28 Feb Secret #7: SEAU Recap – Timber Frame Design (Part 1)
Here are a few take aways (adapted from the presentation by Paul Thorley, SE*) on the basics that you should understand about timber frame design…
This is a two part series on timber frame design:
- Part 1 — the basics: materials, tools, and the related industries
- Part 2 — the rest: connections, analysis procedures for gravity and lateral loads
What is most important?
This question was polled at the very beginning of the presentation: What is most important of the following?
- Understanding the engineering principles behind the design
- Following the governing codes that direct the design
- Clear documenting and detailing of the design
- Proper construction of the design
This is relevant to more than just timber frame design, but all types of engineering. Let that simmer in your mind for a little bit. At the end of Part 2 of the ‘Timber Frame Design’ recap, the answer from 100 engineers at the presentation will be revealed. What ‘wood’ you choose? (corny pun)
What is a timber frame?
Think about what you would expect to see in a ski lodge, a rustic barn or a gathering area at a park. This is essentially what you should expect. These building all have large wood frames (aka timber frames) constructed using traditional building methods that have been used for ages, with the help of some modern technology at times.
Timber Framed Pavilion
(image credit — www.acuteengineering.com)
Materials & Tools Used
One thing to remember with wood is that things are not always as they seem…
The ‘actual size’ of a wood member will vary from the ‘nominal size’ that you are used to seeing for 2×4’s or 6×6’s at Home Depot. When designing with wood, it is important to make sure that you are using the member size that is standardized in the governing wood code (NDS). In the midwest, it is a pretty good chance that you will be using ‘Douglas Fir’ which has great structural qualities.
Prime pieces of wood do not come cheap and some are inherently better than others. The inexpensive timber posts (8×8’s) that you get at Home Depot will likely be ‘boxed heart’ instead of ‘free of heart’ (FOH) since it is cheaper. Boxed heart lumber which tends to twist, turn and warp as it dries out. This can happen up to 30 degrees!
Boxed Heart vs Free of Heart Wood Cuts
(image credit — www.artisanloghomes.com)
There are those out there that do not believe in using automated tools, so they continue to use the archaic technologies to build timber frames. New automated technology, such as CNC machines, allows the cutting of lumber and fitting of connections much much faster and efficient. Regardless of the technology used, white oak dowels are are commonly used for the mortise and tenon connections. White oak is high density and perfect for the high load transfers at the joint, while providing a hidden connection that any architect would love.
Timber Related Industries
There are a few related industries beyond light frame wood structures and timber frames that you may stumble upon as you design in wood:
- Log cabins
- Cross Laminated Timber (CLT)
Log Cabins
As noted previously, log cabins are not ‘timber frames’ but they are made of wood… very large amounts of wood. Log cabins structurally overbuilt compared to your standard wood shear wall with 2×4’s at 18”cc spacing. Since there is so much wood, it is important to remember that wood shrinks radially and not axially. This is good to keep in mind because your windows, door openings, and plumbing have to accommodate that shrinkage. Between winter and fall, you could expect to get somewhere in the ballpark of 5”+/- difference.
Cross Laminated Timber (CLT)
Cross laminated timber is a really awesome wood product/technology that allows for low grade lumber to be used in high grade structural material. Similar in fashion to glulam beams, the strength of CLT is in the layering, bonding and perpendicular layout of the wood layers. It can be machine shopped with accurate cuts… talk about seamless and beautiful structure if the architect lets you expose some of it. While it was recently been recognized in the wood code (NDS), it has been used for years in Europe (10-15 years ahead) and Canada (5 years ahead). It is a system that has only started to make its way into the United States.
CLT Cross Section Showing Layers of Lumber
(image credit — www.apawood.org)
One concern is the glues expel vapors over time. If that’s not your cup of tea then you should check out the “Interlocking CLT” (ICLT) which is currently being developed and researched. It uses interlocking wedges between the layers instead of glue. Research shows that the system performs as a response modification factor ‘R’ between 4 and 4.5. For reference, a typical light-frame walls sheathed with wood panels (wood shear wall) has an ‘R’ value of 7, which means that the ICLT system is more ductile (flexible) than the typical wood shear wall in a home. Research has been done by graduate students at Brigham Young University (BYU) with the support of Paul Thorley, Euclid Timber Frames, and Dr. Fernando Fonseca. For further reading here is one of the research papers on how ICLT shear walls perform: ICLT Shear Wall Thesis
Stay tuned for Part 2… CLICK HERE
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*This information is adapted from the first portion of Paul Thorley’s presentation entitled ‘Timber Frame Design for Vertical and Lateral Loads’ given at the Structural Engineers Association of Utah (SEAU) Conference in Layton, Utah on February 20th, 2018. Paul Thorley, SE is the Founder and President of Acute Engineering, Inc. (www.AcuteEngineering.com). He has engineered hundreds of timber frame projects, published several papers, and sponsored original research in timber frame design including such topics as: tensile capacity of mortise and tenon connections, interlocking cross laminated timbers, and timber frame knee braces in lateral design. Beyond work and research he actively participates SEAU and the Timber Frame Engineering Council (TFEC).
I was not able to cover everything that he discussed, so I will have to periodically come back and revisit his thoughts another day. In the meantime, his presentation can be found here: Timber Frame Design Presentation
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