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pdev67

pdev67

Civil/Environmental
Sep 22, 2018
33
For 20'7" high tall using 20' long studs, Can we make it with 4 top plates connected together with sds screw ? or other options?
 
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Sure... can you make the building shorter or use slightly longer studs? The purpose of the double plate is to provide continuity of the wall at corners and intersecting walls. It also helps distribute point loads on the wall from floor or roof joists/rafters.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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I assume you're trying to use standard dimensional lumber not LSL OR LVL? I'm sure you could do that, but I'd still caution against this. Even the best dimension lumber at 20 feet tall will not be nice and straight. Do the finishers a favour and just go to LSL studs.
 
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Meh...

1) I agree with jayrod on LSL being the better solution owing to the waviness of 20' studs.

2) If this is an exterior wall, your shear transfer at the top of the studs will be quite eccentric relative to the top of the top plates.

3) Five plates total has the potential to introduce rather a lot of shrinkage into the system.

If you do this, I'd at least go two on the bottom and three on the top to balance the eccentricity out a bit.
 
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If this is an exterior wall a quadruple plate could be quite the cold bridge. While you can probably justify it from a technical standpoint, it seems like a silly exercise to make up for the available stock being a few inches too short.

If this project is in the design phase I second dik on trying to shorten the building. The potential shrinkage, eccentricity, and energy efficiency issues don't seem to be worth an extra 3 inches of ceiling height.
 
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LSL studs have limited capacity for shear wall design, so if this is a tall shear wall, you may have to reconsider your wall lengths to get sufficient resistance from the lateral system.

Edit: LSL has lower stiffness as well, so you'll need to check out of plane wind loads.
 
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KootK said:
If this is an exterior wall, your shear transfer at the top of the studs will be quite eccentric relative to the top of the top plates.
Click to expand...

KootK, can you explain this a bit more? Not sure I understand what you're indicating here.
 
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ChorasDen said:
LSL studs have limited capacity for shear wall design, so if this is a tall shear wall, you may have to reconsider your wall lengths to get sufficient resistance from the lateral system.
Click to expand...

I wasn't aware of that. Any chance you could point me to a document or something where I could find out more?

ChorasDen said:
KootK, can you explain this a bit more? Not sure I understand what you're indicating here.
Click to expand...

I see the SDS screws turning the stack-o-plates into a rigid-ish block which is peachy. But, then, to keep that block from rotating, one of two things needs to happen:

1) You need enough dead load to, effectively, prestress the joint. Plausible but a bit sketchy in my opinion.

2) You need to rely on your toe nailing to resist withdrawal loads. Also yuck.

C01_n8hxlq.png
 
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I think he means under face loads, the top of the studs is 4 x top plate thicknesses below the top of the built-up top plate where presumably the roof is attached, and the face loaded shear is taken out of the wall into your roof/ceiling diaphragm.

So you end up with some bending at the connection which for 1-2 top plates is typically ignored. But for 4 top plates it's something you might want to start thinking about reinforcing for with some straps front and back wrapping over the top plate to provide some nominal bending capacity out of plane of the wall.

If it's an option, I'd try going with 2 much higher spec engineered lumber top plates. Alternatively increase the wall thickness until a double top plate worked. You have not said what depth studs you've got for the 20' height.

Lastly putting in some steel here might be a more logical conclusion. If you're pushing the stud span, and top plate spans then the combined deflection of the wall system may getting up there.



 
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Yeah, Agent666 described my concern perfectly.

How do we feel about lap spliced studs these days? I feel that there is plenty to not love about them and I'm not sure how good a job they do of producing a straight stud anyhow. From a purely engineering standpoint, I feel that it's a workable solution.
 
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KootK said:
Any chance you could point me to a document or something where I could find out more?
Click to expand...

Sure, the only LSL stud manufacturer that I am aware of is Weyerhaeuser, as LP (who also supplied Boise Cascade) converted their mill to the production of siding, leaving the only player in the game as Weyerhaeuser.

Weyerhaeuser's code report is ESR-1387, table 5 provides the nailing and specific gravity limits for LSL studs, depending on stud grade.
Table_5_o3c0wi.png


KootK said:
I see the SDS screws turning the stack-o-plates into a rigid-ish block
Click to expand...

