need help for a 3D frame made of L-section beams

[snoopies666]

hi everyone,
the structure is made up of 4 L-beams along its length. These beams are joined together by rectangular frames at regular intervals (each frame is made up of 4 L-section beams).

i've used the BEAM188 element and the "common sections" part. i've used keypoints and then drawn the lines to get my structure.
A force will be applied onto one of those rectangular frames which come at regular intervals.

now how do i join the rectangular frames which come at regular intervals to the 4 long L-section beams, so that the forces and moments get transmitted?

will really appreciate some help...thanks.

 

[GregLocock]

using what package?

If these are beam elements then using the same node will give a perfect joint.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[snoopies666]

hi thanks for replying so fast...
i'm using Ansys 8.1

i've tried using the same nodes but it didnt work.

these are the steps i followed:
1. change title/jobname
2. created BEAM188 element
3. created the required section
4. applied material properties
5. created nodes in 3D space
6. connected the nodes with lines
7. meshed the lines
8. applied DOF and forces
9. tried to solve but was not able to.
got an error which basically says that my model is unconstrained somewhere.
i found that the problem was with the rectangular frames as one of the nodes in the middle of one of the rectangular frames had exceeded the UZ limit.
i presume that occured because the frames aren't connected to the long L-section beams, so how can i make sure they are?

waiting for some replys....thanks.

 

[rb1957]

do you have a "box" beam, ie are your L-sections the edges of a rectangular prism and your frames are spacers holding the shape. and i guess you're loading through the frames too.

this looks simple enough, the trick is analyzing the joint between the frames and the beams.

good luck

 

[rb1957]

from your second post, it sounds like you didn't constrain the beam in space ... where does it contact to the rest of the world ? ... the load comes in from somewhere (a frame) and it needs to go somewhere.

7 meshed the lines (with beam elements, not rods)
8 applied DOF ... ? (maybe sthis is where you constrained the beam in space, remember you have to constrain 6 degrees of freedom)

again, good luck

 

[snoopies666]

Yes, the L-sections are the edges of a rectangular prism and the frames are spacers holding the shape.

The structure is contrained at the two ends (all DOF) and also I prevented the spacers from deflecting in the X and Y direction.

The force is in the -Z direction on the central spacer.

What do i do about that joint between the frames and the beams? I've made sure they have the same node and stuff, but still got nothing. Although I've noticed one thing, that if I make the long L-sections out of many lines (instead of the single lines that they are now), then I can analyse the structure. However, this is gives a very high deflection, far beyond what is expected. So I dont think it correct....

So I guess the question still remains as to what should I do with that joint of the beams and frames? Is there anything else I can do?

btw thanks for all the replies....

 

[rb1957]

it sounds simple enough ...
you've got four L-section beams (the corners of a rectangular prism) in the X-direction.
there are frames (in the Y-Z plane) that hold the shape, and apply the load.

if you have one element between the frames, the FEA will calculate results at these nodes. in this specific case this may not be too bad, 'cause there isn't any change in shear load between the frames. if you divide this element into many and get a large deflection, then i suspect the element is a ROD, and not a BEAM, so it doesn't like moments (and so the results are wrong) or else you've input a very small inertia for the beam.

i don't know ANSYS ... are the displacement freedoms associated with the node (like NASTRAN) or the element ? maybe this is what you mean by step 8 ... not sure on your terminology, but you want all six degrees of freedom at all your nodes (since this is a bending problem, and you want to restrain translation at the ends of the L-section beams (pinned supports) ... possibly they're fixed (and so have moment restraint).

i don't quite understand "but still got nothing" ...

is the model running now ? presumably it wasn't at first, what with "got an error which basically says that my model is unconstrained somewhere"

if it's running what's the displaced shape look like ?

maybe simplify your model, consider one frame (in the Y-Z frame) with a Z load applied (lower midside)and with reactions (constraints; X,Y,Z) at the upper corners. this might help you understand the elements abit more.

Another simple model to try is a simply supported beam, since you can hand calc that (can't you).

regarding the joints between the frames and the beams. if these are the same node, then they are welded together. analyzing these is a slightly tricky matter, but once you understand the internal loads in the L-section beams, then you can qucikly understand the loads being transferred by the joints.

a different matter ... don't over constrain the problem. maybe this was don't to trouble shoot the problem, but i'd suggest releasing the moment restraints at the ends (unless the problem says fixed ends). i'd suggest releasing the X- and Y- freedoms on the frames ... there should be a little deflection, bowing in the sides and maybe you're preventing this.

make sure you've given the elements bending stiffness in both directions, in your model that'd be about the YY and ZZ axes.