Existing Building Basement Addition

[phwgray]

Good day,

I'm involved in a project where a single story building built in 50's had basement expanded for fur storage not so long after the building was built. Current owner was in the process of getting occupancy permit for new tenant and inspector, after looking into basement, advised client to get a engineer to look at it.

Added basement walls were built of CMU and are in good shape. Basement floor is slab on grade in good shape as well. Level 1 slab was a concrete slab that before used to be slab on grade. Surprisingly, the slab didn't have any large cracks on the underside. I designed shoring structure to support this slab as I had doubts of any reinforcement present in it.

Authorities did not accept my certification of the space after the construction of shoring structure because they say there is no evidence that added CMU walls are safe. They are now requesting the information about wall construction such as block/mortar strength, reinforcement, how it connects to basement floor slab etc.

Im curious to know how many of you would flag the wall "issue" during site visit and if you would be concerned of this wall performance. Again, walls are in good shape without any signs of distress or deterioration whatsoever and been there for 60 years.

Thank you

 

[KootK]

Im curious to know how many of you would flag the wall "issue" during site visit

I can't seem me spotting it.

and if you would be concerned of this wall performance.

Not unless something significant had changed recently that might affect wall performance. As you've alluded, the walls have been time tested.

because they say there is no evidence that added CMU walls are safe.

Unless one may consider past performance to be "evidence", I would agree with them.

 

[phwgray]

Forgot to add that load conditions on walls have not changed since basement expansion took place. Also, basement is not occupied and is not even used for storage.

KootK, I proposed reviewer to justify leaving the wall as is without looking more into it based on Structural Commentary L of NBCC but he said he wouldn't accept my Commentary L analysis without giving real reasons why.

 

[KootK]

but he said he wouldn't accept my Commentary L analysis without giving real reasons why.

As I understand it, the nature of Commentary L is that you get to use less conservative factors of safety based on successful past performance. Fundamental structural reliability theory stuff. I don't see how that helps you however as, no matter how you adjust your safety factors, you're still essentially trying to say:

Phi x No Demonstrated Capacity > Reduced Gamma x Loads

also, basement is not occupied and is not even used for storage.

Seal it off for good?

 

[JAE]

Seal it off for good?

What about the folks walking on top of the slab and falling in?

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[oldestguy]

If you are not PE , get a PE to write a statement that, in his opinion the wall is safe. Usually that is all it takes. If you are PE that shoould be sufficient.

 

[KootK]

Seal it in for good with dirt & lean mix concrete.

 

[phwgray]

Regarding the commentary L, it has a risk probability path only that eliminates dealing with structural engineering at all. My thought is that it was put in NBC for situations like this.

Sealing it off would be cost prohibitive. I could collect all the information that reviewer is asking by doing investigative demolition and scanning wall for rebars etc. but owner doesn't want to do anything else and implies that I resolve it with the reviewer without any other work so I'm walking away from the project.

I'm a P. Eng. and provided certificate under my seal saying that place is safe but reviewer refuses to accept it and saying that I have no reasons to state so.

Anyway, I'm still wondering how many of you would flag or worry about these cmu walls being unsafe.

 

[oldestguy]

Well you surely are in a hard place. Would it be possible to bring another PE in who majors in geotechnical engineering? He might do some simple site checks to tie in soil types and potential for changes, etc. and that adds another PE to he mix. Then sit down with the inspector and discuss history of the site as related to other sites and experience of you two engineers. Hopefully he has worked the area before and knows the inspector and has a lot of local experiences and age. At my old age I find that many years experience seems to have some weight when it comes to arguments. Also perhaps the inspector's boss might somehow be asked in (think about how it might not be good also).

 

[gte447f]

You are not in a tough spot, the owner is. I would probably also walk away. The authorities are asking for all sorts of things, yet the owner is not interested in playing ball with them and is not authorizing you to run on their hamster wheel.

