hey friends welcome back to the channel and today we're going to go over some tips on how to perform
a 3D frame analysis without running into instability errors we're going to look at
global deflection and local deflection and i'll show you some steps to make sure your 3D model
will accurately represent what's going to be built in real life to start with i just wanted
to remind you to not forget to add the self weight of the structure i'm just saying that
because some software will not add the self weight automatically and it's easy to miss this step
next it's important to understand the difference between global deflection or displacement like
robot calls it and the member deflection itself so displacement is the distance
from the original position of a point to its final position on the deformed shape while deflection is
the distance from the supports of this beam to a point on the deformed shape so by looking at
the deform shape of this 3d frame we can infer that this structure is trying to tip over and
these numbers are the displacement in the x y z direction so in other words they're not only the
vertical deflection you would expect from vertical loads if you want to check the member deflection
and assess this number against the code limits like spend value by 250 or 360 or 500 or whatever
number you go for we should check the member deflection which in this case four bar number four
is one millimeter and not six or five millimeters as we mentioned before the structure is leaning
towards the y axis so if you have some other structure bracing these columns you could add a
roller support restraining y direction and that would give you more realistic displacements so
here applied supports restraining y-axis movement and that five millimeter displacement is gone
on the other hand if you don't have anything bracing and preventing the frame to tape over
you might need to keep these displacements as small as practicable let's make this structure
a little bit more interesting i'm just going to copy some columns and beams up you can use
this intersect 2 which will divide members at the nodes of every intersection that's a good practice
so you you can ensure every node is connected you can just create a node at every intersection of
bars if your software doesn't have the intersect tool you know that paint connections are cheaper
and easy to fabricate and it's most likely that you will be using them in the majority of your
project so let's apply paint release to the end of every bin in the model and run the calculations
we now get several instability errors
this is something you might run into when you first start modeling in 3d in any software and
what we're going to do here is is read the errors and try to figure out how to fix them
one by one so the first one instability in rx direction in the node 12. what does that mean
if we turn on the local axis notice that the x-axis is the one along the length of the beam
rx is then the rotation about x so what this error is telling me is that the beam is sort of spinning
around its own axis nodes one four and nine are the support nodes so supports use the global axis
therefore it is not rx anymore and it's r z and it looks like it's happening the same
problem which is the bars are spinning around its own axis to correct these arrows let's fix
our z in the supports because we know that in real life or columns
won't twist unless we're causing some sort of torsion in the structure
okay so where our zed is released we're just going to fix that and run the calculations and for the
beams i'll check what robot is releasing for pin connections so let's go to geometry releases
the fixed pane release has its pinged and with rx released therefore we're going to untick this box
so that twisting doesn't occur same applies to the painfix release and paint paint as well
now note that the calculations will run without any instability errors
once the 3d model is running smoothly make sure the bending moment diagrams appear as you expect
them to be for example a simply supported beam would have no moments at the supports
and the peak positive moment in the middle while for a continuous beam we would expect
to have zero moment at the paint connection and negative moment at the side of the cantilever end
have a look at the deflections as we did in the beginning of the video
is this displacement looking as you expected do you think the structure will deflect this way when
under loads if not something in the model might be wrong and you need to fix that before going ahead
check the bar deflections so you can assess the code deflection limits
looking at bar 11 there is a two millimeters deflection if the software you're using doesn't
show the deflection separate for each axis a little trick you can use is to apply supports
at the ends of the bar and these will give you the local deflection of that member
so here we get the same two millimeters deflection
and again we can see that the structure is trying to tape over however we may have some
sort of bracing in the y direction and that's why i will restrain this direction so we get
displacements that closely match reality if you think you won't be able to brace this structure
you will have to make sure these displacements are not excessive
alright fellas this was not a modeling tutorial so anything that you learned here you can apply to
any software the principles are the same if you understand releases supports
the displacements or deflections you can do that to any software
the only thing that change will be the tools or the interface of the software
that's pretty much it and i hope this video was helpful to you if you have any questions just
drop a comment below if you can like the video and share with your friends and i'll see you next time