Raised heel attic truss alignment with wood studs 2

[StrEng007]

I have a raised heel attic truss that is going to support upwards of 30 PSF for storage loading. I would like to sit the truss on top of the double top plate and sheath the wall full height (wall stud and truss heel).

I've already checked my 2x6 studs and they work (very close to unit) at 2ft o.c. assuming the stud is fully braced against y-y axis buckling.

The questions I'm trying to answer are:

1. How important is it that my studs align with the trusses? I'm used to 16" o.c. studs, but the 24" o.c. seems to make more sense if it allows the wall panel edges to align the full height of the stud.
2. Should I switch the roof truss over to 16" o.c. instead? I've got a 24'-6" clear span with an intended load bearing partition at midspan. Doing so would alleviate the close to unity wall stud but add 8 trusses to the package.
3. Will the truss manufacturer be able to make use of this load bearing wall (at center) without the introduction of a vertical web?
4. Is it correct to assume that my wall studs are braced full height against y-y axis buckling (axial load) if I have wood structural panel sheathing on the exterior side only? Or should this unbraced length be limited to the blocking height of the stud?
5. Same thing for unbraced length due to out of plane flexure (wind load). Is my Lux considered unbraced for NEGATIVE wind if the interior stud was left unfinished (no sheathing). Is mid-height blocking good enough to cut this span in half?

 

[KootK]

1. How important is it that my studs align with the trusses? I'm used to 16" o.c. studs, but the 24" o.c. seems to make more sense if it allows the wall panel edges to align the full height of the stud.

It's obviously better structurally if the studs can line up with the trusses. We call that "advanced framing" here. That said, it is also much more common here to not align the studs with the trusses but, rather, let them transfer through the top plates. And, of course, this is made possible by not actually checking the top plates or by telling some wildly aggressive story about how the sheathing helps, splices occur over studs, etc.

2. Should I switch the roof truss over to 16" o.c. instead? I've got a 24'-6" clear span with an intended load bearing partition at midspan. Doing so would alleviate the close to unity wall stud but add 8 trusses to the package.

I wouldn't. That's real $$$ and I would expect some real complaining to be followed by replacing you with another engineer on the next job.

3. Will the truss manufacturer be able to make use of this load bearing wall (at center) without the introduction of a vertical web?

Meh, I tentatively vote no. The middle support won't help with the overall truss action of the thing. It will help a great deal with the bottom chord sizing which is, effectively, a fix-fix floor joist. However:

1) When you consider the realities of camber, truss uplift etc, it may not be the case that the BC will actually be in contact with that interior bearing unless you intentionally build the interior wall a tad high to compensate.

2) This will put the bottom chord in hogging moment in the middle, necessity some LTB bracing. This probably isn't a big deal.

I'd just let them see if they can achieve what you want with a 2x10 or 2x12 MSR bottom chord. If not, then start considering more exotic stuff.

4. Is it correct to assume that my wall studs are braced full height against y-y axis buckling (axial load) if I have wood structural panel sheathing on the exterior side only?

I believe so. I've always found that a bit aggressive but whatever.

5. Same thing for unbraced length due to out of plane flexure (wind load). Is my Lux considered unbraced for NEGATIVE wind if the interior stud was left unfinished (no sheathing).

I don't think so. Although I can see how it would still take the "lateral" out of LTB.

Is mid-height blocking good enough to cut this span in half?

I believe that it is.

 

[RontheRedneck]

A few thoughts and opinions from the perspective of a truss designer:

I've often wondered why more people don't put studs 2' OC under trusses in general. Less lumber used, and more space within the walls for insulation.

Setting the trusses 16" OC will increase the cost of the trusses by roughly 40%. (Educated guess based on experience) And there's no real benefit to it IMHO.

A 24' 6" attic truss is a rather simple design. Nothing to get exited about even if the center bearing was not there.

That little kingpost above the collar ties is a waste of money that does nothing in most cases. (Unless the collar tie is over 10' long)

An attic truss does not need to have a web at a center bearing in an attic truss. But the added bearing will make the floor much stiffer.

