Existing Concrete Structure, No As Built or Existing Drawings, How to verify Capacity?

[IsaacStructural]

I've got what looks to be a series of little projects involving putting some AC panels on existing roofs. When previously discussed, the example projects had various steel bar joist roofs, wood roofs etc. Basically I'm supposed to do some quick calcs to verify the roof capacity and provide a simple letter, with an attachment detail or two, as required.

Now when we go live with a real project, the first project comes in and it has a concrete roof structure with no drawings and only some assorted close up photographs of concrete beams. I informed them I couldn't do a whole lot without some decent field information and I don't have X-Ray vision to see what the reinforcing is. Now the electrical engineer/PM on the project is giving me a hard time and saying local guys don't have a problem with this. I'm not sure if this is because they are able to go on site and field verify, or if they are just cavalier about writing letters certifying roofs they can't actually verify.

the only options I can think are:

1. to analyze the slab and beams as if they had minimum steel reinforcing and see if they would work for the loading (doubtful)
2. calculate the loads carefully and try to show that the panels represent only a small increase in total load (<5%) which might work

Anyone got any other suggestions?

Am I crazy? It is a small fee project, so any kind of destructive testing or X-Ray is probably not going to work.

Kind of want to send the electrical engineer a zoomed in photograph of some random piece of wire and ask him to send me a stamped and signed letter certifying I won't get killed if I pull on it.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[BAretired]

You can't verify the adequacy of a concrete roof if you don't know how it is reinforced. You might be able to indicate that the new loads are minor compared to the total load if you know the load used in the original design, but if you have no drawings, how would you know that?

I think you will need to use non-destructive testing to determine the location of the reinforcement.

BA

 

[TLHS]

You might be able to make some really conservative assumptions assuming minimum levels of steel for the slab. But if you start assuming for things like light weight concrete you're never going to make it work, so it seems pointless.

Are the beams big enough that you'd be comfortable that you're adding a small percentage of load? Maybe look at providing structure to bridge back to the beams and then try to prove the new load is a small amount of the beam capacity given minimum steel amounts.

 

[dhengr]

Some uni’s offer courses in cavalier clairvoyance and x-ray visualizational 3-D mappography. The advanced course allows you to verify the conc. and steel strength without even touching it. You mean that wasn’t part of your curriculum? Make the letter shorter, and you should be able to do it for a lower fee. That will make the PM happy. They always wonder why it cost so much when the letter was only a page or two long. Since they are already paying an insurance premium, why not tell them to get their insurer to certify what they are doing and save the middleman. Otherwise, I agree with BA.

 

[TDIengineer]

Do the panels prevent live load from accumulating on the roof? As in access is blocked?

Why not dip into the 20 psf live load capacity of the roof, I assume it flat? They won't be re-roofing with the panels in place, so capturing the live load is possible. Possibly a letter to the owner as such, informing them.

Also, with a concrete roof that is much heavier, I could easily see 5 psf added and not even worry about it..It when you add these panel to a 15 PSF dead load (panelized wood roof) where it could be a problem.

What is these panel weights per sqft? If you were in california, I would also be concerned about additional seismic mass if the area is large, but since it appears you are in midwest this isn't an issue! Will the panels change the wind load to the MWFRS? i would assume not.. but another valid concern.

 

[bookowski]

I think you're only hope is showing the additional load is negligible, or recapturing some live load as TDI stated. If you know the age of the building you could compare loads then/now - have snow loads been reduced? Roof live?

Assuming reinforcement is useless. You are essentially making stuff up at that point, and with concrete you can talk yourself into anything.

In general I think that signing off on anything that you have only seen in pictures is risky. Regardless of the original capacity how do you know there aren't other issues - corrosion, alterations, damage etc. Whoever is sending you the photos is probably not doing an in depth evaluation and sending you photos of questionable areas.

 

[JStephen]

So- my suggestion- might require some tweaking here! Right now they're all of the easy-going loosy-goosy persuasion and you're being the persnickety one. So you write them a letter saying
"Dear Sirs:
"It appears that the added equipment load will only be a small fraction of the total live and dead loads, and in my opinion, will probably not cause structural problems for the existing roof.
"However, as no structural details of the existing roof and roof reinforcing are available, it is impossible to confirm the adequacy of the existing roof, and in fact, it is not possible to confirm that the roof is adequate prior to the work, or that it met building codes when it was built."

