Re-entrant corner at alcove

[Gumpmaster]

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.

 

[dik]

Any plan drawing/sketch?

Dik

 

[mike20793]

Why not put a joist at each end of the alcove and let it function as a drag strut? I usually look at these types of cases as semi rigid so those walls don't suck up an unrealistic amount of lateral load.

 

[Once20036]

What's your edge condition look like at the walls where you want to resist lateral forces/your end walls? Do you have an edge joist or do you support the diaphragm on an angle attached to the wall?
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.

 

[Gumpmaster]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1509578065/tips/Alcove_z0pdtd.pdf[/url]

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.

 

[dik]

Maybe something like:

Dik

 

[Gumpmaster]

dik, the problem is, at least with a flexible diaphragm, is that your shear walls at the alcove take a large percentage of your building shear (SDS=1.0) and that just can't be handled with such short wall lengths. I'll have to check out what the forces are with a semi-rigid diaphragm.

 

[haynewp]

The alcove creates re-entrant corners in the diaphragm that can tear open and therefore needs collectors. NEHRP and FEMA have documents on this.

 

[haynewp]

Also, I don’t see the alcove walls realistically taking most of the shear.

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.

 

[KootK]

One approach is to use pseudo capacity design principles as follows:

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.

 

[KootK]

And I contend that a good structure today is mostly the same as it was three hundred years ago: a bunch of stuff thoughtfully tied together. That, as opposed to a bunch of stuff deliberately disconnected for arcane theoretical reasons.

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.

 

[dik]

I agree with haynewp... the alcove is proportionally too small to offer significant shear resistance.

Dik