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Regarding effective length for lateral torsion buckling

 
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arun_kunnanchira
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Joined: 18 Dec 2009
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PostPosted: Mon Nov 11, 2013 12:49 pm    Post subject: Regarding effective length for lateral torsion buckling Reply with quote

Dear sefians,

I am little confused over a calculation of effective length for lateral torsion buckling of girder.

I have a structural system consisting of 2 girders spaced at 2.5 m c/c  and spanning 14.5 m. The girders are tied to each other by cross beams spaced at 3.3 m c/c and welded to girders. Top compression flanges are not fully restrained as such.

I would like to know what value of effective length for LTB should be taken for design of girders. Should it be 14.5 m or 3.3*1.2 m=3.96 m as per Cl 8.3.1 of IS 800-2007 ?

Currently i have taken effective length as 3.96m.

Appreciate if some one can clear my doubt.

Thanks & regards
Arun KM



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arun_kunnanchira
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PostPosted: Tue Nov 12, 2013 2:22 pm    Post subject: Reply with quote

Anyone here to reply ?

regards
Arun KM
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debayan
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Joined: 25 Jul 2008
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PostPosted: Wed Nov 13, 2013 10:22 am    Post subject: Reply with quote

Firstly, why can't the tie beams be connected to top flange? Please explore options to tie the top flange.

If top flange is not constrained then theoretically lateral torsional buckling will occur. But having said that a lot of times due to practical considerations it is not possible to completely restrain the compression flange. As a thumb rule, if sections are at least class 1 or class 2 then a lot of practising engineers assume there will be a lot of inbuilt stiffness in the beam and consider it to be safe from LTB. Please keep in mind, it is only a thumb rule. Others are free to criticize this.

On top of that you have a factor of 1.2 and additional Limit state factors. You can also specify two spans for LTB definition (i.e 3.3*2*1.2 instead of full 14m) for a conservative check.

One can also consider providing joist beams (light weight and low depth) which tie the compression flanges as an alternative along with the main tie beams.

Also if the section size is huge then it is better to specify offset in the tie beams to get most realistic behaviour.

Regards,
Debayan
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akos
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PostPosted: Thu Nov 14, 2013 12:13 pm    Post subject: Reply with quote

In my opinion 3.3m span can be considered as effective span. Bracings are connected to the secondary beams. The shear connection in secondary beams (try to give full depth fin plate or angle plates) can restraint the rotation of the web of the main beam even though it is not restraining the compression flange. Hence it will act as a restraint for lateral torsional buckling.

Regards
Akos
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