Basement wall Design calculation 1

[djzzz]

Dear all,
Can anybody give a detailed calculation for the design of basement wall. I am currently working on a project that has a basement floor. Kindly help me out.

 

[CELinOttawa]

That's just too broad of a question. These can range from 10" thick unreinforced residential basement foundations with 4" thick unreinforced slab to 18" thick heavily reinforced six storey propped wall underground parking....

Need to know:

-soil conditions (incl. Design high water table)
-wall height
-intended use
-drainage conditions

 

[djzzz]

I want to know the general calculation. May be one of the projects that you have done. It would be really helpful.

 

[mihmb]

For a single story basement. General assumption is the wall spans vertically with end conditions being pinned at the top and the bottom. One could assume bottom to be fixed, but proper detailing of this connection would need to be done. Depeding on type of construction this may or may not be the case (residential will almost always be constructed as pinned to the foundation, however for commercial/industrial it will generally be up to the desing engineer of record to make the decision)

For this to be true (pinned-pinned), the ground level floor needs to be constructed prior to backfilling.

Now you have a span and end conditions. Determine soil loading (typically I use an equiv fluid pressure based on soil types and surcharge loading). Apply this loading to the wall using standard V&M tables. Design wall based on a 1' strip width. Determine the required reinforcine. And Done.

 

[oldestguy]

Gosh, of all the basement walls I have seen, no one does any design. Just follow the code and it works. Just don't backfill it too early in the curing stages. It's kinda like an open top cardboard box with some outside pressure trying to cave it in. Maybe 3 or maybe 4 edges have resistance to that. A little bending resistance in the wall also. Put a few jogs in the walls and that helps, but not easily figured. What about the building load putting the whole thing in compression against vertical bending? That explains why block walls usually (?) stay, except for next questions.

mihmb: what if there are no floor joists resting on the wall? No friction then. No hinge support for the vertical beam?

Gets into the realm of a flat plate supported on three sides.

 

[CELinOttawa]

Oldestguy: I have yet to see a building code that did not require solid blocking to at least three joists back in order to ensure the "prop" at the top of the wall.

It is very rare to see an actual three sided, unsupported at top, foundation wall in practice. Except for the forensic engineering calls, of course!

 

[mihmb]

The wall that does not see the bearing load from the floor joists still has a joist setting on it along the entire lenght (sets parallel to the wall). The plywood subfloor is attached to this joist, this acts to resist the load as it is essentially a large diaphram. Additionally, floor joist utilize X bracing at some interval (so you have many floor joist resisting the load, utilizing the joists weak axis to resist this load).

This is all true for good quality construction. I am not saying there are times when contractors do not install the X bracing, because I have seen it missing myself.

And there are always exceptions. For example, the stairway in my house which goes down to the basement is on an outside wall. As such, there is a section of my basement wall where nothing ties into the top of the wall.

With all of the being said, it is very time consuming to run the numbers and "prove" that it is pinned at the top. But with proper construction, this works. It just does.

So, for a standard single story basement if you put ACI min steel in the center of the section and the contractor builds the house properly there will be no structral concern.

 

[oldestguy]

In answer to the original question, my intent was to stir up a little, since a real answer covering all aspects probably is just too darned complicated for most of us to cover all possibilities. I think luck comes in also along with the help of corners and what tensile capabilities come from the concrete.

 

[oldestguy]

We've all see a just completed house basement backfilled shortly after placing concrete. Once in a while it fails then, but most seem to get by with it (the luck part). So, as to a design method, I'd look to the situation that is present when that backfill is placed and with no house or floor on the walls yet as being a more realistic design circumstance. A house on it is just an added support factor later on.

 

[JAE]

The wall that does not see the bearing load from the floor joists still has a joist setting on it along the entire lenght (sets parallel to the wall). The plywood subfloor is attached to this joist, this acts to resist the load as it is essentially a large diaphram.

mihmb - Not sure I 100% agree with you here. I've seen so many homes where the parallel-to-joist conditions results in a wall rotated inward at the top and diagonal cracking in the wall near its ends where the orthogonal wall intersect. (i.e. wall is restrained laterally on bottom and sides but not on top.)

