Moment capacity of steel fin plates
Moment capacity of steel fin plates
(OP)
Would anyone be able to shed any light on how to best determine member moment capacity for plates being bent such that the narrow edges of plate are the extreme fibres? i.e imagine where a beam consists of the web only of a universal beam/col when bent about major axis without any flanges present. Buckling of compression edge will obviously govern but I cannot deduce how to quantify moment capacity from standards. Particularly with respect to AS4100. An example of the scenario I’m designing for is shown in sketch attached where a deep plate is cantilevering from eave/knee of portal frame. The plate in question is to have timber laminated to each side of plate which will further cantilever. Said timber will no doubt provide ‘some’ buckling resistance but will be considered negligible in this case. An addition of a top flange to make a ‘T’ is not an option.
Thanks in advance for your help.
Thanks in advance for your help.






RE: Moment capacity of steel fin plates
As a starting point for member buckling I'd follow Cl. 5.6.2 for a segment unrestrained at one end and use the equations for a constant open section to determine the reference buckling moment. I would assume Iw = 0.
There's a little more info in appendix H.
RE: Moment capacity of steel fin plates
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moment capacity of steel fin plates
"It is imperative Cunth doesn't get his hands on those codes."
RE: Moment capacity of steel fin plates
handoflion - Reference buckling moment seems to be fairly straight forward. Iw = 0 agreed. It's actually the effective section modulus that stumps me. I assume i'm dealing with a 'slender section' but Ze still requires input of slenderness limits that don't seem applicable without having one or both longitudinal edges supported (table 5.2). Similar problems occur with calculation of 'b' in section slenderness - clause 5.2.2.
I could perhaps use a conservative value for Ze and not worry about including slenderness limits but what would this be? Simply bd2/6? I can't be comfortable that slenderness limits should not amount to a coefficient of <1.
Thanks for other posts too guys.
RE: Moment capacity of steel fin plates
RE: Moment capacity of steel fin plates
I'm not so sure about this. The extended single plate provisions check local plate buckling assuming that there is rotational restraint at both ends of the plate. If the cantilevered end of OP's plate is not braced rotationally somehow, this method will not capture a global lateral torsional buckling style failure which I would expect to govern.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moment capacity of steel fin plates
But, I don't know if the "fin" can be considered to be a bar. If it is 8" or less, it should meet ASTM A6's definition of a bar. AISC's Steel Construction Manual doesn't have a definitive answer. In the plate products section of chapter 1 it says ". . . flat stock has historically been classified as a bar if it is less than or equal to 8 in. wide . . ." and "There is very little, if any, structural difference between plates and bars."
RE: Moment capacity of steel fin plates
"It is imperative Cunth doesn't get his hands on those codes."
RE: Moment capacity of steel fin plates
RE: Moment capacity of steel fin plates
1) LTB simple span (2.0 x C) with the ends prevented from rotating.
2) LTB cantilever span (1.0 x C) with one end free and the other fixed against rotation and warping.
Regardless, I get the impression that the design method for a prismatic plate just comes down to the classic elastic LTB equation. How to handle the taper properly is an interesting wrinkle though. Base it on the deepest, shortest, or average section? That's one of the things that I like about the method that I pitched above. It actually deals with the taper explicitly.
The analogous tapered beam for LTB evaluation would be shallow at the ends, where faux rotational restraint is present, and deep in the middle where the restraint is essentially warping restraint due to symmetry.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Moment capacity of steel fin plates
RE: Moment capacity of steel fin plates
I have, however, seen equations somewhere with lateral torsional buckling checks for a vertically aligned plates. If it isn't in the AISC spec, it was likely in Ziemian's "Guide to Stability Design Criteria for Metal Structures". If I remember, I'll take a look.
RE: Moment capacity of steel fin plates
Thanks again for all replies