ascent mass timmber
ascent mass timmber

America’s Tallest: Ascent Mass Timber Construction Project – 25 Storeys

Foreword by Ian Thompson, Editor

Introducing The Ascent mass timber project. Allow me to introduce The Ascent, a 25-story hybrid construction gracing Milwaukee’s skyline. Standing tall at 284 feet (87 meters), this timber hybrid masterpiece transcends mere measurements. Its cross-laminated timber (CLT) and glulam components blend seamlessly with a concrete base, creating a resilient yet eco-conscious edifice. It was completed last year and was said to be the tallest mass-timber construction in the world at the time, but for now we’ll settle for the tallest mass timber construction in North America with 260 residential units and 273,000 square feet of mass timber utilisation.

In today’s video we’re diving into the key aspects of this construction project that’s pushing the boundaries of what’s possible with mass timber construction. The project is a testament to the innovative use of mass timber, a sustainable and renewable building material.

In this video, we’ll explore how the Ascent project leverages the International Building Code (IBC) to design structures that push beyond the framework of the code, while still meeting its intent. The 19-story mass timber structure sits on top of a seven-story post-tension concrete structure, a common construction approach in Milwaukee for above-grade concrete structures for residential parking.

The Ascent project demonstrates the potential of engineering smarts in maximizing the use of exposed timber – about 35 to 40 percent of the structure is encapsulated, achieved purely through design. The project also showcases the use of concrete cores as the lateral stability system, a clever way to increase the flexibility and safety of the structure.

We’ll delve into the fascinating world of connections in timber structures, where seated connections and shelf angles are used to create a more distributed connection around the core. This approach provides flexibility in setting elevation adjustability and a cleaner system overall.

We’ll also touch on the challenges and solutions in ensuring the stability of the structure, where the movement of timber and concrete requires careful consideration and flexible connections. The use of shimming for in-field adjustment is a particularly innovative approach used in this project.

The Ascent project also highlights the importance of collaboration, especially with the local fire department. The project has been a collaborative journey, addressing concerns for smoke, temporary conditions, and heat, and coming up with design criteria to meet these concerns.

Finally, we’ll discuss some of the lessons learned from the project, including the need for a strong partnership with contractors, tight tolerances, and the benefits of prefabrication and modularized delivery approaches.

The Rise of Ascent: Innovating Mass Timber Construction

Ascent Mass Timber Project

Video Transcript

First of all, thank you for having me. I’m happy to talk a little bit about our Ascent project in Milwaukee. It’s a mass timber structure that, basically, we have utilized the IBC in the United States here and kind of pushed it through what we think is its next logical conclusion as far as how to really design these structures that go a little bit beyond the framework of the code, but yet still meet the intent of the code.

It’s 19 stories of mass timber on top of seven stories of post-tension concrete, which is quite common in Milwaukee to have above-grade concrete structures for residential for parking. So, but yeah, that’s that’s the Ascent. We’ve we started its design on it really in 2018, and we’re now underway in construction about really five or six floors of mass timber up.

Phenomenal! And you mentioned using the International Building Code, the IBC, so how much in terms of exposed timber, what level of exposure were you able to use through engineering smarts?

Yeah, I mean, that’s a great question. So, unlike some of the prescriptive approaches and even some of the new 2021 versions coming out, what we did is we looked at it more from a pure engineering standpoint and design, and where where we wanted concealed spaces to be encapsulated, and where we wanted corridors to be encapsulated for a host of other reasons.

And we ended up, I think, somewhere close to right around 35 to 40 percent encapsulated, but that was just through pure design. There was no target that we were looking to achieve.

Yeah, I like it. And pushing to be one of the or the tallest timber building in the world, obviously the lateral stability system is vital to get that height. How’s the stability system work?

Sure, I mean, so the lateral system for the building is concrete cores. We looked at doing an all-timber lateral system above level seven, but through some of our negotiations with the Milwaukee Fire Department and some of the considerations for egress during the temporary conditions, we ended up settling on concrete cores.

It gave us some flexibility to have, you know, them constructed prior to most of the timber underway, and give more of a hardened corridor, an egress point for the fire department where we could have two dry stacks basically constructed as we go up. Then they could charge them with water, you know, during the temporary condition and construction if they ever had, you know, event on site with fire.

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So that was some of our logic, and in, you know, in concrete, I would say is very common in Milwaukee. It’s pretty much the way most buildings are built, so it was logical for us to just continue that up, get it off the critical path, free up the cranes. It really kind of dual purpose, speed things up, but also give something back to the fire department.

Yeah, got it. Were they precast concrete cores, or was it all in on in-situ?

