Shear Wall Blocking
This may be a stupid question so I thought I would ask here before asking framing inspector or an engineer. I’ve asked plenty of stupid questions here, but even more of the engineer and inspector.
Blocking the edges of sheathing in a shear wall is to support the edge and transfer edge forces to the framing, correct? So is there any reason why you can’t add the blocking to the outside of the sheathing, and use structural screws or ring shank nails to secure the blocking to the plywood? Using 3 1/2″ nails to fasten blocking through the sheathing to framing puts 1.5 inches of nail into the framing, which is same as nailing flat blocking to 2x studs inside the stud bay.
I ask because we are finally able to reside our house (which was built with siding only, no sheathing of any kind), and I am having the contractors sheath the house, and the specs call for flat blocking at all unsupported edges (we are in the PNW). The walls are 9.5′ tall, so there will be blocking unless they use 10′ tall sheets. The issue is that the walls are not uniformly 16″ on center due to original construction and sistered framing for areas that had rot. You have to measure and cut varied lengths of blocking for each bay. At least that was my experience for the areas I repaired, and I am sure there will be more repairs.
The wall will be externally insulated with trusses and dense pack cellulose, so it would be much faster just to run lengths of 2×4 over the joints and fasten. The truss design is such that the external flat blocking won’t affect the trusses.
Thanks
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Replies
David, blocking the panel edges is to keep them from buckling under load. I think it would work fine to use continuous blocking on the exterior side of the sheathing, but because it's a non-standard detail, you might need a licensed engineer to sign off on it to make the building inspector happy.
David,
I don't see anything in the language of the BC Building Code on shear-wall blocking that precludes it - probably because they didn't anticipate anyone wanting to do so. The only complication I can think of is how you would fasten the sheathing from the outside? You can't nail through the blocking first, then the sheathing. It has to be the other way round to meet code.
Ok, thanks Micheal and Malcolm. If code requires nailing through the sheathing into the blocking, then this won't work. Not going to be able to access the back of the sheathing, as we are not touching the plaster walls. I was curious if screws would work instead as the threads prevent the sheathing from separating from the blocking. Or maybe some sort of anchor system, but that starts to become even more of a pain than cutting the blocking to variable sizes.
David,
Unfortunately the code section includes prescriptive requirements for both the type and length of fastener. As a general rule you can't fasten something thick to something thin.
The blocking needs to be between the studs and have the nail heads against the sheathing. That's how the assembly's have been tested and code approved. https://bayarearetrofit.com/testing-shear-walls/
Once the crew get going cutting custom blocks get less painful.
Yes, it's just the estimator didn't include it in the price quote, so trying to find ways to not increase costs once I point it out to them ( I only like pointing out issues that cut costs). I could add the external support after they sheathed, I just wasn't sure if screw threads where sufficient to hold in the plywood or not. Or if structural screws are even allowed.
No offense to your skills, but experienced carpenters should have no problem cutting blocking to different lengths as they go. There's a few ways to do it quickly.
There are structural screws code approve for floor and roof diaphragms. I don't know of any approved for shear walls.
In new construction tall sheathing is used to be more efficient. Sounds perfect for your project.
Yes, good idea. Thanks
David,
Backing up a bit. There is no requirement to bring your existing house up to current building code seismic standards. The addition to shear-wall design calling for blocking at all edges of sheathing just came into affect in BC a few months ago, so no houses built before that are done that way. If you don't want to block between the studs you don't have to.
I know codes can be very local, and vary in adoption and enforcement, so before ignoring it and just doing what you feel like, you should definitely get an official ok. It's no fun to undo work after the fact.
Besides the code aspect, when it's a structural and especially seismic issue it's better to err on the side of caution. We're overdue for a good shake in the PNW/BC and if it's anything like what the big 300yr quakes have been I'd certainly want to ride it out in an "overbuilt" house instead of a code minimum or historic building.
Yes, good point.
Andy,
I'm sure not advocating d0ing un-permitted or non-code compliant work. I was just pointing out that we have had seismic requirements here for about seven years now, and blocking only got added to them recently. So while it's an improvement, I don't see it as something extremely critical.
I can guarantee David's existing house is full or things that don't meet current codes. Any house built before last year wouldn't pass inspection now. That doesn't necessarily make them sub-standard in any meaningful way.
Thanks Malcolm,
The engineer did specify blocking panel edges for the new addition we are putting in, but I'm not sure about the rest of the house. We added a foundation a few years ago (to replace the rotting post and pier supports), brought up to the seismic codes in place at that time, so I am fine staying within current code when we can. Was just trying to simplify if it was allowed.
The deflection for unblocked shearwalls is calculated at 2.5 times a blocked shearwall. So, the blocking is important to limit deflection of the structure. Deflection will cause damage.
Did the plans also require the installation of hold downs?
This may be obvious, but which axis of deflection are we talking about?
Yes, when we installed the new foundation, it was brought up to code. Engineer called for Simpson STDH14 and HDU8 hold downs. Same for anchor bolts. When sheathed house will be up to 2015 code.
That 2.5 times is for the inplane deflection. It is what was used in the older code documents. In the new AWC seismic provisions (https://awc.org/codes-standards/publications/sdpws-2015) It is more complicated see section 4.3 page 25
In plane is the loads from the floor or roof diaphragm transferred to the walls. Out of plane is wind loads .
I don't like the strap tie downs. I don't like how they are exposed to the elements, make sure to use liquid flashing to cover the thin sliver of steel where it exists the concrete and goes behind the WRB.