PT conc slab analysis without drawings

[BubbaJ]

I have a client who wants to put a large piece of equipment on a 2-way PT elevated slab in a hospital. The only information I have is an X-ray of the floor locating the strands.

Anyone have any ideas on how to proceed with an analysis?

I posted this in the PT forum also.

Thanks.

 

[CELinOttawa]

Nearly impossible without the drawings unless you can identify the stand grade, size, specific location (I mean height in the slab as well), etc.

I do a lot of work on existing properties, but I refuse to modify or evaluate the capacity of an existing PT floor in absence of quality information. Perhaps I'm just not aggressive enough, but this is not a check I'd be prepared to undertake.

With any luck one of the PT experts we have around here will chime in and I'll learn a thing or two. Good luck with it!

 

[CELinOttawa]

Oh, also: PT systems tend to be fairly aggressively designed themselves. They are a reasonably high efficiency system - Why else go to the trouble? That means that they are also very unlikely to have any reserve capacity when you're looking to go past the code stipulated minimum bearing load.

For a large piece of equipment I'd also be worried about punching shear, which you can check conservatively with less information.

I don't think you should be sticking your neck out on this one unless someone like Raft can tell you how and hold your hand. Good luck!

 

[ajk1]

Where are you located? Have you tried the municipal building department? They are usually good at having the structural drawings on file. If you know when it was built, you can contact the post-tensioning contractor and he should have the shop drawings.

If all avenues like this have failed, you can chip into the concrete to locate a strand and measure its diameter with calipers.

If you know the date of construction, you can assess the most probable GUTS of the strand.

An added issue that you must keep in mind is that if these are older strands before the extruded sheath was introduced, they almost certainly have some degree of water in them and corrosion and failure of wires and even whole starnds. Guaranteed. The water got into those type tendons during construction, even if it is in a dry environment in the finished building.

For a number of reasons, whoever used unbonded tendons in a hospital was not a very prudent engineer, in my opinion.

Curious - how much does the equipment weigh, and what is the bay size?

 

[dcarr82775]

You have the Xrays so you know the diameter. If you know the age of the building you can get a reasonable estimate of steel strength. You still need the strand profile, but could have GPR done. I have seen some amazingly accurate plots from GPR that were verified by chipping and Xrays. They folks doing the work had a post processing software that worked really well. They are not nearly as cheap as the local geotech firm, but their answers were spot on.

How big is the equipment?

 

[CELinOttawa]

Dcarr: Would love to know the name of that firm...

 

[dcarr82775]

CEL, I will try to dig it out. They are out of Texas somewhere. I was stunned, had never seen anything like it. They were right almost all the time. Even around the columns.

 

[BubbaJ]

We found original structural drawings...and it wasn't even that difficult. Never trust an Architect to do your recon.

No shop drawings though, but that was too much to ask I think.

Floor is designed for 200 psf (former x-ray storage), the equipment is 225PSF, but on a small footprint. So I need to do an analysis to see what we can get for load spread on the 2-way slab.

 

[hokie66]

If it is designed and built for 200 psf, and the equipment load itself is 225 psf, that part is OK by inspection. No further analysis required. But I agree with ajk1, if this is unbonded PT, I would be concerned about corrosion.