Basement per Contractor

[AZPE]

I continually have issues with contractors who swear that they do basement walls with masonry all the time under the following conditions -

-Walkout basement
-10' ceilings
-2' deep floor trusses sitting on a 4" bearing wall on the inside of the basement
-6" slab over the toe of the footing

The way I see it and anylize it is a 12'-6" tall wall (12'-8" to be block modular) cantilevered wall because the floor diaphragm is not resisted on the other side of the basement because it is a walkout which also means that you have to check for sliding because the slab doesn't terminate at an opposing basement wall.

The only way to get that wall to work assuming 35pcf EFP with masonry is with 16" block, specially inspected, with #6 bars at 8" o.c. that can switch to an 8" block with #5 at 8" o.c. at about 6'-0" above footing!

I can't imagine that that is more economical than a 10" concrete wall with #6 at 8" o.c. that can terminate and lap with #5 at 16" o.c. at 4'-0" above footing.

Both footing designs would be comparable so no difference there.

I have found that masons on residential projects here in Arizona have a mind of their own and don't follow detials well at all, whereas the CIP guys seem to be right on because they do the footings and the wall.

Any thoughts? And if any of you have the time to do a quick check of the numbers I would greatly appreciate it.

 

[civilperson]

Are you assuming 12' of fill against the exterior of this wall? Can pilasters, interior walls or bends in the wall allow you to span horizontally? Could the floor diaphragm transfer the horizontal forces to the side walls to eliminate the cantilever and make the wall a simple span? Details of backfilling can change the horizontal pressure to much more than the 35 pcf or maybe lessen this value.

 

[oldestguy]

Question:

How's come I have never seen a structural engineering analysis taking into account the benefit of weight on block walls?

Take the simple case of a child's toy blocks. Stack them up, say 10 blocks high and push down on them. It takes a healthy sideways push to change the pressure distribution to one where the resultant of the compressive stress and the lateral push vector combined is outide the middle third and they collapse.

Practically speaking a lot of blockwalls are not designed. The masonry work is poor, no sigificant bonding. They just work, due to sufficient compressive vertical stress from the building loads above, keeping the resultant of vertical and horizontal stresses within the middle third.

Can't say this is good engineering, but practically, that's what happens in some cases.

Comments on if this is analyzed by any who frequent here.

 

[PEinc]

A few years ago, I was called about a house that was for sale. The buyer's home inspector happened to notice cracks in the block basement wall in an unfinished laundry room. The rest of the basement was finished so that the block wall was not visible. The basement had a 10' ceiling. The wall was fully backfilled. The owner pulled down some drywall which showed the wall to be extensively cracked. The block wall was unreinforced and ungrouted, with no pilasters or shear walls. I had them reinforce and grout the wall and add tie rods and a deadman in the back yard. So far, so good.

 

[AZengineer]

I agree with civilperson. I have designed similar basements utilizing all of the methods he mentioned.

Oldestguy: I think it is unusual for an analysis not to take into account the weight of the wall, but it doesn't really help that much anyways. It helps in overturning and sliding (friction), but doesn't do much for the actual wall design as it seems you are implying. Take a cantilvered 12" thick concrete wall retaining 10' of 35 pcf backfill for example. The overturning moment is 35*10^3/6=5833 ft-lbs/ft. The resting moment due to the wall weight is 150*10*1^2/2=750 ft-lb/ft. Helps some, but not much - and the resisting moment is less than half that for an 8" wall!

 

[DarthSoilsGuy]

civPE, AZegr, or others-

do you know of a good reference for using these bracing structures in residential construction? We end up looking at each section separately and more or less reinvent the wheel for each wall section. it may be the most appropriate to look at each wall span, but it seems like there should be some sort of design checklist or flowchart already out there for this since it is so so common.

we don't have control of where to place bracing structures since the Arch has everything done when it gets to us.

The way i see it there are 3 ways of simplifying the walls and they all have their drawbacks.

1.cantilever ret. walls - gets shot down by the contractor and owner b/c of cost unless it is absolutely necessary for the walls to cantilever for some time frame due to means/methods.

2. one-way slab or simple horizontal span- counting pilasters/interior wall/& wall bends, but we're never lucky enough for a good spacing to give a simple design back.

3. pinning the top-of-wall with the 1st floor - with walk-out basements side walls could step-down to follow the grade and you're not going to pin the TOW with a vertical stud wall on the sill plate. Also, there are openings for stairs (which make architectural sense to place on the retained fill side so rooms get the basement windows) which takes away TOW pins. Also there is the whole un-pinned diaphragm issue.

It seems to me like there should be something out there already to put it all together.

 

[oldestguy]

AZengineer:

How did you come up with a weight ON the wall?
He didn't tell you anything about the structure above as to spans, floor loading, roof, etc.

I agree the weight of the wall is negligable, but what about the weight ON it, and the resisting ability of a deck on that wall, probably tied in with anchor bolts?