Now, if they say that letter just won't do, you need to reword it, etc etc., then THEY are being the persnickety ones that are holding up the job. Or if that letter is all it takes, then there you go.

That's assuming it IS your opinion, of course.

Another thought- would it tell you anything to measure deflection (if any) when the equipment was added?
And if you do calcs and the roof isn't adequate, what do you do then? And can you do that now?

 

[MikeHalloran]

Take four nice big core samples from the roof, drilled all the way through, in a rectangular pattern.

Throw them away.

Erect a tower/truss/frame from the floor below right up through the holes to support the AC panels. Install gaskets/buckets/whatever for weatherproofing etc.

Problem solved.


Mike Halloran
Pembroke Pines, FL, USA

 

[hokie66]

What are these "AC panels", anyway? What do they look like and how much do they weigh? Regardless, you need to visit the site in order to make a decision.

 

[msquared48]

Unless you are superman and nave X-Ray vision, you will not be able to determine what is there without a forensic study. Good equipment will give you the size, depth and spacing of the rebar, but probably not the grade - use the worst case for that and see what you get for the allowable live load to see if it is in the range of what the structure would have been designed for at the time. You can get a rough idea on the concrete strength too, but be conservative.

Do you know the year of construction and the most probable design code (UBC) that would have been used?

I have to do this a lot, and you can get reasonable results, if the client will pay for the testing. If not, then fail the addition. Do not try to be Mr. Nice Guy here.



Mike McCann
MMC Engineering

 

[ztengguy]

If its a small fee, can you pass and suggest they hire a local guy to do it?

What sort of weight/footprint are we talking about? Is it super heavy, or just alot of them?

 

[Forensic74]

Why not just do the load test procedure in the code and be done with it?

 

[ACtrafficengr]

I don't have any direct experience with it, but at a conference, a ground penetrating radar presentation showed them using hand-held units to find the reinforcing in a concrete bridge beam. He said they can locate rebars with good accuracy, and determine bar sizes within a size or two.

It's probably as close as you're going to get, short of growing up on Krypton or going to Hogwarts.

 

[steellion]

Ground-penetrating radar is relatively inexpensive. It is good at locating reinforcement, but cannot accurately determine the size of the rebar. That said, your best bet would be to just assume code-minimum live load and check the slab accordingly.

 

[IsaacStructural]

Thanks for the advice and laughs everyone.

hokie66, I intended to right PV panels (solar). I have another job that involves a lot of modifications to window AC units, and I guess my wires got crossed. I probably need some more sleep. Why is that the week when work goes ballistic is the same week I'm packing up my hole house for a move on the weekend?

Hopefully I can get enough information to prove the new load is an insignificant increase compared to existing and if needed, I can reduce the roof live load.

Thanks

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[BAretired]

Without testing, you can't confirm that the roof is adequate for existing loads, let alone anything additional. Why is it that engineers are so anxious to accept responsibility for impossible tasks just to keep the client happy? You can be sure that the judge won't have much sympathy for you if the roof fails under the new load and kills somebody.

Either do the necessary testing, strengthen the roof or back off and let somebody else be a hero.

BA

 

[John2025]

Isaac,
My guess is the reason they want you, a long distance Engineer, is that they can't find anyone local who's willing to do what they want.
John

 

[dhengr]

Isaac:
I’ll bet they never really asked you to do any serious engineering on these jobs, did they? You know..., these are just small jobs, and we can’t afford real engineering, we need your fee money to increase our own profits. All we need from you is your signed letter, saying all is well, and a few phonied-up pages of what look like calcs. and engineering ramblings, so we can get our building permit. You don’t really need to know that some of the rafters are already cracked or rotten, and they probably haven’t looked anyway. That will come to light when the roof fails and you can deal with it them, because that company will likely be out of business. Then, please keep your insurance in force, even if the fees we paid you won’t pay your premiums. We will need you to help cover, or take the fall, the bulls we shoot. That’s the primary reason you’re involved, not for proper engineering advice, but as the insurer of last resort, another deep pocket to distribute the blame and liability.

This approach to engineering and the thought that the general public thinks this is how engineering is done, is really frustrating and scary to me. That really isn’t why I got into engineering, or how I want to do it, or what I like about my work.

 

[dik]

Biggest problem with rooftop additions in these environs is the added snow accumulation... Can you frame it with new steel rolled sections located under the slab?

Dik

 

[IsaacStructural]

Project is actually in Hawaii, so I'm not worried about snow, but dealing with wind/storm issues.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)