Steel Frame OMF to SMF 1

[YoungGunner]

BTW, I'm not looking for solutions or alternative ideas - simply asking if this logic checks out.

The height of the frame is around 45ft. It exceeds the 35ft limit for a multi-story steel frame for OMF. It would change the architectural look of the designer's ceiling to do a SMF all the way up and deal with bottom flange bracing on the top beam at the roof. We want to avoid Simpson's specialty product that doesn't require bottom flange bracing. So, can we do an OMF on a SMF (we can do bracing at the floor level easily) that share the same columns? Obviously the R of 3.5 tracks into the SMF, but the goal here is to fall underneath the provision of a 65ft single story OMF by having it stack on an SMF, and avoid bracing on the top beam. Does this logic check out?

 

[KootK]

To qualify for two-stage you do need flexible over stiff. Not likely the OP could qualify with the proposed system

Okay, I guess that you've been assuming that OP meant for the SMF to be stiffer than the OMF. I've been assuming the opposite. I guess that it would depend on the drift limits applied to the two stories. I'd just assumed that, because the lower story was designed to yield and the upper story was not, that would make the upper story the stiffer. I can, however, imagine situations where that would not be tre.

_{That's a horrible idea. What time?}

 

[Deker]

YoungGunner, I think we're all clear on what you're trying to accomplish. Since you are showing two beams, I presume this is a 2-story structure. Correct? If so, you cannot apply the 65 ft height exception for OMF because that only applies to single-story structures. A shorter height limit applies to two-story structures, which you exceed.

What we're telling you is that in order to justify what you are proposing, you would have to perform (and qualify for) a two-stage analysis so that you could apply the building height limit to the upper frame only. While that would technically be code compliant, it's not how the code is meant to be applied. See the article I link above.

Not sure what your constraints are, but there is a way to achieve SMF bracing with an adjacent HSS member that is parallel to the beam without introducing bracing members back into the plane of the diaphragm. See page 26 of this document: Link.

 

[driftLimiter]

What exception exactly are you attempting to use?

My thought is that whichever one you are referencing contains some language that prohibits what you want to do.

The only reasonable conclusion about your system to apply towards these exceptions is: you have two story structure, and the height is 45 ft.

 

[YoungGunner]

Allow me to shed some light on the ASCE wording. With this wording, it is not the structure that needs to be single story, it is the OMF. I'm just trying to create a single story OMF on top of an SMF, all within a structural height of 65ft.

 

[KootK]

Question for all: what do you believe is the theoretical basis for for the height limitations in the first place?

This is the best that I can come up with on short notice and it's only speculation:

1) Taller buildings imply greater societal risk in a very general sense. Not very sciency.

2) Taller buildings would be more prone to P-Delta collapse.

3) Taller buildings usually have more stories and, the more stories there are, the less likely it is that all will participate in the dissipation of seismic energy.

What else?

_{That's a horrible idea. What time?}

 

[YoungGunner]

Adding in the ASCE commentary about this topic as a reference. They just provide justification based past performance. I agree with your thoughts and don't have much to add to it, if any.

 

[canwesteng]

4 - Taller buildings have longer periods as well, so the base shear is proportionally lower. Seems like extra ductility is not a bad idea if you are counting on the structure being "softer".

Re: the stiff portion on top of the soft portion - in that sketch wouldn't you hinge the columns first? Maybe some way to consider the top story as equipment, but I'd think you're applying overstrength factors or forces equal to member yield to the soft structure, so not too much different than dealing with the soft story irregularity.

 

[YoungGunner]

Oooo I just had another thought - OMF don't have a specific hinge point. Theory is you just provide strong connections so those don't break, and any point of the frame will yield - including the columns. With that backdrop, I think it's scary to allow for hinge points to develop in columns for tall structures.

 

[driftLimiter]

@Younggunner - Starting to understand where the confusion is coming from. It does say 'Single Story OMF' it isn't clear whether it means 1 story of many can be OMF or if the whole structure has to be 1 story or less.

Looking at that C12.2-1 Table seems to me to suggest that even though the wording in the exceptions is slightly different, the intent is that the structure has 1 story max. I could be mistaken about this however.

I believe part of the height & story limitations in ASCE 7-16 is to provide well proportioned distribution of the ductility. And it is exactly that ductility which I would be concerned about if I was going to accept your reasoning that the OMF is allowed because it is only one story.

The determination of the story drift I would be especially interested in. Essentially you want the beam of the SMF to hinge at the same level of seismic load as the OMF beam and you need the story drift to be within limits for both floors.

The Cd factor and R factor for the SMF is based on the detailing, lets say you did a reduced beam section, well just because you use Cd from the OMF above, does that really mean you will get that much drift, or perhaps more because of the physical shape of the beam?

 

[YoungGunner]

But the Cd and R factor for the SMF would match that of the OMF. Yes, in complete theory, this is really just an OMF with SMF connections on the lower half. In which case, I guess you could say it is just an OMF that uses option (c) of AISC 341 E6b, where you match the detailing of an SMF. So in that logic, I guess this still stands as a two-story OMF either way.

That table in the commentary is interesting to see, though it doesn't clarify whether that is the max number of stories of the structure or whether it's the max story of the structural system, though in context with everything else we could say single story structures.

I'm becoming more sad as this goes on. I don't have a good way of making this system work and beam bracing can't be achieved at the roof level without changing the ceiling.

 

[kissymoose]

Question for all: what do you believe is the theoretical basis for for the height limitations in the first place?

