precast double tee capacity 2

[WDM]

Greetings,
I have been asked to evaluate an existing building that has 4' wide (stems @ 24" o.c.) double tees that span 27 feet and some span 36.5'. The short ones have 12" deep stems and the longer span tees have 15" stems. They are mid to late 1960's and in Southern NC, near Charlotte. Metromont made them but has no record of them or any load tables for the same. Any ideas or information would be greatly appreciated.
Thank you.

 

[civilperson]

Measure the deflection during a static load test. This will give a approximation of E and I.

 

[WDM]

thank you. that is what I was thinking if necessary.

 

[haynewp]

There is an engineer that works for Metromont in that region, his name is Harry Gleich. I have talked to him before and he would be able to help you if anybody can.

 

[dik]

You will require the strands and strand pattern; they may have 1 to 4 strands with likely 2 point draping as well as the geometry. Have you checked with any local building inspectors or perhaps the original engineer of record to determine if any original shop drawings are available.

If the project is in NC, a long time friend and one of the top 5 architects (world class) I've met in 30 years works with LSV Architects, there...

Dik

 

[dik]

Having the EI information will not help with the load capacity... conc stuff including pre-tensioned DT's is too irregular to use it for strength info. Based on the stem depths noted for the spans, deflection is not likely an issue and may be of little help.

Dik

 

[JLNJ]

Radar or X Ray (I've found radar to be fast and cheap) can give you the strand pattern, but you may still need to do some destructive testing. With no tables to consult, backing out the capacity yourself will be dicey. I don't think the first manual from PCI came out until '71.

 

[Taro]

Even if you can find the size and location of the strands, you would still need to determine the amount of prestress in the strands in order to calculate a load capacity. I'm not sure many testing labs are equipped to handle this. You could assume a conservative value, but this would only be a guess and might not be adequate if you really need to be sure of the answer. It might be easier to perform a load test per ACI 318, Chapter 20.

 

[WDM]

Thanks to all you folks for your valuable input. No resolution yet at my end though.

 

[dik]

As Taro points out... not an easy problem. If you can take a small sample from an end... You will be able to determine the yeild and hence the ultimate strength of the DT.

Dik

 

[Lion06]

Taro-
Are you sure you need to know the pre-stress in the strands to determine the flexural capacity? My understanding is that the prestress is only relevant for camber and deflection purposes and not strength at failure. The tendon stress at failure is not affected by the amount of pre-stress in the tendon. The Ultimate capacity of the TT is fps*Aps*(d-a/2).
Please let me know if I am missing something.

 

[dik]

Nope... strength and stressing is important for elastic properties... The strand strength is important for determining ultimate capacity... with the high strength, sections can be approaching 'over reinforced' condition and with stems at 2' o/c could be problematic.

Dik

 

[Lion06]

dik-
Can over-reinforcing can be checked for without considering elastic properties? Why are we concerned with elastic properties to determine the ULTIMATE capacity?

 

[dik]

From a strength issue, the limit state condition can govern. With the reduced section and reduced concrete cover alotted to pre-stressed concrete there may be serviceability issues that the DT's don't meet.

Dik

 

[mitchelon]

All,

Yes, you need to know the # of strands and the prestress force on the strands to determine its ultimate capacity. You can use ACI equations instead to get an approximation, but it will not be exact. I do not recommend the load test to determine E. The deflection during a load test is equal to the deflection due to external loads (including own weight) minus the camber. Since you don't know the # of strandd and PS, you will not be able to calculate the deflection due to external loads only. Recommend you keep looking for documentation, use non-destructive methods, and destructive if necessarry to determine the strenght of the concrete.

 

[WDM]

The owner wants to store 3'x6' steel boxes (filters)weighing up to 4k (222 psf) on these tees. They may be spread out to avoid overstressing the tees. Also, they need to drive fork trucks on these tees. I'm thinking the they could likely safely store boxes near the ends of the tees but not at the middle. The contractor is pushing for an answer from me and so far I don't know what to tell him. I am very unclear on how to proceed. They are considering adding beams at mid-span to increase the capacity. Doing this I would think the concrete would crack in the slab top over the beams, resulting in possible failure. This building was originally (1960's) a textile mill, therefore, I believe it would have a live load capacity of about 100-150 psf.

 

[kslee1000]

I can sense the pressures you got from the owner and the contractor. It is now the time to tell the owner that it is cost prohibitive and time consuming to carry on the idea to utilize the existing structures to support the loads as described. If this is a temporary need, provide shoring. If it is for long term, try design add'l structures to pick up the loads that exceed the time proofed loads (D+L).

 

[VTPE]

You could always load test using ACI recommendations.