Consultation about Single Tensional Member by LRFD99 on SAP2000 1

[willys8]

Hello COLLEAGUES, this is my first post on forum.
I did a simple study for practice the code AISC LRFD 99 on SAP2000, and I found a problem with the iteration ratio demand/capacity calculated by the module Steel Frame Design. The conditions, loads and sections are well indicated. Also I checked with other codes ASD89, and result was well. Add the file about the result

http://es.scribd.com/doc/233517382/Barra-Simple-Traccionada

there we see that:
phi*Pnt=11181 kgf (daN)
Pu=1600 kgf (daN)
then by the code the ratio is Pu/(phi*Pnt) ; 1600/11181=0.143
but the program show a ratio of 0.072 (a half, of side of Insecurity)
here is for see the load

http://es.scribd.com/doc/233518113/Barra-Simplemente-Traccionada-Carga

thanks for your time, I will wait for your answer.
Wilson

 

[SteeliestFire55]

SAP2000 is using the combined bending + axial check where Pr/2Pc + Mr/Mc when Pr/Pc < 0.2

It's the same equation whether you're using LRFD or ASD. I'm not certain what year this was introduced, but ASD89 used direct combination Pr/Pc + Mr/Mc.

So while SAP2000 is showing you the combined bending and axial check, it is not a computational error, but more of an incorrect reference. SAP2000 should be able to recognize this shortcoming where the axial DC alone can exceed the combined axial and bending. However, such a case will never be critical above a D/C ratio of 0.2. You can try this by increasing the axial load in the member.

The combined provision is meant to be used when you have low axial forces with high bending moments where the failure surface on the P-M interaction is not linear.

 

[willys8]

Thanks a lot SteeliestFire55 , I have seen now, the formula H1-1b in the CODE for ratio <0.2,
greetings.
Wilson