Waffled Concrete Ceilings

You may have better luck at the HGTVPro.com forums

My question would be this: Is the time it would take to do the scan more or less than the time you may loose if you take a guess (even an educated one) at the internal structure and are wrong?

I agree 100%, but unfortunately the time issue isn't my call.

Wow!

You mean to say that there are NO dwg's? Someone must have built it to SOME specifications sheet. An airport? someone b%&s should (must) be chopped off!

AND, If I were you and time issue is not my call, or at least I have got nothing to say,I would simply not take the job. YOU will be the responsible person for any disaster, if, God forbid, happen. The consequences might be too sever.

Wangito.

<....b%&s...>

It is not known whether the Designer had any in the first place.

If the building is a major American airport, there should be drawings available. Check with the building authority in the city. They ought to have drawings. Or find out who designed the building. They will have drawings.

Waffle slabs are the system you are describing. Each rib in each orthogonal direction will be reinforced, probably with bars. Could be pre stressing strand but it is doubtful. Usually there were two bars, one straight on the bottom of the rib and one bent. The straight bar was only required to be 3/4" clear of the soffit of the rib, but in a building such as an airport, the cover may have been a bit more to provide fire protection. I wouldn't count on it though.

The size of bars depends on the spacing of columns. At a guess, I would say the bars are probably 5/8" to 7/8" in diameter. With only four inches in width, I would expect each bar to be centred in the rib, but there is no guarantee of that either.

Hi, Out Of Box Experience!

There's probably no guarantee that the final pourings have been kept to spec either. I can't see that you have any option but to scan, depending upon the pupose for and size of the fasteners you want to put into/through the slabs. If the city/airport can't give you useful specs re the contents of the waffling, I think they are obligated to pay for the xrays, and extend the time of your contract to get them done.

On the other hand, if all you want to do is, for example, hang signage, then inserting a few screws into both sides of the ribs should not normally cause any difficulty. Check out what's already in place to see how it was done.

If the work is more complex, you have to remain cognizant that too many roofs are coming down everywhere, and it's been caused by folks who want to short-cut.

Sorry. I guess on the positive side, you'd be saving both the city's/airport authority's and your own insurance companies a lot of money if a worst-case scenario could be prevented.

In the course of time, the danged iron rebar may eventually cause spalling anyway, but that's, hopefully, a loooong ways off. Try to not expose any.

Mark

The "waffles" are really looking at the bootom view of beams and floor/ceiling. The whole thing has a grid of rebar that was set per engineers spec and should be readily available through the facilities maintenance office. However a standard that is used is that the rebar has to be a min of usually 3" from the surface of the concrete so depending on the weight and type of anchors you are using you may or may not even need to factor in the rebar. Are the mounts such that you cannot adjust a hole a few inches? Dead center of the narrowest part of the beam structure would be the best bet for free space. If you are going deep you migh want to know what might be in the concrete other than rebar. Hope this helps I'll be watching.

Thank you ba/ael and DGC1. That's the kind of information I was looking for. Yes I did get ahold of the original prints, but they are very old. They give dimensions of the waffle cross section but don't mention any internal reinforcements. I found that doubtful ... hence my question here. Waffle slab standards is what I was hoping to find.

The fasteners are actually going into the slanted sides, not vertically into the 4" wide surface. Given the narrow width, I may decide to go straight through with backing plates and nuts on the opposite side. Now that I have an idea of where the bars might be, I might try a few strategies here. First, I believe I'll put a bolt of all thread thru the rib high enough that it will be above any internal bar, which would prevent any worst case catastrophic failure. Second, I will make a V shaped bracket that matches the contours of the rib, effectively encircling the bars once the thru bolts are in place. Third, I'll put in an extra hole in the bracket in case we do hit bar during drilling. I can then stop drilling that hole and still have enough bolts to do the job.

Thank you all for your suggestions. Further comments and observations are welcome.

Drilling straight through the rib at about mid-height is going to miss most of the reinforcement. But if you are drilling close to the column lines, you may encounter bent bars or stirrups.

Bent or truss bars were used as both top and bottom steel. They extend past the column on top and crank down at an angle of 45^o just outside the quarter point each end thus adding to the bottom steel in the middle of the joist. If you hit one or two of these, you will know where they are in the future and be able to avoid them.

Stirrups are vertical bars, probably 3/8" in diameter which are sometimes added near the column lines to improve the shear strength of the concrete rib.

As mentioned by Sparkstation, you could hit other services, so take your time. Good luck.

Hello again, Out of Box Experience

Apology for my late posting of the above.

I started the reply earlier today, then was called away, completed the reply on my return.

In the interim there have been the answers from ba/ael and DGC1.

Kind Regards....

This is a standard type of 2-way slab. There will be strucutral reinforced throughout in both directions (not just shrinkage steel in one direction like some slabs). The reinforcement is calculated based on the proximity of columns and the applicable loads (including seismic loads). Since the reinforcement is done by cross-sectional area of steel to cross-section of concrete, based on moments and steel embedment depth, some designers may choose more smaller steel bars, others might lean towards fewer larger bars to meet the design requirement. It is likely structural and could require an engineer to analyze the additional loads, if significant, to verify that it is within acceptable design loads and does not comprimise the structural integrity within the allowable safety factors (particularly under seismic loading).