No way would I ever design an attic truss with a 30# live load. People regularly do stupid things. God knows what they might put up there. I never use less than 40 PSF.

Running plywood up the ends of the trusses looks good on paper. But getting the contractor and truss manufacturer to agree on just exactly how far apart the walls will be (If the concrete is even poured right) is not always a given.

I always insist that the builder have a plan to put stairs up into the attic. Too many of them think it's O.K. to cut a big section out of the BC and header it off to adjacent trusses.


O.K. - Rant over.

 

[StrEng007]

1) When you consider the realities of camber, truss uplift etc, it may not be the case that the BC will actually be in contact with that interior bearing unless you intentionally build the interior wall a tad high to compensate.

An attic truss does not need to have a web at a center bearing in an attic truss. But the added bearing will make the floor much stiffer.

That being said, should I intended for this wall to be a non-load bearing partition? Or will the truss designer want to use it's gravity capability? I ask because it changes the load along my top plates, and believe it or not, has an impact on the counteracting dead loads for wind uplift considerations (this one is at 165 MPH).

I don't think so. Although I can see how it would still take the "lateral" out of LTB.

I got mixed up by your response. Let me ask the question in another way... assume there is no sheathing at all and the wall stud is bending out of plane. Is solid blocking at mid-height of stud enough to cut the Lu by 50%?

Section 3.3.3 doesn't really say if full depth blocking is considered to effectively provide "lateral support to prevent rotation at intermediate points" along the beam length.

 

[StrEng007]

That said, it is also much more common here to not align the studs with the trusses but, rather, let them transfer through the top plates.

I just checked the double top plate. The only way I can comfortably get it to work is to indeed allow that bearing wall to pick up gravity load.

 

[XR250]

I've often wondered why more people don't put studs 2' OC under trusses in general. Less lumber used, and more space within the walls for insulation.

My siding guy tells me Hardie Plank gets wavy at 24" O.C. stud spacing. I thought about doing 24" on my recent house build.

 

[gte447f]

Generally, sheathing on only one side of wood studs is considered adequate for continuous weak axis bracing of the studs for studs up to 2x6. This is buried in the NDS somewhere. I'm not sure if the NDS includes the part about "up to 2x6", and I'm not sure if the NDS explicitly addresses just buckling under compression parallel to grain, or if it also addresses lateral bracing for flexure, but in practice, I think it is generally assumed to be adequate for both scenarios.

 

[gte447f]

Don't space the roof trusses at 16". You will be a pariah if you do.

Don't space your wall studs at 24". You will be a pariah if you do.

Given the above two statements, it obviously isn't important at all that your trusses align with you wall studs, although in a perfect world we would all like for that to happen.

Use a healthy dose of creativity if you decide to check the top plates for bending.

Don't use the interior wall as a load bearing wall, and let the truss designer do his/her thing to design a simple span. Know in your heart that in reality the wall will probably provide center support and result in a nice sturdy attic floor, and sleep well knowing this. Provide a foundation element (e.g., thickened slab) under the interior wall, even though it is not supposed to be a load bearing wall.

 

[XR250]

Don't use the interior wall as a load bearing wall, and let the truss designer do his/her thing to design a simple span. Know in your heart that in reality the wall will probably provide center support and result in a nice sturdy attic floor, and sleep well knowing this. Provide a foundation element (e.g., thickened slab) under the interior wall, even though it is not supposed to be a load bearing wall.

I disagree with y'all about this. Like RTR said, It will make the trusses cheaper and take some load off the already sketchy top plate so why not use it intentionally? I don't buy that truss camber/uplift could keep the bottom chord from bearing. I think I have seen this occur less than 5 times in my 30 year career. No one around here does any special detailing on trussed roofs where they cross interior partition walls and it does not seem to be an issue.

 

[StrEng007]

I understand the conflicting positions on how to treat the interior wall. Since the wall is available and allows me to get my top plates to work, I'm going to move forward with this as a load bearing wall. I'll mark it as such on the plans, provide the right system for it, and coordinate with the truss manufacturer. Since I haven't encountered this before, all I can do is make my assumptions and move forward until I get to a point where additional coordination is needed.

