CSA S16 (Canadian Code) Laterally unbraced class 4 flange

[Bridge_Man]

Hi,

I am performing an assessment on a girder section (I can't change the design) and in one section both the flanges and web are class 4 sections (Slender).

My question is the following: Section 10.10.3.4 (Class 4 sections) starts with saying "For beams and girders with continuous lateral support" which is not the case that I have (mine is discretely braced), so my first thought is whatever is in this clause might not apply to my case, but when I open the CICS Handbook of steel construction 11th edition (13.6 Bending - Laterally unsupported members) (b) it says that the limit for Mr for class 4 flanges should be the same as the laterally supported members (in terms of effective section modulus) would that still be true for the bridge code ?

Question 2 - Giving that both my flange and web are classes 4 does that mean that I have to use the effective section modulus for MR and then modify it by the moment reduction factor in CSA S16 in 10.10.4.4 ?

Last question: in the AISC there is a local flange buckling check for non-compact or slender flanges, is there such a check in the Canadian bridge code or even the Handbook of steel construction ? or does the effective section modulus kind of replace that check ?


Thanks for your time.

 

[Craig_H]

I think that there are a few issues going on here. You're mixing S6 (bridge code) with S16 (generic steel code), and their nuanced differences are potentially causing some confusion. I'll temper all this with a disclaimer that I am not extremely well versed in CSA S6, so someone with more direct experience may be able to provide better input.

My read of S6 10.10.3.4 and the associated commentary indicates that the code only covers cross-sections that have Class 3 or more stable webs, and only deals with Class 4 flanges that are continuously braced. That would put your situation outside of the code boundaries.

I did once deal with a custom box-girder that had Class 4 flanges and Webs (custom transport equipment). I recall that analysis involving significant amounts of effort using finite element methods to determine the stability of the structure under its loading scenarios. It was not a simple analysis using codified formulae. I performed a buckling analysis on the structure, then used the buckling shape as an initial imperfection for a geometric non-linear analysis to determine factored stresses in the girders. I then compared these stresses to the material properties, modified with the appropriate confidence factors.

I have some difficulty picturing my above method being practical for a bridge application, where your load scenarios and limit states are far more broad and numerous.

 

[Bridge_Man]

Craih_H, I agree that it is not a good idea to mix codes but since I have an I shape and the S6 code holds off talking about discretely braced members with class 4 flanges I though it wouldn't be a bad idea to see what the steel code says. especially since it has the exact same limitations for the effective section modulus for laterally supported members and the moment capacity equations are identical between the steel code and the bridge code S6.

Thanks again for your insight