Ep 115 - Seismic Design of Timber Buildings

21 min 28 sec

In this episode of Timber Talks, we speak with Andrew Dunbar, a Senior Structural Engineer at Structex. Andrew is a leading expert in seismic design for timber buildings, and shares his insights on designing resilient and sustainable timber structures.

During our conversation, Andrew discusses his approach to designing timber structures for seismic performance and sustainability, and provides advice for young professionals interested in pursuing a career in timber engineering.

Whether you're a seasoned professional or just starting out, this episode is a must-listen for anyone interested in the intersection of timber design, seismic performance, and sustainability.


Timber Talks Series 6

Series six of WoodSolutions Timber Talks, provides the latest informative and entertaining information about the best design practices, latest innovations and interesting case studies and interviews with world leading experts in timber design, specification and construction.

Episode transcript

Adam Jones (00:09):

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

Andrew Dunbar (00:16):

Yeah. Cool. So it's great to be here. My name's Andrew Dunbar. I'm a structural engineer out of a company called Techs based in Christchurch, New Zealand. So my background, I guess I started coming out of uni with a masters looking at post-tension timber or CLT steerco. So that was kind of my first look. So that was under Andy Buchanan and the like. So I got a really good insight there and then worked for a few years at Structex as a junior engineer doing a few smaller timber jobs and then took my chance at an OE to Vancouver in Canada and worked at Fast Net over there and got heaps of experience and some cool mass timber buildings and a real insight into what they're doing over there, which was awesome. And now back in New Zealand for the last three years now I suppose, and kicking on with joining back in the Mass Tim Ministry in New Zealand.

Adam Jones (01:31):

Yeah, amazing. I didn't realize that was your PhD under Andy Buchan and that's really cool. I, yeah, it was

Andrew Dunbar (01:37):

Masters so I'm not quite a doctor but

Adam Jones (01:40):

Oh, okay. Gotcha. You could have just claimed that but that's alright.

Andrew Dunbar (01:43):

I Could have claimed that. No.

Adam Jones (01:46):

So what are some of the unique challenges and opportunities design in the state, say slightly toward timber buildings? And I understand New Zealand you might not be able to go as taller as other parts of the world and maybe we can just describe some of the challenges as you go taller and particularly in your region.

Andrew Dunbar (02:03):

Yeah, I guess probably some of the big challenges I think with going taller is certainly the stiffness I guess of timber. It's naturally relatively flexible I guess You see the big trees, trees are made to grow tall but then not made to have someone sitting at the top of them in a treehouse, in a gal force win. So they move and they do that on purpose. So that's probably a challenge with high rise timber buildings where you can end up with quite a lot of structure. So I guess looking back at the progression of the tool buildings kind of started with there was a one in London in a no non-seismic area that was lots of walls everywhere and then the likes of Lendlease in Melbourne I think was next at 10 stories. And again lots of walls everywhere, kind of the honeycomb type feature.


So then moving towards higher buildings and more towards seismic zones. I think Brock Commons in Vancouver at the 18th story student accommodation building was up there. So that one was a bit of a hybrid where they had a concrete or two concrete cause that were doing the lateral system. So I guess the jumping that stiffness challenge by throwing a different material at it but keeping the gravity structure as timber. Yeah and I think the one in Norway, 18 stories or so or 20 stories was all timber but braces and not so much of a seismic zone. So I guess where I'm coming to is I think the stiffness is difficult when you're getting really tall without having just a walls everywhere. Probably one other, another challenge then was you're getting those, the big heights and the big forces I guess is the connections. So you start things starting even governed by connections pretty quickly and sometimes can look a bit out of proportion if you get your system wrong to start with, if you know what I mean. Yeah. And I guess seismic zones also add their own complexities with ductility and how you reliably achieve that and potentially stuff like nailed connections with lots of slip and low post yield stiffness don't quite cut it so much anymore.

Adam Jones (04:58):

Gotcha. So I'm keen to pick your brain a little bit on seismic. Bear in mind that a lot of the listeners are structural engineers but are also a lot aren't structural engineers. So we might sort of speak at that level a little bit. So when it comes to mass timber buildings, because it's start with the positives on one hand there are a lot of positives that you hear about when it comes to seismic zones and what are some of the features that are really helpful?

Andrew Dunbar (05:26):

Yeah, so I guess there's any challenges in mass timber buildings, but there's awesome opportunities as well and I think that's why probably I'm here and you'll hear and hopefully the people listening are here. So in terms of seismic, I guess a big benefit of timber is it's light kind of weight strength to weight ratio is pretty good. So yeah, almost ending up with a quarter of the weight of the building if compared to concrete. So that's directly your seismic forces you the amount you've got to resistors to hold up the building is kind of potentially courted is. There's always a bit of unders and overs but yeah, huge benefits there. Another kind of positive I think is really is not specific to seismic but is the speed of construction. It's just really can be quite fast and that's something we really have to leverage we to get the cost equation in there. Yeah,

Adam Jones (06:43):

A hundred percent. And they all linked together of seismic design with the speed, particularly in manufacturing. One of the things you sort of touched on was steel, so how timber can combine with steel in as a combination as a system to actually work. And you mentioned the idea of ductility and why, how is that important for a seismic zone and is steel doing the work that enables the ductility in sort of a hybrid kind of way?

