Stud alignment in mid-rise wood buildings 2

[mike20793]

How do other engineers handle stud alignment with floor trusses/joists in 3-5 story wood buildings? Since most of my wood buildings use floor trusses and the truss manufacturers typically use 2x4 ribbon boards, I usually spec that the studs must align with trusses and so forth throughout the wall stacks. The double top plate can usually span between studs for one floor load, but any more than one floor, they don't theoretically work. Any thoughts? I'd really be interested in hearing from any west coast engineers since apparently this doesn't seem to be too much of an issue with TJI joists they like to use out west.

 

[FoxSE14]

In the past I have dealt with this by calling for a 2X header cut into the studs right below the top plates, depth = whatever I need to handle the stacked floor loads above. There's a myriad of ways to fasten to the typical wall studs depending on if this is an isolated misalignment, or a typical situation along a wall line.

Don't know how deep you want/need to go with your analysis, but I assume some of your lower level rim joists could also be counted on to help span loads across multiple studs?

 

[mike20793]

Thanks, that's what I've done in the past but we've gotten significant push back with the rim boards at party walls (truss bearing walls). They are difficult to install and get positive attachment to the trusses other than just toenails. There wouldn't really be an issue if I was running the trusses corridor to exterior wall, but that kills the podium slabs. I'm looking for more of a typical wall situation. Some contractors have not had issues (or not reported them), but a couple recently just don't want to line them up and complain when I say to block under the top plate.

 

[FoxSE14]

just don't want to line them up and complain when I say to block under the top plate

Easier for me to say right now: Give them a hearty laugh and tell them one of the two are a requirement unless they have a better idea. They can follow your ideas now, or they can follow them later when they are fixing cracked/failed members.

 

[FoxSE14]

You've sparked my curiosity (I have no experience with podium slabs) - are you saying that the heavy line loads that the corridor walls/exterior walls would impart on the podium will cause nasty issues with the slab (requiring thickness/rebar that is too costly/architect headache)? Is that the general idea? Makes sense.

 

[mike20793]

Yes, that's exactly the case. If you span between demising walls, the loads are distributed a little better and you can get a little thinner with your slab. The downside is you will have banded tendons (if post tensioned) under shear walls and they can struggle with holddown anchor placement.

Your original response was mine exactly: a hearty laugh. I never had any pushback until recently (my last 2 wood buildings). I brought up the issue to Woodworks and they said when you get something that works, let us know (they mentioned they get this question a lot in the Atlantic or Midwest but not really out west). I assumed it was because of TJI joists and the TJ rim board, but this is speculation. They did mention they are going to try and write a white paper in the next year or so about the topic.

 

[FoxSE14]

Great info - thanks for sharing.

 

[dhengr]

Mike & Fox:
Just to throw a couple more wrenches in the gears of our designs...
Non-aligned std. studs are bad enough in this multi-fl. situation, now try to justify jambs at openings if they are not aligned vertically. You do need a continuous, and competent, load path all the way to the found., and you do need to pay attention to shrinkage variations, cross grain vs. parallel to the grain. Secondly, your 2x header under the double top pl. might not be as effective as you think or wish it to be. When the cross grain shrinkage takes place in that header, you are left with a smaller stud end section, due to the header cut-out, causing compression perpendicular to the grain on the top pls. to be a design problem. I think you might be right about the TJI’s and manufactured rim boards. The TJI’s may need web/bearing blocking, but mostly the rim boards don’t shrink vertically, so they act as a better beam to distribute concentrated loads, without settlement issues. We probably don’t give enough credit to rigid sheathing (maybe even sht. rk.) in its ability to help with this redistribution of concentrated loads. It just helps even out the top pl. deflection, or load distrib., to several adjacent studs. We just don’t know how to apportion all of these small redistributions which seem to make the whole system work. But, it does. One significant life saver is the large FoS which we apply the allowable wood stresses, because of its strength variability. Then, once in awhile we find a few pieces of wood acting together which actually approach ‘fully stressed,’ and we get to fix some sht.rk. cracking, etc.

 

[FoxSE14]

I agree dhengr...perhaps if a let-in header is the chosen route, an LVL/PSL or other engineered wood is a better choice because there are no shrinkage concerns with those types. I've got a project on the books now where I will make the change per your recommendation - thanks!

