Stacked Walls Tall Wall

[Signious]

Hi,

I have been requested to review a stacked tall wall design. The build has been done as requested from the previous engineer (stamped drawgs), both I and the inspector fail to see how he planned on this working.

Criteria:
It is a 20'-0" tall wall with an additional 7'-0" of Gable on top.
All framing is 2x6 SPF No2.
7'-0" wide stairwell tall wall.
26psf unfactored wind load

Site Conditions:
-Framed with 2-9'6" walls (12" Oc studs) stacked one on top of the other (double top, single bottom plates) for an effective 3-Ply Spf girt across the middle, no continuity between it and the other plates
-No attachment at either end of the wall to the floor platform or surrounding framing.
-a single 2x6 on the flat above the wall, extending 5" on each side and nailed above the top plates and butted to the first roof truss(attempting to act as a wind girt for the gable I think)
-absolutely nothing connecting the wall to the foundation. No nails to the ladder, no bolts to the concrete.
-no shear wall provisions for the stairwell walls (full height stairwell walls on both sides and both floors. Might be my saving grace in making this work

Am I missing something - or is this just shoddy engineering?
I tried to run it using C&C wind loads, and given the hinge point in the wall it really seems to me that this is pretty crappy design?

 

[jayrod12]

That will never work. If the original engineer actually designed the wall ask to see his calcs.

You may be able to add straps at each stud across the hinges to attempt to strengthen it but there is no way you can get 2x6 spf studs to work unless they are spaced ridiculously close.

My bet is he never even considered it.

Did you think to account for some lateral resistance from the stair landings? (provided they actually frame into the wall)

 

[KootK]

-a single 2x6 on the flat above the wall, extending 5" on each side and nailed above the top plates and butted to the first roof truss(attempting to act as a wind girt for the gable I think)
-absolutely nothing connecting the wall to the foundation. No nails to the ladder, no bolts to the concrete.

These sound like fundamental load path issues, not just capacity issues. Fail by inspection based on the information provided.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JAE]

Yes - ask to see the calculations.
I can't see how it would work unless the (3) 2x6 flat plates at mid-height span across the width and somehow are fastened to the adjacent walls.

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[Signious]

Thanks for confirming this, it is a similar design to what our shop issues - but we throw a nice HGUS hanger to the floor platform across the middle.

Will request calcs from the issuing engineer but I'm not expecting anything in return - it is a standard detail they issue for MANY homes.
I have reviewed 3 of his walls so far out of countless ones the inspector has let slide.

Continuing this discussion - is this something you would take to the professional association? (if he refuses to provide calcs, or they don't pan out)

 

[Signious]

@jayrod12

-Clear 1/2" gap between stair landing platform and wall
-On our designs if we are really close for deflection I will give some consideration for landings, but all the stamped designs for landings I've seen specifically state not to use them as lateral support for the walls.
Probably just the stair guys covering their bacon, but I like to play by the rules for the most part.

 

[Signious]

Did some number crunching, and adding additional 2x6 studs to shorten to 6" OC runs gives max moment of 650lb-ft across the girt.

Tension at extreme fibre going to ~3000#, Simpson MSTA36 into each stud centred across the girt should solve that issue, but it is bloody close.

 

[BadgerPE]

Out of curiosity, how did you come up with ~3000#? I came up with about 2200# of tension force.

 

[Signious]

Total wind load per stud: 26psf * 6/12ft (new max stud spacing on the as built wall) = 13plf

Max moment on stud: wl^2/8; 13plf * (20^2) / 8 = 650ft-lb

max tension force on stud: T=m/d where distance from neutral axis = (5.5/12)/2 = .229ft
T = 650(ft-lb) / .229ft = 2838lbf (tens)

If I have made an error please let me know! haha

 

[Signious]

Ignore the lbf - was doing some fluids work this afternoon and let that slip in

 

[wannabeSE]

I must be missing something. I thought the wall has an unsupported span of approximately 27' with a splice at 20'? The calcs look like a 20' span with a splice at 10'.

 

[CANPRO]

wannabeSE,the way it was described I believe it is a 9'-6" wall/3 2x6 sills/9'-6" wall/2 2x6 sills/7'-0" gable end.

The 2x6 on the flat is overhanging the wall to the inside by the thickness of the truss and is likely there to fasten the ceiling strapping. As you show a 20' span in your calcs, you're considering the ceiling as a lateral support.

An alternative to adding intermediate studs and the tension strap is to cut the 3ply 2x6 at the joint in the 9'6" walls and splice the studs in place.

For the tension strap calcs, I think the strap will be closer to the neutral axis than .229 ft.

 

[wannabeSE]

Thanks CANPRO. I missed that, I assumed they were 9'-6 wide walls that were 20' tall. Now it makes more sense

 

[Signious]

CANPRO:

your right on the money with the description, I'm beefing the ceiling plane up a bit and having them sheet it with type X to get some support out of the ceiling.

I talked to my mentor also and he figures the strap would actually be further from the N.A. given the 5/8 ext. sheathing would act partially as a compression chord for the wall. (my calcs were assuming midpoint of 2x6)

by splicing studs are you saying chop the 3ply up, and sister studs around the hing point? Just clarifying.

 

[dhengr]

Signious:
You may need a 3x stud just to get all the nails in and spaced to prevent splitting. While the strapping might prevent the wall from out-n-out failure, I’ll bet you can expect a fair amount of deflection/movement around the joint/level. Maybe detail a joint and cover trim in any interior sht. rock at the level to tolerate this movement.

 

[CANPRO]

Signious, yes I was suggesting cutting about 2" of plate away from each stud and sliding in a new stud to splice.

I guess I was just being picky with your 'd' value...but I would think if you're considering the tension trap, stud, and sheathing to act compositely, the N.A. would be closer to the steel. I haven't run the numbers so I don't know, just my gut feeling.

 

[OldPaperMaker]

Signious,
You might want to provide a sketch of the elevation & cross section of this wall to make sure that we are all talking about the same thing.

 

[woodman88]

Are the stairs attached to the walls? If so this can be a brace point.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jayrod12]

Woodman, I asked already. They are not intended to act as lateral support. They are detailed with a 1/2" gap between stair framing and walls.

 

[wadavis]

Not a technical response, but this is relevant reading regarding reviewing tall wall designs produced by others.

APEGA Discipline Committee Order Skip to page 94.

Wadavis,
E.I.T.