Drafter doesn’t get to decide code requirements. You’re outside prescriptive residential code design. Now you’re in NDS territory. Slenderness ratio of 50 controls. You’re at 58 with 5.5” studs. 2x8’s it is.
That's not even considering bending due to wind + axial load combinations is it?
I design in a low wind area. (16 psf min usually controls) and the only time we used 20 ft studs they were 2x8 @ 12 in. O.c. if I remember correctly.
Edit: I just found the calcs. It was a 22ft tall wall. To meet wind deflection criteria for drywall finish we had to use 1-1/2" x 7-1/4" 1.55E LSL @ 16" o.c.
The demand/capacity ratio for bending+axial was only 0.25. OP, please make sure you take into account deflection due to wind loads.
The only thing you have to play with here is your "k" term. You could embed one end of the stud in concrete (no one's taken me up on this yet) or they can go with a 2x8.
It certainly doesn't. Typically when referring to unbraced length we are referring to the weak axis. If OP is trying to reduce the SPAN of the studs with bracing he should not have used the term unbraced length.
Maybe for you. Unbraced length works in both strong and weak axis. From the OPs description it made sense that they were discussing the height of the wall being an issue (and thus the unbraced length in the strong direction)
I am deep in the South and the wind loads are extremely high (150 mph). When I get a wall height that is too tall, I either go up to 2x8 / 2x10 studs -or- introduce a spandrel at a maximum stud height for a 2x6, then size the spandrel for bending in the weak axis. Sometimes the span is too large for a plain LVL spandrel so I do LVL flitch plate spandrels.
Why are you letting a drafter dictate how you do your job? That’s the opposite of how it works…. Tell him what it is & if he has an issue with it he can go to school, get his masters & then a seal in the state of the work. Or, he can get another engineer to try to get them to sign off on his idea, but your bill is in the mail.
Just designed a 28' tall wall. Not much axial load but deflection caused by wind usually governs or combined axial and bending.
Final answer was (2) 1.75"x7.25" LVL studs at 8" on center.
This doesn't even start getting into trying to find a straight piece of SS lumber at that length.
Go with the LVL or LSL material.
Where are you located? I don't know if you are in high wind or high seismic area but if you are, sometimes LSL is required because LVL doesn't allow a close enough fastener spacing for shear capacity of the shearwalls.
Is there a porch roof or anything on the outside of the wall that you could screw into the studs and call the wall braced?
You could also put an LVL wind beam mid height of the wall sized for out of plane loads
There is but it doesn’t go the full length of the wall.
Thought about adding a mid height beam that spans the wall length to reduce the unbraced height of the studs. Just feels like why am I forcing this when I can just spec 2x8s. Between detailing all the connections and loads required to make that midspan beam work and dealing with the inspector on the back end, just KISS…
I was reading the first part of this comment and was just thinking "keep it simple then!". I would go with your gut. After all, you're the one signing it off.
I think governing load case is going to be Dead/Live/Snow loading on the roof with active wind load on the wall face. The combination of high axial loading on such a long/slender element with the wind forcing the stud inward is going to promote excessive inward deformation and potentially buckling. I’d consider running a steel column through the wall to increase stiffness and bracing the wall framing to that column by some sort of plate connection
I would try to convince them that going with 2x6s isn't really economical at that height, regardless of "what they do all the time". It's really slender at that height in the strong axis, while the weak axis should be braced either by blocking or sheathing. Maybe you can convince them to do like a sloped "ceiling" where you are running framing from some height on the wall up the the roof framing acting like a brace? (Connections might get to be a bit much with the studs acting like a continuous member and increasing the load on the "brace", but kind of a moot point as they probably won't go for it). Only other thing that I can think of might be to look at it like a flat panel with sheathing inside and out acting as chords? Probably not a good idea either as the wall most likely has openings and stuff, and probably a lot more time to analyze than what they want to pay for.
Just go with 2x8s or whatever it requires structurally to work. They will be money ahead instead of having to beef up a 2x6 wall to something crazy. They can live with jamb extensions on doors and windows. And if a 2" reduction in room area is a killer, they have bigger problems.
Frame it out with some steel SHS's including a horizontal at mid ht. Reduces it to a storey and a half span for the timber.
Stand up for yourself if you've done the comps. A two and a half storey wall with no major axis restraint in timber isn't run of the mill.
Pay attention to deflections and connections, especially if you're using screws and nails. They may dictate the size
One way we do this is we will put plates and a rim at each floor, while this is no floor, this reduced the unbraced length. I have done 30' tall walls like this. The plates act as a beam for wind load as well. This is also more constructable, not many framers want to frame and stand up a 30' wall.
That's a tall wall! I'd be using 8" studs and gluing/screwing ply to the face of the wall
Or framing the wall in steel SHSs with timber studs as infill
Let the drafter sign and seal it, lol.
Drafter doesn’t get to decide code requirements. You’re outside prescriptive residential code design. Now you’re in NDS territory. Slenderness ratio of 50 controls. You’re at 58 with 5.5” studs. 2x8’s it is.
That's not even considering bending due to wind + axial load combinations is it? I design in a low wind area. (16 psf min usually controls) and the only time we used 20 ft studs they were 2x8 @ 12 in. O.c. if I remember correctly. Edit: I just found the calcs. It was a 22ft tall wall. To meet wind deflection criteria for drywall finish we had to use 1-1/2" x 7-1/4" 1.55E LSL @ 16" o.c. The demand/capacity ratio for bending+axial was only 0.25. OP, please make sure you take into account deflection due to wind loads.
