[ANALYSIS] Horizontal reactions from vertical loads

Question:

Hi,

I've been working on a bridge deck model where I want to investigate the horizontal reactions at the supports. I have applied HA type load on the deck and even though I have verified the vertical reactions at the supports, it appears that I'm also getting horizontal reactions due to HA loading as well. It seems to be like braking/traction load is applied at the same time and I haven't been able to find how to remove that. Can you please let me know if I'm missing something? I have attached the model and a screenshot for your reference.

Thanks,


Answer:

Hello ,

The horizontal reactions in your model are due to a combination of factors.

1) Your 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) Your bridge is overconstrained in the horizontal direction and reactions become coupled. That is, you 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 you can either:
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 you  still have correct boundary conditions for any horizontal loads to reflect the true structural system.

Kind regards,
MIDAS UK

Creation date: 8/23/2018 3:34 AM (MIDAS_UK_IM)      Updated: 8/24/2018 12:07 PM (MIDAS_UK_IM)
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