Existing Concrete Pan Joist Questions

[birneys]

I am working on a renovation of an existing two-story concrete building with a full basement (~early 1900's). The owner intends on using the second floor for an open-concept office and wants to use the roof as a social area. I have been told it was originally a department store, so the design capacity should have been reasonable for the second floor, but I am skeptical about the roof. The typical construction (both roof and second floor) is one way CIP concrete pan-joist (4" slab + 10" joist depth) supported by CIP beams and circular columns. The pan-joists are designed as a three span system of ~21-22 feet for each span (confirmed by NDT scans procured by the owner) with two longitudinal bars at the bottom of the joists, one longitudinal bar at the top centered on the joist web, and two longitudinal bars (for negative moment) at the top of the joist web over the supporting beams. The NDT report does not provide the length that the negative moment bars extend into the joist spans. I was also told that the NDT method used could not give me the bar diameter, only the approximate locations of the bars (both depth and horizontal spacing). Does anyone have any advice for how to proceed with determining the allowable capacity of the joist system without further testing (destructive or NDT) or is further testing the only viable option? I ran some preliminary calculations with a 50 psf uniform live load (office), but I'm making a lot of assumptions about bar size (#4), length of the negative moments bars (6'-0" centered on support beam), concrete strength (3000 psi), and 40 ksi steel. If anyone with more experience in this type of scenario could offer advice, I would appreciate some input. Thanks.

 

[JAE]

A few thoughts on your questions:
1. f'c = 3000 psi is probably a safe, good estimate. f'c doesn't affect flexural strength that much but if shear becomes critical you might consider taking some cores.
2. fy = 40 ksi - I think early 1900's might have been less but I'd have to check my CRSI historical document back at the office.
3. Rebar "back then" was sometimes not deformed but either smooth bars or twisted square bars so bond will be less than current design standards assume.
4. Older pan joists systems always...always...have too little top steel and the top steel they do have was always turned down (crank or truss bars) too soon. So negative moment steel won't work under current ACI requirements for top bar development past the point where they are no longer needed. This requires some thought as to post cracking moment capacity and redistribution of moments.
5. Chipping away at some joists can reveal the number, size and depth of bars without too much work - sometimes is a good plan, especially if the joists all are about the same.
6. Consider a load test per IBC (Chapter 17 I think) and/or ACI (Chapter 5?)





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[JAE]

7. Shear sometimes is a problem - stirrup design in older systems always seems to..well...er....suck.


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