Wood bearing wall with trusses

[milkshakelake]

Need help with two details. Feel free to mercilessly tear them apart. My concerns:

1. For the interior bearing wall, I'm putting 3 pieces of solid blocking. I'm wondering if it's necessary. I want to transfer shear wall loads from the shear wall above. This detail occurs in 30ish places and these walls are 4 stories high, so I want to reduce solid blocking for cost. The other reason is for stud wall bearing. But maybe a single piece of solid blocking in the middle (for non-shear walls) and 2 pieces of solid blocking (only at shear walls, not the whole length of the wall) would be enough. Also concerned about eccentric loading if I use 2 pieces of solid blocking, but like my exterior wall detail, I think it's pretty common practice.

2. For the exterior bearing wall, I'm using the top plates as the collector/drag strut with 2500# force. I'm wondering if a rim truss is capable of transferring the load from the sill plate to the top plate, and if I'm missing something in the load path. And if I need shear clips, like Simpson LTP4.

 

[txeng91]

Most of the details I see use a continuous LSL rim board like you have shown on the interior section, which they also use at the exterior wall with A23 clips to the top plate. I typically just use a 2x band joist at the top of the truss on the exterior walls, which you can then use to transfer the shear out of the floor and into the wall sheathing. You could do this same thing at the interior wall and just spec blocking panels or LSL blocking over shear walls. I don’t really see the need for using full depth rim joists with metal plated trusses, they basically have studs built into them at the bearing locations, so there’s no web crippling issue like you would have with I-joists. Maybe it’s to compensate for shrinkage? But I wouldn’t think that would be an issue with floor trusses.

You can also check out this framing system a firm out of Houston developed. They use top chord bearing trusses to reduce the need for blocking.

http://www.scaengineers.com/pdfs/Wood-Framing-Brochure.pdf

Edit: I don’t really do midrise, mostly just residential, so I’m now realizing that continuous rims are probably needed to transfer high axial loads from the studs above since they’re not always aligned with the trusses, so probably a good idea to keep it. To answer your 2nd question. The load could technically be transferred through the sheathing but it’s common practice to use the angle clips.

 

[milkshakelake]

@txeng91 Thank you, that's pretty helpful. I changed it to continuous rims and added the angle clips.

Like you said, the studs aren't lined up so I'd rather not use a band joist. The truss manufacturers try to push that design but I have a hard time getting the bottom plate to work for the axial load, even for a 2 story building, so I avoid it. I'm also not sure about the load path; it would rely on the band joist and bottom plate being spliced properly to act as a diaphragm chord and collector.

I like that type of system a lot, but the contractor fought against it and I went back to platform framing. Something about his guys being used to conventional construction. (shrugs)

About the interior section, would I be able to put blocking on one side (shear wall side) and middle only?

 

[KootK]

1) On some projects, the team has deemed it desirable to build an upset into the truss bearing such that 2X material can be used for the interior rim rather than LSL. It may be just my market but contractors here seem to lose their composure over the need for the engineered lumber rims. Frankly, I don't get it but, if this is an extensive condition for your, this might be worth throwing on the table as a cost savings measure. A supplier once told me that the deep LSL's at the interior conditions tend not to get fastened well because the installers are working from the top and have a hard time reaching down to do it. I don't understand this either as I would have expected the connection to be made working from below. Who knows, maybe they were just trying to frighten me into deleting the LSL.

2) With regard to the truss band joist, and blocking in general, I find that to be a tricky issue. As you said, you often do get bearing issues. That said, a lot of folks don't seem to be deterred by that. And a perennial argument here on the forum is how much we actually care about wood bearing failure given that it's a deformation limit state only.

3) What makes sense for your exterior detail will be, in part, a function of whether or not you'll be using prefabricated walls and how sheathing will likely be arranged at the joint.

4) For your interior condition, I'd be inclined to delete all blocking other than the central LSL. In fact, I'm not sure what the extra blocking actually does for you unless the intent is to help with perp-to-grain bearing issues. I'd expect that you'd only need solid blocking at your shear wall boundary posts. The straight shear in your wall should be able to travel the path: sheathing above --> sill plate above --> LSL --> top plate below --> sheathing below.

 

[XR250]

Why not specify top bearing trusses and make the walls taller?

 

[jayrod12]

hy not specify top bearing trusses and make the walls taller?

Then you couldn't use the pre cut studs.. contactors would freak out in my area if I made them custom cut every stud.

 

[milkshakelake]

@KootK: Thanks for the reasonable and thorough reply, and for sharing your own details.

1) I'll keep this in mind for the future. I'd be concerned about shrinkage of the 2x material, which is the main reason I like engineered rims.

2) Another failure mode would be combined axial and bending. The band joist is spanning between the trusses, thus taking a concentrated load from the stud. It can also take axial loads from being a diaphragm chord. I guess it could work; thanks for the perspective.

3) Makes sense. I'll modify it if they're using prefabricated walls.

4) I was concerned about transferring shear from the LSL rim joist to the top plate. My detail has a single stud wall, so there's little space to attach the Simpson shear clips. The shear clips are 1.5" wide and my bearing area is about 2", so I'm worried about splitting the wood. So I'd need the blocking to attach the Simpson shear clips; I'm not too thrilled about using toe nails to transfer shear. But you're right, I definitely don't need it on the non-shearwall side.

@XR250: I prefer that, but contractor wants to use platform framing.

 

[bhiggins]

Would it be possible to have a pre-engineered drag truss at the wall instead of blocking? This might be easier to install and fasten than blocking. The downside is you would need to use hangers to fasten the trusses.

 

[KootK]

I'd be concerned about shrinkage of the 2x material, which is the main reason I like engineered rims.