Unfortunately, there isn't a whole lot of "coordinate with the truss engineer" before hand. I have to handle a lot of this stuff after the truss engineer is selected and the drawings have already been issued.

Regarding the "advanced" framing system. Since this job is located where very few contractors stick frame buildings, I'm going with a conventional approach. Roof trusses at 24", wall studs at 16", make sure my load gets to foundation. I feel like moving the studs to 24" invites problems and the potential for a longer top plate when the GC misses the concept that trusses and studs should be aligned.

 

[XR250]

I get the whole "advanced framing" craze. Single top plates with everything aligned. Theoretically, it makes sense. It has not taken off here and I'm glad. If I live in a wooded area, I certainly would rather have a double plate spanning 16" than a single spanning 24". I have seen enough trees stopped by a double top plate that I'd rather not take the risk. Another reason I went with 16" O.C. studs on my build.

 

[Eng16080]

I just finished structural plans for a small garage last week with trusses at 24" o.c. and wall studs also at 24" o.c. directly aligned below. I still use a double top plate, though, despite the alignment (assuming it's constructed correctly). I didn't read all the comments above, but I generally think a single top plate is a bad idea (if that was suggested).

I will say, though, for a house (especially higher end), 24" o.c. stud spacing might not be a good idea just in terms of attaching interior and exterior finishes. And in general, it's probably safer to design the wall in such a way that the truss could land anywhere on the top plate, with the top plate designed accordingly (possibly even 3-ply if necessary).

 

[RontheRedneck]

XR250 said that adding a center wall will make the trusses cheaper. It might require smaller plates at some of the joints. But plates are a small percentage of a truss cost. On small attic like this one I'd say it would make virtually no difference.


StrEng007 said ..."coordinate with the truss engineer"...

We're truss designers - Not engineers. I covered that in my "Truss design misconceptions" thread:

https://www.eng-tips.com/viewthread.cfm?qid=494551

I wish more architects and engineers would talk to us earlier in the design process. It can save a lot of headaches in the long run.

 

[XR250]

XR250 said that adding a center wall will make the trusses cheaper. It might require smaller plates at some of the joints. But plates are a small percentage of a truss cost. On small attic like this one I'd say it would make virtually no difference.

Ron,

Seems you could go to a smaller bottom chord due to the decreased span. Can you elaborate on how the bending stiffness/strength of the bottom chord affects this type of truss? Room trusses I have been involved in have the room width limited by the span of the bottom chord between kneewalls.

 

[lexpatrie]

And, of course, this is made possible by not actually checking the top plates or by telling some wildly aggressive story about how the sheathing helps, splices occur over studs, etc.

I feel like I missed this thread or missed this before.

If the calculations calculate out, it's possible. If there's no calculation, you're skipping part of the load path. There's a few articles on that, but I'm not convinced anybody here is actually struggling with that.

I've seen that kind of "wildly aggressive" approach with 65' long trusses at 4' o.c. and a double top plate with metal panels and intermittent wood girts.... naturally this doesn't calculate out.

(For the record I caught KootK's comment as facetious. )

I guess I didn't do an FAQ on this, but here's one engineers "take" without any calculations are all (yay!) Or taking a definitive stance (extra yay!).

To Align or not to Align, Steimle, Structure Mag, Feb 2024.

ETA: Second article but it's by the same author and covers largely the same ground.

Alignment of Bearing Wall Studs and Trusses/Joists in Multi-Story Wood Construction: Structural Design Considerations, Steimle, Woodworks website, date unknown.

I think the woodworks was published first. Woodworks hasn't been forthcoming.

 

[RontheRedneck]

XR2500, you have a point. It might be possible to use a smaller BC size with a center bearing.

It depends on the situation. If these attic trusses go up against a 2nd floor that's framed with 2X12s and the floor heights need to match, that might determine the size of the BC.

In general, I like to limit the span/depth ratio of floor members to 18/1. i.e. a 1' tall floor member should not span over 18'. If the span/depth is the limiting factor, this truss could probably go down a to a 2X8 BC. And that would save some money.

