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sorgjee
Posts: 49
Registered: ‎12-11-2011

Manual LT buckling length in LRFD steel design module

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12-15-2011 12:28 AM

Hi is there any way of setting defined lengths for the LT buckling lengths in the LRFD steel design module? The only options that appear show these to be relative lengths as multiples of the bar lengths.

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Rafal.Gaweda
Posts: 5,070
Registered: ‎04-26-2010

Re: Manual LT buckling length in LRFD steel design module

12-15-2011 12:40 AM in reply to: sorgjee

If I understood you correctly:

 

lt.jpg



Rafal Gaweda
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Active Contributor
sorgjee
Posts: 49
Registered: ‎12-11-2011

Re: Manual LT buckling length in LRFD steel design module

12-15-2011 02:04 AM in reply to: Rafal.Gaweda

Hi Rafal,

 

Yes that was what I was looking for, but it appeared not to be working entirely as hoped. I started setting the LT buckling length, but this was not implented when running the design.

  1. For some reason  the design always sets the Lb as a maximum equal to your Lz. Is this intentional?
  2.  I tried several different combinations, and generally it works assuming you set the Lz manually, the Lb can be set manually, but not implemented as greater than Lz as mentioned in1.
  3. When combining setting Lz through a coeffcient though things get funny. Using Lz = 7xL, and Lb = 7.0m, the design implemented the Lz value correctly, but for some reason set Lb to 4.92m. Any idea about what has happened here? I've tried attaching a screen capture showing the results.

Thanks

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Rafal.Gaweda
Posts: 5,070
Registered: ‎04-26-2010

Re: Manual LT buckling length in LRFD steel design module

12-15-2011 02:48 AM in reply to: sorgjee
Could you be so kind and either send model or show it on your model (real connections, element , behavior) to let me understand why do you need this coeff or LTB length grater then member lenght?
Only one situation comes to my mind: you are trying to check longer member but consisting of several shorter ones without creating supermember.

 



Rafal Gaweda
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Active Contributor
sorgjee
Posts: 49
Registered: ‎12-11-2011

Re: Manual LT buckling length in LRFD steel design module

12-15-2011 03:40 AM in reply to: Rafal.Gaweda

Yes, we got a cranked truss with the chord made up by several members between restraints.

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Rafal.Gaweda
Posts: 5,070
Registered: ‎04-26-2010

Re: Manual LT buckling length in LRFD steel design module

12-15-2011 03:58 AM in reply to: sorgjee

So replace serie of elements by one or create supermember for design:

 

super.jpg



Rafal Gaweda
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Active Contributor
sorgjee
Posts: 49
Registered: ‎12-11-2011

Re: Manual LT buckling length in LRFD steel design module

01-16-2012 07:56 AM in reply to: Rafal.Gaweda

Hi Rafal,

 

Aplogoies for not reply to this one sooner. I've finally got around to doing some screencaps that show my the problems I've been having with superbars.

 

In short, when trying to define lateral restraints for superbars, it looks like the prgramme is only able to identify members that are perpendicular to the z-axis of the members.

 

As you can hopefully see from the illustration below, the restraints from the internal members of the truss are correctly identified for the major axis bukcling and LT buckling. For the minor axis buckling on the other hand, it does not appear to 'see' te slightly angled bracing members that connect to ever second or third chord member.

 

I was hoping there was something wrong with my appraoch, but not been able to find anythign so far.

 

bracing definition.PNG

 

regards,

Even

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Rafal.Gaweda
Posts: 5,070
Registered: ‎04-26-2010

Re: Manual LT buckling length in LRFD steel design module

01-17-2012 02:38 AM in reply to: sorgjee

Points for buckling lenghts should be automatically recognized, but not for LTB.

 

bucklz.jpg

 

Adjust position for LTB (if you need for buckling also in similar way) as shown below:

 

ltb.jpg

 



Rafal Gaweda
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Active Contributor
sorgjee
Posts: 49
Registered: ‎12-11-2011

Re: Manual LT buckling length in LRFD steel design module

01-17-2012 03:10 AM in reply to: Rafal.Gaweda

It is not recognizing anything that is not perfectly in plane with the buckling plane in questions.

 

I made a similar model to yours, and the fist picture shows the beams horizontally from the superbeam correctly picked up as buckling restraint points. The second shows the support point lowered 0.1m out of the plane, and the members are no longer recognised as buckling support points.

 

in plane bracing.PNG

 

 

 

offset bracing.PNG

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Rafal.Gaweda
Posts: 5,070
Registered: ‎04-26-2010

Re: Manual LT buckling length in LRFD steel design module

01-17-2012 03:21 AM in reply to: sorgjee

Yes, I have forgotten to move them out ouf plane.

 

You are right, from help:

 

"Use the  option to automatically calculate buckling coefficients for the member segments between bracings based on the analysis of rigidity of bracings adjoining the analyzed member in the corresponding buckling plane. Values of calculated buckling coefficients for successive segments are displayed in the field under the bar diagrams."

 

http://docs.autodesk.com/RSA/2012/ENU/filesROBOT/GUID-1177E678-E14E-4C8E-8AB2-0A76B44DF34-955.htm



Rafal Gaweda
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