RAM SS Gravity column Design 1

[karthik_m]

The gravity column design interaction equation H1-1a in RAM SS does not account for the Mux/(0.9Mnx) in the unity equation. Why is that ? Please let me know if someone has faced the same issue. Thank you

 

[JoshPlumSE]

The equation referenced says H1-1a per H1.3....

When I look at H1.3 I see the following (14th edition):
"For doubly symmetric rolled compact members with KLz <KLy subjected to flexure and compression with moments primarily about their major axis, it is permissible to consider the two independent limits states, in plane instability, and out-of-plane bucking OR lateral torsional buckling separately in lieu of the combined approach provided in section H1.1. "

I don't believe I've ever invoked this provision myself. But, it seems that's what RAM is doing.

 

[SethGuthrie]

Thanks JoshPlum. See also:

https://communities.bentley.com/pro...aad-forum/176935/ram-ss-gravity-column-design

 

[karthik_m]

@joshplum: I agree that Equation H1-3 governs but the in-plane instability is checked by equation H1-1a and I can't see (8/9)*Mrx/Mcx ( which comes around 0.15) been added to Pr/Pc (0.819) which gives a DCR of 0.97 which is greater than what we get from Equation H1-3.

 

[JoshPlumSE]

I don't really like the way this provision is written as it is more confusing than it needs to be. Take a look at the commentary on this section. Specifically, they point us towards equation C-H1-5a (which is a revised form of H1-1a) and C-H1-5b (which is a revised form of H1-1b).

C-H1-5a:
Pu/Pn + 8/9*(Mu/Mn) < 1.0

Check this equation once with Pn based on weak axis buckling and Mn based on weak axis bending.
Check it again with Pn based on strong axis buckling and Mn based on strong axis bending (using a bending unbraced length of 0).

Finally, you check equation H1-3 for weak axis buckling combined with strong axis bending where Mn is based on LTB.

The Canadian code has provisions similar to this, I believe. It's a little more clear in the CSA code, but still a little goofy. Personally, I prefer the simplicity of H1-1 without the need to invoke H1-3.

 

[JoshPlumSE]

The idea, I believe, is that weak axis bending doesn't really affect strong axis buckling. Similarly, strong axis bending of a beam that doesn't experience LTB doesn't really affect weak axis buckling. But, lateral torsional buckling (LTB) does very much affect weak axis buckling.