Andrew Dunbar (07:13):

Yeah, yeah, sure. So I guess ductility at a high level, please don't shoot me all your other structural engineers out there is I guess the ability for something to go past a yield point or a soft breaking point or a fuse point and hang on and go for the ride for as long as it can. So I guess some of this analogy would be like when you're playing with a paperclip and you bend it, the bending, you bending it is a yield point and then for a while you can crank it back and forward and that's the ductile zone I guess or the ductile behavior part and then eventually you snap it and that's the brittle failure.

Adam Jones (08:05):


Andrew Dunbar (08:05):

So timber itself is, parts of it can be have brittle failures, like a tension you pull on a piece of timber and eventually it just snaps and that's a very sudden failure. There can be slow failures in timber in compression I guess where things just kind of squash. But in general in seismic design we're trying to not have the timber if we don't want the timber to be the failing part. So we need something else to fail. So that's where steel comes into play really. So it's either bending of little things like nails and screws, which can be very ductile or it's something bigger for mass timber, maybe it's a fully steel brace system or a buckling restraint brace or a seismic dissipator connected in there. So quite personally, I quite like hybrid structures. I'd rather something had a bit of concrete and steel in it and which means the timber can remain as the gravity system than it being completely shelved. So yeah, don't not afraid to have purists timber buildings. I think we need a bit of a mix and it might be better overall. There's other strategies we can do to increase resilience with some of those other materials

Adam Jones (09:47):

A hundred percent and it makes a lot of sense. And I guess your master's was somewhat related to this topic now what is the role of post-tension shear walls and is this, what actually are they and then what is the sort of feasibility and when can someone think if it's right for them?



Andrew Dunbar (10:05):

Right, yeah, so post-tension shear wall, it's a system called press lamb. So it's basically you get your big piece of CLT or a structural timber wall and you put a big piece of steel tendon or a steel bar down the middle of it and you stress that up. So that acts a bit like a rubber band so as the wall can rock around and do its thing and this tendon is there to snap it back, so after an earthquake it should be back vertical again. So along with that then you can add some damping elements. So you have some little ductile fuses say so that in an ideal case a big earthquake happens, you're building rocks around snaps back together from the rubber band tendon and your dissipated do their thing, they form their fuses, they yield and we you go and unconnect them and put a new one in and then your building's good to go. So my research came kind of from that and I guess looking at stairwell, cause in particular was the first kind of look at trying to get higher where we, there's always a stairway call or multiple stairway calls in a building and they're often a big group of walls because you need that for fire and everything anyway, so it's a nice big solid structure. So it's looking at yeah, post-tensioning as a way to get high strength connections and good seismic performance.

Adam Jones (11:59):

Yeah. So that's one new technology. Are there any other new technologies, not necessarily just material specific that you see as coming through? Could be modern me methods of construction more generally or anything like that that are having a big influence in the way we build our buildings.

Andrew Dunbar (12:21):

There's certainly a few technologies coming, there's lots of different types of dissipator and there's ones like tecton which is a steel kind of form things. So I did one of those in Vancouver which

Adam Jones (12:34):

Says A what? So can you maybe elaborate on a D dissipator and on how that works and what it is as well? Yes, and yeah, keen has to hear about that.

Andrew Dunbar (12:41):

Yeah. Okay. So it's basically a bit of your shock absorber and on bike and it's as the earthquakes come, you attach these things to the bottom of the walls say so that the walls are rocking back and forward and these things at each end are acting like your shock absorbers for the system. So they're taking a bit of the heat out of it and slowing it down. And that one in particular I guess is so often something has to get damaged to form a fuse. Don't usually get energy for free, but there are systems out there that can do dissipation with not much damage. So I guess base isolation has been around for a long time and New Zealand and Japan and the states with lead rubber bearings and that sort of thing that deform and provide damping and dissipation. But I guess in a concrete wall say that's designed to be ductile, the energy dissipation in those come from the bars within the walls yielding and deforming getting longer. And that energy that that's created from deforming those bars gives you the energy dissipation. The only catch in that is when afterwards you are left with a wall or a building that is damaged and has used its fuse I guess. So we saw that a lot in Christchurch following the earthquakes that large chunks of the city were demolished somewhat from insurance. We were very insured, but partly because of that damage you take

Adam Jones (14:44):

Could, I was going to ask, can you take a peak if it's a reinforced concrete shear well can you really inspect or is it sort of brittle cracks or something? How do you know if it's damaged shot or the fuse is broken? Yeah,

Andrew Dunbar (14:57):

Yeah, it can be, depending on the detailing of the wall, it could be difficult. But in a ideal case, like a lab case, you'd see a big fan of cracks at the bottom of the wall. So you should be a whole bunch of distributed cracks and that would be a good indication that a hinges form there. And it'll be at a location at the bottom of the wall that you'd expect or at the beam column, joint in the beam where it can be hard as sometimes a single crack could form at the very base. So there was a few of those in the Christchurch earthquakes where the wall detailing didn't allow this nice fan cracking to occur and it just lifted up at the bottom and it potentially fractures the bars and then the wall comes back down again because of the weight on it and you just see a little crack at the bottom, but you don't know that that has actually fractured all the bars. So there's a certain challenge there, but there are techniques I guess with scanning and drilling and gets a bit messy to investigate. But it's certainly a challenge and that's why timber buildings are a bit easier. You kind of, everything's on the outside you can see it and that the crack or the rocking has to occur at the base of the wall weather between the wall and the foundation.