Ah, I see now, so you're concerned with out of plane loads, and the transfer of that load from the stud to the roof diaphragm. Makes sense, but I'd have to think on it a bit more, not sure I agree that it is a big issue, as I assume the exterior sheathing goes all way up to the top plate, unless I am misunderstanding?
 
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KootK said:
How do we feel about lap spliced studs
Click to expand...

Are you referring to (2) studs that are shorter than the wall height tied together somewhere mid wall height to achieve the tall weight requirements?

I don't see how you'd engineer it? Moment connections are notoriously difficult with wood due to fastener slip, unless you plan to design your wall with a large internal hinge somehow?
 
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ChorasDen said:
...not sure I agree that it is a big issue, as I assume the exterior sheathing goes all way up to the top plate, unless I am misunderstanding?
Click to expand...

1) Wind load is reversible, of course. So the sheathing only assists in one direction, unless you're laying claim to the drywall as well.

2) I wouldn't classify it as "big issue" either. It's just one of several things that I don't love about the situation that inform my overall distaste for it. A bit like how I don't super hate most hip hop songs but, at the same time, I've little use for the genre on balance.
 
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Fair point, an argument could be made that drywall may be effective (we assume it's effective for stud compression bracing), but I won't be the one making that argument.
 
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I've seen it done, but I don't like it. My previous employer had a "spliced stud typical detail" that I begged them to remove from the library...

In this case, it would go something like: (2) 14' long pieces of lumber with about a 4' lap. You put a cluster of screws at the top and bottom of the lap to resolve the moment and out of plane shear. Then put a couple rows of staggered screws between those 'clusters' to take the axial load. Blocking at the splice elevation to stabilize the joint and sheathing fastening to resolve the eccentricity.

Works on paper, but makes my skin crawl...

EDIT: you could vary the lengths to get the splice away from point of max moment, of course - and alternate high and low to improve continuity and reduce the chances of having a continuous hinge in the wall. Still not a fan, though.
 
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ChorasDen said:
Are you referring to (2) studs that are shorter than the wall height tied together somewhere mid wall height to achieve the tall weight requirements?
Click to expand...

Something like that. Maybe three studs for symmetry. Maybe special blocking to iron out eccentricities in the plane of the wall. Maybe two or three studs up the full height of the thing...

ChorasDen said:
I don't see how you'd engineer it? Moment connections are notoriously difficult with wood due to fastener slip, unless you plan to design your wall with a large internal hinge somehow?
Click to expand...

You'd make the moment connections out of two, well spaced shear connections to get around the fastener slip business. That's pretty common in joist sistering exercises.

Like I said, I don't love it:

1) Difficulty making the things straight.

2) Difficulty with finished to bear but joints if that's the program.

3) Thermal performance.

4) Impact on standardized insulation.

5)....

 
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phamENG said:
My previous employer had a "spliced stud typical detail" that I begged them to remove...
Click to expand...

Per our quarterly, industry bitch sessions... behind every cringe worthy detail there's usually a successful business man. These days, I tend to do less judging and more emulating. Hard ass KootK mostly only exists here now.
 
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Yeah...but this was a very specific "typical detail" complete with screw quantities...and no overall stud length dimensions. Dropped randomly onto a set of drawings it could cause some pretty serious problems.
 
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I'll be honest, I don't like splice detail for wood design. Fastener slip causes all sorts of analysis difficulties. It shifts the bending neutral axis (load doesn't go where we want or expect it to go, load wants to follow the stiffest path, and that is not necessarily the directly nailed shear connection). How do we get the deflection at the splice, I doubt we can we rely on small angle theory for deflection, so we get into some weird considerations with trying to justify the performance of the connection.
 
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I don't share that opinion. If one is going to disavow the use of simple dowel type connections for the transfer of shear and axial load, that rules out a whole lot of fun options. Are you opposed to wood trusses too? Like a lot of engineers, I don't love direct moment on a single fastener group but, even then, I'm up for it if I can scare up some testing to validate the Moment-Rotation characteristics.
 
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