SpaceGass Frame Buckling

[asixth]

Hi guys,

I am using a frame analysis package called SpaceGass. Basically the software is a FE package that only analyses 3-D frame elements, very useful for steelwork. Unlike other FE packages, I cannot delete freedom cases and sometimes when I have modelled insufficient restraint to the plane not under consideration, I recieve an error message stating...

ERROR: Frame buckling has occured for one or more load cases therefore the results cannot be used.

My question is this...

Is this frame buckling based on any one element exceeding it's capacity based on pure mechanics alone or does it refer to my frame buckling as a whole (i.e snap through)?

 

[apsix]

From Spacegass Help;

'The SPACE GASS non-linear static analysis includes a simple buckling check on individual members that is intended to alert you if a member is being removed from the model due to its Euler buckling load being exceeded. However, it is not a full buckling check that considers groups of members or the structure as a whole.'

Does that answer your question?

BTW Spacegass is not a FE package.

 

[asixth]

Thats apsix. I was discussing this with a colleague late last week and I thought that might be the case.

Why can't I say SpaceGass is an FE package. A beam element is still an "element", the structure is modelled as a number of degrees of freedom, a stiffness matrix is compiled, condensed and inverted calculate the displacements?

Just because SpaceGass doesn't analyse plate or solid elements is no reason to say it is not an FE package? Or is it, am I missing something?

 

[apsix]

"Why can't I say SpaceGass is an FE package."
It looks like that you're right. It was my understanding that the FEM by definition does analyse plate or solid elements. I think I was wrong.
From wikipedia;"The direct stiffness method is the most common implementation of the FEM".

 

[csd72]

You may be right, by when most people refer to FEM they are referring to a more in depth analysis package.

Nothing wrong with SpaceGass though, I would take it any day over these useless non-user friendly packages I use over here. In spacegass I could setup a basic analysis in 15 minutes the one that I am using at the moment takes an hour to do the same thing.

Anyway, it sounds like your portal frame is buckling out of plane, make sure that they are all restrained out of plane. If so then check the buckling output to see where the problem is.

 

[apsix]

Yes, I won't claim to be proficient at FEA just because I can use Spacegass.

 

[asixth]

Thanks guys, I'm assuming it is only Aussies who are using SpaceGass, like csd mentioned, the real advantage of SpaceGass over more complex FEA software is it's useability, no question.

I noticed when modelling a 2-D pin-jointed truss that SpaceGass will automatically delete the rotational dof, so if I apply a momnet at a node it will not be analysed. It would be nice if I could manually delete freedoms, the majority of structures I analyse in SpaceGass are 2D.

 

[Denial]

Asixth,

I'm not a Spacegass user, but your desire to apply a moment to a joint in planar truss model would be fundamentally incompatible with the concept of a truss in any structural analysis software I have ever used. Think about rotational equilibrium for the joint in question. The applied moment can only be resisted by bending in the truss members that are connected to the joint, but truss members (by definition) do not carry moment. If you need to apply moments to the joints, your model needs to be a plane frame, not a plane truss.

 

[asixth]

Denial,

I was using the application of a moment to a planar pin-jointed truss as an illustration of how this particular software deletes freedom case automatically (that being all three rotational degrees of freedom and the out-of-plane translational freedom from a 2D pin jointed truss).

For my original post, I was questioning the valididty of a buckling out-of-plane error message that results when performing a two-dimensional frame analysis, assuming my purlings adequatley prevent the beam from buckling out-of-plane under pure compression. Because I have this general restraint, I would like this particular package to allow me to delete the non-used freedom cases for my analysis, being two rotational freedoms and one translational freedoms, other software packages allow this.

 

[hokie66]

asixth,

Suggest you submit your query to SpaceGass technical support. They should be able to help.

 

[apsix]

asixth
I think the use of a 'general restraint' on nodes which are otherwise not restrained can be used to partially overcome your problem. It may still require a few dummy nodes along your beam but at least you won't have to restrain each one.

As hokie said, contact tech support; I've found them to be prompt and helpful.