Adam Jones (16:33):

Well we're talking highly technical things in seismic and also mass timber, it's, it's not taught at university and this is probably across the board, not just for engineers. How did you go about learning? Because there's probably a big hurdle for an engineer listening right now and they're thinking, Hey, I learn concrete and steel, I want to get into mass timber, bloody, what do I do? Where do I start? How would you go about it and what sort of methods?

Andrew Dunbar (17:02):

Yeah, I guess for me I went with the method of doing some study doing a master's, but I was targeting I guess a very specific avenue that I wanted to do timber stuff. And so talking to the right people, so if anyone's out there kind of getting close to the end of their studies, there's lots of research programs around there. So that's one avenue that you can progress. Otherwise if you're kind of in a consultancy, I guess Tim is getting certainly more popular and lots more people are talking about it from a client perspective. So probably my advice would be kind of say yes to in any opportunity that you get.



And secondly, don't necessarily wait for an opportunity, so try and create some, so even if it's within your office, you volunteer to create your carbon tracker spreadsheet or something then, or you just do a carbon track on a concrete and steel building and that often is enough to trigger the client or even just other people in your office to be like, hang on, sure that's a lot of concrete or a lot of emissions. So that can, small things like that can lead to the, oh okay, let's investigate. Maybe this could be a timber beam even and probably start small, don't try and make the purest timber building out there, just switch the floors, put a CLT floor in over the top of steel beams just like a concrete floor that we have here, which is a steel tray with near fill up with concrete. Just do that. They do about the same span, the timber's much lighter and has other challenges with acoustics and stuff, but it can be competitive. So just start kind of small and then snowball. Yeah,

Adam Jones (19:12):

That's awesome. Thanks Andrew. And looking forward with a bit of a blue sky hat on, what do you see as the future of timber construction and construction more generally in the next five to 10 years? Thinking about new tools or where things are going in terms of overall trends in body carbon? And take the question any way you like.

Andrew Dunbar (19:32):

Yeah, maybe I'll spin your question around a little bit and probably say, I guess I don't know if it's the future of timber construction, but probably the future of construction is timber in my opinion. I guess thinking if I had someone say that if concrete was invented today, it just wouldn't get off the ground. It takes a month to cure before you can do anything. You got to have all this special form work, you have to have special trucks to drive it around. It's supercar intensive and it's slow. So it's kind of everything that timber isn't. But I also want to say we need concrete, it's good, it's good in the ground, it's durable, we just pick where we use it.


So in terms of timber construction moving forward, I guess what I hope for the industry or for construction industry is kind of boring stuff like super standardized beams and panels that you just fire out that are super obvious. You go down to your Burnings or whatever and you can buy a lamb beam and it's just super automated. So that with the goal I guess of getting costs down. I know Europe, even for us in New Zealand, sometimes it's cheaper to buy Glue Lamb from Europe and ship it over here than it is to buy it locally. It's not always the case, but that kind of doesn't make sense. Java tree down it and yeah, within 50 ks of view get it fabricated and it, it's cheaper to come from Europe, it it's crazy. So I just want boring stuff. I want standardized connections that just work kind of like steel and concrete have got these catalogs that have been around for a hundred years or something.


Yeah, obviously longer, but it's well known in the industry, kind of what works, how the contractors like to do it, what works for a fabrication perspective. So I know with timber there, there's a lot of that IP out there from the suppliers, but I just want it really easy so we're not having to reinvent the wheel all the times. And yeah, hopefully if we get to that, we get nice and cost competitive event, the more timber we get in the buildings, the better. And then we can play with the fringe architectural stuff as well. There's plenty of room for innovation.

Adam Jones (22:17):

A hundred percent. I love it. That's not boring at all. I'm an engineer as well and that's sort of stuff that excites me the most. If people want to find out more about yourself, Andrew, and some of the things we've been speaking about, where should they go?

Andrew Dunbar (22:30):

I guess you could check out the Structex website and get my email off there. And have been involved in some kind of seminar series in New Zealand and I've got one coming up in Nelson in a few weeks. Or if you've got a cool timber job, hey you want some design advice, then yeah, come and talk to me. But yeah.

Adam Jones (22:57):

All right, nice. Thanks so much mate. That was awesome. We'll leave it there.

Andrew Dunbar (23:00):

Awesome. Yeah, thank, thanks a lot for being, for having me along on here and hopefully people have enjoyed it. We haven't talked too much about concrete.

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