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Wood Diaphragms and Shear Walls for Common Apartments

Wood Diaphragms and Shear Walls for Common Apartments

Wood Diaphragms and Shear Walls for Common Apartments

(OP)
Fellow Structural Engineers,

I am curious about how other structural engineers deal with convincing owners, architects, and other clients about the need for chord and collector elements in wood buildings. I have not designed a lot of wood structures, however, when I have I include chords and collectors as appropriate for a complete load path. However, I see wood apartment buildings being built all the time with re-entrant corners, jut-outs, and other horizontal and vertical irregularities. In many cases I haven't seen the actual drawings, however, in some cases I have and I don't see ways to address these irregularities. My question is is this your experience too? How have you been successful in educating owners, clients and architects that this is indeed required by the code?

I am obviously obligated to follow the code, however, it appears as though some other engineers are ignorant of these code provisions. It is hard to convince an owner that they need these things when they haven't been required to put them in in the past. I have talked with various engineers who have said things like "Oh it's wood, it's fine". As a conscientious engineer who wants to do the right thing, I will have a hard time competing with other engineers that seem to be ignorant of these code issues. I believe that part of that ignorance lies in the fact that I live and work in the southeast, and it seems as though building officials don't really look much at the structural designs. This seems to have led to poor code enforcement and I feel penalized for following the code. In some ways this is venting, but it's a serious concern that I have moving forward and I'd like to see how others have tackled this problem. I don't want to seem to be over doing the design, and I also don't want to throw other engineers under the bus.

Thanks

RE: Wood Diaphragms and Shear Walls for Common Apartments

To some degree I have to say that I am on side with the average Structural Engineer who waives their hands dismissively stating "Oh, it's wood - It's fine.", but this is a matter of judgment on a case by case basis.

There have been very good discussions on this topic on Eng-Tips in the past; I suggest you search for them.

In my own practice with flexible diaphragms, I tend to evaluate as rigid as well in order to find the highest possible loads for the supporting shear walls, and ensure tights spacing for fastening around corners and other areas of stress risers. You can get some very good help in doing this with software as well, but the basic principle is to ensure you're using the highest possible diaphragm loads in your design and provide a robust load path.

RE: Wood Diaphragms and Shear Walls for Common Apartments

It likely behaves like a bunch of three sided buildings that don't have chords.
I certainly consider it when I am designing large rectangular boxes, however.

RE: Wood Diaphragms and Shear Walls for Common Apartments

Generally, there are so many shear walls that the chord forces just do not mount to much and the wall double top plate is sufficient.

Mike McCann, PE, SE (WA)


RE: Wood Diaphragms and Shear Walls for Common Apartments

njlutzwe, Your question deals with a single type of issue that appears in all sorts of other types of construction.

The issue is how do "good" quality engineers respond when "not-so-good" engineers put out poorly designed structures.

The only thing I can point out in your examples - related to the IBC specifically - is that many "apartment" buildings are, or should be, designed under the "light framed construction" provisions of the IBC section 2308. This allows three story structures to be designed using empirical methods like the IRC without needing full engineered load-path designs.

The structures may not always qualify for 2308 (such as high seismic/wind areas, too-tall walls, etc.) but in many cases they do. As a result, you only have "braced wall lines" and specified sheathing/nailing to worry about and no actual wind loads are derived for them.

I did a three-story wood framed hotel building once (fully engineered) and it included a huge number of shear walls, hold downs, etc. - all shown on the drawings in specific locations.
Another hotel right next door was also being built. I wandered over to it and looked at the drawings one day (out of curiosity) and saw on the plans where the engineer had a large note that stated "All walls in the building are shear walls and shall be constructed of...." No hold downs. No collectors. No shear walls extending through the attic to the roof sheathing. I was dismayed. But it was simply using 2308.

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RE: Wood Diaphragms and Shear Walls for Common Apartments

(OP)
All,

Thank you very much for your replies...I appreciate the thoughts. It has given me something to chew on.

RE: Wood Diaphragms and Shear Walls for Common Apartments

My experience of light wood frame construction matches yours exactly njlutzwe. I've been involved with light wood projects on the seismically active west cost of Canada as well and am sad to report that it's only marginally better there.

I recently red this book (Link). It deals exclusively with the kind of wood diaphragm issues that you've brought up. I enjoyed the book thoroughly but oscillated between these two sentiments as I worked my way through:

1) Wholly crap, nobody does this! Ridiculous!
2) Wholly crap, every word of this is 100% technically correct!

I think that light frame wood frustrations that we all share are symptomatic of a broader problem with our field. Structural consulting firms are businesses. Finding and retaining good clients is the key thing at most businesses. Finding and retaining good clients is far more about employing good project management practices than it is about doing sound structural engineering. It's good project management that keeps clients happy, ensures profits, and gets young engineers promotions. In many instances, doing sound structural engineering is actually at odds with good project management practices. It requires greater expenditures of design time, sometimes results in having to disappoint clients, and often adds cost to projects. It's tough.

JAE's approach using IBC is analagous to our "Part 9" in Canada. Prescriptive, pseudo design. I agree, it's a great strategy for staying competitive and remaining sane. I also feel that these design methods simply things that probably deserves simplifying. I have a horrifying related anecdote to share however.

A colleague of mine is currently embroiled in a lawsuit pertaining to a building that he designed using NBCC Part 9. It's experiencing some problems that have nothing to do with the Part 9 design aspects. Unfortunately, the lawyers for the plaintiff seem to be successfully tearing him to ribbons using this argument: "Any structure that warrants the involvement of a licensed engineer also warrants real engineering rather than prescriptive recipe following. Therefore the engineer is negligent in his duties." I don't agree with this at all but that's how it's going down. In legal settings, prescriptive design is starting to look like toilet paper armor.

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

RE: Wood Diaphragms and Shear Walls for Common Apartments

(OP)
Kootk,

Great reply, and thanks for the book reference. I actually purchased that book and it's what got me thinking this way. That, and preparing for the SE exam. Incidentally, Terry Malone, who wrote that book, also works for Woodworks and is a tremendous resource. He's available as a technical resource for non-residential wood structures. So if you email him, he can help further than the book. I don't get anything from plugging him here. I just hope more engineers grab the book like you did. As for the "questionable" designs we all run into, I don't think it's gross negligence in most cases. I think it's just ignorance, and in some cases willful ignorance. I've run into many engineers that seem to prefer being in the dark. At least that's my experience.

Anyways, I really appreciate your feedback, sometimes I feel like I'm on a conscientious engineer island...

Nick

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