They’re all in-situ, self-climbing form systems, so disconnected really from any of the, you know, need to lift the forms manually or with the crane. So, just a self-climbing jump form system.

Yeah, I like it. And there’s to be some big diaphragm loads coming from the mass timber system back to the core. What do these connections look like, and is there any special considerations were thought about here?

Yeah no i mean that’s um that’s a great question from a structural standpoint. It’s really i would say one of the most challenging connections that we had to design in this project.  Right now they’re they’re more of what you would call seated connections where we’ve got shelf angles that go around the core that basically attach to the bottom of the clt diaphragm. 

And instead of having big steel ties coming off the corners that strap back, we had a more distributed connection around the core.  What it allowed us to do is it gave us also some flexibility in terms of setting elevation adjustability. You know for when we’re targeting it to be level and really i think it was it was a cleaner system for us versus maybe some of the more traditional strapping that you might see.

But really a shelf angle almost almost like what you would see for a metal deck on a composite steel office tower . Where you have like a metal deck angle around the core. 

Yeah absolutely and the connection i think you touched on it briefly.  There just that allowing for movement in that connection because we the concrete’s going to move and behave a little bit differently to timber.  So what was what was looked at there to make sure it um you know connects as easily as possible. 

And fast and just so it could be installed fast.  I guess yeah i know it’s a great question.  So a lot of what we’ve done early on uh we actually have designed a lot of tall buildings.  Like some tall some super talls where vertical movements and differential movements over time from creep are really a major portion of how we recommend compensation or vertical correction during construction. 

A typical 25 story building out of concrete, you would say is negligible, but this one is quite a challenge. Timber moves a lot more over time, but you also have then the concrete right directly adjacent to it, which doesn’t at this height.

So, um, the connection itself, um, has flexibility. It’s a vertical slot that has cert… It’s basically an embedded Hilti channel with serrations that allows you to adjust the anchor height. Um, and also the, um, the analysis work that we did, we work to target when, you know, we, we tie into the core what elevation we target level.

And we, we put the adjustment in the columns with our, um, recommended shimming. So, you know, that’s something where if all gets built at the sequence and all the movements go as they, pre, as we predicted, we give “Here, here are the shims that should go in under each column. Here’s an eighth, here’s a 16th over here. Uh, here’s maybe nothing.” Um, and then of course the reality comes in, they start building it and things move a little differently.

And of course, and then we, we adjust the shim, the shimming basically. So, the connection has some flexibility in terms of adjustment, but most of the, um, let’s just say in field adjustment comes through shimming.

And so one of the most important interfaces is with the core and the lateral stability system. Another, I’d imagine, is the, the first level. So, with the transfer deck, um, how was that? And was there any special thought around and designing that? And, um, just so you know to get things moving as quickly as possible…

Yeah, it’s a, it’s a very good um, topic as well. You know, for a while we were trying to get project budgets to align and there’s the fighting argument of “What’s a good column grid for timber versus what’s a good calling grid for concrete parking?” And in the end, we knew that we just needed to have a transfer. So we’re actually transferring out um, I would say maybe 25% of the timber column location, um, over the 19 stories on PT transfer beams at the uppermost level of parking.

Yeah, I never transferred columns on on this scale before over 19 stories, but the reality is there are a third of the weight, um, really because of the fact that timber is so light. So it actually makes it quite feasible. We have about five foot of depth to work with, so the transfer beams are about five feet deep, um, they’re post-tensioned.

Uh, so we are stage stressing them. We’re, we’re stressing uh, about 50%, we stressed about 50% of the tendons initially. And then we are waiting till we get up to uh, about half the height where we get a lot of the crete and self, um, superimposed weights in, um, to counterbalance the post tensioning. And then we pull at the second half, um, so that kind of minimizes the amount of movements we do uh, through stressing.

Fantastic. And are you using a post and beam structure? What was the, the grids that you landed on that was most appropriate? And then also the orientation in, um, in coordinating with services as well?

Yeah, no it’s um, it what we tried to do is get beams basically spanning in one direction. Uh, we wanted to minimize the number of connections. The hardware, you know, adds up in cost, um, and the CLT panels themselves, you know, we knew we could get an efficient layout to take full advantage of the, you know, the manufactured lengths of about 40 to 42 feet, um, and and maximize that five ply, uh, panel span efficiency.

So that the the distance between the beams is only a 20 foot at its worst case location. It’s really slightly under. And that’s so we could really take advantage of fully utilizing the efficiency of the five ply panels that we’re using on the job to keep the fiber down.

