Basement Wall Design 3

[ick2]

I have an 11' deep basement in a residential construction. Do i have to tie the basement slab to my basement wall? up to now i was thinking to use 6" thick "floating foundation slab" but i have second thoughts. Then i guess the wall i can design as cantilever wall using the active earth pressure or a propped cantlever using the pressure at rest.
what are your thoughts on this?

thank you

 

[ishvaaag]

Here if a rim beam foundation or, then, slab (14" in minimum is usual) is used it invariably is joined to the walls (usually 10 to 14"). It may work well maybe with thinner bottom slabs, but better keep some proportion not one element restrict too much the other and then the thinner crack. At rest pressure would be O.K., many here use to use just active and never happens anything, but it must be said that most times minimum steel requirement covers about both for 1 level, and top of wall is here almost invariably restrained atop by the street level floor, and this is not countes usually by whatever the reasons (simplicity of calculation, stage of construction).

 

[MiketheEngineer]

Check with your local codes. May need a complete strcutural analysis.

 

[msquared48]

A free standing or yielding retaining wall takes all the lateral pressure in and of itself.

A basement or non-yielding wall relies on the floor diaphragm to distribute the lateral loads to another mechanism at the top of the wall, such as a floor or roof diaphragm, to prevent overturning.

There is another animal called a counterfort wall that is basically a yielding retaining wall, but a little different in how it behaves.

In that difference lies your answer in that the decision to use one or the other is based on:

1. The sequence of wall backfill with respect to the placement of the floor diaphragm;

2. The preferences, experience and expertise of the contractor, engineer or owner;

3. Whether or not the floor diaphragm can take the lateral load from a basement wall without interstitial walls or other complications.

4. Local geologic and soil conditions.

There are others, but these are a few considerations when msaking your decision.

Mike McCann
MMC Engineering

 

[ick2]

i like the idea of the rim beam. let's say that i design the wall as a propped cantilever with pressure at rest. then i design the footing as a beam spanning between the cross walls subjected to a uniform load (the reaction of the propped cantilever) and the uniform torsion equal the moment at the "fixed end". this way i can keep the slab separate from the footing and thinner.
the other choice is to have a monolithic basement slab of 14" which seems excessive for a single family dwelling

 

[ishvaaag]

Certainly when no mat is required, the walls have some ("rim" beam) foundation and the slab on the ground (here called "solera") is usually kept above the upper face of the foundation of the wall.

 

[msquared48]

The basement wall condition has what we call a "strip footing" at the base that can function as a grade beam. The slab is usually cast separately and from 4 to 6" thick. Once cast though, the wall finctions as a deep beam, so the grade beam is much more flexible and l;ess functional as a grade beam in the long run.

The yielding or retaining wall situation is different in that the main toe and heel steel spans perpendicular to the wall steel. Again, the wall serves as a deep beam too. The slab can be separate, or cast as part of the toe of the retaining wall if so designed and properly reinforced.

Mike McCann
MMC Engineering

 

[ick2]

thanks for your replies.
the walls by archtectural geometry have to be 16" thick and 11' deep.(12.5 ft to the bottom of the wall ftg).so i have decided to design them as cantilever retaining walls. however they are only secure against overturning when all the dead weight is on. (all the four stories built). i guess the contractor has to prop until then however how can i make sure that there will be not transfer of force to the ground floor and the wall will deflect prior to the placing of the floor?

 

[mady2009]

We do the overturning stability of basement wall by including slab......but during construction first the basement wall is erected and then slab and beams are casted......so how we can make it possible that wall will not overturn when there is no slab casted.....because soil is always acting which causes overturning... it may damage our wall during construction.

 

[hokie66]

ick2,

If you have 4 stories of building supported on a retaining wall, it is not a cantilever wall. Design it as a basement wall spanning between the lower two floors. Design the floors to restrain the wall, and prevent backfilling until the wall restraint is complete.

mady2009,

Same advice, wait to backfill until the wall is restrained.

 

[paddingtongreen]

What mady and hokie said. The floors above were vital information.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ick2]

i cannot assume that the wall is supported at the top. too much load to bring into the plywood diaphragm of the first floor.

 

[hokie66]

Sounds like you need a concrete slab for the first floor. The upper floors are not going to assist materially in preventing movement at the first floor.

 

[SirAl]

The attached reference may be of assistance albiet a bit shorter than your wall

 

[InDepth]

I would recommend a cantilever wall. I would also recommend adding notes on sequencing that address that floors shouldn't be attached until the backfill has been placed behind the wall. ...don't want soil load entering your lateral system unless you have designed for it. You still need to attach your wall though if you are planning on using the cantilever wall for shear seismic resistance.

 

[ick2]

thanks all for your answers. i finally convinced the owner and the architect to use concrete on the first floor and thus tie the walls together.

 

[hokie66]

Good decision and good convincing. Now you can sleep well.