At the lab where I work NO ONE is allowed to penetrate concrete without first having it x-rayed, a bit costly, but a lot cheaper than fixing or explaining what happened. "Mom, I'm going to be on TV, look for me at 5, 6, 10, and 11PM."

Just mark where you would like to anchor, how deep and what size. They will give you the rebar layout, if an area is post tension, and any piping in the area.

Good luck

Las Vegas Convention Center tunnel C3 has a suspended pipe system bolted vertically into a similar T-Bar ceiling into the soffit of the beam with wedge bolts. The load is causing cracking in the concrete. Typically reinforcement will be at bottom of each waffle within 2-3" so you are better off connecting closer to the neutral axis horizontally. Use more smaller connections than a larger with epoxy.

Are you a registered engineer? If not, I wouldn't touch it with out the original engineers approval or some other engineer's stamp of approval. Once you drill into it, you own it. The way these things are built is with a rebar grid on the slab portion and rebar on the top and bottom of the beam portion, both directions. If the span is short, then they used rebar, if the span is significant, then they used prestressed or a post tensioned system. A metal detector is a quick way of checking for rebar in the vicinity. If this is a rebar reinforcement, they would have put the tension bars near the bottom with ties running vertically every 12" or so. I would tapcon a hanger to the side about 8" or more from the bottom of the beam portion after using a metal detector to clear the area of rebar.

Propose your idea to the airport facilities manager and get their approval first.

Sry, I'm not a structural engineer, but If you are going to be attaching anything to a pre-existing structure, I would advise doing sonograms and possibly structural xrays and incorporating those tests into the bid.

Since you don't have the actual drawings or specs for the structure, you had damned well better be sure of what you have to work with.

I would assume that the 'waffle' effect is just cosmetic and covering underlying structural elements, but you have to be sure before you can make any estimation.

Follow the first rule of structural engineering..."Do not build on what you do not know!"

So, basically, I am assuming those high 'waffle' ridges are covering structural elements, but since you have no drawings, you have to create new drawings from scratch, hence the radiometry. You have to absolutely know. End of discussion.

Any structural engineer that would 'guess' should be shot in the head. Preferably with a large caliber projectile at very close range.

Been gone awhile, but watching. If you have the original prints they should have a reference to a rebar schedule on them that would give the size and spacing. When the rebar is placed in the form "chairs" hold it away from the plywood or steel forms and leave spots that can be seen specially on older concrete. These spots can be see in rows. How much does the object you are attaching weigh? There are so many fastening systems available and other factors are relevant. Is this to be a permanent mount? Is this object mechanical, in other words does it move or vibrate or create it's own stresses other than weight? Is it flexible or rigid? Will it be disturbed regularly, ie maintained, adjusted, altered? A banner could be epoxied or a small sign could be "tapconned" something heavier may need to be put in with lead or other expanding anchors and other items may need to be bolted firm. Not enought info.

Hello Out of Box Experience,

The attached is a page out of "CRSI Floor Systems by Ultimate Strength Design", copyright 1968. The upper portion is a plan showing a typical waffle slab arrangement.

The lower portion shows typical sections, firstly of the column strip, secondly of the middle strip. The location of these strips is indicated on the plan view.

The diagram is a little hard to read because of its size, but you can see the truss bars and straight bottom bars I referred to in an earlier post.

If the load you propose to hang is substantial or vibratory, you should have your procedure reviewed by a structural engineer before proceeding with the work.

Use X-Ray to determine precise location of any steel before you drill.

I wish to apologize for my earlier remarks. I'm not going to try and justify anything. I handled my incredulity and probably ignorance in the worst possible way. I was called on it, and justly so.

I hope Mr. Out of box experience, will forgive my rash diatribe.

Your public apology was as classy as your private note to me.

Don't worry about it. Thank you.

It's amazing how a simple and perfectly fine question to my peers ended up with me evidently being some kind of idiot after a couple dozen replies to my question. So many presumptions made. So much snowballing. Would be a fascinating case study of social behavior. But ... wrong forum for that.

Thank you to those of you with helpful comments. You know who you are and I appreciate you. I don't need to defend my qualifications, degrees and experience to the rest of you. Let me just say that your assumption are pretty far off the mark. Enuff said about that.

Haven't been available over the weekend to comment. So for those that are still curious, the load is a static load, not particularly heavy. I simply did not want to drill into any rebar. And I didn't happen to be familiar with rebar dispersion patters in a waffle slab. I didn't know if there are standards, or if it is variable. Therefore, I asked. The answer seems to be that it is variable. That was the extent of my question.

From your suggestions, I see ways to accomplish my goal despite the challenges of shoddy as-built drawings, reluctance to go to the time and expense of Ferro scans, and unrealistic time schedules. But .. what else is new, right? Those are pretty much typical issues for any project. Rest assured, you all can walk comfortably in any airport ... even the one I'm working on, and not worry about it falling on your head; at least not areas I'm responsible for.

I have some decisions to make. I'll update you when this project has progressed. I've got to get some work done now.

Thank you again.