This puts the wall into a situation of a beam standing on one end, not a cantilevered retaining wall out in the open.

 

[Machine15]

I think you should re-analyze your system. Why are you designing a cantilevered wall? Just because the other side is wood framed and not full height masonry? Then all wood buildings would not have a shear wall diaphram system. Anyone else agree or disagree with that?

Also, if you were to analyze a cantilever wall with additional dead weight on it. That would only induce P-delta forces and really decrease alot of your capacity.

If you had a simple span, that would increase capacity but not enough to consider it in designing laterally. Not to mention you should porbably only take 60% of that DL.

 

[AZengineer]

DarthSoilsGuy - I'm not aware of any refereces.

oldestguy - I didn't include any weight on the wall. You're right, it would help, but my point is that the moment arm in the wall (d from compression edge to center of reinforcing) is so small that for any truely beneficial weight to aid in bending will approach the compression limits for the masonry retaining wall. In your analogy to a stack of blocks with an axial load, it makes a huge difference if the blocks are 2" wide or 6" wide. I'm not quite sure what you mean regarding the beam standing on end or the deck tied in with anchor bolts...

Machine15 - he is designing a cantilevered wall because there is not an equal and opposite reaction at the opposite side of the basement to counteract the load exerted on the diaphragm. True, you could still design it as a restrained wall and resolve the top reaction into the perpendicular walls through a diaphragm, but it often times isn't an option because of the geometry of the floor plan - i.e. if the perpendicular walls are short in comparison to the restrained wall, the diaphragm shear becomes too large.

I really don't think P-delta effects have anything to do with this problem - it is not a slender beam-column type structure.

I am still trying to figure out what your last two sentences mean...

 

[bylar]

are you using IBC if so look in section 16, 16.8 I believe and for a basement with sand or sand and gravel (not clay) you have to use 60 psf and clay 100 psf.
Why won't they sit the floor on the wall and therefore its a retained wall and not a cantilever!

 

[allisch]

I Agree with machine15.
Normally the houses around here are 24-30'+ wide.
Use the floor as a diaphragm to carry the loads to the sidewalls.
Then design as a simple span vertically oriented.
Take the loads to the floor and then to the sidewalls.

 

[sundale]

The resulting diaphragm shears due to the main floor spanning to the side walls for a "walkout" basement are quite large for the material. Recall that the IBC Chapter 23 allowable diaphragm shears imply 160% stresses for wind and seismic, whereas an unbalanced lateral earth force imposed diaphragm shear should consider at least 90% stresses.

I NEVER do a wood diaphragm in a walkout condition unless the aspect ratio is very favorable. This gets much grief from the contractors, who are typically omniscient. Namely, any wood butcher with a pickup truck and a dog knows more about structural engineering than I will never know.

In reality, the great majority of houses with unbalanced earth forces work just fine with zero engineering because they never actual receive the "design" Rankine at rest pressures. How many times have you seen a 10' vertical earth cut? There should be 400psf or more of pressure at the bottom of this cut, and yet it is resisted by air. This being said however, I have seen about a dozen or so houses with serious foundation problems because lateral earth forces were not considered properly. These had some costly helical pier tiebacks as a structural repair.

I have seen a couple of disciplinary actions from the CO PE Board for ingnoring unbalanced earth forces in a walkout basement...

 

[Machine15]

sundale,
Please tell us how you design walkout residential houses if you do not use a wood diaphram. Concrete? It does not seem economical to build the way you describe.

I know that the walkout house I live in that was built 25 years ago works like a charm still today.

 

[IceNine]

Machine15,

If you design the basement wall as a cantilevered retaining wall and backfill before the wood floor is in place, the wood diaphragm will not see the soil loads.

 

[lornapaul]

Good morning,
I am looking for HELP and if anyone can help id be very much appreciate it.

My concern is with PYRITE. I live in an apartment block in a development in Dublin Ireland. There is alot of houses in our development affected and in other developments in which the contractor used fill from. Regarding the apartment block it was constructed with pads and floor all case insuitue. Bue now 2 years on there is cracking in the floor occuring and white flacky ftuff coming out. I was told that the apartments at this stage would not be affected and there is nothing to be concerned about. They are not going to take up the floor in the basement and remove the pyrite (good or bad?). Byt recently a person in an apartment on the first floor has got tests on his floor done and he was told it looks like there is pyrite affecting the floor. If this is the case they must have used it in the concrete to. My question regarding this test is. Pyrite only has affects if its subjected to water (yes / no) If yes its ok to have it in a concrete floor construction (yes / no)
My e-mail is paul_cadtec@hotmail.com
If anyone can help i realy would appreciate it !!!!!. I have tryed to find out things regarding pyrite and all iv gotten is closed doors and its next to imposible to find anything on it on the NET.
If you know of sites please e-mail me.
Many thanks and do hope to hear from you.
Many thanks,
Regards,
Paul