Unrestrained Basement Wall
Unrestrained Basement Wall
(OP)
Looking to see if anyone has some tips/ideas to design the unrestrained basement wall as seen in the attachment. My initial design was a retaining wall. Like a typical project in the design-build field, the contractor thinks this is nuts and way overdesigned. Per the usual, they did not have this amount of concrete figured in their contract and are super concerned about cost. After talking to their concrete subcontractor (due to lack of belief in the engineers design), the subcontractor believes (2) pilasters extending 4' into the soil as denoted in red will be sufficient to brace this wall. I took this approach and looked at the wall spanning as a continuous beam and used the pilasters as supports. The force at these pilasters becomes very large and to prevent overturning and sliding the 4' length will not work. I assumed this would be the case and the length required becomes extreme and I will hear major push back from the contractor.
I have thought of other possible scenarios that may work:
1. Span wall the 36' between cross walls (unrealistic due to reinforcing and depth needed)
2. Place steel beams on the outside of the wall and somehow tie to the basement wall (beam would span horizontally to cross walls.)
I know my design is not dictated by the contractor that had overlooked this when bidding, however I want to give the most efficient and cost effective solution possible. I am at somewhat of a lost on this. Any ideas would be greatly appreciated.
Thanks
I have thought of other possible scenarios that may work:
1. Span wall the 36' between cross walls (unrealistic due to reinforcing and depth needed)
2. Place steel beams on the outside of the wall and somehow tie to the basement wall (beam would span horizontally to cross walls.)
I know my design is not dictated by the contractor that had overlooked this when bidding, however I want to give the most efficient and cost effective solution possible. I am at somewhat of a lost on this. Any ideas would be greatly appreciated.
Thanks






RE: Unrestrained Basement Wall
RE: Unrestrained Basement Wall
RE: Unrestrained Basement Wall
1) Would you consider the slab on ground to provide sliding resistance at the base of the wall? This may work with the slab in friction or bearing against a wall on the opposite side. (This assumes at least a 4' heel)
2) Think of the wall as a basement wall continuous to its 20' height. If I were to do this, I'd likely design it as a propped cantilever with reinforcing on the inner and outer face. It seems that you can use a 4' heel, as ytyus shows, and based on the sketch with the pilasters
I understand that you want to work with the builder. Even though they sometimes throw out some seat of the pants ideas, I've found that the good ones often have a decent sense of what might work.
RE: Unrestrained Basement Wall
I'm sure you could make the proposed fix work, but would it be cost efficient? (I'm asking not telling)
Under your original design, the vertical steel is the primary reinforcing steel, and the horizontal steel is just designed for temp & shrinkage.
Once you put the two counterforts in, won't you have to increase the size of the horizontal steel significantly since you will now be relying on it for flexure? Can the vertical steel then be reduced in size?
It all kind of seems that you end up in the same spot cost wise?
RE: Unrestrained Basement Wall
Still some good ideas that you have given and maybe a hybrid like you are describing is the way to. Possible steel columns at a specific interval that ties the top of the concrete wall to the precast diaphragm above, like the propped cantilever you described.
RE: Unrestrained Basement Wall
I am on the same page with you on this, cost can be saved here and there but a change in member functionality also affects cost.
The contractor is concerned about the footing and excavation cost. I know the major problem with this project is the gentlemen that bid this thought this was a typical basement and a 12" wall with 2' strip footing would be sufficient. Obviously this is not the case and it really is on him for misreading what was going on. Not saying that wall would have worked anyways, but this is the dilemma I am at. Trying to make the customer happy (especially since we get a lot of repeat work from his company) and not taking on the reputation of over engineering.
RE: Unrestrained Basement Wall
What about something like this
RE: Unrestrained Basement Wall
www.PeirceEngineering.com
RE: Unrestrained Basement Wall
RE: Unrestrained Basement Wall
Another great idea. This was brought up and mentioned already as a viable alternative. However, this was shot down as nothing can be placed on the interior of the building due to usage.
This project like the ones we all dread really tests ones patience with customer/contractors. Give numerous solutions that are viable and get rejected as this will not work per their requirements.
RE: Unrestrained Basement Wall
Yeah, that was always going to be a dud of an idea cost wise. Gracious of you to entertain it though.
I hope that we're not considering trying to span the track for 36' as girt. I see the propped cantilever as being a concrete wall all the way up to the precast elevation. That would work but the economics are probably not better than the cant retaining wall unless excavation was the big cost driver.
I've done this on several occasions using a single concrete beam (girt) located a couple of feet down from the top of the wall. It means:
1) Probably some doweling.
2) A second pour.
3) Enough property line clearance to get it done.
4) Some consideration for building envelope concerns.
