Boundary Condition - Shear Transfer Only for Strong-Back System


I need help with how to properly model boundary conditions which are very common to many specific bridge construction techniques. I am attempting to model Precast Beams which will be connected together in a final condition through a closure pour. In the staged construction these girders will be held together through the use of a "strong-back" system in which only shear transfer will take place (no moment or axial). I have constructed a very simplistic model to attempt to model this behavior. See below for model description. In this model I have attempted to use beam end releases to get the shear transfer only condition however it doesn't seem to be working properly (unstable). I have also attached a picture in order to clarify exactly what the strong-back system looks like.

Model = (1' x 2' x 22' rectangular concrete beam. (2) 10' segments which are separated by (1) 2' segment (splice/closure pour).
Stage 1 = (2) 10' segments are supported at each abutment and at other end by temporary tower with bearings
Stage 2 = Temporary towers removed and beams are connected to each other through the use of a strong-back. Strong-back can transfer shear only. Load on the 2' splice is due to wet concrete
Stage 3 = Strong-back system removed and beams now act as 1 single simply supported member.


Thanks for the clear explanation! The issue was due to inappropriate boundary conditions in this case and not with the beam end releases. It was noted that Ry wasn't restrained at the ends. No doubt the beam would be resting on pin support, but it will have bending resistance due to overhang. Kindly refer the image below.
If we try to model just this beam resting on the pier cap, then its bending and rotation could be obtained as below and this in turn could be used to calculate the rotational stiffness Ry at pier location.

When Ry isn't restrained, there is bending My generated in the beam but absolutely no place to resist this bending and hence there are abnormal deformations. To replicate this, point springs with Ry stiffness have to be provided at pier cap locations.

If required, these stiffness could later be removed to keep the beam as simply supported.
Creation date: 1/9/2018 4:17 AM      Updated: 1/9/2018 2:56 PM
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Strongback Example Photo.jpg
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