Avoid Special/Intermediate Moment Frame in SDC D 3

[DETstru]

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.

 

[thaidavid40]

I gave your question a "star", as I thought it was a great leading question. I too am interested to see how this one plays out, as I have occasionally run into the same philosophical question myself, but I had no definitive insights into the correct answer. Thanks for posting it! Come on guys, chime in!
Dave

Thaidavid

 

[KootK]

I think that you can use OMF if you're under 35' and 20 PSF/35 PSF for cladding and flooring. Based on the project description, you might satisfy that. Another route might be to pursue classification as a non-building structure of some kind.

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.

 

[DETstru]

Koot, I can't use an OMF in SDC D unfortunately.

no simple weak axis moment frame connections under AISC 358

Another reason to get myself out of this debacle...

thaidavid40, thank you.

 

[TehMightyEngineer]

Geotechnical investigation to see if the soil enables you to get out of SDC D?

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[haynewp]

There's an exception when you are in SDC D that you can use an OMF based on the weight and height limits mentioned above. See the table footnotes in ASCE 7.

 

[DETstru]

@TehMightyEngineer
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.

 

[mike20793]

Check out ASCE 7-10 Section 11.6. If S1 is less than 0.75g, you can limit the seismic design categoy to just the value of Sds (Table 11.6-1) if your structure is really stiff. I've only been able to use this on concrete dome structures to avoid seismic design category D, so it likely won't help your case..

 

[sandman21]

Why is the new stair separate from the wood building? You can try to use ASCE ch. 15, you will still need to design the connection of the OMF per the seismic provisions. Personally I would try and match the response of the building that is exiting onto the stairs.

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

 

[Gumpmaster]

If you do need SMF's try to use SidePlate connections. They're pretty simple and allow for weak axis connections.

 

[DETstru]

@mike20793
Unfortunately my S_DS 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.

 

[TehMightyEngineer]

I know the architect wanted moment frames but what about something easier to build like a shear wall?

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

It's frustrating in the sense that, in spirit, I'm sure that the structure satisfies the intent of some of the escape clauses (what's lighter frame than no wall/floor/roof?). But then who wants to seek a special code official dispensation for such a minor bit of work. For what it's worth, my first instinct was also to lean on the existing structure. That probably means a bunch of costly envelope penetrations however.

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.

 

[DETstru]

That probably means a bunch of costly envelope penetrations however.

And a tearing up the interior finishes to drag the load into the diaphragm.

@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.

 

[FoxSE14]

I think there's a real case here to discuss this with the local building department with the goal of talking practicalities with them. As the owner of the building, I would have a real problem when I saw the bill for a SMF steel egress stair.

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

 

[TehMightyEngineer]

What about a braced frame? Heck, what about a special steel cantilever column system?

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[sandman21]

Special steel has roughly the same limits.

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.

 

[SAIL3]

for such a small structure, I would be tempted to use R=1 and OMF.....whatever method one chooses I would check out the possibility of the two seperate structure deflecting independent of each other and thus open up a gap @ the point of egress between the two structures....
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........."

 

[DETstru]

@TehMightyEngineer

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.

 

[shaneelliss]

I'd add a vote to use a braced frame. I'd also echo someone above who said to watch deflections. It is going to be hard to get enough separation to avoid interacting with the building during an earthquake with a cantilever column. Typically you would have to evaluate the drift on the building and then the drift on the stair to make sure there won't be pounding.