Access platform "moment frame"

[cec17]

I have used something similar for small access platforms in the past. Ive gotten comments before in plan review, but rationalized it since we are able to resolve the moment and achieve adequate stiffness / rigidity using this connection (Using ASCE Ch.15, Steel ordinary moment frame with R = 1 and pinned base).

The loads are very small and it works fine from a capacity/stiffness perspective. If I had never received comments on it, I would probably use it without second guessing. But I am curious if anyone here feels differently - if you'd be uncomfortable using this connection in SDC D as an AISC 360 moment-resisting connection.

 

[CDLD]

Can you use a knee brace instead?

 

[lexpatrie]

Do you mean R=3?

 

[canwesteng]

Is the methodology clear for the plan reviewer? Because it won't work as a chapter 12 SFRS, but since you are into chapter 15 with R=1 I don't see an issue.

 

[cec17]

@CDLD this platform is typically used where you can't brace due to piping or other obstacles beneath the platform.

@lexpatrie you may be referring to the Ch. 12 "steel systems not specifically detailed for seismic resistance" which is a building structure, and that system isn't permitted in SDC D. This is Table 15.4-1, steel ordinary moment frame, which is permitted with no height limit in SDC D.

 

[cec17]

@canwesteng yes with some back and forth I have been able to get them on board in the past. It's my experience that usually they hardly know what they're looking at if there's no brace visible, though.

 

[canwesteng]

I think it would have to be on the notes somewhere... otherwise if you say for example call out in your notes that it's steel OMF or OCBF for example, then you have this as a different system, they might comment for that. Perhaps saying platforms designed to the requirements of ch.15 alongside the SFRS callout.

 

[JoshPlumSE]

Is the methodology clear for the plan reviewer? Because it won't work as a chapter 12 SFRS, but since you are into chapter 15 with R=1 I don't see an issue.

I tend to agree. There is zero ductility expected out of an R=1 design. So, if it works like that you should be good. I'd make sure that I analyze it for bending axial and torque. But, provided that it works for all that, I don't have a problem with it. You see this sort of stuff with little industrial stuff.... Essentially, a "field solution" that you have to check for adequacy. Or, something a vendor does to support their equipment (and has been doing for decades because it's easy to fabricate), but gets called out by a plan check guy or another engineer.

That being said, I would NEVER do this sort of thing to support something really important or high profile. An access platform that gets used once a year? No problem. But, if it's going to be used continuously, I wouldn't do it. Nor, would I do it if it supported critical equipment that must be operational immediately after the earthquake event.

 

[TLHS]

Is this short? I've done this for little platforms that are a a few feet tall and a handful of square feet in area. Like, step-ups for equipment, or little walkovers.

 

[cec17]

Thanks for the response all. Yes, this is only for short, infrequently used small access platforms.

 

[GC_Hopi]

Ditto as Josh. Definitely worth doing a quick gut check on the members and loads (no equipment) but I don't get heartburn using it for short, small maintenance access platforms.

cec17: I would put "short" and "maintenance" in the description of the platform as well.

EDIT: Looks like I was late to the party but just echoing the same sentiments.