Extracting bolt loads from cbush elements
Extracting bolt loads from cbush elements
(OP)
Hi everyone,
I'm trying to model a plate with two c-channels bolted underneath it in Patran/Nastran. The plates and c-channels are modeled with shell elements, and the bolts are modeled using rbe2 with bushing properties in order to extract the bushing forces for bolt analysis. Based on my hand calculation for the shearing force on the bolt, the bushing forces (shearing component) I extract from my model are significantly higher than what my hand calculation suggests. I think this is most likely due to the stiffness values I assign to the bushing property, calculated using K=AE/L.
Is K=AE/L the correct way to determine the fastener stiffness value for bushing elements? Does this K value also apply to rotational stiffness? In general is this a good way to extract bolt loads?
Thank you very much in advance!
regards,
I'm trying to model a plate with two c-channels bolted underneath it in Patran/Nastran. The plates and c-channels are modeled with shell elements, and the bolts are modeled using rbe2 with bushing properties in order to extract the bushing forces for bolt analysis. Based on my hand calculation for the shearing force on the bolt, the bushing forces (shearing component) I extract from my model are significantly higher than what my hand calculation suggests. I think this is most likely due to the stiffness values I assign to the bushing property, calculated using K=AE/L.
Is K=AE/L the correct way to determine the fastener stiffness value for bushing elements? Does this K value also apply to rotational stiffness? In general is this a good way to extract bolt loads?
Thank you very much in advance!
regards,





RE: Extracting bolt loads from cbush elements
My preferred method is to use 3 celas elements for each bolt (2 shears and a tension), and use Huth to obtain the stiffnesses for the FE.
The saving grace is that even when you use one of the methods above, in the great scheme of things it makes little difference.
RE: Extracting bolt loads from cbush elements
RE: Extracting bolt loads from cbush elements
jetmaker
RE: Extracting bolt loads from cbush elements
RE: Extracting bolt loads from cbush elements
"Clever of me to use my spine to break my fall like that."
RE: Extracting bolt loads from cbush elements
RE: Extracting bolt loads from cbush elements
"Instead of a RBE2 element, maybe a bar element would give you reasonable results"
I dont think so, with a bar element you can enter values of shear stifness for the elemental Y and Z axis, but i dont know how you could enter a value for the X axis.
RE: Extracting bolt loads from cbush elements
"Sheesh. Some days it just doesn't pay to get out of bed."
RE: Extracting bolt loads from cbush elements
The standard stiffness calcs you mentioned are usual for comparing beams etc, but bolt joint stifnesses are far more complicated. Compare specialised bolt stiffness theories and test results, against the standard stiffnesses and see how they compare.
I'm not saying using a bar element is bad, just that using celas's is better. Though the main thing is to get decent elements to represent the actual bolts and joints, anywhere within 10-15% and its going well!!