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.

 

[DETstru]

I'll definitely evaluate the drift for my stair and size the joint appropriately but I'm not incredibly concerned about a few stair stringers banging into a brick wall. If it were building on building contact, it would be a different story.

 

[TehMightyEngineer]

I've used ordinary cantilevered columns before on a pipe bridge in a low seismic area without much trouble. They have a crappy R factor and crazy detailing on the anchor bolts and baseplates (appendix D for the loss). But, as you noted, you'll have 10x worse details on the SMF beam to column connections, if you can make it work then you might have your answer. The added advantage is you get "free" heavy columns and high wind resistance. That will be a very rigid stairway for non-seismic loads and make a great emergency egress in the event of fire/blast/hurricane/alien invasions/zombie apocalypse.

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

http://www.americanconcrete.com

 

[DETstru]

Can you point me to the alien invasion design section in ASCE 7? Can't seem to find it...

 

[TehMightyEngineer]

I made an inappropriate joke and decided it was too offensive, so deleted it...

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

http://www.americanconcrete.com

 

[jayrod12]

TME, don't be so offensive! But really, I don't think that was overly inappropriate. Be proud of your sense of humour. I laughed if it's any consolation.

 

[DETstru]

I'm now offended that I didn't get to see it before it was deleted!!!

 

[jayrod12]

click on the post deleted link and it will show up.

 

[Once20036]

I read this thread yesterday and while looking through ASCE 7-10 on a different issue, noticed that the bottom of Table 13.5-1 lists "Egress stairways not part of the building structure".
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.

 

[DETstru]

Nice find Once20036, thanks. I'll keep that in my back pocket.
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

 

[TehMightyEngineer]

With a brief glance I seem to recall my understanding of that note was for things like fire escapes on the exterior of a building (or similar). I'd have to read it again and see if there's any commentary on it to be sure, though.

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

http://www.americanconcrete.com