Feature Illustration (above): The Ascent, a proposed 21-story residential tower in Milwaukee, would be the tallest mass timber system building in North America. Courtesy: New Land Enterprises LLP.
by Adolfo Pesquera
The Milwaukee, Wisconsin City Plan Commission last week recommended rezoning for a project that could lead to construction of North America’s tallest mass timber building–a 21-story residential tower.
VBX does not normally report on out-of-state construction projects, but this is such a historic achievement it deserves consideration. The question should also be asked, could such a structure be built in Texas?
It’s already happening, albeit on a much smaller scale. The technology being used in the Milwaukee project is almost identical to that being used for a 6-story office building project in downtown San Antonio.
The Cavender building in San Antonio will have five stories of prefabricated timber panels above a one-story concrete structure. The DLT (dowel-laminated timber) panels used for exposed floor and roof decks will be supported with Glulam post and beam substructure, according to StructureCraft, the structural engineer on this Hixon Properties Inc. project.
The Cavender Office Building in downtown San Antonio, at six stories, will be the first mass timber CLT designed building of its height in the city. Courtesy: Hixon Properties.
The architects are Lake Flato and Boka Powell.
New Land Enterprises LLP is the developer behind the Milwaukee project. Jason Korb, their architect, described the structural aspects of the building in similar terms–the base of the building is to be made of traditional concrete, but the upper 16 floors will be made of mass timber.
Mass timber is an engineered product made by combining layers of lumber into a stronger material, an Urban Milwaukee report said. A mass timber skyscraper is competitive on price with traditional concrete high-rises because of the prefabricated segments and lighter overall weight, allowing for smaller foundations and a shorter construction phase.
“It can go up incredibly quickly. We estimate the 16 floors will go up in four months,” Korb told the Plan Commission.
Mass timber buildings are gaining in popularity among architects, but actually building any has been a hard sell. New Land Enterprises tried to build a 19-story tower in 2009 but never got the financing. A Chicago developer pitched a concept in 2014 for a 35-story tower, also never built.
Building codes have only recently caught up with the technology. In December, the International Code Council released results of an online governmental consensus vote on 14 tall wood construction code change proposals. They will not be incorporated into the International Building Code until 2021.
The changes create three types of tall wood construction:
- Type IV-A allows for structures of up to 18 stories, with gypsum wallboard on mass timber elements.
- Type IV-B allows for structures up to 12 stories, with limits on the area of exposed mass timber walls and ceilings.
- Type IV-C structures can go up to nine stories, with all mass timber exposed but designed with two-hour fire resistance.
Fire resistance is another feature of the new mass timber components. The systems use laminations that char. This deprives a fire from using the lumber for fuel. American Wood Council fire tests resulted in 3 hours and 6 minutes of resistance, well beyond the 2-hour rating.
The attractiveness of cross-laminated, fire-resistant timber (CLT) is finding its way into other Texas building projects, specifically for its fire resistance. But also for aesthetics.
Endeavor Real Estate is building a five-story residential complex in Austin on East 6th Street east of Interstate 35. Texas Architect notes the Endeavor project will be Austin’s first hybrid structure using CLT as a primary structural element.
Endeavor Real Estate’s mixed-use residential mid-rise on East 6th Street will use a structural CLT mass timber system. Courtesy: Endeavor.
“Large prefabricated solid engineered wood panels, sourced from Structurlam in Canada, compose the structural decking in the otherwise steel-framed structure,” Texas Architect said.
“Classified as a IIIA construction type, the entire structure needed a one-hour fire rating. Meeting that benchmark is no problem for CLT, as it performs just like any other heavy timber element would. As it burns, a sacrificial layer of char about an inch thick develops, insulating and protecting the assembly from further degradation.”
There are other advantages, too. The combination CLT modules and steel frame drastically reduce the number of columns needed within each floor plate, compared to an all timber structure.
Still, truly tall structures of the height that architects claim mass timber is capable of may be years away. There are only two tall mass timber buildings already in existence in North America.
Carbon12, an eight-story condominium tower, was completed January 2018 in Portland, Oregon. And the University of British Columbia’s Brock Commons in Vancouver built an 18-story student dormitory in 2016.
Architects also take advantage of the aesthetic value of the structural columns, beams and walls of a CLT system by exposing them as part of the interior and exterior design. Courtesy: Carbon12.