Re-entrant corner at alcove
Re-entrant corner at alcove
(OP)
I have a single story CMU building with a steel roof deck supported by steel joists. The building is in a high seismic zone (SDS=1.0). It's an industrial facility but it looks pretty similar to a strip mall building.
The architect has shown an alcove in the middle of the long walls (~100ft total wall length). The alcove is about 7 ft square and extends the full height of the building, including the roof deck. If you look at it as a flexible diaphragm, then the alcove walls take the majority of my diaphragm shear. The issue is that, considering the door in the alcove, I only have 4' of shear wall in the alcove and can't in any way take that diaphragm shear down to the foundation.
How would this best be detailed? I don't really want a drag strut because I don't want my walls to take the shear. It would be best to cut the alcove free from the diaphragm, but I don't see that's possible. I do have a shelf angle supporting the deck that I could put horizontal holes in. I can't just call it a non-bearing wall.
How do you normally deal with this situation? Is the secret to look at it as a semi-rigid diaphragm?
Thanks.
The architect has shown an alcove in the middle of the long walls (~100ft total wall length). The alcove is about 7 ft square and extends the full height of the building, including the roof deck. If you look at it as a flexible diaphragm, then the alcove walls take the majority of my diaphragm shear. The issue is that, considering the door in the alcove, I only have 4' of shear wall in the alcove and can't in any way take that diaphragm shear down to the foundation.
How would this best be detailed? I don't really want a drag strut because I don't want my walls to take the shear. It would be best to cut the alcove free from the diaphragm, but I don't see that's possible. I do have a shelf angle supporting the deck that I could put horizontal holes in. I can't just call it a non-bearing wall.
How do you normally deal with this situation? Is the secret to look at it as a semi-rigid diaphragm?
Thanks.






RE: Re-entrant corner at alcove
Dik
RE: Re-entrant corner at alcove
RE: Re-entrant corner at alcove
What's your edge condition look like at the alcove?
Are the end walls/alcove walls bearing? What's your roof framing plan look like?
I would try to find a way to detail the diaphragm such that there is no shear connection at the alcove.
RE: Re-entrant corner at alcove
I attached a sketch. Does that help?
There is a ledger angle all around. The deck supports the walls out of plane on the end walls. There is a ledger angle at the alcove.
Once20036, I agree, I should find a good way to avoid shear transfer to the alcove, I just can't think of a good way to do that.
I don't think it's an uncommon condition (having an alcove like this I mean) I just, surprisingly, haven't ever run into it and I was wondering how other folks dealt with it. I've had plenty of alcoves before, but never ones that penetrated through the diaphragm. Those are easy to detail because you can just detail the walls as non-bearing, which I can't quite swing here.
RE: Re-entrant corner at alcove
Dik
RE: Re-entrant corner at alcove
RE: Re-entrant corner at alcove
RE: Re-entrant corner at alcove
A way to handle the diaphragm tearing, instead of collectors to the wall piece, is by putting a chord segment to the left side of the wall within the roof. That would restrain tearing under loads in the left/right direction.
RE: Re-entrant corner at alcove
1) Figure out how much the diaphragm would move were the alcove shear walls not there.
2) Design the alcove walls for that much movement, probably with some connection yielding along the way.
3) Design the other stuff in the area force the max for you expect to develop when all this happens.
Another approach, which I would favor, is to simply accept that the alcove area may see some rough treatment should the design earthquake come to pass. The diaphragm and the walls may get a bit chewed up around the alcove. So be it. Lateral systems rarely achieve the level of load transfer perfection that gravity systems do. And that's just how it is. If we'd prefer to abandon architecture and just built boxes, that's another conversation. Lateral's very much an 80/20 thing in my opinion. If you've actually put numbers to your diaphragm and your drift, and you've put in your chord collectors to maintain diaphragm continuity, then you're light years ahead of the work that our predecessors did. That's your 80. Granted, a hard nosed reviewer from Sacramento may feel differently.
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: Re-entrant corner at alcove
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: Re-entrant corner at alcove
Dik