Improving shear capacity of precast girders. 2

[yakpol]

Hi, guys!
I have a very typical existing three span precast girder made continious bridge. Bridge is on the curve but girders are straight. I am adding some extra dead load and redesigning it for AASHTO LRFD loads. The assessment showed that and flexural and shear capacity of the bridge need to be improved. I will welcome all the Ideas and details.

thank you!

 

[Qshake]

Please give us some details.

It is very difficult to strengthen pre-cast, prestressed girders. In fact, only one method comes to mind right now -external prestressing. However, to do so for the sake of a new analysis with a new code is not necessary. I recommend that you speak with the owner on this issue.

There are many bridges out there which were designed to older AASHTO codes that included only HS15 loading. Naturally when it comes to widening these structures we design the new for HS20 or even HS20-Modified and simply note that the older portion is not designed for the ammended load. Many times, the owner accepts the risk, knowing the character of traffic and the fact that all oversize loads must be permitted on the structure. This is the case with many State DOTs. Usually, the widening is temporary until a more comprehensive bridge replacement program is implemented for that corridor.

On the other hand, with more information it will be helpful to determine if the structure is old and the slab is in need of replacing. If so, then the girders may be replaced easily. Another alternative, for roads off the major highways is to reduce the number of lanes. Lastly, there is always the load limit.

 

[yakpol]

Thanks Qshake!
More details about this bridge: It's actually built 1985 and designed for HS20, consists of 3- 100' precast made continious spans. Girders 73.5" deep with 5" thick web. We need to raise a grade 5' at one end of the bridge. The current solution is to place new deck on the top of geofoam. This bridge is a part of larger project and has to be designed according to AASHTO LRFD code. Increase of DL requires external P/T in the loaded span. Unfortunately, longitudinal P/T does not help to increase shear capacity of section with high V/f'c ratio, and in unretrofited condition shear Demand/capacity ratio around 60%. Currently I am looking at following solutions to improve shear capacity of precast girders:
1. Increase web thickness.
2. Attach external steel bars to the webs.
3. Place steel jackets or strips around girders.

The main trouble in all methods above is inability to attach anything to the bottom flange of girders (full of prestressing strands).

Am I dreaming or some of it has been already done?

4. Replace the bridge.

Thanks for the help.








One of the solution to increase shear capacity is to increase the web thickness of each girder

 

[austim]

I give my vote to choice 2 - 'add steel bars to the webs', assuming that your proposal is to bolt trusses on each side of the webs in the problem areas. Simple to do (providing that you can avoid the P/S tendons when you drill to bolt), reasonably certain in its effect. With girders 16 years old I would not expect any serious problem from residual shrinkage or creep.

 

[Qshake]

Not sure about the raising grade issue. Your going to leave the slab in place and then place geo-foam on top and a new deck on top of the geo-foam? Is this correct, or did I miss something?

I suppose that if anything is done at all adding new bars to a thickened web will be best. A combination of 1 and 2 above. To increase the web and ensure composite action/transfer mechanism should be by shear-friction. As such new bars will need to be drilled and grouted in place. If you have a I-girder I can see use being made of the top and bottom flanges as form ends. Install portals every 5 feet or so to facilitate placement of the grout or high-slump concrete and go to town. This is not a cheap alternate. The labor will knock you out I would imagine. A new structure may be feasible given the new life it would add to the remaining project.