Modelling of bridge bearing devices

Andrea91

Hello,

I am trying to model a two-span bridge deck supported by a central frame pier in Mecway.
The beams are supported on the central pier through bearing devices with different constraints. Specifically:
- The two outer bearings provide vertical support only, allowing horizontal movement.
- The central bearing provides both vertical and transverse restraint, while allowing longitudinal movement.

I would like to know how these bearing conditions can be correctly modelled in Mecway. In particular, which constraints or boundary conditions should I apply between the deck and the pier elements?

Thank you in advance for your help.

kuhl

I guess you could connect the nodes of the frame with those of the deck using constraint equations.
This way you could define that the vertical displacement of the outside deck nodes must always be the same as the vertical displacement of the corresponding support frame nodes.
Same aoplies for the central nodes with vertical and transverse displacement.
I think you will have to define one constraint equation for every node pair and constraint direction such as z1 - z2=0.

Andrea91

I have tried using constraint equations, but they seem to work only partially. In fact, constraint equations appear to transfer forces, but not moments.

To clarify the issue, consider the bridge with one of the two spans removed. Because of the eccentricity between the beam end and the pier axis, each beam should transmit to the pier both a vertical force and a moment, with the latter equal to the vertical force multiplied by the eccentricity.
However, I cannot obtain this moment in my Mecway model.

Other FEA software allows the use of rigid links to connect nodes, which would naturally account for the eccentricity and transfer both forces and moments. I am wondering whether Mecway has a similar feature or whether there is another recommended way to model this type of connection.

MikeMcMullen

I had some luck modeling eleastomeric bearings as a simple elastic support. Multiple elastic supports at the same location could do pretty much what you want.

Victor

You could use high-stiffness beam elements aligned vertically with released rotational DOFs (flexible joint on beam) at both ends to allow movement in only one direction. For free movement in both directions, it reduces to a truss element.

Constraint equations can couple moments but you have to write it into them which is a bit fiddly. See picture and attached file.



Node-surface coupling with the RBE2 option can connect the ends of two beams together but doesn't have options for releasing individual DOFs - only all rotations at one end or nothing.

disla

I would model the finite dimension of the suporting base so the moment due to the excentricity of vertical load will emerge naturally.

disla

"- The two outer bearings provide vertical support only , allowing horizontal movement.
- The central bearing provides both vertical and transverse restraint, while allowing longitudinal movement."

vertical (z)
transverse (y)
longitudinal (x)

Bridge decks are underconstrained in Longuitudinal direction (x) ¿isn't it?

Andrea91

@Victor
I also tried to introduce flexible joint on beams:
- For the two outer bearings, that provide vertical support only, vertical truss elements should work.
- For the central bearing, which provides both vertical and transverse restraint, how can I contraint the transversal displacement only by means of releases?

@disla
The longitudinal contraint is on the the bridge abutment, so to have a statically determinate system.

Victor

Here's an example of a connecting beam (purple) that provides vertical (Z) and transverse (Y) connection but is free in longitudinal displacement (X) and all 3 rotations.

Victor

By the way, if you're willing to write some input by hand (AI can do it) and be cautious verifying the results, you might consider the Mystran solver that's included with Mecway. It has more of these connector type elements from Nastran like CBUSH (general elastic 2-node spring), all 6 DOFs release on beams, and RBE2 and RBE3 with more fine grained control over which DOFs to connect.