There is a reason the codes demand that these walls have blocking sent back into the floor between parallel joists to resist this upper wall end reaction (as CELinOttawa states above) The parallel joist you mention, sitting on the top of the wall for its length, will simply rotate if enough earth pressure is applied to the wall.

As I said, I've seen this numerous times. The houses where the blocking is left off, but no wall movement occurs, is probably due to the earth on the wall never building up its at-rest pressures over time and/or no excessive moisture in the soils.

 

[msquared48]

I totally agree with JAE here, and have detailed these conditions similarly for many years. Somehow, the wall top reaction needs to get into the floor diaphragm through periodic blocking and additional nailing to be distributed to the foundation.

Mike McCann
MMC Engineering

 

[oldestguy]

More stuff. Has anyone used some form of angle on the underneath of a house central "girder" sitting in a pocket of the basement wall? When I've seen cave in,these "girders" punch right through. If so equipped at each end, seems like a good feature.

But what if that "girder" is blocked up with some wood spacers to reach proper elevation?

 

[msquared48]

Beam needs air between it and concrete by code so any restraint of wall would have to be thru connectors.

Mike McCann
MMC Engineering

 

[mihmb]

Jae & MS,

When the floor joists are X braced together, this transfers the load to the diaphragm. I believe that the X bracing I am referring to serves the same purpose as the bocking you are referring to. Please correct me/explian if I am missing something.

I personally have never seen the the joist rotate, however I can definately visualize this occuring.

 

[JAE]

The concern with the X-braces is that they are typically little 1x2 or 1x3 members toenailed into the joists. And usually you only see one or two lines of these per joist span.

That to me doesn't sound like much resistance to lateral at-rest earth loads. Do they help? I would say yes - but not sure how much.

And many times you see the X-bracing start at the first interior joist - not extended out to the outer rim joist.

 

[msquared48]

OK...

The "x" bracing is not used to resist lateral forces, as opposed to blocking which is usually 2X members oriented vertically, and used to transmit shear forces.

What the Bridging does is two-fold in nature:

1. It stabilizes laterally the bottom flange of the joists to prevent rollover, and
2. It spreads out isolated point loads to two or more joists in the floor system.

Bridging, due to the small nature of the members as JAE mentioned, is not intentioned to resist the soil loads imposed at the top of the wall. Considering the angle of the bridging and the increase force due to that, the force would be too great. Solid 2X blocking nailed off to the floor diaphragm works for that situation.

Mike McCann
MMC Engineering

 

[TDIengineer]

@Firo-
If you are looking for some basic general design information because you don't have a soil engineer or soil report, I would go with an "at rest pressure" of 55psf/ft^3. This is pretty conservative and it usually isn't higher than this.

This is assuming the wall has drainage, i.e. no static fluid pressure, and that there isn't a seismic dynamic earth pressure. On retaining walls greater than 15' a seismic pressure should also be considered in high seismic zone.

So for a 10' basement wall, which is designed as simply supported top and bottom, the triangular pressure would be 550 psf at the base varying to zero at the top. Mmax = 0.128Wl or 3.52 k-ft.

Some soil reports show a uniform design pressure when designing "at rest" condition of anything from 20H to 40H. If you go with the higher 40H, that would be 400 psf with Wl^2/8 yielding 5 k-ft.

You should also check shear friction at the wall base along with out-of-plane bending.

Hope the above helps.

 

[JAE]

I'm not sure I'd just go with 55 pcf. I've seen geotechs recommend up to 85. You really need to determine what type of soils you have to do this right.
Typically for houses, the city will have some type of standard to use.

 

[mihmb]

TDI, what is the basis for "On retaining walls greater than 15' a seismic pressure should also be considered in high seismic zone."

The 15' is new to me. Can you please clarify?

 

[XR250]

The IBC list equiv. hydrostatic pressure for various soils. 55 PCF is pretty darn high.
I typically use 35 psf for granular soils.