Shear Wall Hold Down Distance Apart

[CurlyQue]

When designing a wood shear wall with gypsum board or wood diaphragms, is there a maximum distance the hold-downs can be from each other? If a shear wall is 20' long versus 100' long, the uplift is more for the 20' than the 100' so longer shear walls give you better leverage to prevent overturning. However, 100' long walls seem more prone to buckling than 20'.

Thans

 

[AELLC]

Seems to me the SE of record used this 4' panel method as a way of simplifying his calculation effort at the cost of construction economy.

 

[CurlyQue]

RFreund and AELLC,

The SE of record maintains that this 4' panel method is stronger than a 30' or 100' long traditional wood shear wall. Again, his primary statement is that this method won't 'crumple like a piece of paper'. It is more expensive because of more anchors and because the SE insists on OSB for all shear wall panels.

I am trying to understand why this method is stronger.

When Oldrunner says it is the same as tilt-up or pre-cast concrete, it is. Pre-cast are typically 8' panels. Concrete panels are typically 8" thick with 2" of insulation. Construction would seem to dictate that one would have weld plates at each 4' section if these walls are used in shear. But one could also have weld plates welded together to take advantage of the moment arm and reduce the uplift too like a traditional shear wall does. It seems the SE of record is seeing that wood walls could and should be done similarly.

The SE provided a hand calculation sheet for a 40' wide load area. He calculates the total overturning moment and divides that by the amount of panels (12 for a 48' long shear wall). He then divides the reduced moment by 4' to get to his uplift. What I don't understand is why that wouldn't be calculated by summing moments instead. When I do that calculation , I find that the wall will take 3 times the load which is, curiously, how much the wall is supposedly over-designed according to the second engineer who was enlisted to evaluate the first. I see that you might also be able to take this one step further and sum moments around one point for every 16" stud space and reduce this overturning to little clips like stitching a pocket rather than having big grommets at the corners of a pocket. Levi, though, does both.

Then engineer definitely does like to simplify. I'm not sure earthquake was a consideration given that Indy is in the lowest category. All drawings were hand sketches and the leasing building utilized 2x8 diagonally at 10' on center to tie down the roof to the shear walls. The truss manufacturer said he never saw such a solution. The options were to provide a wood diaphragm on the bottom chords or move the 2x8 to the top of the bottom chord.

The second engineer does forensics and says the first engineer causes all sorts of red flags to be sent up for him. He then provided a 150 page, computerized report with one leading paragraph of 7 lines that says the design works but is only utilizing 33% of its capacity for wind; 66% for earthquake (apparently earthquake is at least equivalent to wind). The only recommendation was where the engineer had to use 3/4" plywood. The second engineer said to use 16d rather 12d at 4" o.c. The contractor's response was expected; "that's a lot of nails"

One of the advantages to this 4' panel method, as I see it, is that it moves the boat-anchor concrete chunks from the hold-down locations to all along the thickened slab which is the way it was designed. These thickened slabs are 36" wide by 18" thick with (4)#6 continuous.

What I see is that 8":4' is stronger than 4":4' and 3 1/2":4' with OSB is stronger than with gypsum board.

If the wall is braced laterally top and bottom, then, from a beam-column viewpoint, its L/r would be the same for a 30' or 100' length (in plan) as for a 4' section. So how does the engineer see his method as being stronger? If the wall is not braced laterally top and bottom, then I can see how a 4' section is stronger than a 30' or 48' section. The beam-column 'web' would cripple or 'crumple' like the engineer says. So, I assume, the engineer sees it this way.

So how is this method stronger? Or is it? It apparently works ... even more than it needs.

 

[oldrunner]

Curlyque:

Your certainly have a unique problem on your hands. From your descriptions and arguments, it appears that you may have not taken a timber design course when you went to college. I would suggest that you obtain almost any edition of the Design of Wood Structures by Breyer and review chapter three (Behavior of Structures under Loads and Forces). Having a peer review by another firm was a good choice. I would also wonder if the EOR has previously designed a building similar to what you are building.

As to the stiffness of 25 - 4 foot wall elements versus 1 - 100 foot long wall element; just considering any thickness of sheathing, the moment of inertia of the 100 foot element is greatly (say massively) greater than the collection of the short width elements.

Having witnessed panel racking tests of gypsum board panels when I was assigned to the research division of a prestigious structural engineering firm, I personally try to avoid to ever using sheet rock for a shear wall.

Was the design of this building reviewed by the local jurisdiction in order to obtain a building permit? Recently I worked a case where this wasn't done and only the contractor was provided hand sketches and calculations. The EOR in this case was found at fault for the subsequent problems. The structure had to be demolished.

