Bridge Model with Truss and Beam-Column Elements OR End Moments Release

[NetoConrado]

Good afternoon,

First, I created a Linear Elastic model of a bridge on OpenSees, using all elements as Beam-Column elements. It worked, OpenSees perform the analysis and the results (Internal forces, reactions, displacements) match the results obtained using SAP2000.

However, I am trying to run an analysis with Beam-Column elements in the deck, and the remaining elements as truss elements. This consistently gives me a singularity problem when I run the model on OpenSees. However, the problem is solved with no issues on SAP2000.

The only thing I changed in my model was that I changed those selected elements to be truss elements (I verified the list of elements with SAP2000, and they match).

Is there any trick to make this work?

another alternative I would be interested in, if anyone knows how to do it, is to try and release the End Moments on the beam-column elements.

Can anyone help me with any alternatives?

thanks!

[mhscott]

You should fix the rotational DOFs at the "truss only" joints.

[NetoConrado]

You should fix the rotational DOFs at the "truss only" joints.

Thank you, how do I do that? Do you have any reference?

[NetoConrado]

Oh I assume I should just actually use the Fix command? I will try that.

[NetoConrado]

Yeah I tried fixing the rotational DOFs of the nodes that connects only Truss elements (Fix NUMBER 0 0 0 1 1 1)
the problem persists

if there is any other alternative or something I might be missing I would appreciate the assistance

thanks!

[mhscott]

Also check supports with only truss elements connected.

[NetoConrado]

Also check supports with only truss elements connected.

I did fix the rotations of the supports with only truss elements
the problem persists
I checked the list of nodes (with only truss elements) with SAP
I am out of ideas to check if I fixed the moments of the truss only elements...
any suggestions are appreciated

[selimgunay]

If you have distributed forces on the truss elements, that could be the problem.

[Ashok]

I think you should also fix out of plane DOF for 3-D truss elements.
For example if the pushover load is in x-y plane the for truss end nodes the fixity should be - fix(0,0,1,1,1,1) - arrest z movement
if the load is in x-z plane then fixity should be - fix(0,1,0,1,1,1) - arrest y movement