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.

 

[JAE]

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

 

[msquared48]

50 psi minimum for basement or non yielding walls.

Mike McCann
MMC Engineering

 

[XR250]

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)

 

[msquared48]

Excel:

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

Mike McCann
MMC Engineering

 

[XR250]

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

 

[TDIengineer]

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.

 

[XR250]

@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.

 

[mihmb]

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!

 

[msquared48]

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

 

[XR250]

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.

 

[msquared48]

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

 

[XR250]

here you go....

 

[jayrod12]

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

 

[msquared48]

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

 

[XR250]

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.