i don't know i got the your question right or not. Have you tried the tie constrain option . An other possibility is to use cohesive element may be that will help. 

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Have you set a general contact or a sutface-to-surface contact.

Use thie s-t-s contact. The surface defined as master should be the lower surface of the upper beam. Please use a finer mesh for this surface with respoect to the slave surface.

Let me know.

Franco

@Muhammad Afzal: no I only tried the interaction option; (I guess) with a constraint the relation between a point and surface is described. How can I use this as an interaction between the surfaces?

@Franco Concli: I have chosen the upper surface of the lower beam as the master surface. I have tried changing the master/slave surface but I still get the same results. It seems like the end nodes (see figure) have a constraint that they may not change from each other in vertical direction, except for slip in horizontal direction.

For the Contact Property I have chosen Tangential behavior: frictionless because I want to allow slip in horizontal direction (for now), normal behavior (hard contact and augmented Lagrange) and cohesive behavior

Also if I refine the mesh this problem still occurs. I have uploaded my model, maybe this is easier to watch the model and its constrains.

For that kind of problems  in Step module (1) turn on geometric nonliearity (Nlgeom) and (2) use "nonsymmetric" Matrix storage in Step-Other