mass timber connections
Mass Timber Connections: A Review

Exploring Mass Timber Connections: Efficiency and Innovation in Modern Construction

Mass Timber Connections

Foreword by Ian Thompson, Editor

Mass timber connections are a critical aspect of modern construction, playing a pivotal role in the overall design and efficiency of a structure. In this informative video post, we delve deeper into the world of Mass Timber design, with a specific focus on these mass timber connections.

Traditionally, metal plated connections have been the norm, but their installation time and cost can pose significant challenges, especially in complicated assemblies. This brings us to the concept of wood to wood connections, an alternative that promises efficiency and cost-effectiveness, but also requires installation accuracy that can raise its own challenges on site as well.

In this video, we showcase Heartwood in Seattle, one of the first type 4C Mass Timber buildings in the United States. It’s a testament to the potential of mass timber connections, as it employs a two-hour fire rated all-wood beam to column to panel connection, bringing a new perspective to the industry.

This video also explores the modern adaptation of the mortise and tenon joint, an age-old technique that has been reimagined to meet today’s requirements, including fire rating standards and seismic events.

This is a relatively short video packed with loads of useful insights into the world of mass timber design and construction. This is my construction method of choice and one I’ll be using on my next house project.

Innovative Mass Timber Connections Unveiled

Video Transcript

One of the first type 4C Mass Timber buildings in the United States to be completed, Heartwood in Seattle ushers in a new era of timber housing as the industry experts in Mass Timber technical design assistance and education. Woodworks has supported nearly 200 tall Mass Timber projects in design, with most tall Mass Timber projects requiring two-hour fire resistance ratings for structural members and mass timber connections. The topic of fire design is at the forefront of many project conversations.

In this video, hear from members of the Heartwood team as they explain the design, fabrication, and construction process of a two-hour rated all-wood beam column panel connection.

“We ended up with a mortise and tenon joint, which has been used for hundreds of years. We really worked hard to elevate exactly the details of how that works to make sure that it works with all of the fire rating requirements for two-hour rating, the modern ways that we fabricate wood, which is no longer with a hand chisel but with a CNC machine, and also that it can handle drift in a seismic event, that that mortise and tenon joint is going to hold itself together.”

“My name is Adam Youngward. I’m a structural engineer with DCI.”

“Hi, I’m Taylor Cabot. I’m a project manager with Timberlab.”

“Hi, my name is Lauren Magasco. I’m a digital construction engineer with Timberlab.”

“So this is the Heartwood two-hour mass timber connection. Today, I’m going to kind of lead you through how this all gets assembled. looking at two-hour mass timber connections, it basically means that you know if there’s a fire in a building, it’s going to stand for at least two hours. The connection, especially in a two-hour rated scenario, the mass timber connections will drive the member size way more than anything else.

And so you need to understand exactly how it’s going to come together, and your beam and column sizes will come based on the approach of those mass timber connections. It’s really important that the structural engineer and the architect and the contractor are all brought in early, and we’re all talking about how this building is going to come together. It creates the framework for the entire design.”

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“The first thing that happens on site is your concrete pad goes down. We’re going to use a steel base plate that’s going to be leveled, and it’s going to bring the inconsistency of the concrete slab into alignment and allow for our timber, that needs very precise positioning and plumbing to happen.”

“So the steel is placed on anchor bolts, and then we set our first glulam column down on that. That’s held in with epoxy rods, so you get a completely clean looking column, you see no fasteners at that point, and the base plate is hidden inside of the wood to protect it from the heat that would happen in a fire.”

“After this column is set, we’re going to brace it. We’re going to put two braces on either side. We’re going to plumb up this column, get it exactly into alignment of where it needs to be. At that moment, we’re going to take our beams, slot them in through this mortise and tenon connection, and have them bared directly on the beam.”

“Typically, this would have been the moment when you would have had a beam hanger and you would have been putting in bolts or drift pins and fasteners to hold this together. Because we’ve changed to this wood to wood connector, we don’t have that issue. This just simply slots in, and we’re off to the next beam and we’re off to the next beam.”

