K bucling factor for truss 3

[Mohamed Maher]

Dear all

I want to get the k factor for the attached truss by using sap 2000
The model consisting from column and trusses arround it..and repeated .the covering is fabric but not shown in the model

Kindly find in the attached photo from the model

Thanks,
Maher

 

[rb1957]

1) don't post files without extensions.

2) The typical procedure is to determine the eigenvalues of the structure. I don't know if SAP does this.

another day in paradise, or is paradise one day closer ?

 

[Mohamed Maher]

Do you mean attached there is already

 

[BAretired]

The trusses appear to be braced at specific locations. For out-of-plane buckling, I would consider the buckling length to be the length between braces. I would use a k factor of 1.0 for the slenderness ratio kl/r.

BA

 

[Mohamed Maher]

those lateral bracing are hinged connections to the main truss is it possible to consider it as preventing buckling?

 

[BAretired]

In a word, yes! Or perhaps I should say possibly. I think I would need a little more information before committing firmly to an answer. A plan view would be helpful, as well as some idea of the purpose of the whole contraption.

BA

 

[Mohamed Maher]

Buckling to be prevented for example in columns for shades should be by cross vertical bracing and for rafter should be cross horizontal bracing.. by same concept should we add cross to this lateral bracing

 

[MotorCity]

For the design of a new truss with web member ends welded to gusset plates, I would conservatively use K=1.0 assuming pinned ends. If this was an existing truss that I was checking that was marginally overstressed, I would try using a K = 0.85 or 0.90 to take advantage of the fact that welded gusset plates offer very little rotation at the support like an ideal pin does.

 

[BAretired]

Buckling to be prevented for example in columns for shades should be by cross vertical bracing and for rafter should be cross horizontal bracing.. by same concept should we add cross to this lateral bracing

You are asking this question at the wrong time. Do your analysis with brace points where you think you need them, then decide which, if any, braces need cross bracing.

BA

 

[Mohamed Maher]

I'm not the designer I'm just review this and the designer depend on those bracing to prevent buckling and in case they are not prevent buckling the members will be unsafe on high ratios

 

[BAretired]

I have marked a few locations on the photo with green lettering. I have some questions:

1. Does member BA continue beyond A to a pinned support? Or does it stop at point A?
2. Does member BC tie into the column in the foreground?
3. Does Point D tie into the column beside it? It does not appear so.
4. Is Point F held against lateral movement?
5. Are all six trusses fastened to the column at Point E?
6. If so, what prevents rigid body rotation of all trusses about Point E other than the connection at E, and the torsional resistance of the column at E?
7. Can you provide a framing plan of the structure?

BA

 

[centondollar]

This structure seems more than complicated enough to warrant a thorough calculation. To answer your question: have the designer do an eigenvalue buckling analysis, at the very least, to confirm adequate capacity. Simply adding bracing (providing unknown, possibly small and insufficient, stiffness) at discrete points in a complicated frame/truss structure does not constitute design against loss of stability in the form of various possible types (flexural, lateral-torsional, torsional) of buckling.

 

[Mohamed Maher]

@centondollar

That what I believe in also that we should make buckling analysis.
I should do it and provide him the results
So do you have any idea how to do this on sap2000

 

[BAretired]

It is a complicated structure, but trying to solve it by a buckling analysis strikes me as the wrong first step.

The first step should be to examine the drawings to determine whether or not the structure is adequately braced. I believe several braces will not be effective if built as depicted in the photo, but the framing needs to be studied in a little more detail. To do this, a framing plan and possibly a few details are needed.

Unfortunately, the OP seems reluctant to provide the necessary information to conduct a preliminary study; furthermore, he has no idea how to carry out a buckling analysis.

BA

 

[BAretired]

those lateral bracing are hinged connections to the main truss is it possible to consider it as preventing buckling?

Upon further consideration, I agree that the bracing shown on the model is not adequate to prevent buckling.

BA

 

[271828]

SAP2000 does linear eigenvalue buckling analysis. Don't just turn it on and run it for this structure, which is way too complicated for a learning exercise, especially if it will be a real structure with people under it.

Start with a model of a simple column and see if you can reproduce the Euler buckling load. Then model a small and simple 2D truss to see if you agree with the results. Work your way up to the structure at hand.

Also, you should be able to tell from visualization and judgment whether the arrangement is stable. If you can't do that, and rely on the program to tell you it is stable, then that is unsafe practice in my opinion.

 

[BAretired]

Also, you should be able to tell from visualization and judgment whether the arrangement is stable. If you can't do that, and rely on the program to tell you it is stable, then that is unsafe practice in my opinion.

Wiser words were never spoken!

BA

 

[Brad805]

Break it into substructures. From what I can see the bracing is not sufficient. Am I correct the fabric is tensioned and wind loads are the primary load?

 

[Mohamed Maher]

Yes Brad it's tensioned fabric and wind is the primary load..do you work in tensile structure

 

[Mohamed Maher]

@2 what you get that I need judgement from program to check stability!
I just want to make buckling analysis to get the critical buckling length then get the proper k factor ..that is all I hope you get what I mean ..the hole structure is stable of course