Temporary Supports for Box-Beam Bridge Pier Cap

[Noob_Engineer]

Hi all,

I'm designing a temporary support system to support a box-girder type pier cap for a simple span bridge, using (2) support posts and a cap beam across the top of the post to bear under the box girder. I designed tube stiffeners to be wedged on the insides of the box girder, adjacent to the webs to prevent buckling of the webs from the jacking force/reactions at the location of the support.

I am seeking help just to verify that my thought process is valid and I'm not missing something in the design consideration.

Here are my design assumptions/thoughts:

1. In reality the bottom flange of the box girder won't really be taking any loading, or it would be very little. I think most of the loading will go into the webs & stiffener tubes. In the same vein I am not designing the cap beam for a uniform distributed loading from the reactions because I don't think the bottom flange will really be loaded.

2. I am locating the support posts to be directly under the stiffener tubes because I am making an assumption that the stiffener tube will take the full vertical reactions, and this will eliminate any shear in the cap beam.

Any thoughts and input on this would be greatly appreciated. See attached a rough sketch.

Thank you!

 

[Lomarandil]

Agreed. Load will be delivered into your system at the webs and stiffeners.

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just call me Lo.

 

[jorton]

I've used details similar to yours in the past, but only on existing I-girders. For new stuff we usually get erection stiffeners added in the shop drawings if they are needed. It will be tough to place the tube steel if it is cut to exact length and each end is finished to bear. Will it be retrieved after? It might be tough cutting in such a confined space. Is it too late to get the fabricator to weld in bearing stiffeners? Or maybe a diaphragm so you can use a single point of support?

I agree with your design approach except for P/2 at each post. I would design it so that it will not fail if either post experiences P rather than P/2. It can be difficult to get a box to sit down uniformly on multiple supports. I don't know the overall geometry of the boxbeam or the temporary support conditions, but you may want to think about the relative stiffnesses of all the members involved. I typically use a large factor of safety when designing bridge shoring because in practice you will see it sitting hard in some spots and not in other spots. The geometry and locked-in forces can change throughout the erection process depending on member cambers, cross-frames fit details, temperature, sun, etc. Add in a small elevation errors at a few bearings or temporary supports and it gets worse.

Also, you may want to think about the girder erection sequence if this shoring will still be in place when girders are placed. Usually the contractor wants to place all the girders in one span before moving to the next. That will likely put a lot more load into one prop than the other.