Ep 138 - Advancements in structural modelling of timber buildings with Zhiyong Chen

Intro  (00:07):

Timber Talks is brought to you by WoodSolutions. Stay up to date with the latest in timber, the building material that is strong, safe, and sustainable. Here is your host, Adam Jones.

Adam Jones (00:20):

One of the key processes of designing safe and efficient timber buildings is the structural modeling process. However, this does require deep understanding and expertise in order to execute effectively because timber is a material with very different characteristics to concrete and steel. So, in this episode, we're speaking with a world leading expert on this topic, Zhiyong Chen, a senior scientist at FPInnovations about the structural modeling of timber buildings. Now by the nature of the topic, it does get into some technical discussion, but I found it really riveting because it is fascinating when you're hearing from some of the most renowned technical experts on topics like this. So, without any further ado, here is my conversation with Zhiyong.

(01:02):

Well, thanks so much for coming on the podcast. Zhiyong, can you start by telling us a little bit about yourself and what you do?

Zhiyong Chen (01:09):

Thank you, Adam, for the invitation. I'm a senior research scientist in the building systems group at FPInnovations in Canada and also a member of ISO/TC 165 and Timber Structural Committee and members of the Wood Structural Committees of Intelligence Society for Civil Engineering and are members of the technical task group of the Kenya Standard for the engineer design in wood CSOD 6. So, I conduct research by testing, modeling, or both, to investigate the seismic performance and the bio resistance of timber structures to support the code and standard implementation of timber systems. And meanwhile, I'm also an external graduate faculty similar to adjunct professor at the University of Miami in US. So, I co-supervise graduate students there and I also have some cooperation with the Kenya University. So, I also advise Master and PhD students at the UBC, McGill University and also Universities of the Waterloo. So, my expertise lies in the advanced and practical numerical monitoring of infrastructures under different actions like the seismic, wind, fire, moisture, and also the time effect. So, this is basically who I am and also what I do.

Adam Jones (02:35):

Amazing. Well, can you start maybe just by telling us a little bit about the differences when you're modeling and looking at timber, and how it might be different to concrete and steel?

Zhiyong Chen (02:45):

This is a good question. The main difference of this material and structures lies in two levels. The first level is material level as we know that still actually is anisotropic material. And reinforced concrete actually is a composed material and timber is anisotropic material. So, modeling of timber at the material level is more complex, and the timber actually have different elastic properties and strengths in different grain directions. And even though in the same grain directions, the strengths, and also the failure mechanism of timber will be different. So, that's usually we need to have a suitable constituent model to submit the timber correctly. Another level is the system level because every material have their unique mechanical characteristics. So different kind of the design philosophy or strategies will be applied to the design of different structure systems to optimize the use of material.

(03:56):

So, the structural behaviors and also the modern emphasis or different structure system will be different. Here I want to share the two examples. The first one is the shear wall systems. The concrete shear wall system actually is governed by the cracking of concrete and also the building of the rebars. And the system is usually similar with the beam column elements with the fractured plastic at the bottom. But for the timber sheer walls usually is the behaviors governed by the connections at the bottom and also between the panels. And so, we use weights to weigh the panel with the shell element and the spring elements for the connections. This is the difference between the concrete and the timber for the sheer wall system. The second example I want to share is the base frame system. For the steel-based frame systems, it is governed by the kind of bases, which is actually new under tension buckle under compressions.

(05:02):

So, when we model the steel-based frames, we need to pay attention to the model of the dyno bases. But for the timber structures, the timber-based frame system actually is governed by the end connections of the panel braces, not the diagonal basis. So, for the timber-based frame, we need to pay attention to the modeling of the end connections to make sure that we set the model, his model to represent the behaviors of the timber connections. So, this is how we see the difference among timber, steel, and concrete structures. In terms of the modeling emphasis.

Adam Jones (05:39):

That's an incredibly dense, awesome, enlightening answer there. And so, can you start by introducing the modeling guide briefly? And I'd just like to say that because you're a humble man, but it's a 740-page document and I know some people in Australia who have been part of it and I think you've been able to get the world's best engineers on this topic come together and collaborate for a beast of a document that is taking the industry forward. So firstly, thank you on behalf of the world for putting that together and secondly, over to you and talking about what this guide is.

Zhiyong Chen (06:14):

Thank you, Adam. And also, we are very happy to share the honour and share that and describe with you guys. I think we can go back to several years ago I think we conducted a survey and then we found out that most of the engineers, they're familiar with the modeling of the steel and concrete structures, but they're not familiar with the timber structures, especially mass timber structures. And nowadays the timber structures increasingly require the demonstration of the performance and equivalence of them and no matter if they decide for the prescriptive or performance-based procedures. And so that's why there's a huge need for develop this modern guide. And so, at the beginning of 2020, we initiated in global cooperation, we invite more than one hundred experts on different area to work together to generate this guide and this guide actually has 10 chapters. Basically, we can divide into three main sections, introduction, modeling, and analysis.

