03 Mar Secret #8: SEAU Recap – Timber Frame Design (Part 2)
Here are a few take aways (adapted from the presentation by Paul Thorley, SE*) on the rest of 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
Timber Frame Connections
The are a lot of connection types out there including: scarf joint, spline, half lap, dovetail, tension rod, split rings and shear rings, log screws, etc. The list could go on for days. The two that stood out to me are the ‘knife and gusset plate’ connection and the ‘blind mortise and tenon’ connection.
Knife Plates and Gusset Plates
The most important thing to remember for knife plates and gusset plates are that wood dowels and steel DO NOT go together. Just think about what material a saw or an ax are made of and it will be an easy way to see why steel bolts are used with steel plates. The main difference between knife plates and gusset plates is that a gusset plate is exposed and will be on both sides of a structural member, while a knife plate is located in the center of the structural member. Both connection types are not covered in the code (NDS), but some of the principles of the code can be applied. The NDS only allows for 1/4″ steel plates on ONE side of a structural member.
Timber Frame with Steel Gusset Plates
(image credit — www.vermonttimberworks.com)
Blind Mortise and Tenon
A very common connection type is the ‘Blind Mortise and Tenon’ joint as shown below. This connection allow for a hidden connection that is an architects dream. It is a connection that has been used for many years and it hasn’t changed much despite the technology growth. This connection has 3 failure mechanisms: dowel failure, shear failure of the tenon perpendicular to the grain, and mortise compression failure. Data collected has shown that the failure mechanism is near the limits for all of the 3 mechanisms. The reason that this connection has been used for so long is that it is an efficient design. For further reading here are two research papers on how mortise and tenon connection perform: Timber Frame Connection Thesis and Mortise and Tenon Connection Thesis.
Timber Frame with Mortise and Tenon Joint
(image credit — Paul’s Presentation)
Analysis Procedures
One thing to remember with wood is that the connections are great in compression but not in tension. Where a bearing surface is present, wood members can transfer the loads as needed in compression. When looking at gravity and lateral loads, special attention is required at the tension connections.
Gravity Analysis
Not all members of a timber frame are needed in the analysis since not all of them are structural. Where this occurs, it is okay to turn on or turn off those members to make them active in the analysis. Another analysis hack when looking at timber frames is to use a roller at the top in the vertical direction. Timber frames are statically indeterminate truss structures, which cannot be solved by analysis software unless the roller is added to provide stability in that direction. WARNING: Only use this when looking at GRAVITY loads! This will not affect your gravity design, but it would affect your lateral design.
Lateral Analysis
There are a few different things that you may need to think outside of the box for the lateral design. The points of interest for a timber frame are the base of the frame, and the connection at the truss to the post. These can be seen in the example figure below. Connections at the base are difficult with timber frames, so engineers have to get creative. Here are a few ways that the moment restrained connection is designed at the base: proprietary base connections, steel boot wrap (not ideal due to water proofing concerns), or a steel tube up the center of the post.
Since the connection at the top of the post cannot naturally be a moment restrained connection, the knee brace is added for extra rigidity in the connection. Research data is currently being done for timber frames using knee braces with the following sponsors: Dr. Judd (UW), Dr. Fonseca (BYU), TFG, TFEC, and Acute Engineering. This research will help to determine the seismic design criteria values for future design codes.
Timber Frame Example
(image credit — Paul’s Presentation and Euclid Timber Frames)
What ‘wood’ you say is most important?
As mentioned in ‘Part 1’ of the recap, this question was polled at the very beginning of the presentation with the following options:
- 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
The true answer is that it depends on who is answering the question. Is it an engineer, plan reviewer, contractor or owner? The majority of the 100 engineers at the presentation said “understanding the engineering principles” then the runner up was “clear documenting and detailing”. This was a trick question because all of these are important. As engineers it is important to understand the principles behind the design, but it is also important to know the code, and detail well so that the contractor can construct it properly. The code provides direction, but it is often a minimum design criteria. Engineers must use proper ‘engineering judgement’ when making decisions.
If you missed it, go back to Part 1… CLICK HERE
Subscribe to my newsletter to receive monthly notifications for new ‘Secrets for the Modern Engineer’. I will notify you of new posts on the website, as well as other updates to help you further your career.
Keep on keepin’ on.
Life’s a garden… Dig it!
If you have any additional topics or subtopics that you would like me to cover, please contact me at josh@thehipsterengineer.com
*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
No Comments