Thanks Josh ..
But ,, talking about Cb ,, When I leave blank de Cb value at Risa 3d , the results show me a value of 2 , I am worry about if that means that I am lack of steel , I mean , needs a better shape even the UC values are below of 1 and the relative deforation member ratio is above n=360 .. ??? Will you help me to uderstand this topic better . I am from mexico and do frabicate steel structuras . Iam learning fron Risa 3 demostration version and I am very happy with this program .. Thanks again
In AISC, the beam buckling equations are put together assuming that the moment in the beam is constant throught the length of the beam between segments. In reality, obviously, this is not the case as our bending moment diagrams vary greatly.
The Cb factor is a way to determine the increase in bending moment capacity that would result from the fact that the bending moment diagram varies. For a linear moment diagram with a zero value at one end, the Cb value would be approximately 1.67. Therefore, a value of 2.0 is not unreasonable.
I should point out that RISA does allow you to enter in a user defined value of Cb for those cases where you do not want the program to calculate it automaticallly. If you enter in this value as 1.0, then you would be conservatively ignoring this effect.