“The biggest challenge was probably the fire rating. In tall wood buildings, you really need to think more collaboratively. And so figuring out ways to show the city that we are meeting all the fire rating requirements, for from the beams and columns were pretty simple, the connections are a little bit trickier.”

“The fire rating for type 4C is a two-hour rated primary structure, which means two-hour rated beams and columns and two rated panels. We’ve done fire ratings on a lot of different projects but type 4C really kind of elevates that to the next level to make this a two-hour mass timber connection, we needed to oversize all of the wood associated with with a Char layer right now.

All the columns and beams are upsize in order to maintain what we call a Char layer there’s a certain thickness for a one hour Char layer and then there’s a certain thickness for a two hour Char layer and for three hour and so on and so forth. All of these columns and beams that are exposed have been upsized in order to maintain a minimum structural integrity so as you can see when you get up here, this lower column.

This is actually the only size that we need this column to be to structurally hold the building up so we’ve increased, we’ve added these added layers here as char protection. These can be damaged by fire and we have not affected any of the structural integrity of this column. We’ve done the same exact thing for this beam. This beam does not need to be this wide or this deep to support itself.

But we have oversized it to allow for the Char layering to happen inherently. When you build a building like this you need to provide gaps between all of the pieces this is both for fabrication tolerances. What is a natural material? It’s expanding, it’s contracting, but you also need them to allow for the pieces to come together easily in the field. Unfortunately, these gaps expose this connection to fire, and we need to ensure that those gaps are removed.

One of the ways that we do that is by utilizing fire tape. It can be a challenge because we have lacking data that really supports how we approach fire rating. It’s usually surrounding a UL assembly, and in wood charring, you don’t really deal in UL assemblies anymore.

You have to deal with calculation and char rates and how the wood actually behaves in the fire. It’s a little bit more of a performance rather than a one test pass or no pass type of scenario. So it takes a little bit more creativity and thoroughness to make sure that you’re covering it from all angles because it’s going to be exposed.

We use this fire tape, so it’s an adhesive tape that really just sticks to the beams and then there’s places in the column that it sticks to. It basically expands to about three times the thickness of it that really like locks in so that no smoke can kind of get through any of the gaps within the glulam column and beam or the glulam column and the CLT.

We wanted to maintain that clean, really clean and nice look for the public and the users of the building. This fire tape is routed into the beams and routed into the columns. Without the tape, these mass timber connections would not work, and this is really part of the advancements that we’re making to utilize these materials to allow us to build how we traditionally would have, but do it in the speed that we need.

Once all the beams are on and all the columns are on, it’s time to set our CLT. The CLT is pulled directly off the truck. It’s set on top, and this is the moment that we start making our mass timber connections. We’re going to screw down through the CLT and into the beam. We’ve now locked the entire bottom connection together.

Once that has happened, we can go ahead and pull the braces off of the column and utilize them on the floor above. Once the next glulam columns come, we pull them directly off the truck and they rest on the column below. You can see that we’ve held this column up off of the CLT. We only want this column to bear on that column.

We don’t want the column to bear on the CLT in place a loading. This also does double duty of allowing water that might happen during erection from rain not to wick up into the column and cause discoloration or damage to it. By utilizing this method, we were able to install the entire floor plate in five days.

The mortise tenon connection has been used for hundreds of years. The way that they built it was by hand tools, and so we wanted to find ways to modernize it in the speed at which it could be fabricated and then could be constructed. And so allowing the all wood connection really allows them to move really fast.

Just a few screws in the field allows them to place beams and columns extremely quickly. We saw on-site that we could install all of the beams on a floor in half a day that typically would have taken two to three days if we had to make all of those steel connections happen.

Get on site and see it go, see one whole level go up in one day, that’s just amazing. This is the reason why getting the taller wood buildings is so exciting to me, is that it can really compete with steel and concrete on all fronts. I think that people like being in these buildings more, and they will work. On paper, considering WoodWorks can help.

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Hartwood on WIN