Contact problem

Hi..

     My model consisits of two steel plates. Top plate has curved bottom and bottom plate is rectangular. Top plate is resting over the bottom plate. There are 3 steel pintels between the bottom face of top plate and top face of bottom plate to prevent running of top plate. Sliding is allowed between the contact surface of top and bottom plate. Pintels are fixed to the grooves made in the bottom face of top plate and frictionless hard contact is defined there.

Loading - Vertical pressure of 5MPa on top face

               Lateral displacement of 1.5mm on the side face of top plate

BC-        Bottom surface is fixed.

Steps- 1. Vertical load

Step-2.    Lateral displacement

When I am doing the analysis results for very low value of vertical load are coming fine for both steps. But as I increase the vertical load to the above mentioned value the RF2 value in step 2 starts decreasing after reaching its peak value in 7th increment. According to me it should attain a constant value of RF2 as soon as sliding starts between the two surfaces. 

Any help regarding this is highly appreciated

Thanks

Neha