 

[mike20793]

Good point about the shrinkage in the plate causing slack in between it and the header. It seems like it's easier for the contractor to put additional studs or stud packs in rather than trying to install the header, especially if there is several in a row. TJI's cause trouble with HVAC ducts around here; that's why they have all been trusses. Even though the TJIs can have holes cut in the webs, its extra labor and another step they have to think about in advance.

I would add in that the contractor on one of the projects allowed his framer to cut holddown anchors because "they were in the way of the sill plate"

 

[dhengr]

Mike:
Fox gets my point on the let-in header. I meant the 2x header shrinks in width and pulls down, away from the two top pls., which are now sitting on a smaller stud end in bearing. And, the top pl. now has to deflect before it starts loading the header. Although, the two top pls. will shrink cross-grain too, but more or less as a double thick unit. Be sure that they do a good job on those let-in cuts, so the top of the stud and the header are in the same plane under the top pls. The double top pls. are there for the bending strength btwn. studs, just what we’re talking about, and to provide longitudinal continuity down the length of the wall and at corners; staggered butt joints, spacing of these joints and lapped at the corners, etc. If I were using the let-in header on all the bearing walls (all walls in one direction, some consistency in plan orientation, for the framers) I might consider the header and only a single top pl. Stagger their joints for continuity and maybe some straps around the corners to what would likely be a double top pl. condition.

 

[FoxSE14]

All this is jogging my memory a bit. Around two years ago on a remodel job we found they had a single 4x6 instead of double top plate. I had never seen that before, but Mike it could just be the thing that solves your issue and keeps everyone happy. (If it's not too late)

 

[mike20793]

I guess I did misunderdtand. I make them install the header vertically, not flat, under the top plate with hangers. There won't be any real shrinkage except in the double top plate. It takes more than just an extra flat member to get it to work for 4 or 5 floors of stud loads. These are usually issues in 2x4 walls so the only thing I've got going for me is there are typically 4 studs every 16 inches so the clear span of the top plate is only 10 inches.

I'd like to push towards the rim board option but I don't know how you get any real positive attachment of the rim board to the trusses. I know a deeper ribbon has been discussed here before but I've been told by a truss manufacturer those are only used to align and prevent rotation of trusses. I'd hate to use it since I don't know how they would splice it.

 

[mike20793]

Maybe it's just a contractor competency problem and aligning everything isn't really very difficult to do. I've only had push back on my last 2 projects but had no issues on the previous dozen or so.

 

[XR250]

I'd like to push towards the rim board option but I don't know how you get any real positive attachment of the rim board to the trusses.

You really don't need any as the top plate will only see un-aligned loads from one floor of trusses. The load from above will go into the rim board and get distributed out evenly to the studs. You could always add hangers to the trusses if you are worried about it. Make sure you use a rim that actually has spanning capbilities such as an LSL or LVL member.

 

[mike20793]

I get what you're saying but if you look at the LSL rim board installation guide it requires more than toenails. Not to mention shear walls need the positive attachment to do their thing.

 

[RFreund]

So you are saying that it is an issue to align the trusses at a common bearing wall? I am having a hard time visualizing the problem. Is it because there is not enough room for both joists to bear on the wall? Can you make one truss top chord bearing and the other bottom chord bearing so that the top chord bearing truss bears on the bottom chord bearing truss? This will require blocking between trusses to transfer shear forces.

EIT

http://www.HowToEngineer.com

 

[mike20793]

RFreund,

With floor trusses and a standard 2x4 ribbon board, trusses need to (or should) bear directly on a stud pack. The top plate can span between studs for one floor's load, but if they line up for multiple floors above and are off on one of the lower floors, then the top plate won't work for the build up of load. Rim board can be used to evenly distribute the loads from the walls above, but there is concern about proper installation at party walls. I was just curious how other engineers handle it. Making them align all the way up in tilt up wood walls requires thinking about truss locations ahead of time and that's something framers are not good at.

 

[XR250]

What is the issue at party walls? One sided connection of rim board to the plates?

 

[mike20793]

Yes, if you look at the installation requirements of the rim board, it requires nailing through the rim board into the ends of the trusses or joists. Easy to do for one wall, but hard for the other. There's also an issue with the attachment if you're using the rim board in a shear wall to transfer loads.