Query re Appropriate Restraints

LHartley

Attached is a model of a simple box girder truss. The displacement results are disappointing if I have set the model up correctly, however that is not my main concern. I will play about with the component sizes and the bracing configurations
I want the truss to be supported at each end by a set of two machinery skates per end - a bit like an overhead gantry crane. My question is have I set the constraints correctly. Thanks in anticipation.

disla

¿Are you considering steel as the main material for the truss?. You have a Young Modulus of 200Mpa. ¿Don't you mean 200 GPa.?

LHartley

Disla - thanks for getting back to me on this. Done it again - I suffer from zeros deficiency! Yes - 450 mpa steel is what I want in the model.
I also meant to say constraints as opposed to “restraints”.

Victor

Regarding constraints, if the skates are free to roll/slide in any direction on the floor, you should only constrain the vertical (Y) direction to represent them. That leaves rigid body motion in the other 3 degrees of freedom (translation and rotation in the XY plane) so you should also have 3 other point constraints which won't carry any load but allow a unique static solution. See picture.

LHartley

Thanks Victor. My idea is for the skates to be set in channel tracks which in turn will be fixed to a concrete floor slab. Therefore the truss will only be able to travel backwards and forwards along the z axis.

Does this change the constraint arrangement?

Apologies for what may seem naive questions. I am not an engineer so I am trying to gain a better understanding of this subject.

Victor

If the effect of the tracks is just to guide it and they apply negligible lateral (X) force, then it doesn't change anything because they skates are probably still free to slide small distances laterally in their channels as the truss deforms. You might need to get a clearer idea of whether they actually behave like this though or also analyze the case where each point has an X-direction constraint representing the skate jammed against the channel.

LHartley

Thanks Victor - you have answered my question. I appreciate your help