How to model Boundary Condition to Transfer Shear Only for Strong-Back System?

If Ry (rotational stiffness about global transverse direction) wasn't restrained at the ends it will create the problem. 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.