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Walkout basement foundation design

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jimtheengineer10

jimtheengineer10

Civil/Environmental
Apr 28, 2012
159
I was asked the other day about a foundation for a walkout basement. I have never designed one before and I don't have to at this point but I would like to be able to design one if I am asked to in the future. Below is an example that I am working through. I have attached a picture to help you understand my text below.

The north, west and east walls will be conc. walls and the south wall will be wood. The floor joists will run north-south and will bear on the north and south walls (there will also be a 2x4 stud wall in the basement that the joists will bear on).

Should the north wall be designed with a cantilevered footing?

Should the east, west and south walls just be typical 8" concrete walls?

I am sure I will have more questions but I will start with this. Thanks.
WalkoutBasement.jpg
 
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You can design it however you would like to. Just make sure it doesn't fall down. I believe that IBC chapter 18 has some guidance on this matter. Have you looked there?
 
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Typically, check the shear through the floor diaphragm and transmit that to the E/W walls.

Should solve your problem. And then you don't need a North retaining wall
 
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When the Main Floor can be considered a diaphragm, I agree with Mike.

Sometimes, say in a bi-level, the floor is not a continuous diaphragm so you either make the north wall a retaining wall or generate some other lateral resisting elements (interior shear walls running north-south).

Also, if applicable to your location, the south wall foundation has to be carried below the frost line.

BA
 
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My floor system would be considered a diaphragm right?

Mike/BAretired, what are some ways to transmit the shear to the E-W walls?
 
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You do need retaining walls where you are retaining earth, but these walls may be propped at the top by the floor, so not cantilevered walls. Whether or not your floor is considered as a diaphragm depends on how you build it.
 
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The floor will be floor joists bearing on top of the foundation wall with 3/4" t&g subfloor glued and nailed. Is this considered a diaphragm? If not how would it need to be constructed to be considered a diaphragm?
 
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@jimtheengineer: Ref ASCE 7-05, Section 12.3.1.1: Diaphragms of wood structural panels or untopped steel decks in one-and two family residential buildings of light-frame construction shall be permitted to be idealized as flexible.(That's a lower bound) You may refer to NDS Specs and Chapter on Wood in the applicable Building Code for design and construction requirements for diaphragms.
For your walk-out basement, the diaphragm would be subjected to sustained loading from earth. A lot depends on the layout of the structural walls and openings in the floor.
 
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If your plan view is to scale, your diaphragm appears to have a length to width ratio of about 2. That is probably getting near the limit for a wood diaphragm and if you have a stair opening, its effect must be considered.

The diaphragm requires chord members on the north and south edge designed to carry wL2/8 where w is the load per unit length from the earth pressure and L is the span.

The diaphragm must be connected to the east and west walls to resist the full shear resulting from the lateral load, namely wL/2 each end.

You really ought to read up on the design of wood diaphragms before embarking on this design.

BA
 
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Attached is a picture of the example basement layout. You can see there are two interior walls that the floor joists will bear on and the only opening in the floor will be the stairway.

So what I need to do next is to calculate Vcp and Vnp and the deflection as per the pdf SteelPE reference above right?

 
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I forgot to attach the picture in my last post so here it is
Clipboard01.jpg
 
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Lot's easier to just design the basement walls as retaining walls. You will end up with some decent connections to transfer the shear to the east/west walls. You will probably also need to block your diaphragm if assuming the top of the concrete walls are braced against overturning. As suggested, it would be a good idea to read up on diaphragms, look at your required connections, etc.

Why not extend the concrete up another 12" and hang your joists off of a ledger connected to the concrete?
 
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Ignore the diaphragm and design as a retaining wall. If you want to use it you need to remember to provide anchor bolts and a manner to get the soil reaction from the top of the concrete up into your diaphragm and then get it back out into the concrete walls each end. Your normal nailing and such will not even come close to handling the soil reactions, which since they are sustained loads need a duration factor of less than 1. I looked quickly in NDS and all the diaphragm tables are for seismic and wind loads. These would have to be significantly reduced to a sustained load level, but I couldn't find where it spells out what to do (I must admit I didn't look real hard).
 
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