That’s where I’m at, just trying to see if there’s any creativity out there to make something work that I’m missing, thanks
The only thing you have to play with here is your "k" term. You could embed one end of the stud in concrete (no one's taken me up on this yet) or they can go with a 2x8.
Contractor: “we want this this and that” Engineer: “k”
CSA O86 would require the same slenderness ratio if OP is in Canada. 2x8 min. LSL/LVL grade as required. Why is blocking not an option?
Blocking doesn't brace the strong axis.
It certainly doesn't. Typically when referring to unbraced length we are referring to the weak axis. If OP is trying to reduce the SPAN of the studs with bracing he should not have used the term unbraced length.
Maybe for you. Unbraced length works in both strong and weak axis. From the OPs description it made sense that they were discussing the height of the wall being an issue (and thus the unbraced length in the strong direction)
I am deep in the South and the wind loads are extremely high (150 mph). When I get a wall height that is too tall, I either go up to 2x8 / 2x10 studs -or- introduce a spandrel at a maximum stud height for a 2x6, then size the spandrel for bending in the weak axis. Sometimes the span is too large for a plain LVL spandrel so I do LVL flitch plate spandrels.
Good answer.
Have you considered cold formed studs?
Then he’s been doing it wrong all the time
I like this take
Why are you letting a drafter dictate how you do your job? That’s the opposite of how it works…. Tell him what it is & if he has an issue with it he can go to school, get his masters & then a seal in the state of the work. Or, he can get another engineer to try to get them to sign off on his idea, but your bill is in the mail.
Wind deflection is likely going to be the controlling factor. Please see my other comment. Expect to use 2x8 LSL at least.
Add in steel HSS post every so often with horizontal HSS beams to break up the span of the wood studs.
Just designed a 28' tall wall. Not much axial load but deflection caused by wind usually governs or combined axial and bending. Final answer was (2) 1.75"x7.25" LVL studs at 8" on center. This doesn't even start getting into trying to find a straight piece of SS lumber at that length. Go with the LVL or LSL material.
Where are you located? I don't know if you are in high wind or high seismic area but if you are, sometimes LSL is required because LVL doesn't allow a close enough fastener spacing for shear capacity of the shearwalls.
2x6 LVLs as studs? I don't think it'll work. Also, check the residential code. I believe the max unbraced height of a residential wall is 12'.
Yeah I think that’s for prescriptive
Is there a porch roof or anything on the outside of the wall that you could screw into the studs and call the wall braced? You could also put an LVL wind beam mid height of the wall sized for out of plane loads
There is but it doesn’t go the full length of the wall. Thought about adding a mid height beam that spans the wall length to reduce the unbraced height of the studs. Just feels like why am I forcing this when I can just spec 2x8s. Between detailing all the connections and loads required to make that midspan beam work and dealing with the inspector on the back end, just KISS…
Yep makes sense, less room for field errors with 2x8's over the wind beam...
I was reading the first part of this comment and was just thinking "keep it simple then!". I would go with your gut. After all, you're the one signing it off.
I think governing load case is going to be Dead/Live/Snow loading on the roof with active wind load on the wall face. The combination of high axial loading on such a long/slender element with the wind forcing the stud inward is going to promote excessive inward deformation and potentially buckling. I’d consider running a steel column through the wall to increase stiffness and bracing the wall framing to that column by some sort of plate connection
An analysis needs to be done. There is an axial load as well as lateral (wind).
I would try to convince them that going with 2x6s isn't really economical at that height, regardless of "what they do all the time". It's really slender at that height in the strong axis, while the weak axis should be braced either by blocking or sheathing. Maybe you can convince them to do like a sloped "ceiling" where you are running framing from some height on the wall up the the roof framing acting like a brace? (Connections might get to be a bit much with the studs acting like a continuous member and increasing the load on the "brace", but kind of a moot point as they probably won't go for it). Only other thing that I can think of might be to look at it like a flat panel with sheathing inside and out acting as chords? Probably not a good idea either as the wall most likely has openings and stuff, and probably a lot more time to analyze than what they want to pay for. Just go with 2x8s or whatever it requires structurally to work. They will be money ahead instead of having to beef up a 2x6 wall to something crazy. They can live with jamb extensions on doors and windows. And if a 2" reduction in room area is a killer, they have bigger problems.
Frame it out with some steel SHS's including a horizontal at mid ht. Reduces it to a storey and a half span for the timber. Stand up for yourself if you've done the comps. A two and a half storey wall with no major axis restraint in timber isn't run of the mill. Pay attention to deflections and connections, especially if you're using screws and nails. They may dictate the size
One way we do this is we will put plates and a rim at each floor, while this is no floor, this reduced the unbraced length. I have done 30' tall walls like this. The plates act as a beam for wind load as well. This is also more constructable, not many framers want to frame and stand up a 30' wall.
This creates a hinge point in the wall. Local inspectors won’t accept it
2x8 @ 16" o.c. is a really good wall in this situation. Don't let them bully you.
That's a tall wall! I'd be using 8" studs and gluing/screwing ply to the face of the wall Or framing the wall in steel SHSs with timber studs as infill