total wall deflection would be 0.8" to 1.0" according to the various CRSI cases. Try to figure out how much of this should occur later on, after the diaphragms are installed. It might not be enough to spit at. Detail your system to handle the deflection. It might not be that hard to do.

What kind of backfill and drainage system are you planning to use? I know I'd feel better about it if I had granular backfill going back at an angle, plus a drain at the bottom. I'd be a lot more scared with clay behind the wall.

 

[COEngineeer]

Do you think 10" thick wall with 3' footing is too much? Without doing calc I think that what I will end up having if I were to design it with no wall longer than 20' (if so, I would put a counter fort or buttress). I just finished a job similar to yours. They just finished backfilling the walls and they put a string accross to keep an eye on wall deflection. Everything looks good so far.

 

[concretemasonry]

You may want to check the availability of higher strength masonry units and other sizes.

Many markets routinely stock 14" and 16" units. The 16" units can be very efficient if partially reinforced/grouted since they allow a very thick wall with slightly less material.

You can also use higher strength units. Many producers only stock one or two different strengths, but other may be produced. A f'm=3000 psi can easily be obtained. I have seen f'm=4800 psi hollow 8" prisms made with block having a 8000 psi net compressive strength.

You may have a difficult time finding a testing laboratory capable of testing without severely cutting the samples down.

This information does not address the actual design concept and analysis of the deflection and floor/wall interaction. It does, however, point out that there are many materials possible.

With the substantial horizontal loads, it is unfortunate that the hollow core plank could not be used economically to maintain consistant long term material properties and not rely on a wood diaphragm.

Dick

 

[bylar]

coengineer
Are you talking about designing the wall as a twoway slab?
I haven't tried that yet but it seem that I did that several years ago and came up with results a lot greater than what you did.

 

[IceNine]

I'm curious as to how you resist the sliding force at the bottom of a 20' basement wall in a walk out basement?

 

[COEngineeer]

yes but two way slab has distributed load. I use triangular load(soil pressure). You can either model it or use a table. If it gets too much, then break the span with counterfort/buttress. Also, the moment about 3'(just a guess) from the base is going to be really big. I usually just use more dowels than the vertical rebars. I make the dowels long enough to take care of the big moment near the base. (a few of engineers I work with always use the same amount of vertical and dowel but I thought with my approach I reduce the vertical steel quite a bit). You can also vary the number of horizontal reinforcement from top to bottom but I didn't do that because I don't want to make it too confusing. Also, I put the reinforcement in the middle of the wall only (to take care positive and negative moment and I really don't want to use reinforcement at ea. face)

To veteran structural engineers out there, do you think my approach is good? The PE I work for liked it and stamped my work. Am I taking too much risk for not worrying how much load is taken by the diaphragm and design the wall to stand without the the diaphragm?

 

[concretemasonry]

IceNine -

The IRC requires that the basement slab be a minimum of 3.5" and poured directly abutting the foundation wall and on top of the footing.

You can use this to provide sliding reisistance if necessary (unless you have a big pool in the basement).

Dick

 

[IceNine]

concretemasonry-

I would imagine in a daylight basement, the slab wouldn't offer much sliding resistance.

 

[concretemasonry]

IceNine -

It all depends on what direction you are talking about. A concrete slab 3.5" inches or more thick can offer significant diaphragm action in both direction, but may depend on the geometry. It is a very handy and predictable engineering tool.

 

[IceNine]

concretemasonry-

I agree, with the right geometry it may work.
But, the reactions at the ends of the concrete diaphragm will be very large for 20' retained height. Plus, you would have to provide dowels from the floor slab into the side walls or footings, which I've never seen.

Most of the houses I deal with that have daylight basements are much longer in the direction perpendicular to the slope, so the diaphragm becomes very long.