 

[Eng16080]

I wish more architects and engineers would talk to us earlier in the design process. It can save a lot of headaches in the long run.

Yeah, I wish we could! But for most projects it seems the design is completed before a contractor is selected and before a lumber supplier and in this case truss supplier is chosen. Most truss designers aren't interested in doing consulting work without there being a guaranteed job. Plenty of times, I've avoided using trusses altogether due to the lack of resources and not knowing if a particular design might work, which isn't something I want to leave to chance at the 11th hour.

As a structural engineer, I feel fairly competent designing mostly anything (or being able to figure it out with all the resources we have). The exception to this is prefab. trusses and PEMBs.

My resources for prefab. trusses include previous designs I've been involved in, basic rules of thumb, and information from this site (plenty of which is from you).

 

[StrEng007]

I was all good with the discussion here, then remembered there was an outstanding question

"assume there is no sheathing at all and the wall stud is bending out of plane. Is solid blocking at mid-height of stud enough to cut the Lu by 50%?"

 

[RontheRedneck]

Eng16080 said: "Most truss designers aren't interested in doing consulting work without there being a guaranteed job."

I'm sorry you've had that experience.

Every place I've worked at, we've been glad to help out. I figure if we can work out details ahead of time, it benefits everyone. And someone else might be doing the same thing on a job that I might end up getting. So it works out in the long run.

 

[lexpatrie]

StrEng007 - gte447f answered that question about bracing the stud. Your 'quote' of the OP (of yourself) doesn't seem to be what you actually wrote, however. It's a paraphrase.

From the top:

1. How important is it that my studs align with the trusses? I'm used to 16" o.c. studs, but the 24" o.c. seems to make more sense if it allows the wall panel edges to align the full height of the stud.
I'm not sure I've seen a sample calculation of a top plate bending and shear check, though it should more or less be routinely done, it's a topic that is largely skipped over. If anybody knows of calculations showing it, I'd appreciate being made aware.

Provided you check the load path, the double top plate for conventional framing (i.e. the prescriptive in the IBC and the IRC), one could argue that the spirit of the IBC is not being violated here, OR (more prudently), calculations for the top plate are necessary to ensure the stresses are sufficiently low.

How detailed one gets in a double top plate check varies widely among practitioners from:

a) no check or calculation whatsoever (engineering "judgement"), which I deeply dislike, (this is negligence, it just might not cause a failure, or the framers may add studs under the trusses and save the engineer from a collapse investigation and nobody will ever know, hence the engineer "did nothing wrong"). This is the "calculation" where the studs are 16" on center and the roof trusses are 24" on center so the stud loads are 24/16 x the truss reaction. No analysis of the top plate is performed at all. You are skipping part of the continuous load path here.

b) various presumptions about load distribution, some of which are totally unrealistic (Steimle, above, mentions some consider the (2) 2x6 as a fully composite 3" deep member, without connecting them properly for the interface shear). It simply cannot be justified as a valid approach, there aren't enough nails for that). This approach isn't valid as it isn't via "generally accepted principles of mechanics", hence it's wrong, and it's unconservative, and it's potentially unsafe.

c) Other approaches are clearly conservative (presuming the top plate is non-composite, one has a splice between the two studs and using only one in bending with zero end moments at the studs (effectively a single top plate that terminates at each stud), and the truss load midspan of the 16" or 24" space between studs. That's clearly conservative but is perhaps so conservative as to be unrealistic.

d) similar to c) above with "Adding" end moments (this is murkier, but there is some reality to it as a top plate will generally not be 16" or 24" long except in odd situations). One could also look at a single top plate, say, six feet long, or four feet (if you specify on plans), and allow a more detailed model of multiple point loads and multiple supports, still using a single top plate. Or, if the plans are more specific, perhaps a full length of the structure model that shows the cuts in the plates where you designate them, then a double top plate (non-composite) with free ends at the splices, could be entertained, I think this is equally unrealistic because either the details won't be on the plans, or it's unlikely they will be followed by the contractor.

