I have a problem when I model with Quad elements, sometimes the different elements fit together directly and sometimes they don’t.

what do you need to think about when you model with Quad elements so that they are stuck with each other? Do I need to use constraints and if so how?


If you model the SARs so they share a boundary, they should connect automatically.

Generally the quads will share nodes.
Constraints are an added complexity and you shouldn’t need to use them unless:

  • You want release some degrees of freedom (e.g. bending)
  • The quad borders have a distance in between them
  • or some other special case

Maybe post a picture/file, if you want specific help

I want to model this tunnel, and I want a corner that I have drawn with support lines, how would you do it?

I’m assuming you are using quads for concrete and that the “support” is a haunch, so I would:

  • Avoid restraints if you can (always)
  • Model the SARS along the centre of the element
    In your example the bottom slab is modelled at the upper surface, you’re underestimating the length wrt bending.
  • Estimate the thickness increase of the wall/vault with a max ratio of 1:3 per side, so the centre line diverts with a 1:6 ratio

So the system lines/surfaces of the quads would follow the green lines below: