AASHTO Std Specs (LFD): axial compression with (flexural-)torsional buckling?

[man_of_steel]

Hey guys, I'm currently working on a bridge load rating using Standard Specs (17th Ed.) LFD methodology. Need to provide axial-force rating factors for steel cross-frames and lateral bracing members, most of which are either T or single-angle sections. But Std. Specs don't really offer much guidance for computing the compressive resistance of singly-symmetric or unsymmetric sections... Does anyone have experience with this calc?

The basic LFD compressive resistance calc (eq 10-150) is straightforward and easy - too easy, it felt like, considering that modern LRFD has a whole bunch of additional checks for (flexural-)torsional buckling (FTB) failure modes depending on your cross-section, including specific provisions for single-angle members. But all the Std. Specs say about it in Section 10.54.1.1 is a footnote at the bottom of the page that just reads: "Singly symmetric and unsymmetric compression members, such as angles or tees... 'may' also require consideration of flexural-torsional or torsional buckling. Refer to the Manual for Steel Construction, Ninth Edition, 1989, AISC." (Not even referencing a specific chapter? Could they have been more vague? lol)

So then I'm going through the 9th Steel Manual (which is entirely based in ASD, not LFD) trying to track down info on FTB. Chapter E gives a general equation for allowable compressive stress F[sub]a[/sub], then the associated commentary provides an additional clause for considering (F)TB. That section has you compute an effective KL/r for singly-/un-symmetric sections, which you substitute in for regular KL/r in the F[sub]a[/sub] equation from before. So I went through all that, but then... what do I do with F[sub]a[/sub]? Do I plug it back into LFD eq. 10-150 in place of F[sub]cr[/sub]? If so, do I keep the 0.85 factor in front of the F[sub]cr[/sub]A term? Subbing an ASD allowable stress into an LFD equation feels incompatible - and in fact, if I do just plug in the substitution, the compression capacity gets cut almost in half... For comparison: I also have to provide LRFD ratings, and the LRFD capacity for all the tee and angle members is much more in line with the LFD capacity as calculated by the original equation (P = 0.85F[sub]a[/sub]A).

So what am I doing wrong? How am I meant to use that F[sub]a[/sub] value from AISC? Am I missing something here? Or do I not even bother, and just ignore (F)TB effects and stick with the basic eq. 10-150 for all sections?

 

[BridgeSmith]

Generally, for an LFD rating, the crossframes are considered secondary bracing members only. They are not considered in the structural model for load distribution, and therefore are not analyzed or checked as part of the rating.

 

[bridgebuster]

Are you rating a curved girder bridge? If not I agree with BridgeSmith there shouldn't be any need to rate cross frames. We rated a curved girder bridge recently but we didn't rate the lateral bracing since it wasn't a primary member.

Anyway, look at the Bridge Evaluation Manual, Part 6B.

 

[BridgeSmith]

Thanks bridgebuster. I was going to suggest the MBE as a reference, but mine's at the office, so I didn't know what section to point to.

 

[man_of_steel]

Appreciate the input guys! I am quite familiar with the MBE and I've been consulting it throughout this process, yes. Re: curvature, it's a bit of an unconventional situation: three of the four spans have all-straight girders... except for the right exterior girder, which is curved. And then the fourth span has 12 girders, the right 5 of which are curved at varying radii. It's a weird bridge lol.

Given the unusual mixed framing, we're conservatively considering the cross-frames as primary members, at least in the bays between curved (or one curved / one straight) girders. And per LRFD 6.7.5.1 (ref'ed by MBE 6B) bottom flange lateral bracing is always considered primary if it's permanent. And as best I can understand from my PM, the DOT wants to see rating factors for the cross-frames regardless, even if some of them can be considered secondary; essentially, whether or not these ratings are technically code-required, I still have to do them because my boss said so lol.

So then back to my original question: (F)TB in tees and angles... what do I do here lol? How are the referenced AISC provisions meant to be implemented into LFD? Or do we generally just ignore any torsional buckling effects, and simply stick with the basic flexural-buckling capacity from eq. 10-150?

 

[bridgebuster]

Maybe FTB isn't a concern. Traditionally, for cross frames and lateral bracing we only consider axial forces. Granted, the axial forces in the frames are determined from moments but to me they're not really flexural members.

The AISC 9th says refer to their 1986 LRFD spec. It seems to imply you use a hybrid LRFD & LFD for the ASD spec. I also looked at the old US Steel example for curved girders (LFD). The example only checks for local buckling and critical buckling. I would stick to Eq. 10-150.If the members aren't showing any signs of deformation, most likely the original design approach was correct

 

[man_of_steel]

Thanks @bridgebuster! Yeah I'm inclined to agree that we're just meant to stick with eq. 10-150, because it really doesn't seem like anyone gets into those footnoted AISC calcs. I posted here because I was having a lot of difficulty finding any AASHTO LFD-specific design examples, and the ones I could find - including the AASHTOWare BrR method of solution documents - do appear to just ignore FTB even for tee and angle sections. So I was hoping someone else had done this calc before, or could at least confirm for me that we've collectively decided as an industry that we're just not doing that calc with LFD lol.

Admittedly I don't know the field conditions of the cross-frames on this bridge, so I can't attest to whether they're exhibiting deformation IRL. But from the finite-element analysis I'm doing, several of the frames are rating < 1.0 - in some cases, their calculated capacity isn't even enough to resist the DL force alone, which is pretty dismal. And if more members would rate similarly deficient if we considered FTB, we wanna make sure we're capturing that so that we can inform the DOT, you know? But if ignoring FTB under LFD is generally condoned, then we'll just roll with that