(07:17):

In the introduction section, we briefly introduced the scope and also the structure of the guide. And also, we point out the difference among the timber, steel, concrete structures in terms of the structur behaviors and also the modeling emphasis. For the modeling section, we talk about how to properly model timber structures at different levels. At the components level, connection, assembly, and the whole system level. And also, we provide specific modeling considerations for the audience. In the third section analysis, we talk about how to analysis infrastructures under different scenarios and actions. For example, we have the polygraphs collision and also the wind-induced response and also the seismic response analysis. And this publication actually is the first of its kind in the world. And also, it brings together the experience obtained from the most recently finished timber projects and also the latest developments of the modeling in timber structures. So, it drives a lot of the advanced and practical content shared by the experts from the academic area and also the engineer area.

Adam Jones (08:34):

So, what are some of the load resisting systems that are available for engineers when designing timber buildings?

Zhiyong Chen (08:40):

Oh, that's a good question. Load resistance actually is the structures that take and transfer the load of force to the ground… to the earth. And usually, we have two main types of the structural systems. One is the gravity load system another one is the lateral load system. In terms of the gravity load system, which actually it takes and transfers the gravity load from top to the bottom eventually to the foundation. For timber structures usually, we have the post and beam to transfer the gravity loads in terms of the lateral load system, which actually transfers the wind and seismic loads to the earth. For timber structures we have several types of the shear wall systems, we have the platform type and balloon type shear wall systems and the braced timber frames and the movement resistant frames. Most of best ones include the post tension, sheer wall system and also pole tension movement frame systems, which is our core system which has been invented in New Zealand. Sometimes the gravity systems and the lateral resistant would be separate in the building. So, one part just take the gravity load, another part would take the lateral load, but sometimes the same structure would play both roles to take the gravity load and lateral load. So, it depends on the design.

Adam Jones (10:04):

That's great. So, you mentioned one thing was seismic and that's obviously a very important topic and I believe around the world there's new building codes that are upgrading to pay better attention to seismic and it's commonly said that timber is good in seismic regions. So maybe can you tell us how does timber perform in seismic locations and how should we be modeling to adequately show the performance of timber and seismic locations?

Zhiyong Chen (10:31):

Yeah, in most of the areas we have a lot of the restaurants and people in actually that area have high seismic zone. So, it's a very important topic. And when the earthquake comes, the ground motion will shake the building and the building has to dissipate the energy by either fail or collapse, or the energy dissipators. And timber structures have been usually been designed smartly for the seismic-based design methodologies in which the timber connection would be designed for the energy dissipators to dissipate the energy from earthquake and other part, for example, the timber part would be capacity protected without any failure. So, the timber connection will play a key role in the seismic performance of timber structures. So, we need to model them correctly and properly. And timber connections are actually kind of different from the concrete and steel connections because they have stiffness and strength adaptation, and also the pinch effect, which are unique to the timber connections. So, we need to select  the most suitable model to simulate a behavior of the timber connections. So, this is something that we need, this is the major consideration we should put on the modeling of infrastructures in terms of the seismic response and assessment.

Adam Jones (11:49):

Gotcha. So, is that for timber connections and is steel, I mean steel in timber connections, how does the steel perform and is that better performing than concrete structures in your opinion?

Zhiyong Chen (12:01):

I think different connection have different behaviours, but for example, for the steel structure, steel connection, it behave have full and for the timber structures the first load would be full, but later on will have pinch effect decreased a little bit. So, in terms of the deformation, I think timber connections performed very well as well. So yeah, mhmm.

Adam Jones (12:24):

Good to know. Great. So about disproportionate collapse, that's something else that I guess it's got different focus in different parts of the world it seems where some places it highlights it in a very big way and in other places maybe not as much. So yeah, what is disproportionate collapse and how should we approach that?

Zhiyong Chen (12:45):

Disproportionate collapse, also known as the progressive collapse, which is s phenomenon in the building when low-code damages happen, for example, failure or remove of load bearing elements and then it caused the structure or entire collapse of the building. And this is a new topic for timber structures and if we want to prevent this type of collapse we would like to increase the load balance of the system. And there are usually six main methodologies for design analysis and also strategies to improve the load balance of timber structures. The first one is the high-force design approaches. Here we don't simulate the whole collapse explicitly, but we will put a lot of the requirements for the design and make sure that the building will not collapse during this time of the event. The second one is the alternative load passing design methodologies with static analysis.

