I want to know the releases.

[shweyeewin]

I understanding is pin connection which doesn't take any moment, only shear & bracing take lateral. So I model one space frame which is all pin connection: so I do all releases & bracings in X & Z direction: which release in My & Mz. But the bracing is assigned as truss, always show "instability". But the bracing is assigned as beam release: My & Mz, it doesnt appear like"instability warning". But my senior engineer says it is false. So I want to know why?
I want to know how they different?
And I want to know that can I release in both X & Y direction?
He says that if I release in both direction, the joint will be instability.
But I do not only release, I did both releases & bracing : so why ????
I want to know detailing if someone online: plz explain me quickly & detailing.
Have a nice day....

 

[IngDod]

Greetings, think of a common roof joist.. wich is in essence a truss as you describe.. in real life.. would the trust be able to rotate around its top chord?.... No... a tipical joist is restrained again such rotations... If it wasn't it would be unstable as i might just rotate and fall down (accidents such as this have happened during erection of joists before they are properly installed). I assume you are using a program such as SAP or ETABS.... The way to avoid this instability is to restraint the first and last diagonal of the truss against bending in the "minor direction" in sap2000 and Etabs this is M2. This gets rid of the instability problem and in my opinion approaches the real behavior of a truss. Trusses should be modeled using pinned connections for the web members but you must always provide lateral stability. Try to picture the real structure when you are modeling using a computer program. Does you program allows animating the structure? if it does you will clearly see the truss rotating around is top chord unless you apply the proper releases and restraints. Feel free to attach a sketch if you need further clarification.

 

[shweyeewin]

Dear IngDod,
Thank you for explain.
Plz check my model and tell me which is false...
My connection is not good enough so I only attach JPEG not staad model.
Thank you very much...

 

[IngDod]

I am not familiar with staad so I wouldn't want to give you bad advice. Your structure seems to be pinned everywhere... I am confused.. Is the program telling you your structure is unstable or is your senior?.

 

[AELLC]

Look at the pinned nodes I have circled - should those be deleted?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[IngDod]

Indeed those nodes look rather strange... I thought they could be perhaps due to the way the program displays the releases; but it may very well be the source of your problem. In my first post I was talking about trusses, but in this particular structure I dont see the need to restraint the secondary moment of the diagonals as they are pinned at both ends. If you can animate your model in staad you will inmediatly see where the problem is, as you will see the unstable member moving in a way that is impossible in reality.

 

[AELLC]

If you are unsure about them, remove them one at a time to see what happens

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[AELLC]

The next step, if unsuccessful, is to change each release to fix, one at a time

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[AELLC]

I don't quite recall, it has been a long time since I used frame analyzers - there is a difference between:

1) "pin" everything at a node

and

2) individual releases at the end of each member

Are you following the rules?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[shweyeewin]

Thank you very much for all generous reply engineer.
I get the answer from my senior engineer.
He explain me that different between moment frame & simple frame(pin-braced frame).
Moment frame: takes all moment connection & pin braced frame doesn't take moment n only shear n bracing takes lateral displacement.
So I can release beam which moment in both direction (y & z: My & Mz) & bracing is assigned as Truss but the support is need to be "fixed but: release at Mx & Mz" Coz: he says when I assign beam as moment releases in My & Mz & then truss which is the properties of Mx, My & Mz releases so support need to be "fixed but" but release at Mx & Mz. If I did support as pin the instability at support will occur coz all of the member does not take the moment.
Another way is we can do bracing as release in My & Mz & support can do as pin.
But he said that the first way is more true than second.
And I ask I can do only one bracing & it is also dont occur instability in Staad in attached picture.
But he said I can do it is not a good structure coz two bracing: when occur at lateral; one takes tension & one takes compression.
That all are I taught from him today.
If sth wrong reply me again plz.
Thank you very much for all.

 

[JLNJ]

Is it a matter of torsional instability? In most steel frames, we don't need to worry about torsion, but your analysis program doesn't know if you are interested in torsion or not.

If a member is truly pinned at both ends, it is still free to rotate about its longitudinal axis. Some programs let you release the moments but "fix" the member against torsion. If you choose this modeling option you still need to make sure this assumption makes the structure act as you want it. (RISA has a "BENPIN" option which pins the member in bending but fixes it against torsion).

Another common instability comes from all members in a node being pinned. In this case, it is the node which is not stable. A node needs to be fixed to something. Usually you can model it such that there is no moment transfer in the node, but the node is still stable. This is accomplished by fixing the node to one of the members coming into it while leaving the other members pinned.

Being curious about errors and instability is a good thing! A "clean" and well-conceived of model should not give you any errors. Too many engineers (especially young ones) will ignore errors which they cannot firgure out, believing that the forces in the members are close enough for design purposes.