tubes and cosmos analysis

I have a working process kind of problem - I know what to do but don't
know the best way to do it.

I'm creating a tubular space frame and using SolidWorks 2007 for
drawing/editing/viewing the space frame but would also like to do a
very simple FEM analysis (possible through COSMOSXpress I assume) on
the entire space frame.

Regarding FEM, I'm not interested in absolute values or maximum load
or similar - only interested in an analysis which can point out weak
areas in the construction that need attention.


Today I have a rough 3D sketch.

Next step was to convert each line into a tube by using Structural
Member from the Weldments toolbar.

But what's next step? How do I go proceed from here?

 

Forget CosmosExpress for something like this. Although the analysis of
frames is typically given as a college level programming exercise
because beam elements are among the simplest to program, this
functionality has been missing from SW from the beginning. Cosmos/M
can handle beam elements which makes it far quicker than SW when
doing this sort of problem.

TOP

 

TOP is correct, you'd need to use beam elements --
which Xpress does not have.

For a quick/dirty analysis with the tools you do have, you
might model your frame with solid rounds -- and not tubes.

The stress results from this would of course be totally
meaningless but you may gain some insight from the
displacements -- ie where your structure might benefit
from gusseting, etc.

Be careful of sharp inside corners -- which stinks because
1) a weldment will have lots of these and 2) thats where
your gussets would be. Use some "engineering sense"
and try to guess where the critical points are likely to
be. Fillet these joints with a radius at least equal to
your intended wall thickness.

Try to run as fine a mesh as your machine will handle
(don't know how much control Xpress gives you here
either).

Finally, you'll have no buckling information -- so keep
an eye out where tubes meet at a "T" or where the
displacement analysis bends them significantly from
compressive forces.

Far from ideal -- but again, may give you some insight.

-Tony

 

Thanks for all input.

So what I hear is that COSMOSXpress can't handle my structure. Í have
no problem
using solid rounds for my model - but as I'm sort of a beginner I'm
not sure how
much extra work that will mean (and how do I practically do it -
sounds like little
bit more work than using a simple 3D sketch).

Don't know if COSMOSWorks is available for me, will check.

Any possibility to export the endpoints of the 3D sketch to another
program
where simple analysis can be carried out? I find it easy to design and
change
the 3D sketch (and later stage add other elements) in SolidWorks but
what I
hear is there is no simple way to do any kind of load analysis (not
even the
very simplest ones).

 

Tony,

Actually, I hadn't thought of this solution. If Hendrick is after
deflections what he might do is this:

1. Calculate the moment of inertia of the tube.
2. Back calculate the diameter of solid bar that has the same moment
of inertia.
3. Use SW weldment functionality to build the space frame from solid
bars.
4. It just might be possible to run this in CosmosExpress for global
deflections and hot spots if Hendrik can get the boundary conditions
right. St. Venant and some long thin members might come in handy
there. There will be a lot of elements in any event. The stresses
won't be right because c/I will be too low. You might be able to
factor that up to get the stress in your tube though.

TOP

 

Whats everyone doing up so early?

Top, I'd have to give that some thought but working backwards
may get him (her) into the 20-30% range (error, that is). I guess
it boils down to just how big his model is and how "approximate"
is "approximate".

Hendrik/BMW -- the easiest way to model this is to still use
the weldments feature. You can edit the 'profile templates'
(don't remember what they're really called). Find the template
for the tubing size you want -- open & edit -- delete the inside
circle (tube ID) and save it as a new template (Solid_Round_XXmm)
in the same directory as the other templates.

When you pull up the weldment feature, you'll have a new
profile in there -- the solid round you just saved.

Be sure to clean up the ends where 3 or more members meet
up as you may have some sharp corners that Xpress definintely
won't like.

-Tony

 

It Hendrik is doing the space frame then I wouldn't be too concerned
about the corners except qualitatively. If it is a vehicular space
frame it is going to be plenty strong. The concern will be where the
plastic hinges will form when in a crash and in rigidity which is
calculated from loads and deflections. If a particular corner looks to
be a problem then model just that connection with appropriate loads.
Unfortunately pulling those loads off of CosmosXpress will be a
problem.

TOP