12" prestressed concrete floor joist load span tables

[dchirco]

Good morning all,

I was wondering if anyone has span load data for 12" prestressed concrete floor joist spaced at 4'-6" O.C. with a 5.25" composite slab. I am analyzing a building that was designed in 1983. A scan or link for allowable superimposed service load span tables would be appreciated. I've attached a picture of the floor framing plan for reference.

Thank you!

 

[msquared48]

Why don't you back-engineer the system for the live load used?

If you have the steel and concrete allowables, the analysis should be a simple problem.

Mike McCann, PE, SE (WA)

 

[dchirco]

Mike,

I have the design live load; however we are looking to increase the loading to an area of the floor. This new loading will be suspended from below the floor, so I would like to understand if there is any reserve capacity in the system. Hence the need for span load tables.

 

[jayrod12]

Perhaps a few of the pre-stressed designers could elaborate, TehMightyEngineer for one, but I was always under the impression there is rarely much reserve capacity in their systems. similar to Pre-eng metal buildings, the money is made on saving materials.

 

[TehMightyEngineer]

I'm not as experienced with prestressed as you give me credit for jayrod, but thanks! Our plants currently only do thin, flat pretensioned slabs for bridge projects and they're mostly cookbook. I'm trying to change that now that we've freed up some time on the stressing bed. Regardless, I'm definitely experienced in precast, and I can tell you that the biggest considerations in both prestressed and normal precast are availability of forms, shop space/time, and repetition. Making the same piece 100 times is much cheaper than making a number of individual pieces. Prestressed throws in a small twist as limiting the number of strands or the amount of concrete in a piece obviously helps keep costs down, but often multiple pieces will be cast in one stressing and thus repetition generally saves more time/money than reducing materials to a minimum.

All that said, you're entirely right that unless required we'll take things right to 99% of the required design strength. On balance, given the OPs plan where he has irregular column line spacing and prestressed joists with a composite slab it may be the case that certain areas have additional strength that wasn't considered when the prestressed joists were selected. I would be surprised if the majority of the floor has significant extra strength though.

While the plan you posted could use a few more pixels, it appears that you have much of the information you would need to establish a rough idea of the available strength. I would agree with the others that calculating the expected capacity is the way to go.

Edit: I should also mention that economics of scale play a big role as well. For custom jobs without stock items the above holds true. For anything that's a stock item we 100% will reduce the materials involved to the minimum acceptable amount as repetition, shop time/space, and number of available forms are now a mostly fixed quantity; as materials is the biggest variable left to play with to try to maximize profits.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[EZBuilding]

Typically the precast joist is designed by the manufacturer's engineer as a pinned - pinned boundary support condition element. Often, the Engineer of record will provide mild reinforcement at supports to account for service level cracking. In the past, I have accounted for the "fixity" provided by the mild reinforcement in an effort to get a joist to work.

At my old company, some of the old timers had binders with span load tables of this system from around that timeframes. I no longer have access to those - but the link below is of a pre-caster that is active in my area. I checked and they do not have your framing configuration.

http://www.spimiami.com/uploads/1/2/9/4/12943339/s.p.i._joist_load_tables_2008.pdf

 

[Ingenuity]

I can not read the drawing details you posted with any clarity - I assume that it does NOT specify the strand info (dia, #, spacing etc) within the joists?

Without such info (and assuming the precaster is no longer around to assist) we have usually done some limited site investigation (scanning, probing, maybe a core if necessary, etc) to determine the precast reinforcement then calculated the flexural and shear capacities from that - assuming the owner wishes to pay for such work.

 

[TehMightyEngineer]

I notice on the drawing it says P.S.I. Beam Schedule. Could this be these guys?

http://www.prestressservices.com

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[rapt]

In the time you are taking to find tables, you could calculate it! You will have to for the additions anyway.

Remember the suspended loading will have to be carried to the top by your connections, not just into the bottom of the floor members.

 

[Ingenuity]

In the time you are taking to find tables, you could calculate it! You will have to for the additions anyway.

But in most cases in the US, precast/prestressed concrete is a delegated design so the EoR's drawings don't have much info like strand dia, spacing, forces etc, so the OP probably does not have such info to do the necessary calcs.

 

[hokie66]

What makes you think this is prestressed? The picture is fuzzy, but it looks to me like it is reinforced with bars, and the schedules may have the bars called out.

 

[dchirco]

Link
Link

All,

For those that are interested I've provided Dropbox links with the referenced framing plan and general notes in a higher resolution. The typical precast joist section clearly calls out prestressed tendons.

The general notes list the live loads; however, since I don't have access to an original architectural floor plan I cannot determine which areas were designed for hospital ward live loading (40psf) or corridor live loading (80psf).

Back-calculating a capacity will not really help as I am trying to analyze the floor for reserve capacity to utilize for an additional concentrated live load point load of 1250lb (unfactored) anywhere along the joists span suspended from below the joists; therefore, whatever live load I use to determine the capacity will be used up in my additional load analysis.

A comparative analysis will give an increase in moment and shear of 9.9% and 9.4% respectfully; therefore, I believe this amount of increase calls for either knowing the exact capacity through span load tables or performing an extensive and costly analysis of the material strength of the joists for such a relatively small addition.

If the span tables give a higher allowable superimposed service load than the worst case 80psf LL and collateral DL combined then I could utilize the excess capacity to determine if the joists can handle the additional point load by finding an allowable moment & shear and comparing those to my worst moment and shear due to the LL, DL and point load.

Ultimately, if there are no load tables available, I would be inclined to specify member reinforcement based on my comparative analysis...which would probably be cost prohibitive as well as possibly infeasible to construct do to limited plenum space and M.E.P. interference.

Thank you all for your help and input!

 

[TehMightyEngineer]

It looks like there's two precasters in the Miami area, but none with the name matching Prestressed Systems Inc. Your best bet at finding the original design would be to contact these two precasters and see if one of them absorbed Prestressed Systems Inc. and if they have records (a long shot to be sure). They'll also have a much better idea of local practices.

I don't think the analysis is as costly as you think, unless of course it is entirely outside your original cost you gave the client. If that's the case I suspect you can explain to the client that you need to figure out what is in the prestressed joists for an additional fee or a large amount of costly reinforcement will be required.

For a preliminary estimate you can likely make a lot of conservative assumptions and still arrive at a reasonable estimate for the capacity of the prestressed joists. As the joists have variable lengths in some spans but are all the same spacing and depth I suspect that the precaster varied some of the prestressed strands to accommodate this span/load variability but each bay is likely fairly repetitive. I suspect you'll find that many joists will have the extra 10% you need.

Once you have a rough idea of the expected capacity that shows that you expect the capacity you need then you can likely sell it to the client and do a field investigation of the joists using a combination of rebar scanners, cores, and some selective destructive investigation to determine what joists are repetitive and what the strands are likely composed of and their location.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[TehMightyEngineer]

By the way, attached is a span table from a Florida precaster that's probably similar, but not a direct match for your system. This could be used for at least a jumping off point.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com

 

[Ingenuity]

...with a 5.25" composite slab

Based upon the DROPBOX links you provided (with better drawing resolution) the slab thickness is called up as 4-1/2", which is about 15% less slab SW than 5-1/4" - all helps!

 

[dchirco]

Alas, the tape measure doesn't lie...

 

[Ingenuity]

I thought you were going to say that. Oh, well.

But your overall composite joist depth now increased from 16-1/2" to 17-1/4" so that is 5% depth gain for ultimate capacities.