Rigid nodes

[thomasdw]

I have multiple elements connecting in one node. How can I fixate the angle between the several elements?

I tried using multiple nodes on the same location, and creating MPC, but this doesn't seem to work.

 

[rb1957]

are you rotating your model ? how ??

can you rotate the applied loads ?

 

[thomasdw]

Hi,

I am sorry for the lack of clarity in my first post. The best way to understand my problem is as follows:

I have a an arc of 10 connected bar elements. The elements on the ends are fixed in translations but no rotations. The structure is loaded under a singular load in the middle of the arc. Then, after analysis, the structure deforms. That is fine, except that I want the two bottom elements to not deform relative to one another. I hope the attached image helps.

Thank you for answering!

 

[rb1957]

do you have beam elements or rods ? i think beams should have enough end fixity to achieve most of what you want.

i guess there are good reasons why you're modelling such a simple structure ... it'd be easy to solve by hand ...

 

[btrueblood]

...then the bar from A to B will need to support bending moments, thus is a beam, not a bar. Switch to beam/frame elements.

 

[thomasdw]

The structure I'm modelling is not just these bars. It is an airplane door under cabin pressure that I suspend on several rods or beams over the top and bottom side to allow some movement due to expansion of the surrounding fuselage (of which I have no model). Initially I modelled de boundary conditions directly on the upper and lower side of the door but this gives unreasonable deformations.
If I model the suspension rods/beams tangent to the door it will act as the fuselage.

Currently these rods/beams do not do what I intend them to and I want to solve this by forcing the rods/beams to stay tangent to the door's surface.

P.S. I find it difficult to clearly explain my intentions, but thanks anyway for your quick reactions!

 

[rb1957]

ok, so now we've got a door (we've travelled some way from the original post).

your door carries pressure loads, reacts these at the door stops (probably in a radial direction). no doubt there is a door seal, compressed by closing the door. this could be neglected or accounted for (work through the mechanism to see how much (little?) load is applied to the door from the handle ... maybe remove this load from the applied pressure) or modelled with springs.

becareful imposing your vision of reality on reality ... reality has a way of biting back.

 

[thomasdw]

rb1957,
Thanks for your replies, I tried to model a simple version with beams, and that worked!

I am aware of the problem with reality .

FYI: I'm modelling a non-plugged cargo door, so no reactions at the doorstops. Only at the hinge line (on top of the door) and 5 latch-hooks at the bottom. The door is loaded under a pressure normal to each element.

Again I attached a picture of the result I was looking for!

 

[rb1957]

ok, you're reacting the pressure at the hinge and the latches (and you've got the completely different case of ground gust, where i expect your strcuture will be a lot "floppier").

i assume you're using a linear solver ? (ie not non-linear)

where are you getting large displacements ? i'm assuming you've got a detail mesh, with nodes in the middle of a skin bay unsupported by frames/stringers. i suspect that these guys are "misbehaving" ?