basement wall analysis - counterforts, tank anlysis, etc. 1

[structineer]

I believe that I have read through all the previous posts regarding basement walls and I still have some questions. I am working on a 10,000 sq-ft residential home with a daylight basement. The total dimensions are roughly 85'x35'. The 85' sides have jogs in them. The 35' is a straight run with the other wall being the walk-out side. We do have retaining walls that are perpendicular to the basement walls at both side walls at the walk-out basement side. I am attempting to analyze this as a rectangular tank because the final construction will not allow transfer of force to floor structure and they want to backfill early. I have gone through the PCA rectangular tank design method and I have also created a finite element analysis using STAAD. My question is has anyone analyzed a wall like this taking into account that the bottom is not a mat slab, but a strip footing? The builders are used to seeing 24" strip footings. So, my thoughts on analysis is to release the moments at the base because the strip footing cannot take this load. Without a slab extending to the opposite side wall, my strip footing will experience some localized higher bearing pressures I believe and will be somewhat subject to sliding. I will only have the passive resistance of the soil because they will surely backfill before pouring the slab on grade. I know that in order for the wall to slide, the entire wall has to go, so with the jogs and the ability of the wall to span horizontally, sliding becomes less of a factor in my opinion.

I have considered counterforts, but after running the analysis in STAAD, the counterforts help stiffen somewhat, but the force is so high on this wall that the counterforts move with the wall based on the displacement output. That makes sense to me because counterforts are really most helpful for the wall design, not the footing. This tells me that the PCA tables aren't 100% applicable because the counterforts do not provide the equivalent resistance as a true side wall.

I am heading in the direction of designing the wall somewhere between a fixed base and a pinned base with a larger strip footing. I ran the wall as a true cantilevered condition and my footing would need to be 7.5' in width for stability. My thoughts are that I look into the PCA charts and compare the factors for moment at the base of the wall with my forces and reduce the moment in the base of the wall, effectively reducing the required size of the footing. In order to do this, I would need to select a b/a ratio that makes sense. My wall is 12' tall from top of footing. I was planning on providing counterforts at 8' o.c. This would give me a b/a of 0.67, however as stated earlier, I would need to select a more practical b/a ratio. I'm thinking of going conservative to b/a = 2 and seeing what my moments are at the base.

Any experience or thoughts on this subject are greatly appreciated!

 

[oldestguy]

Are you giving any credit to the building weight? Enough weight and you have no tensile stresses.

 

[oldestguy]

Sorry, I missed the early backfilling requirement. However, run a rough cost estimate for a safe wall system along with a time of construction for all the reinforcement placement and the like. Delays required also add to cost if weather coming on is a factor. Once this info is in hand, the owners may change their minds. Also, if corners are cut with the design, consider the cost of removing caved in wall, mud removal and starting over. Your insurance???

 

[structineer]

This is a wood framed house with brick, but that could not overcome the lateral forces from the 13.25' of backfill from bottom of footing to top of wall.

 

[structineer]

I definitely don't want to cut corners. I just know what they typically build and was trying to find a way to prove/disprove. I can analyze counterfort walls easily. I can analyze rectangular tanks easily. However, this is somewhere in between. I doubt many people have gone through the trouble that I have gone through for basement walls

 

[dhengr]

Structineer:
You gotta get serious here, you need a real retaining wall design here, maybe with a shear key on the underside of a much wider footing, if you think sliding is a problem. You also need some sort of return walls at the corners of the open end. That’s the price they pay for what they want, they are driving all the difficulties here, and they should do a little cost analysis of the best way to do this, or design around it, or do it another way. They can’t just say..., but this is the way we’ve always done it, 24" wide strip footings, 10" conc. walls, etc., and then also stick you with 10-11' of clay backfill, must backfilled before there is any support at the top of the wall, probably using a D8 cat for the backfilling and compaction. And, the idea that you should spend hundreds of engineering hours, likely unpaid, to save them a few bucks on rebar and conc. is not fair to you.

 

[structineer]

I have uploaded an image of the model that I created. One image shows the plate elements that make up the structure. The other shows the outline of the displaced shape (in green). This model has enlarged foundations. This model also shows stress contours for Mx, My, Base Pressures, etc. I am not looking at this as spending lots of hours for one project. I am trying to solve a problem that is real in the residential industry. being that I stated it was a problem, it is interesting how basement walls typically do not "collapse". I rarely hear of a basement wall having issues. I know several builders who provide 2' wide strip footings for 12' walls. I know architects who show these types of foundations and never turn on a calculator. I know this is not a true "retaining wall". It is somewhere between a true cantilever and a tank. I don't feel like the counterforts truly provide the fixed conditions that are used in the PCA tank method. I do however believe this stiffens the wall and increases the overall moment of inertia of the structure. This FEA should provide the information that I am looking for if I model it correctly.

 

[structineer]

2nd picture provided

 

[oldestguy]

Yes it is rare that a "standard" wall caves in, but they do happen and lean and it usually can be predicted if all the facts were known ahead of time. Curing time?? No contractor likes to keep forms on or wet down the exposed walls. Backing away never hurt once you look back on it later. I've seen a lot of walls that couldn't take it and never one designed like your links show. This day and age insurance companies also don't like insuring someone with a claims history.

 

[structineer]

Following up on this. My model shows that the wall still behaves enough like a cantilevered wall to rule out the use of the smaller, typical footings that are used on residential projects. These smaller footings may be fine for walls that are braced by the first floor framing (not that the framing is adequate to take the load), but they are not for open top tank-like structures. The bearing pressures at the toe are similar to what I found when I analyzed the wall as a cantilever type.

 

[rlflower]

First of all, I would strongly suggest having a soils engineer involved if you haven't already. He/she may help you achieve favorable bearing and sliding resistance capacities. Secondly, I would suggest sitting down with the client and the contractor to work out some cost viable alternatives. Compare the cost of a mat slab to the cost of a network of grade beams to stiffen the slab and to provide axial loading to the opposing wall (this is a design option I have employed in a similar residential. Third: where is the water table, and do we have snow/freezing conditions to consider? Remember that the water table is seasonal and can fluctuate considerably. A sump pump design may be necessary. Fourth: when all the above is "nailed down", give careful consideration for the potential of rotation of the basement walls in response to load and how that will effect the attachment of the structure above to the top of the basement wall. In other words, allow the structure above to slip horizontally, transverse to the wall below.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[dhengr]

Structineer:
Actually, I have seen a fair number of basement walls collapse or just roll into the basement, both during construction and some years after the bldg. was completed. And, I’ll bet Oldestguy has too. For a number of reasons; no top support or inferior support at the top, no support at the bsmt. slab, no reinf’g. or not enough, no lateral bracing when the wall was being backfilled prior to the 1st fl. diaphragm being in place, poor or no outside drainage. There are plenty of things that a dumb/lazy contractor can do here, that are beyond your control, so your notes, specs. and details better cover these.
Your counterfort walls are not a very practical solution or much help, but are very expensive what-with all the additional forming, rebar and conc. Put that same rebar and conc. into a real retaining wall design for the found. walls, and save the extra forming. As for Archs and their details, they make lot of purty pitchers that don’t work, all the time. The newest guys in their office do the drafting and details, and the more experienced guys are too busy picking brick and tile colors to check details that they still don’t really understand. You need a shelf for the brick veneer anyway, so make that a 16" wide by 24" deep horiz. beam at the top. The contractor won’t like that either, but it may help with your problem.