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How to remove instability in Staad for steel structures.
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a121s
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PostPosted: Tue Feb 05, 2013 8:23 am    Post subject: Reply with quote

Dear bharath

If you provide node at bracing intersection then how can it transfer the axial forces ?.. you are allowing moment in them by providing the node. Bracings are provided to allow only axial forces. the axial forces must be transfered from the top of column to the base directly without intermediate load. so do apply the cross bracing at the columns as truss members from member specification page. you just check the truss example which I have attach.

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SHISHIR S. KADAM
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Bharathhassan
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PostPosted: Tue Feb 05, 2013 9:06 am    Post subject: Reply with quote

1. keep the top and bottom chord member as non-truss element only.

Did in model


2. Do not give the wind load as point load at nodes, make it UDL and apply it on purlins.

Applied only self weight , Removed all loads.




I have checked your file and I find that application of loads is only a major cause of instability,

I removed all loads , tried only for self weight



rest other minor things i have discussed in my earlier post. Kindly make use of the example file which i have attached and correct the points discussed.

AS per your suggestion corrected all other minor things , But am unable to remove instability..


I will Refer your File and apply loads as UDL on the purlin [wind and LL ].

Thanks

Regards

Bharath


  
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Bharathhassan
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PostPosted: Tue Feb 05, 2013 9:15 am    Post subject: Reply with quote

If you provide node at bracing intersection then how can it transfer the axial forces ?you are allowing moment in them by providing the node.

Ok , But the length of the member becomes more the members will fail in slenderness. And in your model you have given as a tension member it can not be only tension there may compression also . I can do one thing while designing bracing members i need to provide half of Ly lz   .


Bracings are provided to allow only axial forces. the axial forces must be transfered from the top of column to the base directly without intermediate load. so do apply the cross bracing at the columns as truss members from member specification page. you just check the truss example which I have attach.
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a121s
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PostPosted: Tue Feb 05, 2013 9:42 am    Post subject: Reply with quote

Hello Bharath,

Dont confuse yourself, what you said is correct about slenderness follow the simple steps below.I gave it as tension member , but in your case give it as truss member.

1. find the axial force in the bracing member.
2. divide it by (0.67* yield stress), u will get the AREA of the member
3. for the steel table find the suitable section having minimum area from the step-2, take higher than that area
4. calculate the permissible stress based on the slenderness ratio and compare it with stress acting on the column
5. permissible stress should be more than the stress acting on the column.
Your problem will be solved.

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