Avoid Special/Intermediate Moment Frame in SDC D
Avoid Special/Intermediate Moment Frame in SDC D
(OP)
I have a new project that falls into Seismic Design Category D. It's an exterior (non enclosed) stair addition to an existing four-story wood framed building (from the 1960's). They are demolishing the fire escape and adding this usable stair instead (so it is a primary exit stair). In the US, pacific northwest.
The stair will be structurally separated from the building for both gravity and lateral loads. Four columns (2 at the outside corners, and 2 within a few feet of the building). The stair will be your typical channel stringers with slip connections to prevent them from acting as braces.
The architect would like it to be a moment frame in two directions. Technically, this frame is in SDC D, so I have to do either in Intermediate or Special MF. The loads are so ridiculously light on this frame that I would love to just do R=3 and not deal with seismic detailing requirements.
Is anyone aware of some provision in ASCE 7 that allows one to skirt the IMF/SMF requirement for very small or lightly loaded structures? I know they have some provisions to lax the requirements for "light frame" construction but that is not what I have.
The stair will be structurally separated from the building for both gravity and lateral loads. Four columns (2 at the outside corners, and 2 within a few feet of the building). The stair will be your typical channel stringers with slip connections to prevent them from acting as braces.
The architect would like it to be a moment frame in two directions. Technically, this frame is in SDC D, so I have to do either in Intermediate or Special MF. The loads are so ridiculously light on this frame that I would love to just do R=3 and not deal with seismic detailing requirements.
Is anyone aware of some provision in ASCE 7 that allows one to skirt the IMF/SMF requirement for very small or lightly loaded structures? I know they have some provisions to lax the requirements for "light frame" construction but that is not what I have.






RE: Avoid Special/Intermediate Moment Frame in SDC D
Dave
Thaidavid
RE: Avoid Special/Intermediate Moment Frame in SDC D
One thing to keep in mind is that there really are no simple weak axis moment frame connections under AISC 358. That may frustrate your efforts depending on the path that you take.
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: Avoid Special/Intermediate Moment Frame in SDC D
Another reason to get myself out of this debacle...
thaidavid40, thank you.
RE: Avoid Special/Intermediate Moment Frame in SDC D
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
I thought about this route but considering the site class I've been given in some nearby areas I'm not confident that I could do better than Site Class D. Might be worth a try though, I could ask a geotech for their opinion. Thanks.
@haynewp
Unfortunately the exception to allow OMF's in SDC D only applies to single story structures or light-frame construction.
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
ConXtech makes a system which allows moment frames into each direction. They have a tower similar to what we are discussing used for Burning Man. You also have the options of designing a box column
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
Unfortunately my SDS is too high for that to help, still stuck in SDC D.
@sandman21
The stair is separate from the wood building because I have no drawings of what the framing is. If I were to take load into that building I'd have to prove the lateral capacity didn't increase by much and I'd have to get the load into an old wood diaphragm. Both of which probably won't work.
I'll look into ConXtech. Maybe one of their reps can tell me if the system is beneficial for such a small job.
RE: Avoid Special/Intermediate Moment Frame in SDC D
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Avoid Special/Intermediate Moment Frame in SDC D
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: Avoid Special/Intermediate Moment Frame in SDC D
@TehMightyEngineer
The architect (& owner) want to know what a exposed steel stair will cost. I can suggest a shear wall but unless you enclose and finish the whole thing it'll look pretty silly. I can't say with certainty but I think a heated, cooled, lit, brick facade, roofed, and finished interior stair would cost more than the SMF.
Sounds like I'm going to do an SMF for a tiny little stair. Seems ridiculous but it is what it is.
Maybe I'll call the AHJ and see if they'd let me get away without.
RE: Avoid Special/Intermediate Moment Frame in SDC D
My goal would be:
1. use the stringers as braces in one direction, OMF or R=3 in other.
or, 2. OMF or R=3 in both directions
RE: Avoid Special/Intermediate Moment Frame in SDC D
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Avoid Special/Intermediate Moment Frame in SDC D
You would have to show that the addition does not increase the D/C ratio by more than 10%, 3403.4. I would doubt that the stair exceeds 10% of the weight at s floor, let alone all the floors. Provide the columns for gravity and tie the stair the existing. If you dont have drawings submit an add service for field review and details to tie the stair to the building. The penetrations into existing structure are cheap compared providing a frame at an R of 3 let along SMF/IMF.
RE: Avoid Special/Intermediate Moment Frame in SDC D
the codes usually have a blurb at the beginning of the code absolving them of liability and deferring to the judgement of an experienced engineer....I interpret this as saying the engineer can use his professional judgement in deciding wheather a particular part of the code is applicable in his particular case...
ASCE 7-10 has this clause on page two..."ASCE does not intend, nor should anyone interpret, ASCE's standards to replace the sound judgement of a competent professional, having knowledge and experience in the appropriate field(s) of practice, etc........."
RE: Avoid Special/Intermediate Moment Frame in SDC D
I never thought of doing a cantilevered column. Even if I have to use heavy, big columns, there's no way it's more expensive than a SMF. The savings on welds would pay for the columns a few times over.
I've never used a steel special cantilevered column system before but I imagine the only real detailing requirement is the base connection. I'll look into it. Sounds very promising at this point. Thanks.
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
Now if you said "zombie apocalypse" then I'd say it was a Florida specific code. ;)
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
RE: Avoid Special/Intermediate Moment Frame in SDC D
While it might be tough to say that the stairs belong under chapter 13, "Seismic Design Requirements For Nonstructural Components", it looks like the table pretty directly addresses your situation.
RE: Avoid Special/Intermediate Moment Frame in SDC D
Though I'm not sure exactly how to interpret it. By "stairs" do they mean only the stringers, treads, landings or the structure that supports the stair as well?
Regardless, I decided to go with the TehMightyEngineer's cantilevered columns suggestion. Less pieces, no field welding, simpler erection. The only drawback is the columns are larger and heavier (but not by much compared to the MF). The architect didn't even mention the column size so I'm in the clear
RE: Avoid Special/Intermediate Moment Frame in SDC D
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com