High out of plane seismic wall anchorage to wood roof

[strucbells]

Designing a small CMU building with a wood framed roof in a very high seismic area. Sds = 1.68g, Ie = 1.5. This is an essential pump station facility, hence the higher importance factor. Using 8" fully grouted CMU, the out of plane wall anchorage force is really high, around 1.25*Wp.

Typically we are dealing with Sds = 0.75 or less so our usual detailing practices are coming up short. I'm also more experienced in steel and concrete than wood, so I may be missing something obvious.

I can get the load from the top of the CMU (raked bond beam) through the anchor bolts and the 3x top plate, but getting the load into the outriggers from the top plate is a challenge. Usually we specify simpson A35 clips between the top plate and the outriggers but this results in a ~10" outrigger spacing which seems too tight. Beyond that, we use coil straps to get the force from the outriggers into the roof diaphragm and the rest of that load path and subdiaphragm design works fine.

Should I be looking at alternate detailing here? Is 10" or less spacing of outriggers normal practice in high seismic regions? Anyone have standard details or cartoon sketches of one they can share?

 

[dold]

Try looking at simpson PA/HPA/PAI/MPAI straps. These avoid crossgrain bending in the ledger, but it sounds like your purlins might be on top of the wall as opposed to framing into the face of the wall to a ledger? Not sure what you mean by outriggers? Are you talking about purlins/joists? Care to share your current detail?

 

[strucbells]

We have a roof overhang of 2'-0" typically so the MPC wood trusses in the other direction and the outriggers shown need to extend beyond the CMU wall, so it's not a ledger condition.

Detail is a little rough/in progress but looks something like this:

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1714757772/tips/outriggers_wusf7v.pdf[/url]

 

[driftLimiter]

Could also look at embedded anchors with holddown hardware to the anchoring roof members.

This reference has the workflow your trying to accomplish and demonstrates anchorage very well.

Link

 

[Celt83]

Make sure you are adhering to SDPWS 4.1.5

 

[strucbells]

@Celt, yeah it's a one story building and I am meeting the subdiaphragm aspect ratio and load path requirements. the coil straps (nailed to the outriggers + blocking beyond that as reqd to reach the depth needed) will provide the continuous ties to the subdiaphragm chord.

Decide to just go with tightly spaced outriggers (with sim blocking beyond to the subdiaphragm depth). Looks a little funny to me but maybe this is normal in super high seismic with wood roof and cmu walls.

@Driftlimiter I have a slightly older version for the 2018 IBC but haven't reviewed it in a while, is there any particular example in there that you are thinking of? I briefly looked at running straps embedded in the CMU (the product dold linked to) to the underside of a deeper piece of blocking nailed to the outriggers but this seemed pretty ugly.

 

[driftLimiter]

How about some heavier clips than the A35s? HGA10 comes to mind the LS series as well has more capacity.

Section 5 of the Example 4 in that document is what I was referring to. Since your bearing on top of the wall and trying to take the load out there it is a bit different.