Envelope #8: Spot these common challenges in wood lateral system during schematic design
Andy Lin
May 05, 2022
Happy ‘it’s-almost-Friday’ day! This is Back of the Envelope, where I help you become 1% smarter each week.
In today’s email, I will quickly summarize what I learned from a recent webinar called “Common Challenges in Wood Lateral System Layout.” It was presented by Terry Malone, Senior Technical Director at WoodWorks, who has decades of experience designing structures.
The webinar’s goal was to help us recognize things that we need to watch out for during schematic design (so that we can be prepared to face the challenges… or propose alternatives to the client).
Let’s dive in:
💭 Terry’s thought process
📝 Common challenges
🤔 My non-technical takeaway
(Estimated read time: 4 minutes and 10 seconds )
Terry’s thought process
Terry has a standard process that he goes through when looking at a wood building:
1/ First, using PDF markups, overlay the shear walls from floor to floor so you can see how they stack (or not). Use different colors to represent shearwalls on different floors.
2/ Next, locate the diaphragm boundaries to get a sense of the load paths.
3/ Based on what you see, identify potential irregularity and load path challenges, such as offset walls, large openings, cantilever diaphragms…etc.
4/ Lastly, form opinions and develop possible solutions based on what you’ve identified.
Here is an example of what that pdf might end up looking like:
(Source: WoodWorks/Terry – link at the end)
In a nutshell, step back to look at the building from a global perspective before pulling out your calculator to crunch the numbers.
This will help you better understand how the building might behave or perform and help you recognize what you could potentially improve.
Common challenges
Now, here are some of the lateral design challenges that come up frequently according to Terry (or maybe common “misses” by engineers?):
1/ Shear wall horizontal (out-of-plane) offset (i.e., non-stacking wall)
If you are curious and want to open your handy ASCE 7, this is Table 12.3-1 “Horizontal Structural Irregularity Type 4.”
When this occurs, all supporting members need to be designed for overstrength.
(Or convince the client that “stacking the walls” is the way to go.)
(Source: ASCE41-17)
2/ Shear wall in-plane offset (continuous and discontinuous)
Similar to the last one but offset in-plane (“Vertical Structural Irregularity Type 4” per Table 12.3-2).
It is straightforward-ish when the shearwall boundaries come straight down like this:
(Source: Common Challenges in Wood Lateral System Layouts)
It gets tricky when the walls don’t line up nicely (see below) – in this case, the headers and the jambs would need to be designed for overstrength.
(Source: Common Challenges in Wood Lateral System Layouts)
3/ Large diaphragm openings
We should pay attention to how diaphragm forces transfer through drag and chords when we have a large opening.
But what is a “large” opening?
According to the recommendations by FPInnovations (Canadian non-profit wood people, link at the end), if the opening has any of the following, it is considered large:
Opening depth > 0.15 x (Diaphragm depth)
Opening length > 0.15 x (Diaphragm length)
“Distance from diaphragm edge to the nearest opening edge” < 3 x max(Opening depth or opening length)
“The diaphragm portion between opening and diaphragm edge” exceeds the maximum aspect ratio requirement (i.e., check the diaphragm aspect ratio around each side of the opening against the SDPWS requirements.)
(Source: SDPWS 2015)
If any of the criteria above is true, it is recommended that you should do a detailed analysis (the FPInnovations document has examples on how to do this. Again, link at the end).
4/ Discontinuous chord
Essentially, watch out for corners.
Terry dives into this in more detail in a different WoodWorks document called “The Analysis of Irregular Shaped Diaphragm.” Check out the link at the end to learn more.
(Source: The Analysis of Irregular Shaped Diaphragm)
5/ Cantilever diaphragms
This is allowed by code, but you must meet the SDPWS requirement for “Open Front Structures” (Section 4.2.5.2).
(Source: SDPWS 2015)
6/ Flexible vs. semi-rigid or rigid diaphragm (and global uneven stiffness)
So technically, the code allows you to idealize a diaphragm as flexible if you meet the criteria in ASCE 7 12.3.1.1. This means the analysis is relatively simpler compared to rigid or semi-rigid diaphragms.
However, according to NEHRP Seismic Design Technical Brief No 10 (link at the end), most light-framed wood diaphragms behave semi-rigidly in reality.
The implication is that if you have a building with uneven lateral stiffness on each end, you could have excessive drift on one end even though you technically designed it to meet code.
(Source: ASCE41-17)
So what to do?
You could run a back-of-the-envelope calc treating it like a rigid diaphragm (or cantilevered beam). Based on that, estimate the drift, then stiffen up the lateral system based on educated guess (ahem, I mean engineering judgment).
7/ Combined lateral system
Lastly, according to ASCE 7 12.2.3.3, if you combine different lateral systems (e.g., wood shear wall + moment frame), you should use the lowest R for the direction under consideration.
Don’t need to follow that if you meet all of the exceptions though:
Risk Category I or II
Two stories or less
Light-frame construction or flexible diaphragm
Non-technical Takeaways
Well, that was a lot of technical info. Here are some of my non-technical thoughts:
1/ If it feels like you are overthinking things, you are probably not. 90% of the time, we are not designing simple rectangular boxes, and stuff could get pretty complicated, so don’t feel bad. You are not the only one.
2/ Being able to identify these challenges early on in the design means you could potentially get the client to change some of them by offering alternatives.
3/ And if you are very early (e.g., proposal phase), you might be able to even bake that into your fee, knowing that more effort will be required later down the road.
Well, that is all – thanks for reading!
Any questions? Let me know and I’ll address them in future emails. (You can either reply to this email or click on one of the feedback links below, which will take you to a page where you can add some comments)
Yours truly,
Resources
1/ “The Analysis of Irregular Shaped Diaphragm” (link to pdf)
2/ “Seismic Design of Wood Light-Frame Structure Diaphragm Systems” (NEHRP Tech Brief) (link)
3/ WoodWorks Events Archive with link to pdf (link)
4/ WoodWorks Free project support (link)
5/ FPInnovations/WoodWorks recommendation for large opening (link)