World’s tallest mass timber building tops out in Milwaukee

The Ascent building in Milwaukee is a radical change in how to build a downtown tower, with the 25-story building rising 90 metres and eclipsing the previous world leader, the Mjos Tower in Norway, by one metre.

And the result is that the tallest building in the world has taken less time to build, is more environmentally friendly to construct, and excited tenants are planning to stay longer.

Developer New Land Enterprises spent years refining the project after announcing it in 2018. Following initial concept design, additional floors were added twice to the building as further engineering was undertaken.

Led by general contractor CD Smith, construction workers assemble large pieces of wood together like they’re building a LEGO toy.

A just-in-time supply chain delivered truckloads of mass timber to the site, which was provided by two different European suppliers, each piece being designed for a specific location in the building. If a pipe needs to go through a floor the corresponding hole was pre-cut into the beam in Europe.

Each column had corresponding male and female metal joints to connect with the adjoining pieces. The width of the load-bearing pieces shrinks as the building climbs higher, reflecting the decreasing weight they need to support.

The developer is among many investors and firms across the world that are optimistic about the future of mass timber.

“It’s warmer, more beautiful, it just feels good,” said New Land managing director Tim Gokhman of the apartment’s interior aesthetics during a tour of the building. Wherever possible, the mass timber is exposed in the walls and ceilings. A wood-grain-finish floor is installed atop a radiant heating system and insulation layer, accenting the White Spruce mass timber.

But for the first few months of Ascent’s construction, it looked like virtually any other tower. A six-story parking structure was built on site from concrete.

Now the concrete, which stopped when it reached the seventh-floor pool, has given way to structural mass timber.

While two stair and elevator columns currently stand at 17 floors above the ground, the structure of the building above the parking garage is supported entirely by mass timber.

During construction, peering into a partially assembled joint you could see five perpendicular, horizontal layers that form the floor deck pieces (known as cross-laminated timber or CLT) from supplier KLH Massivholz.

The columns and beams that come together between each floor deck are from Wiehag and use a different mass timber strategy. Known as Glulam, the timber that makes up those pieces are glued together, but unlike CLT all of the layers go in the same direction to transfer the weight of the structure.

The two product types are often used together to create a structure that is lighter than concrete, faster to assemble and fire resistant.

Should the building ever suffer a fire, the thickness of the pieces is intended to cause the structural pieces to char, not burn. The principles of this methodology can be seen in century-old wood buildings that burn like Trinity Lutheran Church. Made from dense, old-growth lumber, the structure of historic wood buildings doesn’t bend and collapse like steel. Mass timber is engineered to offer even greater fire resistance.

The materials New Land selected underwent a successful, three-hour fire test in early 2020.

Working with mass timber changes the type of supplies needed at a job site. An example is storage of a vast range of different screws, some as long as one metre.

The largest screws impose a logistical challenge. CD Smith project manager Chris Johansen said each cordless drill can only drive eight of the screws before a new battery is needed. Approximately 1,000 screws are needed per floor. As a result, the screw storage and the top floor each have sizable battery charging stations.

The structure was rising as much as a floor a week, but dependant on a fragile supply chain. Each floor deck came in eight shipping containers and the columns and beams in another seven.

The containers arrived on the East Coast, clearing customs in Chicago and are temporarily stored at Milwaukee’s port. Port Milwaukee equipment was used to remove the pieces from the container with a semi-trailer used to bring them to the job site.

In an ideal world, the containers would arrive multiple weeks early in Milwaukee. In a pandemic-altered environment, they sometimes arrived the day of construction.

“A struggle for us on the install side is to manage all of that,” said Johansen.

Using mass timber involves lots of upfront planning. In order to make sure the right conduit holes were cut into the pieces, every subcontractor had to negotiate its needs using a 3D model of the building. In addition, New Land had to bring in many more project partners than usual.

Building Design was led by Korb + Associates Architects. The Portland office of general contractor Swinerton supported the mass timber component of the project, and Thornton Tomasetti provided structural engineering services.

Article by courtesy Jeramey Jannene of Urban Milwaukee.