Doing my best as someone who lives in SDC-B-ville. The seismic effect on a structure from the ground we represent with Ss and S1. The effect of that ground motion up throughout the structure is...complex, and dependent on the size (especially height), stiffness of members and connections, and distribution of weight throughout. The effects from ground acceleration Ss and S1 on the base of a single story OMF will not be the same as the effects at the base of a single story OMF sitting on a structure.

 

[JoshPlumSE]

Question for all: what do you believe is the theoretical basis for for the height limitations in the first place?

That's exactly what I was going to comment on your earlier post.

Honestly, I think you'd be shocked at how 'non-science' based some of this stuff is. If I remember correctly, the basis of the 35 ft height limit started out as a fire rating thing. I kid you not, an old "grey hair" once claimed to me it was the largest height the fire companies could reach with their normal ladders / engines. It got codified and was never changed. And, is now used for things that are totally unrelated.

I can't swear that this is true. But, that's what I've heard.

That being said, I'm not necessarily buying the OP's argument. I think the only way you'll know for sure whether this will be approved or not is to contact the city engineer and look for their interpretation. If this were a non-building structure, then I'd have other suggestions. But, for a building structure, there has to be better options.

What's the rest of the lateral force resisting system? I might detail it ALL as an OMF, but argue that this is just for construction simplicity and that you are relying on the rest of the structural system (through diaphragms, drag struts and such) to actually carry the lateral loads.

 

[jerseyshore]

Is an engineer even going to review this? No building official I have ever met has a clue about these types of things. Maybe it's different in higher seismic areas, but around here they don't even look at ASCE7. And even most engineers around here wouldn't know anything about these types of limits. Again, probably different in higher seismic places.

 

[driftLimiter]

@jerseyshore, Not sure what your getting at here. It shouldn't really matter if the plan reviewer is going to catch this or not. We aught to design according to the requirements of the building code shouldn't we? Especially given that the OP is aware of the requirement, it would be a dereliction of duty to simply ignore it on the basis that the plan reviewer wont catch it. These are pretty easy things for reviewers to catch IMHO. there is a simple table in ASCE 7 that limits structural systems by height, perfect for reviewers to come and say that a system does or does not meet a requirement.

Sure if your not practicing in SDC D or above, maybe these things arent so much of a concern. But for seismic design this is critically important. Like what has been said here and throughout, the R, Omega, and Cd factors are heavily based on industry consensus, research etc. If we start breaking the rules about height limits, that drawings into question the ductility of the system because the systems are only permitted up to certain heights.

I had a professor who said, you can do what you like to ignore or hide mistakes, but the earthquake doesn't care, it will find them.

 

[KootK]

4 - Taller buildings have longer periods as well, so the base shear is proportionally lower. Seems like extra ductility is not a bad idea if you are counting on the structure being "softer".

I see your point but I actually roll the other way on that one. Taller buildings do have longer periods and lower base shears. But the reliance on softness in this situation is really elastic softness. In the extreme, very tall buildings often do not need to go plastic at all in order to achieve the roof level displacement required to respond to an earthquake safely. Plasticity does indeed increase a buildings period but that only kicks in after the available, elastic displacement is exhausted. I could be wrong with this though... you've definitely got me pondering.

Re: the stiff portion on top of the soft portion - in that sketch wouldn't you hinge the columns first?

I feel that is something that is designer controlled. I'd be expecting the designer to enforce some version of strong column - weak beam.

 

[KootK]

I think it's scary to allow for hinge points to develop in columns for tall structures.

In general we do not allow hinges to develop in our columns. That's the whole "strong column / weak beam" thing. The major exception is roof beam to column joints where column hinges are often allowed. It's okay in that situation because, presumably, the roof level alone would not weigh enough to create a big P-delta demand and pancake that story. It can also be quite impractical to try to enforce WBSC where there is no upper column coming into the joint.

Oooo I just had another thought - OMF don't have a specific hinge point. Theory is you just provide strong connections so those don't break, and any point of the frame will yield - including the columns.

I don't agree although I admit that it's a point of confusion both for me and in the literature. I believe that OMF are expected to yield in the same places that SMF and IMF are typically expected to yield. The difference is that the amount of member rotational ductility required is kept low enough that it doesn't require special attention.

 

[KootK]

I'm becoming more sad as this goes on. I don't have a good way of making this system work and beam bracing can't be achieved at the roof level without changing the ceiling.

Why don't you:

1) Describe your roof level situation and exactly what you're up against there for constraints and;

2) Take of our handcuffs and allow us to propose alternate solutions?

We're full of killer ideas. Just give us a chance.

Yeah, I get it: you wanted to limit the scope of this discussion. I think that we're past that now however.

 

[YoungGunner]

I believe that OMF are expected to yield in the same places that SMF and IMF are typically expected to yield.

I refer to the commentary of AISC 341 for my thought process for OMF, though after a re-read it becomes more clear that inelastic behavior is not encouraged in an OMF, though there may be some.

 

[KootK]

Re: the stiff portion on top of the soft portion - in that sketch wouldn't you hinge the columns first?

I might be getting your drift on this one now. It is true that:

1) My proposed frame could not form a complete mechanism without yielding the columns at locations in addition to their bases (or axially yielding a brace / beam above).

2) OP's frame could not form a complete mechanism without either:

a) Developing plastic hinges in the OMF beam or;

b) Developing plastic hinges in the columns at locations additional to their bases.

This alone may kill OP's scheme as an ELF design. You'd have to do it with a fancier method like performance based design.

 

[YoungGunner]

I do appreciate everyone's posts on this thread. We have moved a different direction than what I was suggesting that meets code requirements. Great conversation and thoughts!