Precast Single Tee

[vvaccare]

Good morning community,

I was hoping to throw this out there and see if I could get any insight from some of the more experienced precast engineers. I currently was dragged into a renovation project where the original EOR left, forcing me to take over. The existing building was built in the late 60's/early 70's, but the original drawings are not available for review. The building has been modified several times over the years. The area in question originally served as a single story building with roof loads. Sometime from then to now, the building has been added onto and now the area is serving as an office space with corridors. As of today, the client wants to raise the elevation of the floor by adding a new concrete slab topping, and placing terrazzo flooring on top.

The original framing system is a 12" precast single tee system. I have not had any luck finding any information for any 12" precast single tee members. Originally, I thought that it was a t-beam, but there are visible joints every 48" o.c. I've attached a sketch with the field measurements of the member for reference. The issue is these members are spanning 34', and based on the information I've been provided from the field they can only locate one #5 bottom bar. Apparently they found this #5 bar from a nearby tee that was cut/abandoned/modified (see picture). The picture shows a groove where an old reinforcing bar may have been placed, and based on the measurement they are claiming it to be a #5 bar. Looking at the picture, it does appear that there used to be strands at the top, but I have no idea if this is a correct assumption. I've instructed the project manager and contractor that further test should be done to locate any potential strands so I can better analyze the member.

Just running the calculations for a basic reinforced T-Beam, I can't even get the member to work with the live loads it has been experiencing over the years. So it goes with out saying I have severe doubts and reservations about adding any additional loads to these members. Unfortunately, all the information and pictures I have received are from the contractor and the project architect. I have been trying to arrange a site visit to inspect and observe the mess myself.

I was hoping that someone could possibly provide some basic information on any old precast single tee's. Any insight would be greatly appreciated.

Thank you.

 

[TehMightyEngineer]

No way they're spanning that long on a single #5 bar. That's gotta be a prestressed or post-tensioned rod of some sort.

When I go to lunch I'll see if I can dig up some historic info on single tee systems but I suspect I'll not turn up anything specific. I'd suggest a site visit is definitely in order with some minor destructive investigation to determine exactly what these beams are reinforced with.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[BridgeSmith]

Is the 'bar' in the bottom of the stem possibly a prestressing strand? The groove for the reinforcing looks smooth in the picture, like a debonding sheath at the end of a strand.

 

[vvaccare]

I definitely agree that there is no way that a conventional cast in place tee could span that far with just one #5 bar. Thank you TME, any further insight for single tee systems would be awesome. Everything I have ever dealt with has been double tee's.

I think you may be correct HotRod10, I am leaning towards thinking that the "bar" is probably a prestressing strand. As of now though, the only go by I have is the measurement from the architect for the diameter of the groove.

Without a doubt a site visit is a must for me. So I'll be off to Miami in the near future...

 

[BridgeSmith]

I would expect a debonding sheath for a 1/2" prestressing strand (the most common size until the last decade or 2, when 0.6" strand became the norm) to be about 5/8", and it would not be at all surprising with a concrete web width that small that they would need to debond the strand at the ends to avoid blowing out the concrete.

 

[TehMightyEngineer]

Looked through slideruleera's website and turned up nothing. However PCI's website has some single tees in their historic PCI design handbooks:

https://www.pci.org/DesignTablesAndCharts

Only 8 ft and 10 ft wide sections but gives you an idea that these were somewhat standardized and likely there is some information out there on the design of them. Might be worth picking up a 1st edition of the PCI design handbook if you can find one.

Judging by the 8 ft single tee; you're looking at a pile of prestressed strands in the stem, possibly one harped. Rebar scanning one of these single tees should locate them enough to analyze and exposing one or two of them destructively should be enough to confirm the diameter. Combined with some reasonable estimates of losses and strengths you should have everything you need to run the numbers on these after your field inspection.

If you can't locate a 1st edition PCI handbook it might be worth an email to PCI to see if they can find more data on the single tee you have.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)