Precast twin tee double span 1

[luedkelincoln]

Greetings all:

I have a project with precast/prestressed 25" x 8'-0" double tees with a current span of 56'-0" that had an original design capacity of 75 psf. We would like to increase the capacity to 125 psf to 150 psf. One option that we would like to consider is to place beam and column supports at 36'-0", creating 2 separate twin tees. This will result in a 36' span and a 20' span. Twin tees are light-weight concrete and have a 3" light-weight composite concrete topping. Our currently proposed plan is to shore the tees, cut the topping slab and tee flanges, support with new beams and columns, and install a joint to eliminate the potential for action as a 2-span. Assuming the existing bottom reinforcing is adequate in the twin tee member(s), and shear at bearing is good are there any other items I am missing that I would need review? Has this been done successfully by anyone else?

Thanks,

 

[BridgeSmith]

Sounds like a lot of work. If the twin tee section will not accommodate enough prestressing for the higher loads, would you be able to consider using a 27" tri-deck section or a pair of bulb tees?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[jayrod12]

I'm expecting that this is an existing installation and the goal would be to avoid having to remove the existing Tees and replacing them. Also, since it is existing, there is likely minimal info on the original supplier and therefore you'd be hard pressed to ask anyone anything.

I don't necessarily see an issue with what you are proposing. As long as there is enough length to develop the strands fully for each span, I don't think it would be a problem. However, if you are cutting through the topping slab, and the double tees, what are you doing for diaphragm continuity? You're now cutting that diaphragm in half.

 

[JAE]

If you cut down existing tees to a shorter length, you may have a major problem!

The concrete stresses in a double tee are limited for various conditions - one of which is the axial compression in the bottom of the tee due to the prestressing.

For a tee that is 56 ft. long, the compression/tension in the bottom is the net difference between the self-weight tension and the prestress compression.
The self-weight tension depends on the 56 ft. span.

If you cut a tee shorter, you now will have much less bottom tee tension. This could possibly result in the tee "exploding up" as the bottom concrete crushes under a too-high compressive force.

You have to check this.

To check this you need to know the amount of prestressing force applied.

So be very careful with this.

 

[jayrod12]

Oh crap, thanks JAE, that also reminded me.

My experience with double tees is the prestressing strands at each end are fanned out to spread out the compressive stress. At mid-span these tendons would be grouped together to provide maximum efficiency. If you cut them at the mid-span, now you've got major compressive stresses occurring right at the bottom of the tee, which could lead to crushing of the concrete at the bottom of the tee.

 

[luedkelincoln]

Thanks all. This is an existing condition and just a small portion of the overall floor plan with reference to the diaphragm. I will check that. We have the original precast supplier on board with old files that will show us "exactly" what was put where with respect to the strands, drape profiles, etc. We are planning on determining the in-place compressive strength with a Swiss hammer. From the above posts and more research, it appears that we will lean toward not cutting full-depth to maintain the compressive resistance at the bottom of the section(s). Once the exact strand locations, loads, support location is determined, we may be able to cut into or replace the topping slab for the length required to add Grade 60 reinforcing steel and/or carbon fiber reinforcing to the upper part of the section (flange and topping) to take care of the moment now created at the top as a result of the newly made 2-span condition. At the very least, I am considering a tooled or saw-cut joint in the topping slab to control the crack location should one develop.

 

[dik]

I've deleted my post... it was my understanding that with DT's strands were straight, not 'harped'.

Dik

 

[BridgeSmith]

At the ends of the beam, there is typically much more transverse reinforcement (stirrups) in order to handle the the transfer of force from the strands to the concrete through the development length of the strands. If the strands are cut or otherwise detensioned in the middle of the girder, it's unlikely the concrete will be adequate to confine the strands, and will likely split.

I'm a novice when it comes to prestressed design, so hopefully someone can set me straight if I'm 'off my rocker'. Would it work to add the support without cutting the girders?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JAE]

dik - sometimes they are straight. Just depends on the design and the fabricator.

 

[BridgeSmith]

To add a little more specificity to what JAE said, harped (or draped, as they're sometimes, somewhat misleadingly, called) strands are used to reduce the tension in the top of the section, and/or limit the camber in the section.

If the twin tees that are to be cut have harped strands, the tension at the top of the section will need to be checked, particularly for the conditions at the time when the girders are cut, before the additional dead load is placed.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

JAE & Bridgesmith: thanks...

Dik

 

[r13]

Do you know what criteria was used for pre-stressing, did self weight was included? If so, the tendon force might have already diminished quite a lot after the double tees had set in place. But it is prudent to construct the middle/new support, and fasten the tees to it, before work on the top slab. My concern is, once the double tees is cut to make two simple spans, then how to control the shear? I doubt there is adequate shear reinforcement near the proposed cutting location.

 

[dik]

Other than the first couple of feet, it's my understanding that prestress loss is relatively little with pre-tensioning... generally where prestressing is not needed anyway. I'd still check with the precaster to see how the strands were placed and proceed accordingly.

Dik

 

[PROFR]

It is not recommended to cut a pre-stressed element. If adding a concrete topping is possible try any of the following ideas:
1- Use extra topping to add more strength. (It might work if the extra capacity needed is not significant)
2- Use the new inner support to create a beam with one overhanging end condition. Use topping to add extra steel for the negative moment. (Shear might still be a problem and bearing as well).
3- Use FRP Strengthening. (Most likely it will be the only solution).

Good Luck

 

[luedkelincoln]

Thanks again all. We were able to get all the original calcs, strand layout, etc. We are going to add the support, reinforce for shear at the new bearing location with carbon fiber, and cut a shallow control joint only in the topping slab. All stresses worked out.