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Wood Wall Anchorage Questions

Wood Wall Anchorage Questions

Wood Wall Anchorage Questions

(OP)
I have a few questions as to how people typically transfer lateral loads from a wood wall into a wood diaphragm (out-of-plane loading) when you are at an end wall (floor joists, trusses, rafters running parallel to the wall). I'm mostly thinking about platform framing.

1. Does everyone detail blocking in the last joist bay between the last joist and the exterior rim board? Pretty sure you have to do this otherwise you get a hinge point at the joint between the top of wall and bottom of rim board.
2. What spacing do you use for this blocking and do you extend it for the first few bays or just the first bay? I think if you wanted the joist blocking for the next couple bays to be doing anything in the case of a wall pulling away from the building(wind suction), you would need to provide a tension strap connecting the rows of blocking. Otherwise, how does the load get to the rows of blocking past the first bay?
3. Do you provide specific nailing requirements for the sheathing down into these blocking pieces?

Thanks, and any other useful information related to this topic is appreciated. I'm trying to make sure I'm covered with what I show for detailing without going overboard on things that the framer will likely miss or complain about.

RE: Wood Wall Anchorage Questions

1. Last 1 or 2 bays depending on the magnitude of loading
2. 24" so it matches with the plywood screw lines. The sheathing will provide your tension tie? Realistically the blocking is there more for something to provide more screws, therefore a higher diaphragm strength at the start of it.
3. If I feel it's required. and when it comes to floors, glued and screwed only, no nails.

RE: Wood Wall Anchorage Questions

When the out-of plane loads are due to minor wind loads I call out for full depth blocking at 4-ft o/c.
When the loads are higher, as resisting soil pressure, or in a high wind speed area, I treat the blocking as joists, meaning I do calculations, and figure connections of sub-floor to blocking, and blocking to wall top plate below. I also make sure the aspect ratio of length of depth of the blocking is stable. If is not, I add blocking over two or three bays.

Eric McDonald, PE
McDonald Structural Engineering, PLLC

RE: Wood Wall Anchorage Questions

(OP)
jayrod12,

Why are you trying to get a higher diaphragm strength along this wall? Is it for lateral loads in the opposite direction (loads parallel to this wall)? Because I could see that as an added benefit since your diaphragm shears will likely be highest along this wall for that case. However, I'm interested in lateral loads in the opposite direction (perpendicular to this wall). I don't see a specific reason why you're diaphragm would need to be stronger along this wall for that case.

The reason I think you would need to strap the blocking going multiple bays for loading that pulls away from the structure is based on my understanding of what this blocking is doing. A reaction at the top of the wall goes into the blocking pieces at the first bay, and that reaction gets distributed into the sheathing through nails/screws. If you can get an adequate amount of fasteners in that first blocking piece, then I think the load will be distributed up into the diaphragm and you're good. If you need more nails/screws, then you can add another row of blocking and use that to distribute the remainder of the reaction up into the diaphragm. You would need to strap to this next row of blocking because it is acting as a drag strut. If the sheathing itself could work as your tension tie transferring the remainder of the reaction it would require that reaction is already up into the diaphragm and therefore the second row of blocking would be unnecessary.

Appreciate your input.

RE: Wood Wall Anchorage Questions

Not sure what I was thinking when I equated diaphragm strength to the tension tie feature. For normal wind loads, even high wind loads, I'd think you should be able to provide enough fastening in the first space. I've only provided it past the first bay at main floors resisting soil pressure, which is always in compression.

RE: Wood Wall Anchorage Questions

One thing nobody seems to consider when determining how many joist bays of blocking to use is the couple produced by the load coming in at the bottom of the block.
Let's say you have a foundation wall with 300 plf inward load on a 2x10 block that is 16" long. The uplift at the second joist is 300 x 9.25/16 = 173 plf - which is alot - especially for a sustained load from earth pressure. Even going back 3 bays gives an uplift of 300x9.25/48 = 58 plf - still a fair amount which can cause a hump in the floor. I have seen this in the field a few times in my career.

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