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Basement wall Design calculation

Basement wall Design calculation

Basement wall Design calculation

(OP)
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.

RE: Basement wall Design calculation

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

RE: Basement wall Design calculation

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

RE: Basement wall Design calculation

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.

RE: Basement wall Design calculation



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.

RE: Basement wall Design calculation

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!

RE: Basement wall Design calculation

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.

RE: Basement wall Design calculation

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.

RE: Basement wall Design calculation

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.

RE: Basement wall Design calculation

Quote:

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.

RE: Basement wall Design calculation

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

RE: Basement wall Design calculation

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?

RE: Basement wall Design calculation

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

Mike McCann
MMC Engineering

RE: Basement wall Design calculation

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.

RE: Basement wall Design calculation

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.

RE: Basement wall Design calculation

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

RE: Basement wall Design calculation

@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.





RE: Basement wall Design calculation

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.

RE: Basement wall Design calculation

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?

RE: Basement wall Design calculation

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

RE: Basement wall Design calculation

35 is too low for at rest pressures. Maybe for active pressures with granular soils.

RE: Basement wall Design calculation

50 psi minimum for basement or non yielding walls.

Mike McCann
MMC Engineering

RE: Basement wall Design calculation

The IRC lists 30 psf for sand and gravel. My experience has been as long as you get some rebar in the wall in the right place, the connection to the floor diaphragm is what slips or fails. the IRC should give better guidance on this for Builders. I generally stop my joists 4' from the basement wall and ladder frame them in instead of blocking. (where the wall is parallel to the joists)

RE: Basement wall Design calculation

Excel:

Do you turn your plywood floor panels too? Seems that might affect the floor diaphragm capacity, vertically and laterally.

Mike McCann
MMC Engineering

RE: Basement wall Design calculation

Everyone here uses Advantech which I do not believe cares which direction the joists run - so they typically do not change it.

RE: Basement wall Design calculation

At mihmb-
This was a California Building Code requirement, 2007 CBC, Section 1806A.1, and the trigger was 12' not 15' and for a Cantilevered wall, not a basement wall.

I reviewed the current 2010 CBC, and it has similar wording but not the 12'. If you are on the CBC, take a look at section 1807A.2.3.

Basically, regardless of your code if it was my design and I was in a high seismic zone and designing a cantilevered retaining wall, I would do a secondary check of a seismic load combination using 0.7E but the full dead load, so D+0.7E and then check wall sliding and overturning to a 1.1 factor of safety. This is what 1807A.2.3 indicates in a nut shell.

Granted the "A" chapter of design only applies to DSA and OSHPD projects, but I still think it is a good idea. I have seen some large retaining walls suffer damage post earthquake. What that tells me is, there is an earthquake load combo that should be investigated in high seismic zones for cantilevered retaining walls.



RE: Basement wall Design calculation

@MSquared:

If you are using 50 PSF/FT min, how are you attaching to the floor system? That is about 500 plf shear load with 8 ft. of backfill on an 8'-8" wall or 665 lbs at each 16" O.C. joist. That would require an anchor bolt about every 12" and some fancy Simpson hardware. A far cry from the 6 ft. O.C. bolts the IRC requires.

That is what makes no sense about basement walls. If that amount of load actually existed in practice, most of them would fail at the top connection. I imagine there is enough give in the system that active instead of at-rest soil pressures prevail.

RE: Basement wall Design calculation

TDI Engineer,

What methodology do you use to check seismic soil loads against a wall?

The reason I ask is because I recently had a to perform such an analysis for an existing cantilever retaining wall. I was lucky enough that I could use some very conservative loads and the wall was still adequate. But, I was wondering what methodologies are out there for this type of analysis.

Most of my work is in a low seismic zone, so I have not previously encounted this situation.

Thanks!

RE: Basement wall Design calculation

Excel:

That's where you drop the joists/blocking and hang them off the plate, allowing the thrust to go directly into the floor joists or blocking, distressing the anchor bolts.

Mike McCann
MMC Engineering

RE: Basement wall Design calculation

OK, so here is another one. Let's say we have that 500 plf inward load at the top of the wall and they are using 24" floor trusses. Say we block for 3 bays (or 6' if they are 24" O.C.) The uplift at the end of the last block is 500 x 24/72 = 166 plf. That must be resisted by the last truss that the blocking is attached to - good luck! Even with 2x10's @ 16" and say you went back 3 bays of blocking, the uplift at that last joist would be 500 x 9.25/48 = 96 plf. Much less, but still significant enough to cause a bow in the floor. Never seen that bow happen, however. Which leads me to believe the actual wall pressures are much less than calculated.
My house has 11 ft, of backfill in some spots, but my joists frame into a band attached to the side of the concrete wall. The load goes right into the diaphragm.

RE: Basement wall Design calculation

Uplift?

Where to you get any uplift if the joists and blocking are hung off the sill plate with top bearing joist hangers? The lateral force goes directly into the joists and blocking, and into the floor diaphragm through the floor nailing.

Can you attach a sketch of what you are talking about?

Mike McCann
MMC Engineering

RE: Basement wall Design calculation

Wow, now that's a calculation I've never seen done before.

RE: Basement wall Design calculation

OK.

The joists and blocking are not dropped in the detail. I am saying that the plywood would be nailed directly to the top of the sill plate and there will be no uplift.

Mike McCann
MMC Engineering

RE: Basement wall Design calculation

I agree that is the best way to do it, but try to convince a builder to do it that way (at least in my neck of the woods). All they see is extra CMU or concrete costs.

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