Vertically loaded slab giving horizontal reactions

[pilesmakesmiles]

I'm modelling a chamber in STAAD.pro. It is a symmetrical cruciform shape with symmetrical walls with 8 pinned supports in a symmetrical arrangement. I have not applied any horizontal loads to the structure, only vertical loads on the slab (and no self weight on the walls).

I have applied a constant load to the slab to simulate the self weight, but I am getting horizontal reactions at all the supports (approx 1% of the vertical load) and the vertical reactions are all different. I would have expected zero horizontal reactions and symmetrical vertcial reactions.

If I remove the walls from the chamber, my horizontal reactions at the supports are all zero as expected.

Does anybody know why this is happening?

 

[structSU10]

you develope a membrane action in the plate elements between the support nodes. if you release the X and Z in all but a couple spots near corners, you can get to have zero horizontal reaction.