My issue with the Steimle articles is they don't present calculations and don't take a more "that's never going to work" stance on the composite 3" deep top plate as a model.

If you have top plate bending issues, adding a stud beneath the truss is fairly common and is sometimes done along with (a). No analysis but add a stud. This may not be sufficient for very large truss reactions because the stud itself cannot take the entire load, leaving the stud buckled and the double top plate trying to resist the remainder of the load.

Overall, I'd be highly suspect of somebody trying to use the sheathing to assist the top plate because the sheathing generally is a shear wall and those nails should be saved for the lateral load.

2. Should I switch the roof truss over to 16" o.c. instead? I've got a 24'-6" clear span with an intended load bearing partition at midspan. Doing so would alleviate the close to unity wall stud but add 8 trusses to the package.

I'd expect five out of five engineers would not do this. If your approach calculates out, the load path is continuous and requirements are satisfied, the spacing has been accounted for properly and life-safety has been met. This truss is also somewhat modest span. It's possible you can get a joist design from the IRC for this span, that's part of my argument here.

3. Will the truss manufacturer be able to make use of this load bearing wall (at center) without the introduction of a vertical web?

If you designate it as a bearing wall and make sure on the shop drawings that they used it. I'm not sure it will do that much good, because that's a fairly open span and it will potentially ridge if you use that as a bearing wall. If you really want, calculate your design both ways, I kind of doubt this interior bearing wall will affect your exterior wall design, however.

4. Is it correct to assume that my wall studs are braced full height against y-y axis buckling (axial load) if I have wood structural panel sheathing on the exterior side only? Or should this unbraced length be limited to the blocking height of the stud?

I'd say that most engineers would probably use this as braced in the 1.5" direction without even really thinking about it. You have a wood structural panel on the outside so the NDS mentions this as acceptable (gypsum board is, too, but "alternative" wall panel stuff like Therm-o-Ply aren't recognized as bracing.

Look at 4.4.1 in the 2024 NDS.

And: 3.3.3 regarding bending, and compression.

Design of Loadbearing Tall Wood Studs for Wind and Gravity Loads, American Wood Council, Showalter and Koch, circa 2017. (Youtube says 7 years ago, so I had to do math).

I'd also point you at Design Considerations for Sawn Lumber Studs, Partain, Structure Magazine, May 2012, but I don't see the PDF link on the web site.

If you're going to consider it unbraced, I think the blocking would be considered a brace point, for positive and negative wind. There is unbraced length and effective length, however, so it's not quite so quick. I expect most engineers don't check their wall this way, but I think it makes physical sense that the sheathing on the outside is on the tension face for negative load, and the negative load tends to be larger than the positive load anyway. Application of MWFRS versus C&C loads is murkier, but using full loads and C&C wind would be simple and conservative.

5. Same thing for unbraced length due to out of plane flexure (wind load). Is my Lux considered unbraced for NEGATIVE wind if the interior stud was left unfinished (no sheathing). Is mid-height blocking good enough to cut this span in half?

I think this is the question you think isn't answered. I think gte got it, but perhaps not explicitly, but see my answer to #4 above. I feel like my impression of the NDS was a lot more "all in" on using sheathing or gypsum board as bracing the studs, the language has gotten less specific or I'm just remembering it wrong. It is perhaps in the Commentary, (1997 NDS Commentary, view only)

Side note: I'm not sure how much good exposed insulation will do you in this scenario (mentioned earlier as "easier" with 24" stud spacing) as far as R value or heating costs. To me a lot of these R values are predicated on the whole assembly, so the gypsum board and the vapor barrier are integral to the values. I'm not an ASHRAE guy, however, that's just my first thought there. As another side note, I think pre-cut batt insulation is available for both 24" stud spacing and 16" stud spacing, so while fewer studs would reduce the framing factor and improve the R value, the labor is slightly higher (more pieces for the 16" wall), there's not that much labor lost or gained either way, and I presume 24" insulation costs more or you get fewer pieces, but whatever.

Yeah, wall of text, I know, but I'm too tired for a KootK style scribble or uploading excerpted images from the NDS.