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?

 

[woodman88]

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.

 

[jayrod12]

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.

 

[Ingenuity]

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?

 

[jayrod12]

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.

 

[Ingenuity]

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)?

 

[jayrod12]

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.

 

[DaveAtkins]

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

 

[Signious]

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.

 

[Signious]

also, jesus wadavis - how did you know what my professional association was

 

[Ingenuity]

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.

 

[DaveAtkins]

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

 

[jayrod12]

I strap each face as well.

 

[Signious]

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)

 

[DaveAtkins]

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

 

[Ingenuity]

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!

 

[Signious]

Thanks for the explanation Dave

 

[woodman88]

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.