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Does this qualify as an OCBF
2

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

RE: Does this qualify as an OCBF

I would not call it an OCBF since the brace cables don't connect to the column base. It's not materially different from a K brace since the brace could hinge the column at the work point in my view.

RE: Does this qualify as an OCBF

I vote no on the OBCF. I'm guessing that this falls into the "Other systems not listed" category.

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

I seem to recall something about cables and other tension only members not being allowed in OCBF but may be thinking of something else.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

RE: Does this qualify as an OCBF

I'm not completely familiar with all the US system definitions, but this would behave differently in a seismic situation from a conventional braced frame. You're generally counting on buckling of the members in compression to help absorb energy. That doesn't happen with cables, plus tying into the column half way up means that you might deflect the middle of the column enough to cause a p-delta style failure. You'd need to take that into account.

RE: Does this qualify as an OCBF

Tension only members are not allowed for SCBF, OCBF they are allowed

RE: Does this qualify as an OCBF

Thanks sandman, that's what I was thinking of.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

RE: Does this qualify as an OCBF

EBF is the only one that allows K bracing, so that's my vote

RE: Does this qualify as an OCBF

Agree with canwest's 2 posts. It's not an OCBF because it relies on flexure of the members (columns) for stability. In a concentric frame all of the forces need to be resolved at the nodes.
Treating it as a K brace sounds reasonable to me.

RE: Does this qualify as an OCBF

I have designed similar tension only brace systems for super tall warehouses (45-ft single story), as they do not like braced frames that protrude below 18-ft or so. Moment frames are inefficient at such a high unbraced height. I have always classified them as "steel systems not specifically designed for seismic resistance".

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.

Quote (codyjb)

If not an OCBF how would you analyze?

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

IMO it has some of the similarities and concerns(seismic) that are associated with K-bracing and may not be allowed in certain seismic catagories.....

RE: Does this qualify as an OCBF

I've had a revelation. In terms of fundamental behavior, I think this is best characterized as a cantilevered column system. Consider the similarities:

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

I'm going to have to disagree with you on that one KootK. While I agree that the elastic forces will be similar to a cantilevered column many of the provisions for cantilevered column systems are specific to a LFRS where lateral resistance is provided by fixity at the base of the column. Low R values, requirements of high axial capacity, and overstrength requirements all follow for these systems. I believe these would over-penalize the original system and potentially push the structure into another SDC.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

RE: Does this qualify as an OCBF

Closest to a knee braced moment frame, is my vote.

RE: Does this qualify as an OCBF

Quote (TME)

While I agree that the elastic forces will be similar to a cantilevered column

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.

Quote (TME\)

many of the provisions for cantilevered column systems are specific to a LFRS where lateral resistance is provided by fixity at the base of the column.

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.

Quote (TME)

Low R values, requirements of high axial capacity, and overstrength requirements all follow for these systems. I believe these would over-penalize the original system and potentially push the structure into another SDC.

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.

Quote (sandman21)

Closest to a knee braced moment frame, is my vote.

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

(OP)
Thank you all.

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.

Quote (MacGruber22)

Why do you need to know if you can classify it as OCBF in order to analyze it?

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

If we are talking about AISC 341, your system is NOT a OCBF. F1.4b K-type braced frames are not permitted, yours is a K-type brace. This is why F1.2 limits the eccentricities to less than beam depth but says nothing about columns. Afraid you are going to have to redesign.

RE: Does this qualify as an OCBF

Quote (Cody)

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.

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

obviously, it is neither a classic moment frame or K-brace....but having the max mom/deflection occur near the mid-point of the col would push me towards considering it a K-brace....when I don't have a clear understanding on the ultimate behaviour and attendant consequences, I usually choose the conservate route.....

RE: Does this qualify as an OCBF

Quote (KootK)

Not just similar but, rather, practically identical from the perspective of the column.

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.

Quote (KootK)

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?

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.

Quote (KootK)

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 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.

Quote (KootK)

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.

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

The proposed cable braced system does not meet definition of an OCBF in AISC 341's glossary (page xliii) because the columns are subject to bending.

Quote (AISC 341-10)

Ordinary concentrically braced frame (OCBF). Diagonally braced frame meeting the requirements of Section F1 in which all members of the braced-frame system are subjected primarily to axial forces.

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