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

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

(OP)

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

So what am I doing wrong? How am I meant to use that

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*, then the associated commentary provides an additional clause for considering (F)TB. That section has you compute an_{a}*effective**KL/r*for singly-/un-symmetric sections, which you substitute in for regular*KL/r*in the*F*equation from before. So I went through all that, but then... what do I do with_{a}*F*? Do I plug it back into LFD eq. 10-150 in place of_{a}*F*? If so, do I keep the 0.85 factor in front of the_{cr}*F*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 = 0.85_{cr}A*F*)._{a}ASo what am I doing wrong? How am I meant to use that

*F*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?_{a}
## RE: AASHTO Std Specs (LFD): axial compression with (flexural-)torsional buckling?

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

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

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

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

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?

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

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

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

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