Steel beam Connection

[kariz1384]

hi,

I am having difficulties with stability type 2. I have used diaghram one-way slab with steel beams as pinned connection, i.e. fffxxx in one end and ffxxxx in another end. I have attached a screen-shot of connection, where Robot says stability type 2. I really appreciate, if someone could help me.

Thanks

[Artur.Kosakowski]

You may consider the following steps:

1. Remove all RX releases and run calculations. Display torsional bending diagram and check where you have significant torsion. Apply Rx releases only in these locations.

2. Use elastic releases with small stiffness: http://forums.autodesk.com/t5/Robot-Structural-Analysis/Steel-design/m-p/3573540

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[kariz1384]

Thank very much for your answer. I have followed your first instruction, but it did not help, unfortunately ;-(
Then I followed your second instruction, which required that i have to turn on the DSC algorithm in the setting, then I did not get any stability problem at that node.
Then I thought I will keep the releases as it was, i mean fffxxx and ffxxxx and just turning on the DCS algorithm to check, what happens. I found out that instability problems suddenly disappear. I have to mention that in my model I have used Rigid Link. So I was wondering, how chome the DCS algorthm make the instability disappear.

[Artur.Kosakowski]

The only reason I can think of is the excessive difference in stiffness of elements reported as instability of type 2 rather than 3. 

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[kariz1384]

Hi,
I have just defined some trusses (see attached screen-shot for the method) and in my model, which I have turned on the DSC algorithm, I get this warning (see attached screen-shot). When I turn off the DSC algorithm, I do not get the warning. A truss isn't like an element with ffxxxx and ffxxxx releases? I do not know, why I get that warning.

[Artur.Kosakowski]

The warning should be displayed in both situations ( I have checked on Robot 2014SP4  and it is). If you decided to use a truss bar then it means that this element cannot transfer any bending (has got no rotational d.o.fs). If so the definition of additional releases for rotations is excessive.

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[kariz1384]

thank you very much for your answer, by two situations, you mean; a truss bar and a element with ffxxxx end releases? 
Ya, i want to define a few of the element to only take axial load, no bending moments. So I should not be worry about the warning. 😉

[Artur.Kosakowski]

To put it plainly - if you assign the truss attribute to a bar do not apply releases. This element will transfer only axial and shear forces (no bending).

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