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

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

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

(OP)
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)

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

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

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

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.

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

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. smile Otherwise, I agree with BA.

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

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.

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

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.

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

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?

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

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

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

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.

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

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

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

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?

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

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

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

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.

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

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.

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

(OP)
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)

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

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

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

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

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

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.

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

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

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

(OP)
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)

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

Write the letter saying based on your knowledge, and calcs, the rook isnt ok for the additional load, and the project shouldnt move forward. I be somehow they find the drawings, or they agree to pay for investigative work. Your not lying, or gambling, and you did your job.

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

To say the roof is not adequate to carry the additional load is just as bad as saying it is. The fact is that you do not know. It is best to tell the client what needs to be done to make the roof acceptable.

BA

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

And if you do not know exactly where the building is in Hawaii, don't assume that there is no snow load. There is seasonal snow on Maui...

Mike McCann
MMC Engineering

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

Do not assume that there is no snow in Hawaii as it does depend exactly where the building is. Next to the coast - no snow, but on the mountain tops, particularly at the observatories, there is - I believe that is on Maui at Mt. Haleakala? Do not quote me on the spelling... :)

Mike McCann
MMC Engineering

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

(OP)
That is fair point Mike. I do have a location on the building and it is not in the mountains. I'd like to just walk away from the work (or not taken it in the first place), but the contact is from within the company, so I have to walk it back more slowly. I'm thinking we'll find a way to pass it to a local, who should have done it in the first place.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

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

Isaac:
It should scare you a bit, that your company or someone within the company, thinks so lightly about the significance of real engineering, and what we do, and what we can and can’t do, that they would push you to stick your neck out this way. If you can’t see it, inspect it, test it, or be given original design docs. and a reliable condition report, you don’t have a thing to hang your hat on. If your company is willing to put you in that kind of position, just to sell product, whatever the contract, or whoever the contact, and their ignorance of what real engineering is, you better watch out. They’ll as likely hang you out to dry, if anything goes wrong, you know... he should have told us he couldn’t do that, etc. And, at that point all you’ve got is your knowledge, experience and P.E. licence, and only you can take good care of those. Your company should just not be putting you in this kind of position, and you should have a talk with your boss about this.

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

It's not your fault that the Owner doesn't have records for his building. Don't take on responsibilty - leave it in the hands of the Owner. You might feel bad for the Owner and want to help him out, but it's his building, not yours.

A load test is probably the only way (unless you are under the 5% increase), but that will be time-consuming, intrusive, and expensive.

My experience with ground-penetrating radar has not been good when you need to do real analysis. To find out where the rebar is when drilling a hole? Yes. To determine exact size and location? No.

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

A contrary view, if I may. I don't think these solar panels are very heavy. It would be a different manner if a lot of air handling units or similar were being added. Any concrete roof that I have ever designed would have no problem supporting some glass and aluminium frames. I would be more concerned about the integrity of the roofing. But saying that, I would still require a site visit to confirm the conditions for myself, especially since the building is in Hawaii.

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

I tend to agree with Hokie66 provided they are not ballasted panels. Ballasted panels use pavers or bricks for anchorage that keep them from blowing off the roof. They don't require any penetrations in the roofing. I have seen typical panels with frame weigh-in around 3psf, while ballasted panel weights run all over the place - 6 to 20 psf. (Disclaimer, I don't have a lot of faith in ballasted panels, but the PV company has somebody else stamp those plans and provide calculations that often times include wind tunnel testing of typical arrays on typical buildings)

Also, it is very unlikely that the roof will ever see the design live load concurrent with the solar panels. I have seen some engineers justify the additional loads by stating the roof live load will not occur at the panel.

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

Ballasted panels just increase the load on an already questionable structure.

Dik

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

I'm not sure about trading the roof live capacity for the panels. If maintenance is carried out on the roof at some point, the panels may be stored vertically in an area on the roof while maintenance operations (replace roofing membrane for example) is being carried out. Potentially exceeding the live load capacity of an affected area. If the roof was new, would you design it for a case considering only the roof live, only the panels, or both at the same time? My point is that things get stacked in areas on roofs during re-roofing and this is now something else up there that would have to be accounted and that is not clearly addressed how to deal with by Code (the combination is a function of D+RLL, many would consider the panels to be a dead load).

