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How to remove instability in Staad for steel structures.
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Bharathhassan
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PostPosted: Mon Feb 04, 2013 10:56 am    Post subject: How to remove instability in Staad for steel structures. Reply with quote

Dear All


How to remove instability in Staad for steel structures. [Warnings]

Am designing a small ware house structure its span is 12 mts spacing of trusses is 6.0mts apart, its dimensions and spacing of purlins,bracings, please find attached fig-1.jpg file.
The loads considered, Deadload = 30 Kg/m2 [ electrical flexures 20kg/m2 and sheet 7 kg/m2+ self weight of the structure assigned in Staad ]
Live load considered75 Kg/m
[Wind loadsapplied as per IS 875 Part -3
[Mono slope roof calculated as per IS 875- part applied as a nodal loads].
The purlins [purlins released at the end allow to bending] and bracings are molded in Staad. [Please check attached file].

I had tried many iterations / trails by providing different types of bracing at different locations [even tried by changing truss pattern also] to avoid instability, finally i could able to remove most of the instabilities by providing bracing s [the pattern and direction of the bracing please check fig-2] .But still I am unable to remove instability in one place /at one node/location please check fig-3 for location of instability. But at that location the deflections are within the allowable limits.

Please help me to remove instability / are please suggest me in which direction I need to provide bracings to avoid instability.

The same model I tried by removing all purlins and plan bracings there were no instabilities. Please check fig-3. And the deflections are also under the limits.

My questions / doubts

Should I model purlins ,plan bracings ever thing in the Staad or only truss members .
If I modeled everything how to remove instabilities??

Regards
Bharath
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PostPosted: Mon Feb 04, 2013 11:23 am    Post subject: Reply with quote

Hello Bharat,

Try out the following thing in your model:

1. Do not give the top chord and bottom chord member as truss element i.e., allow the B.M in those member. Because its a continuous member and we do not break it at each node, only intermediate members are taken as truss element.
2. Do not give a "node" at the bracing intersection.
3. remove the plan bracing
4. Check out with the purlin orientation with that of the top chord member (i.e., Beta angle)
5. Check out with the redistribution value you have considered.
6. give alternate bracing to columns starting from 8.18 m
7. Check with the sections you provided, is it I-section or S-section, S-section is according to Indian code, so just verify it
8. Check with the orientation of intermediate members (beta angle).
9. The top and bottom chord are taken as double angle and not single angle.

Looking at your drawing I can point out these many corrections, if you can give the staad model we can help you. I hope my views are correct and I wish the experts to provide some suggestions on to it

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SHISHIR S. KADAM
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PostPosted: Mon Feb 04, 2013 11:28 am    Post subject: Reply with quote

Hello Bharth,

Ya you need to model the purlin (not as a truss member) by using channel or angle based on your requirement. put the plan bracing as per requirement (as a truss elements). Rest all other points I have posted in the above post

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Bharathhassan
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PostPosted: Mon Feb 04, 2013 12:00 pm    Post subject: Reply with quote

Dear SHISHIR S. KADAM


Thanks for reply

please find staad files .


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Bharath
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Bharathhassan
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PostPosted: Mon Feb 04, 2013 12:11 pm    Post subject: Reply with quote

Dear sir

1. Do not give the top chord and bottom chord member as truss element i.e., allow the B.M in those member. Because its a continuous member and we do not break it at each node, only intermediate members are taken as truss element. .

I will try it

3. remove the plan bracing


If i remove i will get more instabilities

Japanese sections used .


Please find attached staad model


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Bharat
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Bharathhassan
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PostPosted: Mon Feb 04, 2013 12:14 pm    Post subject: Reply with quote

please find attached staad input file
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Bharathhassan
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PostPosted: Tue Feb 05, 2013 4:59 am    Post subject: Reply with quote

Dear SHISHIR S. KADAM

As per your suggestion i have modified the staad model but i am still unable to remove eliminate the instability .
As per your point 1 if i make top and bottom chord member allow to bend the member sizes will increase .

Please find revised model[staad input file ] as per your suggestion .  

Thanks

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BHARATH K R
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PostPosted: Tue Feb 05, 2013 6:14 am    Post subject: Reply with quote

Kindly do find the attachment of a very simple industrial building

1. keep the top and bottom chord member as non-truss element only.
2. Do not give the wind load as point load at nodes, make it UDL and apply it on purlins.
3. A part of small load of DL and LL has been applied on member 276.
4. Model the purlin as non-truss member and apply all the loads on the purlins.
5. No load has to be applied to any other truss members except the DL. Rest all other loads has to be applied to purlins only.

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

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SHISHIR S. KADAM
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PostPosted: Tue Feb 05, 2013 6:45 am    Post subject: Reply with quote

I have one doubts about the model

1. why hinge support is provided at other end ?

1. Some members properties of bracing and other members are also missing and try to provide same member property for same structural use.
2. Avoide the node at the bracing intersection.
3. Check with orientation of the various members.
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SHISHIR S. KADAM
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Bharathhassan
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PostPosted: Tue Feb 05, 2013 8:12 am    Post subject: Reply with quote

1. why hinge support is provided at other end ?

Its existing column .

1. Some members properties of bracing and other members are also missing and try to provide same member property for same structural use.

ok  

2. Avoide the node at the bracing intersection.

Then i need to provide A bracing

3. Check with orientation of the various members

[B]Ok


Thanks for your valuable suggestions

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Bharath
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