I do not know whether I would "flag or worry about" the CMU walls, but if it is as you say (there is nothing visually apparent to suggest a problem), then probably not, and I don't understand why the authorities are so concerned either if it is as you say. The building has performed satisfactorily for 50-60 years without anyone being concerned, there is no evidence of recent changes, no signs of deterioration or distress... what is the problem?

On the other hand, why did you design the shoring system for the first floor slab if it also had been performing satisfactorily for 50-60 years without anyone being concerned, there is no evidence of recent changes, no signs of deterioration or distress?

 

[oldestguy]

Does the roof slab bear on he wall? If so that may be the main factor as to why it stands OK. With sufficient loads, there isn't any bending of significance from the backfill loads.

 

[hokie66]

phwgray,
Why do you think the Level 1 slab was originally a slab on grade? Sounds screwy at best.

oldestguy,
Axial prestressing has to be very large, i.e. not practical, in order to overcome bending on a wall. Have done the numbers, and it just doesn't work.

 

[oldestguy]

OG again. My comment about vertical load effect did not say that is the only resisting factor. It does do some good along with bending resistance horizontally and vertically if that "roof" connects to the wall even slightly. Putting numbers to these side benefits is not easy, so any "designers" neglect these benefits. So there sits a wall with no bonds bet ween the units, yet it stands. Since the average engineer doesn't know how to evaluate those benefits, he says it is not safe. A poor excuse for not knowing the full facts in detail.

 

[hokie66]

Yes, I do, as I have decided it doesn't help enough to matter.

 

[oldestguy]

Hokie: My comments refer to the poster or even his assistants. Preparing a set of calculations to address these small benefits that combined to hold things in place might say it holds,but verifying the degree each factor applies is something anyone can argue against. Again, a difficult situation and help from here is pretty scarce.

 

[phwgray]

The main floor concrete slab has irregular surface on its underside. My assumption about basement extension being dug up after the building construction was proven by environmental site assessment report. I couldn't know for sure that there is no reinforcement and I also knew that local authorities are sometimes overly conservative. Also in some locations slab is as thin as 3". Putting it all together and the fact that shoring structure wouldn't cost a lot, I decided to shore it up. Regarding the top of wall support, there are wood shims that transfer slab load on to the wall.

Appreciate your feedback people.

 

[oldestguy]

You might also consider substituting grout for those wood shims, for a more permanent fix. I'd also put some reinforcing between the floor and the roof, giving lateral support to the wall at the same time as helping hold up the roof. I'd gt a geotech to come up with what forces the wall has to resist from the outside when deciding on wall bracing. A belt and suspenders job that.

 

[nonplussed]

To the OP, this is really not your problem. It is the owner's problem to resolve. I assume your contract with the owner was along the lines of: "as a professional engineer, to inspect and assess the existing basement walls and slab and provide a report on the safety etc". If you have done this then the owner needs to pay up and go away. There is no reason to kowtow to the whims of a bureaucrat. However, if you stated in the contract that you guarantee local authority approval then that is an issue and you should never write that since that is beyond your control.

You have provided, as a PE, your expert judgement on the safety of the basement and remediation works. How you came to this judgement, whether by experience and visual determination, or calculation, modelling etc, is really none of the government's business. You are the PE and take full responsibility for your decision. While it is good to listen to concerns by local government and third parties and take the concerns into account, giving in against your better judgement sets a bad precedent. The local authority could very well continue to reject every assessment you make until you are forced to recommend complete demolition, at which point the owner could sue you for gross conservatism.

And don't say you are 'walking away from the project'. You are not 'walking away'; you have fulfilled your obligations as a PE and need to be paid accordingly.

 

[oldestguy]

non plushed: As yet we do not know if any contract is present between owner and PE and if written what it says. He indicates rather touchy local conditions and the need for walking carefully. Give him a break.

 

[nonplussed]

oldestguy, I agree with everything the OP has written and simply say that he/she is not in the wrong here and that the problem lies with the local authority.