×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Stacked Walls Tall Wall

Stacked Walls Tall Wall

Stacked Walls Tall Wall

(OP)
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?

RE: Stacked Walls Tall Wall

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)

RE: Stacked Walls Tall Wall

Quote (OP)

-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.

RE: Stacked Walls Tall Wall

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.

Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies

RE: Stacked Walls Tall Wall

(OP)
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)

RE: Stacked Walls Tall Wall

(OP)
@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.

RE: Stacked Walls Tall Wall

(OP)
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.



RE: Stacked Walls Tall Wall

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

RE: Stacked Walls Tall Wall

(OP)
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

RE: Stacked Walls Tall Wall

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

RE: Stacked Walls Tall Wall

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'.

RE: Stacked Walls Tall Wall

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.

RE: Stacked Walls Tall Wall

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

RE: Stacked Walls Tall Wall

(OP)
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.

RE: Stacked Walls Tall Wall

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.

RE: Stacked Walls Tall Wall

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.

RE: Stacked Walls Tall Wall

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.

RE: Stacked Walls Tall Wall

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.

RE: Stacked Walls Tall Wall

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.

RE: Stacked Walls Tall Wall

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.

RE: Stacked Walls Tall Wall

jayrod12

Sorry my bad. I was thinking that to attach the stairs to the walls to creat a brace point may be an option. But my mind and fingers were disconnected.

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.

RE: Stacked Walls Tall Wall

I agree with you. That's why I asked the question. I would be tempted to modify the stair connection and design slightly to account for some contribution.

RE: Stacked Walls Tall Wall

Quote (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

Please excuse my stupid question (I have not designed in timber for more than 20 years) - so you have a 20' clear span stud that is 'spliced' mid-height with steel straps? Is this how such studs are typically connected for moment splices in high exterior wall situations?

RE: Stacked Walls Tall Wall

Generally, we don't splice the studs mid-height. The standard method is to have continuous studs for the full height. Thanks to engineered lumber this is now possible in increasing heights. I've done a few 24 foot tall walls using engineered studs. They are things of beauty when constructed correctly.

RE: Stacked Walls Tall Wall

jayrod12,

Thanks for the reply. Engineered lumber does indeed make sense for such applications.

For the OP's situation where traditional dimensional lumber is used and it IS spliced somewhere over it height (presumable due to availability of longer studs), how does shear get transferred across the metal straps (assuming that nominal shear capacity is required even at max moment)?

RE: Stacked Walls Tall Wall

The way I've looked at it, The straps are there to provide tensile (and maybe compressive) continuity across the splice. The shear transfer is a function of the nails and sill plates and I guess a bit of beam action for the straps.

It's unconventional and had this thing been designed accordingly in the beginning using dimensional lumber you generally make your sills continuous and turn them into wind beams.

RE: Stacked Walls Tall Wall

Are you using steel straps at both the interior and exterior face of each stud? I think you should. Then the moment arm between straps would be the full 5.5", and the tension in each strap would be 1,418#.

DaveAtkins

RE: Stacked Walls Tall Wall

(OP)
David-

Im not following you with strapping both sides - it is a moment connection, not uplift so I think the strap on the outside face of the stud would only be in tension when there is suction acting on the wall.

I agree the ideal solution would be to tell them to tear the wall down and use engineered lumber - but I have ran the numbers, and ran this solution by my mentor (35+ years in wood design) and he agrees that it is an adequate solution. I would never design this from scratch, just too damned mickey mouse for my tastes.

RE: Stacked Walls Tall Wall

(OP)
also, jesus wadavis - how did you know what my professional association was :p

RE: Stacked Walls Tall Wall

Quote (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.

That was a fascinating read about Mr. Gill - naming and shaming.

RE: Stacked Walls Tall Wall

I was confused by the moment arm you used to calculate tension in the strap--why only 1/2 of the stud depth? Also, since the stud will see both pressure and suction, doesn't it need a strap on each face?

DaveAtkins

RE: Stacked Walls Tall Wall

I strap each face as well.

RE: Stacked Walls Tall Wall

(OP)
I worked out the actual neutral axis (of the combined external sheeting, 2x6, and strap) to be 3.9" from the inside face of the wall reducing max tension to 2,000#

Im still not following how adding the second strap to the outside face of the wall would increase the moment arm to the full stud depth.
I was always told moment arm is distance from neutral axis to your chosen point, with the opposing force acting on the opposite side of the N.A. (see attachment)
Adding the second strap would bring the N.A. closer to the outside face, but by a fraction of an inch only.

I agree to strap each face in the case of a max suction on the wall.

For anyone interested in a good (yet troubling) read, look at the PEG article on pg. 87 (determining suitability of a slab-on-grade for heavy vehicles by comparing the point loading to the compressive resistance of concrete, no flexural / shear / geotechnical analysis)

RE: Stacked Walls Tall Wall

Now I understand your mistake.

The moment arm should be the distance from the strap to the centroid of the compression, not the neutral axis (think concrete design). The compression force must equal the tension force (otherwise there is not equilibrium).

If you add a strap on the outside face that can take the entire compressive force, the moment arm would be the distance between the straps (think how you would splice two wide flange beams together).

DaveAtkins

RE: Stacked Walls Tall Wall

Quote (Signious)

For anyone interested in a good (yet troubling) read, look at the PEG article on pg. 87 (determining suitability of a slab-on-grade for heavy vehicles by comparing the point loading to the compressive resistance of concrete, no flexural / shear / geotechnical analysis)

That was a troubling read. Damn, $45k Formal Hearing costs!

RE: Stacked Walls Tall Wall

(OP)
Thanks for the explanation Dave

RE: Stacked Walls Tall Wall

The problem with using straps on both faces of the studs is that straps buckle so do not transfer compression. Also, straps can not be attached in tension so you will may get a slight bow in the wall.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources