number of elements to use for one structural element
number of elements to use for one structural element
(OP)
I have to model a truss and perform a dynamic a analysis in nastran/patran. Since the real structure is built from metal tubes welded together, I will use bar-elements for (almost) all structural elements.
My question: Does it make any difference whether I use one bar-element for an entire tube, or should I splitt up each (long) tube in a number of bar-elements?
Rob
My question: Does it make any difference whether I use one bar-element for an entire tube, or should I splitt up each (long) tube in a number of bar-elements?
Rob





RE: number of elements to use for one structural element
Personally I would use beam elements for this case (by saying "bar" elements, do you mean beam elements?). These are tried, tested and trusted, and can give predictable and easily checkable results. Moreover, in the case of static/dynamic analysis, it should not matter if you use only a single beam (for a single tube), as the model should pick up the features of the mode/displaced shape. However, if you feel that the mode shape looks slightly "coarse", you could always introduce a few more - this introduction will not increase run-time by any significant amount (beam eleemnts are very simple in formulation, and require very little CPU time), but in the case of static results the more beams you have for a single tube the more the results will be a pain in the u-know-what to check and post-pro! You would only ever need to use more beams if you were intending to perform some form of dynamic contact analysis (mesh size here = fine mesh, then quadruple it!) where propagation wave times become important.
Hope this helps
-- drej --
RE: number of elements to use for one structural element
RE: number of elements to use for one structural element
I was somewhat unclear about the elements I use. I started out with beam element, but I got the tip to use rods, because the tubes in the structure are basically not designed to carry lateral loads. What will be the differences in a static and normal modes analysis?
RE: number of elements to use for one structural element
RE: number of elements to use for one structural element
pj
RE: number of elements to use for one structural element
Mode of vibration of truss and beam is almost different. The natural vibration modes of truss are only governed by its axial stiffness/mass while those of beam model (Timoshenko beam) are governed by its axial, bending and shearing stiffness. It's clear that axial stiffness of element of the same cross section is significantly higher than beam element. So its natural frequency is significantly higher for truss model than beam of the same properties. In my view, stiffness of truss element sujected to nodal load is exact since its shape functions come from complementary solution of differential equation (EAu')' = 0 ; u = A+Bx (linear). Therefore no matter how we increase the number of elements, the results should be the same.
RE: number of elements to use for one structural element
It's clear that axial stiffness of element of the same cross section is significantly higher than its flexural stiffness.