And then the beams span, I think in the worst case condition about 25 to 28 feet, um, so we’re, we have a pretty efficient, um, beam blocking for, for the glulam beams that support the CLT.

I imagine it’s a pretty uh, pretty pretty sizable glulam columns toward the bottom. Is it what size we were talking about?

Yeah, some of them get to, you know, right, a little bit over three feet, you know, so I get up to about 40, 42 inches, um, it’s mostly where we get along the perimeter where they become a little rectangular, um, on average. The bigger ones on the bottom are roughly around that three foot dimension, um, you know, really maxing out, um, actually the woods all from Austria. So, we’re maxing out the European spruce, uh, the GL 32 grade that we have in, in the columns.

Yeah, and does it, I’m, I’m taking a stab in the dark here, but towards the bottom where you got higher loads, does, does the gravity loads govern more so than the fire case given the, the loads? And then you know, the fire is actually redundant or does the fire actually still govern when we get, you know, so tall in these buildings?

Yeah, it’s, um, it really is governed by the gravity case, um, the way that we handled the column design is a bit unique. So we, when we did the three-hour column fire test, sum, you know, we were basically just proving that the char rate from, you know, two hours to three hours, you know, can conservatively be extrapolated using the provisions that we have in our codes that really go up to two hours.

And we consider that on the columns actually as sacrificial. So it’s a little, we’ve treated it a little bit differently than on the beams where, you know, you can actually take an amplified rating, um, you know, based on the extreme event. We did look at the amplified rating for the columns, but we’re considering the char a bit more sacrificial in the design, uh, for a cent, um, and mostly that was due to timing of when we got our testing, uh, results versus when we had to procure wood.

Yeah, and uh, I think you mentioned twice that having the Milwaukee Fire Department as part of that. So were they part of the discussions early? And and bringing them along for the whole journey was, what was that like and what was that process all about?

Yeah, I, the best way to describe it is it was a wonderful collaboration, um, from day one we engaged with, uh, the fire chief and their fire department in Milwaukee. They were very, um, proactive, you know, we, for even my own edification to understanding what their actual concerns are, was even very different than some of the preconceived notions, you know?

Considerations more for smoke, considerations more for temporary conditions, what’s the heat actually like compared to a fire in a more conventional reinforced concrete or steel structure. And really trying to identify, um, almost a design criteria approach to addressing these concerns, uh, we, we had the luxury of doing with them, um, since early in the schematic process.

So, they were integral. I would say in Milwaukee, that, that was the big differentiator is that we, we had a great partner and we were able to bring them along step by step, you know? Multiple times, you know, we, it’s not just one meeting at, you know, half a dozen meetings with the fire department throughout the design process. So, it was, it was a great, it was a great collaboration.

Yeah, it sounds like it. And, so what, what were the lessons on the, lessons learned on the project? So you’re, you’ve installed the, the transfer deck now, you’ve five levels into it. What’s the, yeah, the lessons learned? And also the, the change in speed now they’ve gotten going at level five, is that, has that increased and, um, in the efficiencies right now?

Yeah, I mean, the lessons learned to me are, are really around the industry like that really doesn’t exist yet for the actual, uh, delivery of these projects, you know? First of all, all of our timber is being procured through Europe, you know, mostly because of supply chain issues and in control of the, the timber supply chain and the quality control that you can achieve through the manufactured process already established in Europe.

In the U.S, we’re way beyond the rest of the world, Europe, Australia, you know, as far as this supply chain, that’s one thing that I think to recognize. The other is really, you know, here with a project of this magnitude, but also doing it North America, having a very strong partnership with your contractor is, is critical.

Like we, we are on site every day, uh, which is very unusual for structural engineers, you know? We’re doing the special inspections on the job for IBC, but we’re also engaging with them regularly to help solve their problems. It’s something that is, you know, not as common.

You don’t get involved too much, um, but it’s, it’s required for this type of work, one that’s a little new. But you know, for the, for the trades and the unions on site, but, but two really is that this industry is, um, it’s very different. It’s a heavily manufactured, uh, product that doesn’t really have too much adjustability in the field.

So that tolerances are tighter, the solutions are much more customized, you know, in order to solve challenges when things are slightly off dimensionally or where you need to make a field fix on the fly.

Yeah, I would say that the lesson learned is that you can’t treat this like a standard structure. You need to have, you know, very, very good interaction with the contractor during construction, really to be their advocate and help them get through the process.

And what was the process like from the building information modeling, getting the to a level of detail from what the structural engineers model? And, uh, through to the, the fabricator, what was that interface like, um, and you know, coordinating that so it’s just “hit print” for the, for the drawings to go?

