Joint Reactions & COG

[BigTank]

I am looking at a simple platform-type structure whose column joint reactions and calculated center of gravity don't seem to make sense. It appears that I am not understanding what mass this COG belongs to.

The electronic help file as well as the General Reference manual describes this COG as:

"The last line provides the center of gravity (COG) for the applied loads. This "COG" is based on the load components acting in the VERTICAL direction. If there are no vertical loads in the combination a "COG" will not be calculated."

If this is true, would not the COG lie inside the the bounding dimensioned by the X and Z coordinates of the loads (Y being the vertical ordinate), and the magnitude of the largest loading equivalent mass? In this structure, when self-weight of the structure is included as a loading in the solution, the coordinates of this COG fall well outside even the structure's bounding box...how can this happen and the structure not be rolling around the shop floor?

I am trying to determine about what point the moments reported in the Joint Reaction are acting. Obviously I'm missing something, but I cannot find exactly what it is...

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Fitter, happier, more productive

 

[JoshPlumSE]

BigTank -

Your understanding is essentially correct. Not sure why it would not fall within the geometric bounds of the structure.

I remember that we corrected a bug with this some years ago. Something about either thermal loads or NEGATIVE loads applied in the vertical direction having a detrimental impact on the calculation. But, that fix was done quite some time ago and I don't recall all the details. What version of the program are you using.

 

[BigTank]

Josh,

The software help says 8.0.2, but the General Reference manual says 'Version 7.1' with a copyright year of 2008.

I am, in fact, loading the structure with negative valued joint loads. It sounds like I might have the bug bugging me! Can you confirm this, by chance?

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Fitter, happier, more productive

 

[JoshPlumSE]

There is nothing in the RISA-3D release notes about it.

You might try removing the negative / upward vertical loads from that load case and re-solve the model. Then see if the location of the CG makes sense. I believe it will. That will confirm the issue is related to the negative loads.

After that, it's mostly a question of what effect the negative loads will have. I ran some tests and those negative vertical loads will impact the COG calculation. I've done some calculations and they appear to be correct... at least for the simple cases that I looked into.