 

[CurlyQue]

Oldrunner,

Just for the record, I did take structures in college; otherwise I wouldn't have a clue about what an engineer was talking about. I am not an engineer, obviously. I'm trying to understand why this specific engineer considers his design (more expensive, more than required) to be stronger. It is, but the question is how or whether there is any merit whatsoever to his method (madness).

I will review Breyer. Thank you. Perhaps it discusses what the person at AWC talked about with edge forces and what the shear panels actually do.

I understand the I-value is going to be bigger. That's an argument for the traditional method but yet at some point the 'web' of the beam has to be thick enough or it cripples, right? You can't have a W section of one size and replace the web with paper (or gyp or osb) and have the same beam. Yes, in that example, the I value goes down but the buckling is more a matter of the thickness of the web and the length of the web; otherwise what are web stiffeners for? In fact, the I-value of a gypsum board W shape is the same as one of steel, correct? The I-value is strictly a function of the geometry and the distance squared of all those pieces, correct? I understand that shear walls are not true beams. What the EOR, the designer of this 'masterpiece' is telling me is that a shear wall can be viewed as a beam in section. I can see how it is like one with the hold down points being like the T and C chords in a W-shape.

OSB is 'extra' cost. Gypsum board is used in a building anyway so it doesn't add cost and it's allowed by code.

In Indiana, designs are not reviewed by the AHJ. There are no structural engineers on staff at the State nor the City. Reviews are not farmed out. Indiana relies on its design professionals. Calculations are not required to be submitted.

I appreciate your experience with a prestigious structural engineering firm. That's the kind of experience I'm trying to tap into in this forum so I can understand this specific engineer's approach. I've talked with two other engineers regarding this design.

That's why I'm questioning. I want to understand. I'm doing what I call triangulating the truth. One person sees an object (design) one way; I see it from another perspective; and yet another views it from a unique perspective. What the 'truth' is is somewhere in between, the commonality or the common denominator of all the perspectives.

Does anyone know why this Engineer believes his method results in a stronger wall? It's more expensive, it's much stronger than it needs to be, but is it stronger than a traditional wood shear wall with hold-downs on each end given that OSB is the shear panel material. It seems like a pipe dream which is costing the owner and the design-builder money and distress and is not reflecting too well on my company. We chose an engineer and he gave us this design. Obviously we won't use this engineer anymore.

Thank you

 

[oldrunner]

How sad that there is no oversight of the engineering in this state. Is this why we see so many buildings trashed in the big winds? My view in the big scale of all things in nature is that wood buildings are assemblies of sticks, pins and paper like panels.

By the way, the modulus of elasticity of sheet rock (gypsum) is hugely smaller than that of the modulus of elasticity of steel. (sophomore strength of materials course at a really good architectural-engineering school)

 

[RFreund]

I can understand what Engineering #1 is saying. Basically he is saying two things:
1.) Even if you sheathing panels over lap at a stud (typical construction). The panels will still act as separate panels and therefore he is designing them as such.
2.) If you have a very long wall meaning a deep web in the beam/column analogy you could be subject to web-buckling (like paper). To solve for this he is breaking them up into panels.

My thoughts are - if the AWC or APA was worried about this there would be something in the NDS or similar publications. I think that the diaphragm above and below as well as the studs which are acting like web stiffeners will brace the wall.
Although it seems like the engineer has put some thought into this and maybe he knows better than I do. Also with all the holddowns you have many points slip although small they may add up. As far as strength goes - I think they are both equally strong if you consider that the code lists allowable strength values in terms of sheathing thickness and nail pattern. Length is not a factor. However I would almost prefer the continuity of one long wall....I think.





EIT

http://www.HowToEngineer.com

 

[CurlyQue]

oldrunner

Yes, things can always be better.

However, an AHJ isn't going to care one way or the other whether one's design is too much or too costly; too little, probably. The problem being discussed isn't whether it's going to blow down; it's been proven it won't by another engineer ... without review.

Thanks for the education. Yes E is a function of strength of materials. I-value is not. EI makes you right. I alone makes me right ... so to speak. I know that much. Do I get credit for that, professor?

I see your superior education and experience with a prestigious engineering firm have served you well. I would expect nothing less from an engineer.

Do you care to humble yourself and put your superior intelligence to the problem?:

Why is a 4' panel system stronger, better than one with hold-downs and boat anchors on each end.

At least one engineer believes so.

Qurly

Sorry, but sometimes egos get my ire. For an oldrunner, humility is obviously not your strong suit.

I've already admitted that I'm not an engineer.