Prestressed Girder Replacement

[FM1]

Hi everyone,

i am working on replacement of only one of the prestressed girders of a multi girder 50 year old bridge!!

the new deck load is going to be less than the existing
the new deck concrete is as strong as the old girder concrete
The DILEMA:
to arrive at the same LiveLoad deflection
to TRY and minimize the or if possible eliminate creep in new girder so the interaction with thw existing in the long run wont be detrimental to the deck slab

How and what would you guys do and what else must be considered

suggestions are much appreciated

 

[TpaRAF]

Greetings, FM1 :

Usually, you would remove a portion of the deck slab, from the coping/overhang to the centreline of the first INTERIOR beam. The existing transverse deck reinforcement is salvaged, then extended into the new portion of the deck to tie into the new reinforcement.

While this "deck splice" over the girder should act as monolithic, it will probably act somewhat as a hinge. I wouldn't be too concerned about interaction between the old and the new portions of the deck slab.

If you try to "tune" the new girder to match the deflection (by adjusting shielding, strand locations, etc.), the actual deflection will probably vary from the theoretical deflection. This is because of actual concrete strength, humidity, curing rate, and delays in erecting the beam.

Generally, the exterior girders should be designed the same as the interior girders. AASHTO LRFD §2.5.2.7.1 states "... the load carrying capacity of exterior beams shall not be less than ... an interior beam".

So, with all else being said, use a design which provides the same moment & shear capacity as the interior girders.

 

[manual90]

If you are using box girders placed adjacent to each other, use post-tensioning in the lateral direction; otherwise use sufficient diaphragms to ensure the bridge acts as a single unit thus preventing differential deflection.