Existing 30" Precast Concrete Tees 1

[Rweave]

I am working on a building from 1966 that has roof framing that consists of 30" Precast Concrete Tees @ 8'-0" o.c. that is spanning approximately 68 feet. Does anyone know where I can get some information on the capacity of these tees? Any rules of thumb for being able to make cuts in existing framing and not jeopardizing the tees?

 

[JAE]

Does anyone know where I can get some information on the capacity of these tees?

I don't think there's much you can do as the number, diameter, material grade (probably 270 ksi) and location of tendons would be unknown.

Best thing for cuts in the floor is through flanges near the ends where flexure is minimum.

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[jayrod12]

What's the opening for? I've got 32" double tees under roof loading spanning 61 feet and all of the pre-casters I've chatted with say they're likely maxed out already.

 

[TehMightyEngineer]

Almost all our pieces are designed to right at 100% capacity, unless we have pieces made for one job that get left over and then used on another job. However, that doesn't mean you couldn't cut the flange as long as it was able to take the reduced compression area and slight shift in the neutral axis.

As JAE said, it will be difficult proving that it will work without knowing the what the tendons are.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[JedClampett]

In most areas, there's only a couple of yards that make prestressed. Single tees (I'm assuming single tees as it's not clear) might narrow it down further. Call them up. They might know the project or have records or even know that their competitor did them. If you get a chatty engineer he might even have some suggestions on your problem.
I've got the 2nd Edition PCI Design Hnadbook (circa 1975) and while they don't have 30 inch tees or double tees, the ones on either side of that (32" and 24") seem to easily span 68 feet, depending on the tendon pattern and topping.

 

[Rweave]

I just got email back from a precast manufacturer here in Buffalo NY. He gave me a load table from 1971 but it looks like it is a double tee setup, is this typical or would they have single tees that space at 8 feet o.c.???? Arch is just looking for a quick ya or na about putting these roof top units up on the roof and punching holes. Sounds to me like I might not have the answer they need and we should just avoid this situation? I appreciate all the replies. I figured being more toward the end of the span would be best for these openings considering the low moment and high shear at that point. I might just make a stop over to the project location and take a look at the roof member dimensions and go from there. Again thank you everyone for the information.

 

[TehMightyEngineer]

If you could find the original loads that the precast tees were designed to hold then you could configure a reinforcement system that spreads the load from the RTUs out such that the tees will only see the original design loads. I wouldn't give up hope just yet (but this definitely isn't a "yes or no" question like the arch wants).

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[JedClampett]

Rweave, with precast members, anything might happen. I've seen double tee roofs, walls, parking garages, etc. Single tees can be used for whatever works. From what I know, as long as you stay away from the web, you can cut the deck, within reason. But I'm not sealing your drawings.
Maybe the local manufacturer has an engineer who seals his drawings who might give you some information. Even if the architect has to pay for it, it's cheaper than spinning your wheels.

 

[jonkirk31]

I'm an engineer for a precast manufacturer and can say with a fair amount of confidence that cutting the flange in the outer quarters of the span (i.e., near the ends) should not be a problem at all in terms of overall flexural or shear strength of the member, but you have to be careful about what you're cutting for other reasons. The biggest thing to be avoiding is cutting out flange-to-flange connections if possible for diaphragm strength, and making sure that what's left of the flange reinforcing you are cutting is still going to be developed enough to support loads around the new opening. Current practice is to use WWR in double tee flanges for transverse bending of the flange, so cutting cross wires can be bad. But probably back in 60s and 70s would be using regular deformed rebar. Obviously that's not an easy thing to confirm so probably just try to be careful on how the RTU's are supported - get a support or curb system in there that gets the loads directly over the tee stems and doesn't rely on flange flexure.