(13:46):

This is the most common and the popular one to investigate the load requirements of the timber structures and also other buildings. We have the linear, non-linear, and pushover analysis with our study of dynamic response. The third one is the alternative load passing methodology about 50 dynamic analysis. The fourth one is the key elements, here we decide the load bearing elements as the key elements, so we would make sure that it has enough resistance to carry the specific prescribed parcel loading and survive from that event. The fifth one is the redundance approach. So, in this approach we try to have more load element to shear the force. So, when one fails, they still have the second or third one to transfer the loads. So, it is how the redundancy approaches apply to the timber structures. The sixth one is the compartmentalisation approach. Here we divide the building into several sections and each section actually have their own robustness, so each would be robust enough to prevent the whole collapse. So basically, there's six ways for the design analysis and also strategies to improve the robustness of the structures.

Adam Jones (15:06):

Great. So, it's an incredible piece of work you've put together. It's taken us forward. What are the next steps on this topic?

Zhiyong Chen (15:14):

The modeling guide actually is a great success, and it provides them guidance on the modeling and analysing timber structures. But at the same time, we talk to the engineers and researchers, and we hear that we need more. For example, we need the input data for the modeling design and analysis of timber structures, and also, we heard that now we are learning how to do the modeling it would be great to have the code standard to make sure that we do it in the standard way to make sure that the analysis result is reliable. So, for the next step, I think we will have the two things can be done. One is to develop a database to support the design, modeling, and analysis of timber structures. And the last one is the standardisation of the modeling of timber structures.

Adam Jones (16:02):

Great. As we're wrapping it up now, J, if we look into the future for you, what gets you excited by the future of the industry and where everything's going? So, it's an open question and just in general, what is getting you excited by the future?

Zhiyong Chen (16:18):

I think now we are going to areas we have the computer and simulation and modeling and also we have the performance-based design methodologies. In the future, we are not going to design the building for the specific code. We are going to evaluate the building by performance criteria. So, in that case, actually we need the computer to help us to evaluate the performance, to demonstrate their performance under different scenarios like seismic wind, collapse, and also other kind of the fire events to make sure that we build the buildings economically but also provide the same safety level. And this I think will bring us to digitalization era  so we build digital teams, not just show the people how we construct it, but also we show the people how we perform in a way safely to make us confident to have this kind building built in such a way.

Adam Jones (17:16):

Yeah. Well, it's a beautiful future and some been great to speak to you today. If people want to find out more about yourself or if people want to download the guide or learn more about this topic, where should they go online?

Zhiyong Chen (17:28):

Thank you. If your audience want to read more about the modeling guide or know more about this. You are welcome to visit the FPInnovations website, it be web.FPInnovation.ca/modelling or you can send me an email if that if you cannot get the right address, you can send me email by to me at [redacted]

Adam Jones (18:01):

Yeah, that's awesome. Just out of curiosity, how many people work at FPInnovations? And the reason I ask is like you come up with some massive documents, the very good literature you come up with. So how many people work at FPInnovations, do you know off the top of your head?

Zhiyong Chen (18:16):

We have roughly three to 400 people, but FP innovation actually is nonprofit and company institution actually, we do research actually for the cycle of the wood from the gene to plant the trees and cut the trees and ship to the manufacturers and then to make the products and ship it to the construction site. And then for the code implementation. Our group actually is the building system group at the end of the cycle. We have experts on different areas. So, we have to be people on the seismic wind, fire, acoustic vibration, and also energy. So, we have expertise on these different areas and at the same time we would like to have a close cooperation with the people around Canada, North America, and also the world. So, we would like to work together to generate something with high impact. So, take the example of the modeling guide, We have our we have experts from FPInnovation, but we also have the experts around the world. No matter is a professor or engineer and wherever they would like to share their knowledge and share the experience, we would love to collaborate with them and then to generate something of high impact. And so yeah. I hope this will answer your question. 

Adam Jones (19:34):

Absolutely does. It's a beautiful note to end. Well, so, thanks so much for coming on the podcast. That was super enlightening and entertaining, so thank you.

Zhiyong Chen (19:42):

Thank you, thank you, Adam, for having me.

Outro by Adam Jones (19:56):

Now, it's pretty well known that timber is something that isn't necessarily taught in a very big way when it comes to university. However, WoodSolutions Campus is here to help. There's a lot of different topics and courses in there. One, for example, does a structural engineering topic, another one for architects. There's other courses for apprentices and trainees. So, I highly recommend going to the website for WoodSolutions Campus. Just put it into Google and you can go and check it out.