Yeah, that’s a, it’s a great question. I think that’s where this industry really needs to evolve a bit more, um, the delivery process is similar to steel, yet you don’t have the same parties at the table, you know? Some examples are, steel buildings have delegated design for connections, so the fabricator has their own engineer to kind of solve and coordinate, you know, the finicky connection bolt clashes, um, the challenge you have on timber is you have one structural engineer, you know, that really owns the design of everything, including the connections.

And understanding that interface and, you know, creates a challenge for on the fabrication modeling side. So I think where you’re going to see this industry go, um, is push a little heavier either on the structural engineer to get more into the actual fabrication modeling moving forward, or you’re gonna see, you know, some hybrid approaches that look at design build.

Um, I think just, uh, to really kind of take a step back and and get it back down to a more streamlined delivery process, um, it’s a little different, right? Because, yeah, you know, the, there’s no fabricators engineer for any of the connections because you have to own the connections due to their fire ratings.

Yeah, I do feel like that it is a bit of a gray area for a lot of projects out there and, um, so which makes it interesting. You, you mentioned how you got a, you designed for big panels and fit very efficient, uh, grids for timber, um, so it’s a brilliant DFMA design. Uh, what sort of productivity are you hoping to, which, or have you achieved? And you’re hoping to achieve as you move into the higher and higher levels and you get the economies of scale?

Yeah, we’re targeting about a floor a week. It’s a, it’s a good size floor plate, um, right around twenty thousand square feet, um, and really I think given the, um, cycle time of the decking and columns, the week makes a lot of sense, um, you could probably push it a little faster. But the benefit as far as getting it, um, encapsulated with the, with the exterior wall and the enclosure, um, you would just start probably going a little faster than needed. So, right now we’re targeting about a floor a week.

Yeah, and you prefabric, doing additional pre-fabrication off-site, so for the facade system or, or anything on-site, um, attaching plasterboard or is that happening on site?

Parts of it are, uh, yeah, modular construction for the window wall system that goes on the tower. Some of it’s stick built on the lower floors. But in general, I would say, anything you can do with these projects to have prefabricated or modularized delivery approaches helps, you know?

I think this is actually a much more precise way of building these, uh, buildings, you know? Your tolerances are tighter, um, you know, than you would have for a conventional concrete or steel, or maybe closer to steel tolerances. Let’s just say then versus concrete. So you have, I think, a lot more opportunity for prefabricated modular delivery to save time and save money.

Absolutely. Well thank you so much John. It’s been phenomenal chatting. He has just, last couple of questions, uh, what, what do you see is the future of timber construction after dealing with this?

Yeah, it’s a great question, uh, I don’t really have a good answer for that one. I think it’ll be interesting to see how the industry evolves. And um, you know, I’ve been in the industry long enough to know that there, there can be trends, um, and there can be fads. Um, and but also, I think it’s opening people’s eyes up also to another, uh, product to deliver actually very high quality buildings.

Um, there’s something to be said about being in these mass timber buildings just from, you know, just a human, uh, human perspective, you know? They’re very uh, warm, they’re very um, uh, let’s say welcoming, um, just when you walk in. And it’s hard to deny that that’s that really doesn’t, uh, come out when you go in one of these.

So I think you’re gonna see more. I think it’s, um, going to be also driven too in particularly in North America where, you know, our codes are still evolving, um, but also the industry has a long way to go to really be able to supply the demand that I think what we’re seeing right now, um, especially on the manufacturing side, you know?

The, you can get the wood, you can get it down to the forest, um, but really getting the production of the actual glue lam and getting the CLT manufacturing really going here at a level that can support, I think the demand is going to be a challenge in North America in particular.

Yep, totally get, yeah. And, so thanks so much. If people want to find out more about yourself, about the project, or anywhere people, you, anywhere people should go, um…

Yeah, is there anything you’d like to plug?

Yeah, I mean, if you go to our website, uh, thorntontomacety.com, we’ve got, you know, a project page up and we usually keep it fairly current. But also, Newland Enterprises is regularly providing updates on the construction that’s going on. They’re the developer of the job, really the, I would say, the heart and soul of the project that really, I would say, double down on delivering something unique and providing a good step forward for the industry and the world for that matter.

They’re providing regular updates, our partners there. And also, you know, I’m sure you’ll hear more as these conferences continue. We’re continuing to push what we know out so that I can benefit the industry, um, we’re not holding any information back, you know, including the testing that we did, you know? The first three hour fire column tests that we did, we did for multiple species of wood with the American Forest Products Laboratory and we’re releasing all that information publicly so everybody can use it. So I think there’s a lot of things coming, they’ll come from different sources, but it’s going to be good.

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