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?

 

[RontheRedneck]

I was thinking today that camber got mentioned a couple of times.

The first truss plant I worked at in the 1980s cambered trusses. Every truss plant since then has not.

I don't know what percentage of plants use camber. Just wanted to mention that it's not universally done.

 

[XR250]

Ron,

Any chance you can run a typical room truss with and without a center bearing and post the results?

 

[RontheRedneck]

Ron,

Any chance you can run a typical room truss with and without a center bearing and post the results?

What exactly is "typical" to you? We do a lot of variations of attic trusses.

 

[Celt83]

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%?"

Similar to cold-formed construction yes mid-height blocking could be considered a brace point for axial compression and/or flexure LTB. To serve that purpose there needs to be a load path for the bracing force into the diaphragm and out to to the lateral system either by diagonals or specific design of jamb stud assemblies to resist the brace force.


One side sheathed NDS addresses and states can effectively provide weak axis bracing for axial compression.

Sheathing on one side will not provide flexural LTB bracing for either wind push/pull depending on the sheathed face, so you’ll take a hit on the bending stress allowables.

Double top plates in my experience can work for a single levels worth of load when the calculation considers the top plates composite and continuous over PL/AE stud springs and the studs are 16” o.c. or less. Usually you’ll see a detail showing cripple studs between floors and a note to carry studs down to foundation/transfer element which tends to mean inserting studs into the assembly.

Be careful with stud spacing beyond 16” many shear wall assemblies specifically require 16” or less stud spacing.

Don’t change the truss spacing to match the stud spacing it is much more cost effective for the studs spacing to change to match the truss module if you are going for aligned bearing, so for 24” truss spacing you’d go down to a 12” stud spacing then make sure you have notes regarding studs and trusses to align and then be prepared for them not to be.

Also see this thread: https://www.eng-tips.com/threads/interaction-diagrams-for-wood-stud-walls.435497/

 

[Celt83]

This one may be controversial but make sure you include the minimum 5 psf lateral live load in the wall stud design.

 

[Celt83]

The central wall you likely don’t need it in this case but if you use it be careful of unintended uplift reactions under gravity loads due to the continuity, unbalanced snow, and/or load patterning. These unintended gravity uplift reactions will in my experience usually exceed the net wind uplift by a considerable amount so usually require heavier hardware and potentially more explicit load path continuity.

 

[XR250]

What exactly is "typical" to you? We do a lot of variations of attic trusses.

IDK, lets say 36 ft. span, no raised heel, 16 ft. room, 8:12. I'd be curious if a 2x6 BC ends up working.

Celt83, "The central wall you likely don’t need it in this case but if you use it be careful of unintended uplift reactions under gravity loads due to the continuity, unbalanced snow, and/or load patterning. These unintended gravity uplift reactions will in my experience usually exceed the net wind uplift by a considerable amount so usually require heavier hardware and potentially more explicit load path continuity."

I can't imagine this being an issue in a room truss. Maybe we will find out if RTR can run this.

 

[Celt83]

For this configuration it may work out to no net uplift was meaning more in a general sense in wood need to be careful when you add continuity and look out for it generating uplift.

Don’t want to start an argument by saying this but the thing that makes wood a deceptively difficult material is that from a design perspective in a lot if instances you are only going to get the load paths you explicitly define/detail and the default load path in many instances with wood is direct bearing so uplift generally requires special attention to resolve. The industry as a whole in my opinion tends to lean pretty heavily on the “inherent redundancy” of wood construction which I feel that from a pure by the numbers perspective with wood you should be detailing down to the nail which doesn’t happen because wood is cheap and by extension fees are bottom of the barrel.

 

[XR250]

he industry as a whole in my opinion tends to lean pretty heavily on the “inherent redundancy” of wood construction which I feel that from a pure by the numbers perspective with wood you should be detailing down to the nail which doesn’t happen because wood is cheap and by extension fees are bottom of the barrel.

My fees are not bottom of the barrel but I sure as heck don't want to detail everything down to the nail no matter how much the fee.. No builder wants to look at a set of plans like that either.

 

[RontheRedneck]

XR250 - I'm going to attempt to post a design based on what you asked. Since the forum software is different, I don't know how it will go.







In your post you said something about a 2X6 BC working. Surely you didn't mean that? It's not even close to being possible.

I made the room 15' 10" rather than 16' 0". That way the narrower face of T&G plywood will work out, and they won't have a narrow strip to fill in.

I used a 2X12 BC so the span/depth ratio is less than 18.

We don't do a lot of 8/12 attics. The forces in the TC are high enough I went with 2X8. Lots of bending at the wall vertical and collar tie.

You'll notice there's no kingpost above the collar tie.

This is how I'd send this truss to the plant to be built. Others would do things differently.

 

[XR250]

XR250 - I'm going to attempt to post a design based on what you asked. Since the forum software is different, I don't know how it will go.



View attachment 195

View attachment 196

In your post you said something about a 2X6 BC working. Surely you didn't mean that? It's not even close to being possible.

I made the room 15' 10" rather than 16' 0". That way the narrower face of T&G plywood will work out, and they won't have a narrow strip to fill in.

I used a 2X12 BC so the span/depth ratio is less than 18.

We don't do a lot of 8/12 attics. The forces in the TC are high enough I went with 2X8. Lots of bending at the wall vertical and collar tie.

You'll notice there's no kingpost above the collar tie.

This is how I'd send this truss to the plant to be built. Others would do things differently.

Ron,

Thanks for running this. The idea was to run it with a center bearing to see how that affected bottom chord size and also how the support reactions at the ends changed. Can you re-run it with a center bearing? Thanks

 

[lexpatrie]

I'm curious, do the newer truss design software iterations handle these bending moment regions? (One would be tempted to presume given the program outputs something, but I'll ask anyway), as trusses go, this one isn't strictly a truss.

 

[RontheRedneck]

Ron,

The idea was to run it with a center bearing to see how that affected bottom chord size and also how the support reactions at the ends changed.

I didn't get that from your post. I'll see if I can do it today.

 

[RontheRedneck]

I'm curious, do the newer truss design software iterations handle these bending moment regions? (One would be tempted to presume given the program outputs something, but I'll ask anyway), as trusses go, this one isn't strictly a truss.

Not sure what you mean by "these regions".

Attic trusses are obviously not fully triangulated. There are high bending moments at the TC joints - Especially on low sloped attic trusses.

The program we use has been developed over the past ~30 years, and is used nationwide. They wouldn't seal attic truss designs if they weren't done right.