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]

I vote no. As I see it, you're basically forcing a strength / stiffness irregularity into the system that will result in a disproportionate share of the ductility demand occurring over a small portion of the building height. In this sense, this scheme is the opposite of good seismic design practice. At the least, I'd be seeking a dispensation from the AHJ before rolling forward with this.

[sub]That's a horrible idea. What time?[/sub]

 

[YoungGunner]

you're basically forcing a strength / stiffness irregularity into the system that will result in a disproportionate share of the ductility demand occurring over a small portion of the building heigh

As an honest seeker of what you are presenting, how would this be different than putting an ordinary moment frame over a special concrete shear wall? Or a shear wall on top of a special moment frame? Or any system with a different R above another system?

 

[sandman21]

You are trying to do a two-stage analysis without trying to meet the requirements of the system. As currently drawn your height for the OMF still exceeds the limits set forth in the code.

 

[KootK]

Fun. First, let's turn this around and try it like this.

Does this logic check out

What is your logic? And I don't mean the kind of logic that is "how do I massage my situation to be code compliant" but, rather:

1) What do you interpret the underlying intent of the height limitation to be from a seismic perspective and;

2) How do you see your scheme satisfying that intent by alternate means?

I'm not accusing you of trying to do anything sneaky here. We all play this game. At the same time, you didn't explain your logic in these terms in your original post and I feel that doing so is important to properly evaluating the situation. What story are you trying to tell?


[sub]That's a horrible idea. What time?[/sub]

 

[YoungGunner]

You are trying to do a two-stage analysis without trying to meet the requirements of the system. As currently drawn your height for the OMF still exceeds the limits set forth in the code.

Conversely, I would try to meet the requirements of each system. I would design the upper connections per OMF standards. I would design the lower connections for SMF standards. The lower R of the two and the more stringent drift would be applied to the SMF. Mix-matching systems above another system is allowed in code, provided you do meet these requirements.

And I am meeting the guidelines for an OMF, because by making the OMF a single story frame, it is permitted up to a structural height of 65f (considering that the OMF is only the upper half).

 

[YoungGunner]

What is your logic?

This is a seismic D category structure. Thereby, OMF is not permitted except in the exceptions.
1) The first exception of ASCE is it can be a single story OMF, up to a structural height of 65ft. I can't accomplish this structure with this provision because I need a two story frame (definitely need the lower beam).
2) The second exception of ASCE is it can be any configuration of OMF (two, three-story even) up to a structural height of 35ft. I fall outside of this exception because my two story frame exceeds 35ft in height.

The idea, therefore, is to still have a single story frame to meet the 65ft limit by stacking it onto a SMF. To me this doesn't seem any different than if I were to make the base level concrete and put a single story OMF on top of it (which I can't because windows). But the weird part with this structure is sharing columns - does it get weird for an OMF and SMF to share the same columns? Would we feel differently if I made it a pinned bolted column splice connection and it wasn't a continuous column?

 

[driftLimiter]

You cannot consider the top of the SMF to be the 'base' of the OMF unless you do a two-stage analysis.

If you just read the definitions of the terms in ASCE 7-16 CH 11 you can see this.

BASE = Level at which horizontal Seismic Ground motions are considered to be imparted to the structural
STRUCTURAL HEIGHT = The vertical distance from the base to the highest level of the seismic force-resisting system of the structure. For pitched or sloped roofs, the structural height is from the base to the average height of the roof.

Does not check out IMHO.

And I am meeting the guidelines for an OMF, because by making the OMF a single story frame, it is permitted up to a structural height of 65f (considering that the OMF is only the upper half).

no

 

[KootK]

@YoungGunner: you're still just throwing code legalese at this. I'm asking you to explain your understanding of the theoretical reasons for the code limitations and how your scheme meets the intent of them from a theoretical perspctive. The "why" and the "how", not the code dump.

[sub]That's a horrible idea. What time?[/sub]

 

[sandman21]

Sure you can mix but you have not changed the height of the structure. The OMF is not a single-story structure ontop of another, your two-story building has an OMF that exceeds the limits. You would need to show that the first story is a separate structure, e.g., a two-stage analysis.

