Tapered Beam with Reference Point Offset

[sam.lee3FKPJ]

Hi, I'm using Nastran In-Cad 2017 to model a thin square panel with reinforcing ribs. I have defined the ribs as beam elements (thin rectangular cross-section), with the offset set to a reference point at the top of the cross-section. The depth of these ribs varies across the panel, so I would like to define a tapered beam. However, it seems that you can't do this if the offset is set to a reference point. Do I have to offset to the shear center to use a tapered beam or am I missing something? The cross-section of these beam elements is 0.15" x 1.5" so I think it will make big difference to mesh to the center of the beam instead of the top edge.

Thanks!

Sam

[John_Holtz · Accepted solution]

Hi Sam,

I agree with your findings. It looks like a tapered beam can only be specified when using the "Offset To > Centroid" or "Offset To > Shear Center" but not with "Offset To > Reference Point". But if you use one of those for the tapered beam, only a constant offset distance can be specified, and that will not work with a tapered beam. (Actually, the beam can only be offset from the mesh location when using "Centroid".) 

In this situation, it might be easier to model the rib using shell elements. 

Technically, a beam can be offset a different distance at each end of the element if you wanted to edit the Nastran file to enter this information. The problem is that the offset of "end A" and "end B" is different for every element, so there would be a lot of calculations to do and a lot of data to enter into the file. In other words, you do not enter the offset at the beginning and end of the sketch line and the software calculates the offset for each beam element (based on the mesh size). 

[sam.lee3FKPJ]

Thanks, John. That answers my questions. I will probably end up modeling the ribs as shell elements, like you suggest.

Just so I understand, the offset to reference point is to a fixed distance from centroid?  I thought it was to a point on the cross-section geometry (i.e. the center of the upper edge on my bar section). That might be a helpful update in the future to allow modeling of tapered ribs as offset beams. 

[John_Holtz]

Hi Sam,

You asked a good question ("the offset to reference point is to a fixed distance from centroid?") and one that I did not think to investigate. My suspicion is that the answer is yes: the reference point is probably considered to be a fixed distance from the centroid in the current software. 

I suggest that you add your idea of a tapered beam with reference point offset to the Nastran In-CAD Ideas page. If you could include an image to explain your setup, that would help other users understand the benefit of the idea (and therefore encourage them to vote for it).