I have generated 3D frame using RSA 2013 professional. When i verify the rafter beams under ultimate limit state, the results shown are instable. And when optimized under ULTIMATE it provides very large dimension beams. whereas when i used SERVICEABILITY METHOD, it is fine.
Please help me, how to verify beams in 3D generator.
Please find attached 3D design file
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ULS and SLS check are two different verifications,. As far as I can see the problem you have got is the exceeded slenderness limit. Looking at the buckling Z part you can see that unrestrained buckling length for 140 section is over 14m which way too long weak the axes if this section. Have you set the buckling parameters correctly?
Assuming it is a 2 span beam then either you set buckling Z (and LTB) coefficients as 0.5L or you should define an intermediate restrain at this point and then set these coefficients as 1.0L for each of the spans.
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