Certifying existing building

[demayeng]

We've been asked to look at a suspended concrete floor in an existing building (built around 1980) to advise on load capacity. There are no existing drawings available.

Out of interest, how would this process occur? If the client does happen to find some drawings, I could analyze it no problem, but how do you know if it was ever built to plan? How do you know if all the steel went in correctly? We could probably test the concrete quality.

And do we assume responsibility for the whole building if we were to check the slab? Seems like a process that is fraught with liability compared to designing a new building. If we said the slab was OK, then next week a bracing failure flattens the building and we're the last engineers to look at it, surely we would be on the prosecutor's list? Checking and signing off the whole building would be a mammoth task.

ps this is all hypothetical as I have declined to do the work, but I'm just curious as to what others would do in this situation.

Thanks!

 

[JedClampett]

Get some waterbeds to simulate the load they want to achieve (plus a factor of safety) and test it.

 

[WARose]

It would mean checking the building top to bottom (including connections). After all, you don't know what the assumed dead or live load is for the floor so you cannot assume anything. So we are talking a lot of field work. In the past, I've outsourced a lot of the field measurements because it saves the client money: my rate for pulling tape is the same as doing engineering work in the office.....and there are a lot cheaper options available.

As far as the slab goes, you can use ground penetrating radar to get re-bar locations and approximate sizes. At about that point in time, they were using 40 ksi re-bar....but some testing would have to be done.

I don't know how badly you/your company needs the work......but you may have saved yourself doing a quote for nothing in this. A lot of times, I've done these sorts of quotes and the client nearly passed out over the cost. (And we didn't get the work.) And they found some semi-retired yo-yo that will seal anything for $20 to approve whatever they wanted to do by just looking at it. I just got finished putting together a quote on a roof modification where there are no drawings and the cost of the field work alone was too much for this guy.

 

[Canuck65]

I have had to evaluate the load capacity of existing reinforced concrete slabs in the past. Every situation can be unique, but in general I follow something like the list below:
1. Ask/look for drawings, wherever you can, local building department, building owner and I have even been lucky identifying the original design engineer sometimes and have given them a call. (Also, for a 1980 building, I would generally assume that it had been professionally designed at the time of construction - no guarantee though)
2. Review the original/current occupancy of the building and correlate with the building code in force at the time of construction to estimate a probable design live load.
3. Site visit to determine how the slab is performing. Is it in pristine condition? Does it have flexural cracks everywhere? Is there environmental related deterioration?

At this point, you should have an idea on where this is going ... but more investigative work required.

4. Covermeter survey top and bottom in localized areas selected to be representative of the standard condition. This will give you rebar spacing, concrete cover, bar diameter and slab thickness.
5. You can usually estimate a reasonable concrete compressive strength based on the original design code, or a few concrete cores could be taken for compressive strength.
6. Do the calcs. See if the calcs come in somewhere close to your assumption from above.

At this point, if the slab is in good condition, one usually has sufficient information to provide an answer to live load capacity of the slabs.

7. If still uncertain, load testing as per the applicable code can be completed, but this starts to get quite expensive. I have only had to resort to load testing perhaps a dozen times for various reasons, many more projects were completed without load tests. In my experience, one can get a much, much, higher live load capacity from load testing a two way reinforced concrete slab than calculations would permit, so proceed with caution on the interpretation of the results.

 

[MrHershey]

Would echo WARose's last point. We get asked this every now and then and I'm perfectly happy to give them a ballpark up front because they usually pass and go find someone else.

Also probably should check with your insurance carrier. At least in the US many/most insurance carriers will not cover you to 'certify' anything. They'll only cover up to the 'degree of care' which stops short of making any guarantees like 'certify' has been interpretted to imply. If you do have to 'certify' something, you'll likely want to sneak in language that reigns this certification in to apply only to the items you can actually control, like certifying your calculations are correct based on the assumptions you made. Can't leave it broad enough that it could be construed as you making guarantees on stuff you didn't even look at or possibly didn't even know about.

 

[JStephen]

Occasionally, in my line of work (tanks), a similar case pops up. The problem is that, ultimately, the owner is wishing to come up with certifications that would normally be furnished with new construction. Well, you can take the existing structure, and there's some things in it you can measure or test or X-ray or whatever. But other aspects, you simply can't "certify" after the fact. For example, with a weld, you can measure it or do radiography or UT, but you can't actually say it conformed to the welding code just by looking at the existing weld.

I see in IBC 106.1, a clause that design live loads above 50 psf are to be posted on the story in question. Is that the ultimate goal of this exercise? If so, is a floor that is "evaluated" the same as one that is "designed" for a particular loading?

My experience on the use of language- is that nobody pays attention to how you word it, except maybe a court. If you "certify" 50.0000 psf or write a report that says "I'm guessing it's good for maybe 50 psf, plus or minus 49 psf but haven't really given it much thought", it won't matter, somebody's gonna post that sign that says "50 psf" based on your statement.

 

[demayeng]

Thanks for the replies

Seems like it's a bit of a grey area for engineers, and probably best to avoid this sort of work if possible.

Only do them if you have to, and make sure you allow enough in the quote to be as comfortable as you need to!

Load testing would be great, but sounds expensive.. and possibly dangerous!

 

[msquared48]

Only for the waterbeds...

Mike McCann, PE, SE (WA)

 

[Canuck65]

demayeng,

Load testing, done properly, is not dangerous. For all the reinforced concrete slabs I have been involved in testing, the entire underside is shored up to within a couple of inches of the underside of the slab. If total failure occurs, the slab settles down onto the shoring. Often water is used for the load, so that it can be evacuated quickly in the event of a failure (as opposed to concrete blocks, sandbags, granular, all of which I have also used) I have never had this type of failure happen. Deflection gauges are mounted to the shoring to measure the deflected shape of the slab, plus also monitor the rebound after the test is complete. It can be expensive, but I do find the process interesting and in several cases the load testing has benefited the owner greatly.