I´m trying to simulate the behaviour of RC frame under lateral load, where at the top of the columns additional vertical load was applied. I have modelled the frame with 2D elements.  Coulumn width is 20 cm. Problem is that boundary conditions on the top of the columns should allow horizontal sliding and not vertical moving upward, and also the horizontal surface at the top of the column should not rotate.
I have tried to model the steel beam above the coulms at put the contact elements in between steel beam and column tops but this solution failed because i cann´t apply vertical load directly to the contact element. I have also tried to put the steel plate between column top and steel beam with contact element at the top of the steel plate and v. load at the top of the column, but that solution caused v. moving downward (which is logical) but also rotation of the horizontal column surface during the lateral load action. My question is how to prevent the rotation of horizontal (top) surface of the column and to allow horizontal sliding and vertical moving dowward?

Thank you for you patience!