Why there is an horizontal reactions while Appling vertical loads in the grillage?
The horizontal reactions in grillage model are due to a combination of factors.
1) grillage is set with a Centre-Top offset so that the plane of the nodes and finite elements is at the top fibre of the beam sections of the real deck. This creates an offset between the nodes where the reactions are defined and the neutral axis of the deck.
2) bridge is over constrained in the horizontal direction and reactions become coupled. That is, grillage have two supports in the X-dir, and two support in the Y-dir. While this is common constraint condition, when this is combined with an offset deck, then an interesting effect is observed where the vertical load generates horizontal reaction.
When the beams are loaded vertically and they want to deform, the top fibre wants to shorten and the bottom fibre to extend. However, this top fibre is restrained by the support, so instead of the top fibre node moving backwards, the whole girder is pulled out forward. This creates a tendency in the deck to rotate, but this is resisted by the Dy supports at the NW and NE corners. Thus, resulting in two X-dir reactions and two Y-dir reactions.
To avoid this:
1) redefine the beam sections offsets so that the main girders have no offset. This will leave the neutral axis of the main girders aligned with the support nodes and the beams will not try to pull the deck when loaded;
2) or remove on of the Dx or Dy supports. However, make sure bridge still have correct boundary conditions for any horizontal loads to reflect the true structural system.