Does this qualify as an OCBF
Does this qualify as an OCBF
(OP)
Trying to settle a disagreement. Does the frame below classify as an OCBF? The horizontal and vertical members are structural steel, the diagonals are steel cables. If not an OCBF how would you analyze? I know this is rudimentary I'm just trying to remain unbiased in stating the question.
Thank you
Thank you






RE: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
Conceptually, I think that the system would have a lot in common with on OMF. If you could slip it past your code reviewers, I'd be game for R based on OCBF but design as OMF.
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: Does this qualify as an OCBF
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
Treating it as a K brace sounds reasonable to me.
RE: Does this qualify as an OCBF
What is going on at the base of the columns? Looks like WF cribbing. The framing at the top also looks a bit odd. It looks like you have a horizontal cable system instead of a true diaphragm. My vote is with KootK.
Why do you need to know if you can classify it as OCBF in order to analyze it?
"It is imperative Cunth doesn't get his hands on those codes."
RE: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
1) The column is the only member that participates flexurally.
2) Only the column would participate in dissipating energy under seismic.
3) Decent displacement ductility.
4) Longish fundamental period.
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: Does this qualify as an OCBF
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
Not just similar but, rather, practically identical from the perspective of the column. I would argue that comparable elastic force distributions represent the most convincing evidence of similitude between the real scenario and cantilevered column concept. Does not the characterizations of all of our seismic systems come down to elastic force distribution with design tweaks to allow for localized plasticity at the bending hot spots? Fancier system excepted of course.
This statement is also true for the structure being considered here. The only difference is that the column fixity is at the top (zero slope point really) rather than the bottom.
I submit that all of these things absolutely should apply to the system being designed, precisely because the limitations inherent in a cantilever column system will also be present in OP's system.
That was my vote originally but I've since come to see it as flawed in some important respects:
1) In a knee braced moment frame, the beam will participate in flexure. Here it will not.
2) In a knee brace moment frame, you could force plasticity into the beams. Here it will be in the columns.
3) In a system with knee braces as large as they are here, you'll get much less displacement ductility that you'd expect from a moment frame. It would be a bit closer to a concentrically braced frame in that respect.
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: Does this qualify as an OCBF
I agree I too would not classify it as an OCBF. The designer's argument is that nothing in AISC 341-10 Chapter F requires the bracing to connect at the column bases, that it must just intersect concentrically with the centroid of the columns. Further D1.4a allows for the application of a moment to columns between the fixed ends as long as it is not neglected in the column analysis.
We need to know the classification because it determines the R and Ω values required. The structure was already completely engineered as an OCBF and to go back and classify it as something else will cause delays.
FYI the bracing can't go to the ground because this structure will be surrounded by decking and equipment that the bracing can't go through.
RE: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
There's also nothing in the code stating that your designer shouldn't eat tomato soup with a fork. That omission shouldn't be construed as an endorsement of that strategy however.
I take "concentric" to mean that all significant axial and shear forces meet at at common point within reason. Your designer's frame would not meet this definition because the column and brace axial loads do not meet up with the base plate shear at a shared location.
I'm inclined to disagree with the characterization of OP's system as a K-brace. I my mind, a K-brace system is one which starts off concentric but loses that characteristic part way through the load history as a result of compression brace buckling. With the tension only braces in the mix, and the brace points coming in high on the columns, OP's brace wouldn't ever be concentric at any point in the load history.
I'm convinced that OP's frame needs to be envisioned as some kind of moment frame, be it conventional, cantilevered column, trussed, or "other".
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: Does this qualify as an OCBF
RE: Does this qualify as an OCBF
I'm inclined to agree. I used the term "similar" because I haven't gone through your permutations fully to confirm the results but I agree that it does appear accurate.
I would agree with this generalization to a point. However, the LRFS system values (R, Ω0...) are prescriptive, historic system values and I personally don't think it's reasonable to consider a system that has fixity at the base combined with the interaction of the anchor bolts, footing overturning, and baseplates to be comparable to a highly eccentric braced frame with cables just because their elastic modes are similar. At best it seems conservative; at worst the OPs system might have design deficiencies if designed using the system factors of the cantilevered column system.
I would agree but failure of the cables would have to be considered and their failure would be catastrophic. While this is roughly similar to the failure of the anchor bolts/footing/base plate in a cantilevered column I believe it to be different enough that the system values are not comparable.
I will concede that I believe you are correct; it will be similar given that it's a highly non-redundant system where the members carrying gravity loads also must support high inelastic forces. That said, I believe the performance will be slightly improved or similar (so perhaps same R value) but the overstrength and deflection amplification values seem like they could vary wildly in either direction compare to a cantilevered column system. Thus, I still stick to my original hypothesis that using the provisions for a cantilevered column is too far outside the standard practice to be justifiable.
I suppose my argument is less whether it responds like a cantilevered column system, and more that it doesn't act exactly like what the code considers a cantilevered column system.
Edit: Star for your last post because it's well reasoned; but also because you sound like you're almost saying what I'm saying above, only in opposition to a K-brace rather than opposition to a cantilevered column.
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Does this qualify as an OCBF