Yeah. On the other hand, you have two or three shrinking 2x's in the truss so one could almost say that a 2X rim is leveling out a discrepancy that already exists.

The band joist is spanning between the trusses, thus taking a concentrated load from the stud. It can also take axial loads from being a diaphragm chord.

With that system, I'd still be telling the story that the shear makes it's way down to the double top plate and that is the chord. Of course you have the usual issue of whether or not loads and structures do as they're told (they don't). Unfortunately a rigid adherence to that with wood structures will drive one insane.

I was concerned about transferring shear from the LSL rim joist to the top plate. My detail has a single stud wall, so there's little space to attach the Simpson shear clips. The shear clips are 1.5" wide and my bearing area is about 2", so I'm worried about splitting the wood.

If it's a 2x6 wall and truly 2"+, I'd not bat an eye at the A35. For a 2x4 wall I agree, it's a problem. That said, I do toe nails all the time. I fear that with blocking and LPT4's, you may not look favorable relative to your competition. Perhaps you can get away with a bit more if this is more of a commercial setting. You can also get a way with more in NYC where property values tend to utterly dwarf other concerns. I'm not sure about this but some contractors may not like having to install the LPT4 prior to the gypsum.

 

[milkshakelake]

@KootK Again thanks for the input, it's all very much appreciated.

About toe nails, if the calculations are working, I guess I can specify them. I shy away from it because everyone is using LTP's and A35's these days, so there must be a reason. But I just need to have confidence that my calculations are correct.

About the band joist, I might be nitpicking here, but I don't see the load path between the band joist and the top plates. I'm not sure what to think about in real life. But theoretically, the load path isn't well connected. The diaphragm shear loads the band joist -> which loads the plywood -> which loads the studs and top plates together. So it's like the plywood is transferring the shear with intermediate nailing. I don't think much of the load would go through the truss end bearings because those would twist.

@bhiggins I use those when the drag truss is parallel to the floor truss. If it's perpendicular, the hangers would make it too expensive.

 

[txeng91]

I fear that with blocking and LPT4's, you may not look favorable relative to your competition

As I said, I’m not in the mid-rise game but I’ve done some QC inspections and probably have about 10 or so plans I’ve collected. Most everyone around here uses deep prefab floor trusses to allow HVAC to run through. I usually see either the LSL or prefab blocking trusses, but either way I almost always see the Simpson A clips for the connection to the top plate. I kind of like it from a QC/QA perspective, you can see the clip and you know the connection is there. Also I guess engineers may just be generally deterred by using toenailed connections in anything they’re explicitly detailing due to the natural “jankyness” of the connection. The same reason everyone specs hangers on hangers for ceiling joists, even on short spans spans that could easily be taken care of with a couple of toenails. Obviously there’s going to be a variance between NDS spec for toenails and how they’re actually installed. The only guarantee is that they will use at least 5 times as many toenails than needed. Whether they’re creating the strongest wood to wood connection in existence or ripping the wood to shreds and making the connection useless is up in the air. I swear I’ve seen 2x12’s tacked into place with no less than 20 nails waiting for the next guy to come around and put the hanger in. People love to shoot nail guns, that will never change.

All that said I don’t really have an issue using them for wind loading. Not many engineers are going around using clips from studs to top plates, which really isn’t that much different, except no one cares enough to detail it. My take is they’re fine with relatively low diaphragm shears, especially at exterior walls where the sheathing is tying everything together anyway. When the shears get to the point where your closer than 4” on the toenail spacing, then I would switch to clips.

 

[txeng91]

Milkshakelake, sorry just saw your post. Your right on the load path for the chord forces, and maybe there’s some wind loading case that would add unaccounted for shear in the sheathing to top plate connection in some locations by transferring the chord forces through the wall sheathing. Usually once I start getting to this level of analysis on load paths in conventional wood framing, I remember it’s a bottomless pit that will eventually drive you insane as KootK alluded to. There’s been times when I focus so much on load paths for wind loading that I’m not paying attention to other things that are way more likely to be an actual problem. If you’re in high winds or in production work for mid-rise where’s load paths are simpler and things are more repetitive, I would applaud your attention to detail. But when I’m doing more custom buildings outside of high wind areas, I usually end up going with whatever keeps the construction as simple as possible, even if I have to have to overkill some things and make engineering judgement calls in lieu of precise detailing and analysis of every connection. So in this case, assuming the axial loading from offset studs isn’t an issue, I’m fine with specifying edge nailing from the wall sheathing to the band joist and the top plate, which should take care of it, but I wouldn’t blame you for going with the rimjoist and clips either.

 

[milkshakelake]

@txeng91 Yes, I agree. And now that I think about it, if the walls are panelized, there's usually a separate piece of plywood between the top plate and the band joist, which would take care of the shear. If it's not panelized, your way would still be okay; like you and KootK said, I'd go crazy if I try to overthink it. I'm just concerned about a shear failure in a mid-rise. Historically, we know a lot about failure points of single houses, but wood mid-rises are new enough that I've read almost nothing about their lateral failure points and things like drag struts. But I know what to do now. Thanks for the advice.

 

[KootK]

It's always good to consider your options but, given the scale of your project, I think that I would personally be doing the LSL rim, both interior and exterior. In a house I feel that most of this stuff is unwarranted BS but, in a more commercial structure, you'll have higher loads and probably a lot less redundancy of load path. As wood sort of goes "commercial" on us, my concern for lumber splitting at 2X etc gets amped up.

I don't recall, is your interior wall 2x6? In that case, you've got a 2" ledge for your 1.5" A35 clips and I'd not bat an eye at that. I think that ditching that extra layer of blocking will go a long ways towards normalizing the interior detail relative to your competitors.