I've had to do this behind long staircases where I end up with the two story condition for a stretch. It can take some selling to get contractors on board but, in your case, that should be easy since they've already rejected the conventional solution in search of something creative.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Unrestrained Basement Wall
Reinforce the soil backfill to again reduce the soil pressure.
In any case, the contractor will need to install a more expensive system. Rest assured, your numbers are real and compromising a building's foundation should not rest on your shoulders.
RE: Unrestrained Basement Wall
RE: Unrestrained Basement Wall
Dually noted. There are some walls that are restrained and others that are not. Don't know how this was overlooked by the our customer when he bid it.
MotorCity,
Fully agree with you. That's why I had designed a retaining wall with large heel; this was rejected.
I can get the pilasters to work, but they become much longer than the 4' that the contractor is proposing. When I did my design, I assumed the wall spans across the pilasters (continuous beam) and the forces become rather large at each pilaster. Connections between this perpendicular connection will become ugly and I am sure the client is not thinking of this either.
Does anyone have any comment on the propped cantilever design that "kipfoot" mentioned? I modified his approach by suggesting using steel columns to connect the top of the wall to the precast deck/diaphragm above.
I would just like to thank everyone that has chimed in so far. I really appreciate your suggestions and hopefully a high-bred (as we continue to discuss) of these will work for the contractor. Hope.
RE: Unrestrained Basement Wall
Interesting that you have done this with concrete. Is this a concrete beam outside of the basement wall? How are the (2) connected together; epoxy dowels from the beam to the basement wall?
RE: Unrestrained Basement Wall
It is a concrete beam outside of the basement wall. Assuming that space isn't a big deal, I'd want.
1) Nice and stiff. L/d < 15.
2) Slope the top convincingly for drainage.
3) Watch your positive moment bar anchorage at the bearings.
4) Maybe void form underneath if you've got frost or expansive clay.
Yeah, drill and epoxy or dowel bars bent inside the form if the contractor prefers. It's really a version of a bottom loaded beam which, of course, brings up some detailing issues. Adhesive should also make sense for a permanent tension application.
I've yet to talk anybody into doing it but my dream connection would be U-bar dowels wrapped around horizontal wall bars and provided with form saver couplers at the outside face. Vert bars inboard of the horizontal.
Yum.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Unrestrained Basement Wall
RE: Unrestrained Basement Wall
RE: Unrestrained Basement Wall
All the ideas are great, but all the extra labor/materials in formwork is probably going to end up costing more than a couple extra yards of concrete. Just sayin.
RE: Unrestrained Basement Wall
Did not give that design guide a thought. Have used it before, but wasn't thinking of this project as a tank.
I fully agree with your point. There are so many ways to strengthen this wall, but in the end cost is the key factor and I am with you that small amounts of concrete and excavation outweigh non-traditional design methods that the contractor is not familiar with.
In the end, I believe it is our clients lack of knowledge with this irregular style basement that has led to the issues I am having trying to meet his criteria. The individual keeps resorting back to the saying, "not in my budget." We have all been in this situation I am sure and all it does is make design professionals look like we are being over conservative in design. This is by far not the case and I hope that clients like this come to this realization. Obviously, we are all design professionals and we strive to be contractor friendly while also providing a viable engineered solution. As we continue to move forward in our professions, I hope all of us can continue with this notion.
Reverting back to the original unrestrained basement wall, our customer has finally agreed to my design of a retaining wall after pointing out to him the costs associated with different design approaches we have presented.
I am very appreciative to all that have contributed to this discussion.
Thanks
RE: Unrestrained Basement Wall
Do you think that maybe asking the client to view this thread and the comments might help a little?
RE: Unrestrained Basement Wall
Great thought. I know there have been some comments from this client about talk in his office to use a engineering firm from a different city to get a second opinion. Which means hoping for a better solution (cheaper). Maybe this can portray to him that I am not "incompetent" and looking out for not only myself but his reputation as a quality contractor in the long run.
RE: Unrestrained Basement Wall
RE: Unrestrained Basement Wall
I think the "cheapest" solution will be to run a couple of steel "wind columns" full height (as you mentioned).
If they could live with the 4 ft high pilasters, the full height wind columns might not be too much more intrusive.
Agree with the others, 36 feet is too far and they definitely will have a problem if something is not done to brace the wall.
About the "not in the budget...." situation - perhaps the owner would be open to the notion that the extra cost is a legitimate extra. IF the contractor had included it originally, his number would have been higher and they would have had to pay for it anyway. Only counter argument is if his price (without it) was compared against someone who did have it in their budget. From the sound of things, nobody had it in their budget.
Seems to me that whoever gets the savings could also get the risk and in that case, they deserve to know what that risk vs.benefit is.
Just a thought.