Repair of prestressed T-beams

[charliealphabravo]

I've encountered these prestressed double T-beams (60ft span, ~36" deep legs) that have cracks at the bearing. They have experienced long-term water intrusion and corrosion of the bearing pads/embeds/reinforcement. There is also corrosion and spalling of the cast-in-place corbel and bearing plate embeds as you can see and some poorly executed shoring.

Does anyone know if the prestressing strands will be harped or if they will be straight through the bottom chord? I have several concrete repair resources but I don't think I have ever seen a repair for this particular situation. I won't be on for the repair but I'd like my recommendations to be informed.

TIA

 

[Agent666]

Every double T I've seen has had strands straight through level at the bottom because of the way they are actually constructed on a long flat bed. They stress the strands over multiple units, put in stop ends to block out for the individual units before concreting. Then once cured they release the strands and cut them between the units. The method makes it hard to deviate the strands within each individual unit.

 

[BridgeSmith]

We haven't specified any double tees, but for the other prestressed girders that we do specify (bulb tees, tridecks, etc.), some harped strands are fairly common. It depends somewhat on the loading conditions, but they're used more often for controlling the long term camber of the beam. It's probably a good idea to make the conservative assumption regarding harped strands, if you don't know what was done. You'd have to get a look at the end of the web to know anything for certain.

 

[Agent666]

.. or just get someone to scan the side of the webs to determine the exact arrangement including any end stirrups, number of strands, etc.

 

[KootK]

It's pretty hard to say if the strands would be harped or not. I vote for finding out for certain via some investigative field work if any proposed repair scheme might compromise the prestressing.

There weren't any great photos posted of the bearings themselves but, at first glance, it appears that the problem may have more to do with the corbel than the TT's. It appears that the bearing ledge is too narrow and loads are being delivered too close to the edge of the corbel, producing your classic, steep angle, corbel shear failure.

The bearing plates for these things typically serve as anchorage for some important horizontal and vertical reinforcing. While that function is very important, the bearing plates typically are not working very hard in performing that function. If you have significant section remaining in the bearing plates, you might be able to get by without remediating those. Obviously, you'd still need to put a stop to whatever building envelope problem is causing the corrosion.

What would you guess the available bearing width to be here for the TT's?



HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[charliealphabravo]

The corbel is 6" deep according to plan and I measured it to be about 6.5". In actuality the bearing plates embedded in the corbel are somewhat shorter with actual bearing length as low as 4" and with edge of steel located right at the face of the corbel.

The beam ends should be accessible from the opposite end with not much trouble. I agree that should answer the harping question.

 

[KootK]

The vertical surface that we see behind the TT ends is a CIP diaphragm pour, right? The TT's extend into that some distance?

It appears that the ledge failure surfaces do not cross into the reinforced part of the corbel (assuming that it's reinforced). If that's the case, it may present some opportunities for cost effective repair. If you've still got 4" of TT bearing after losing the cover concrete, that may well be enough with some minor repair for appearance and fireproofing.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[charliealphabravo]

The vertical surface behind the double Ts is a foundation wall for the primary structure. The corbel is poured integral with the wall. I haven't verified the location of the diaphragm (floor) beyond. The double Ts sit on the steel embed plates with a gap between the wall and the ends. It so far looks to me like the corbel/ledge failure represents displacement of the embed plates or possibly corrosion/expansion of the rebar along the top of the corbel.

I will know more when I get the plans back from the scanners.