 

[Deker]

The point sandman is making is that you can't treat the upper frame as a single story without meeting the requirements for a two-stage analysis. The height limits are prescriptive - there is no arguing your way around them short of using alternative means / PBD.

The same would apply for other systems, as well. You can stack special systems, but you wouldn't be permitted to stack OMF on a special concrete shear wall that exceeds the overall building height limit without satisfying the two-stage criteria.

 

[KootK]

I'm not sure that I buy the two stage arguments:

1) I thought that two stage only applied to flexible stuff on top of stiff stuff. This is the reverse.

2) While OP may be trying to treat the top of the SMF as the "base" for height determination , I do not believe that he is trying to treat it as the "base" for dynamic considerations, as the two stage procedure does.

[sub]That's a horrible idea. What time?[/sub]

 

[KootK]

While OP may be trying to treat the top of the SMF as the "base" for height determination...

OP can clarify but I'm not sure that he's even attempting to do that. I think that the intent is simply to say:

1) This is now, fundamentally, a special moment frame therefore;

2) This system complies with the height limitations (of a special moment frame).

[sub]That's a horrible idea. What time?[/sub]

 

[driftLimiter]

While I understand the OP is attempting to do a classic Vertical Combination of framing (ASCE7 16 12.2.3.1) and not a dual stage analysis,
He is also trying to suggest that the structural height can be considered differently for each of the combined framing elements.

The only way the code allows this is via a dual stage analysis.

I 100% agree with your point 1) Koot, this is not an arrangement that would qualify for two-stage anyway. But if it could, then the OP could continue measuring structural height the way he has suggested.

 

[Deker]

Setting aside behavior, a two-stage analysis is a code-compliant way to accomplish what the OP wants to do. I doubt it will work for single story OMF on SMF with shared columns. As for whether a two-stage analysis could / should be applied to this condition, this article offers a good discussion: Link.

 

[KootK]

...a two-stage analysis is a code-compliant way to accomplish what the OP wants to do.

That sound nuts to me (the concept, not you). Can you direct me to something in the code on that such that I might review it? The more specific, the better. Clearly I need some re-educating.

Stiff on top and flexy on the bottom is basically soft story, right?

[sub]That's a horrible idea. What time?[/sub]

 

[YoungGunner]

Does this provide clarification in what I'm intending for "structural height"? I'm not trying to restart the height at the top of the SMF, I'm trying to ensure that I can use a single story OMF within the 65ft limit. The hope is an SMF below will help accomplish that by essentially remove the lower story of the OMF, same if I made that lower story concrete or whatever. I'm not trying to do a two stage analysis here, which I wouldn't think is required for simple vertical combinations of LFRS.

 

[Deker]

I doubt you need re-educating. To qualify for two-stage you do need flexible over stiff. Not likely the OP could qualify with the proposed system. The article I linked above discusses how a two-stage analysis could justify something similar to what OP is proposing, but also discusses how that is likely a misapplication of the code.

 

[MotorCity]

From the article Deker provided above......

 

[KootK]

Thought experiment. Suppose OP's situation were as I've shown it below and:

1) The soft story irregularity was accounted for and did not trigger a "no-go" code limit.

2) Story drift limits were applied at the lower story such that member and joint ductility levels were maintained to the usual values.

From a theoretical perspective, why shouldn't this be a SMF considered to be the height of the roof?

Thus far, the only reason that I can think of is that we generally want all of the stories in a building to participate in energy dissipation and, in this situation, the upper story does not. That said, that doesn't sound like a big deal to me given that we're talking about a structure that only has two stories and the upper story is a roof. Roof's often cannot satisfy strong column provisions and thus, if the upper story yields at all, it certain to be at significantly different point in time relative to the lower stories.

Viewed in this light, the upper story tends to just feel like "load" on top of the SMF to me. A load that induces overturning of course. Kind of like large mechanical unit.

[sub]That's a horrible idea. What time?[/sub]