The panels are picking up new wind, the amount depends on the angle of the panel to the roof of course. I don't know the building height but if single story it may be a decent new percentage. There are requirements in IBC Ch 34 for existing elements picking up more than 5% new load.

I understand that more expensive pachometers can tell the bar diameter as well. That plus concrete testing is what I would look at.

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

Might only be 3-4 psf more, but how do we know it hasnt been used already ....perhaps there is a array of pipes hanging from the ceiling, last engineer said, eh, its only 3-4 psf. Yada yada yada....I would find a way out.

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


"especially since the building is in Hawaii." Yea, right Hokie... lol



Mike McCann
MMC Engineering

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

Whats the outcome of all this? Any update?

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

First Measure the depth of the slab and calculate the Dead Load (assume the density)
if the additional load for the AC panels is within 20% of your dead load then most likely possible

Anyway Since its AC panels most probably the load is negligible if compared to the carrying capacity of the slab.

good luck



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

(OP)
It isn't all resolved yet, but I managed to get a higher up structural engineer to go out and visit the projects. He has the authority to handle the situation and won't allow it to come to us signing for something we aren't comfortable with, so I the ship has been leveled and we are back on course. Most likely he'll be explaining to the Electrical Engineer that we can't do the work for the proposed fee range and the company will reconsider if these projects are really something we should be pursuing.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

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

I don't know if this is relevant to you or not.

We do similar projects like this one every once in a while. They are mostly residential structures and consist of someone going onsite and taking measurements of the existing roof. We then usually take a quick look at the roof for the revised loading. In my area we are subjected to snow loads and I always receive the "but the snow is going to slip off the roof" argument which I follow up with "but the code doesn't say that" followed by a days worth of head butting. We usually supply an attachment diagram so they know to "uniformly distribute" the load on the roof. In a few instances we have had them take off the solar panels an attach the racking in accordance with our diagram. I would say about 30-40% of the time we have a contractor reinforce the roof which I imagine will disappear once we are allowed to use ASCE7-10.

I have often seen advertisements for structural engineering jobs at solar panel companies. I imagine a bulk of the work is fighting management to OK roofs for solar panels. Not something I would wish on my worst enemy.

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

Quote (SteelPE)

...we have a contractor reinforce the roof which I imagine will disappear once we are allowed to use ASCE7-10

Why is that?

EIT
www.HowToEngineer.com

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


Quote (SteelPE (Structural) )

16 Jul 13 14:46
I don't know if this is relevant to you or not.

We do similar projects like this one every once in a while. They are mostly residential structures and consist of someone going onsite and taking measurements of the existing roof. We then usually take a quick look at the roof for the revised loading. In my area we are subjected to snow loads and I always receive the "but the snow is going to slip off the roof" argument which I follow up with "but the code doesn't say that" followed by a days worth of head butting. We usually supply an attachment diagram so they know to "uniformly distribute" the load on the roof.

But, the "new" solar panels and panel frames WILL "trap" the uphill snow loads and prevent those weights from sliding off the roof. So, the total load (ice + snow + panels) will be on the roof for longer periods of time. The melted snow will also get trapped and re-freeze into heavier point loads rather than wide area general loads.

I question the "pounds per square foot" assumption too: A solar panel will "concentrate" its loads into a very small area where the lift (airfoil) loads and pressure (gravity and airfoil vibration) loads are attached to the roof itself. So, the total loads won't be spread out like an average snow load, but pinpointed into bolt hole sized areas.

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

Question away. When we are allowed to use ASCE7-10 using Pg to design the rafters change to Ps for a roof slope greater than or equal to 7 on 12 (I'm sure the jurisdictions will increase the snow loads accordingly). And I know that a beam with a uniform load placed on it has a higher moment than one with equally spaced point loads. Ice buildup may be a problem but until it is addressed in ASCE 7 I have no idea how to account for it.

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

Equally spaced point loads produce the same bending moment as a uniform load if the number of spaces is even, slightly less if the number of spaces is odd.

BA

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