In January 2019, Preston Cole left his post as Commissioner of the Milwaukee Dept. of Neighborhood Services and became Secretary of Wisconsin’s Dept. of Natural Resources. It was a step up for the 25-year veteran of public service—a forester by profession—who as the city’s top building official had reformed DNS by fostering a developer-friendly environment.
For the building team seeking his required signature on a variance for an unprecedented 284-ft-tall mass-timber and concrete high-rise, Cole’s departure from DNS was both untimely and unsettling. “We had a scare when Commissioner Cole got a new job,” says Jason Korb, principal architect for the local firm Korb + Associates Architects, which is designing the 25-story code outlier, named Ascent.
In May 2018, with Korb’s initial concept for a 19-story mass timber apartment building in hand, the Ascent team approached Cole to feel him out about the project, which would break the city’s 85-ft height limit for wood. “If he wasn’t into it, no way would we go forward,” says Korb.
A Milwaukee booster, Cole was all for a showcase for tall mass timber in the forest product state’s biggest city. “I liked their energy,” Cole says of the Ascent team.
Green Building Material
He was so enthusiastic about the innovative use of the green building material that in July 2018 he directed the DNS commercial-building code variance committee to find a way to get the tall building, with its combustible wood frame, “across the finish line” without compromising public safety.
In January 2019, the Ascent team had no way to know, but Cole’s departure would not throw a wrench in the works. Erica R. Roberts, Cole’s successor as commissioner, also was excited about tall-timber innovation.
On July 24, two long years after the Ascent team first presented the concept to the DNS advisory committee, Roberts granted the variance for the building—which had grown to 25 stories and 488,800 sq ft. The protracted approval time was necessary to thoroughly investigate the project, says Roberts. “I hope the steps taken by Milwaukee can be used by others” to reduce approval time, she says.
Construction began in September. If finished as planned on July 1, 2022, Ascent will break height and size records both for pure timber and composite timber and concrete structures anywhere. Though only by 4 ft, the building will exceed the height of the world’s tallest wood building—Mjøstårnet, a 122,000-sq-ft mixed-use building in Brumunddal, Norway, completed in March 2019.
Cole’s departure had added more uncertainty to an already risky but attractive development due to wood’s appeal and renewability. The U.S. has only one 85-ft-tall wood building, Carbon 12 in Portland, Ore., completed in 2018, and no modern wood building taller than 85 ft—the historical height limit for a structure made from a combustible material. The restriction is in great part due to concerns about fire fighters’ vulnerability in a blaze.
No U.S. Precedents for Grand-Scale Timber
There was no permitting path for a timber building that would exceed the height limit by nearly 200 ft. There were also no U.S. precedents for grand-scale “supertall” timber and concrete building development, financing, insurance, design, detailing, material procurement, fabrication and construction.
The 19 floors of wood-framed residential space, which will sit on a six-story structural concrete parking podium, will contain just over 80,000 cu ft of glue-laminated timber fabricated into 1,150 glulam columns in 17 different sizes and 1,320 glulam beams in 50 sizes with 36 different lengths. Ascent also will contain 336,000 sq ft of cross-laminated timber (CLT).
That’s a tall order, especially as, initially, no one on the Ascent team had any in-depth experience with mass timber, let alone supertall mass timber. “Had we known all the challenges, we probably wouldn’t have started” the journey, says Tim Gokhman, managing director for Ascent’s local co-developer, New Land Enterprises, which is in partnership with Wiechmann Enterprises.
Tim and his father Boris, who co-founded New Land in 1993 with fellow Ukrainian immigrant Walter Shuk, were no strangers to uncharted waters. They had escaped the Soviet Union, which controlled Ukraine, in 1990, when Tim was 10 years old. The pioneering of tall timber, which they considered a market differentiator for downtown residential development—based on wood’s beauty and growing popularity as a renewable building material—paled in comparison to their journey to the “new land.”
Still, “from a developer’s standpoint, we both understood and misunderstood how big a process this was from the code standpoint,” says Gokhman, who joined his father and Shuk at New Land in 2002, not long after college.
Milwaukee’s collaborative variance model helped smooth the approval process. “The project team was very responsive to and understanding about concerns raised,” says Jeff Zagar, a DNS operations manager and committee member. “They developed and incorporated the appropriate studies, tests and evaluations and utilized industry experts, as would be expected for a cutting-edge project.”
The variance hinged on the support of two top fire officials, invited to serve as guest advisors on the committee. “Without the fire department’s acceptance and approval, the petition would have been denied,” says Mark Gardetto, a DNS supervisor, fire protection engineer and one of 10 on the committee, counting the fire officials. Beyond keeping building occupants safe, “we must also ensure that safety is not compromised for our emergency responders,” Gardetto says.
The fire officials’ nod was not a slam dunk. “We found ourselves questioning our own suspicions and the seeming lunacy of building a high-rise out of wood,” says Aaron Lipski, the Milwaukee Fire Dept.’s acting chief. “At the end of the day, once we studied the problem from the basis of actual information, utilizing … live-burn test results and internationally studied data, we were able to overcome our own doubts and not object in any fashion to the variance.”
Unknown to the Ascent team going in, Lipski and especially Erich Roden, acting deputy fire chief, had some knowledge about mass timber through affiliations with UL and the International Association of Fire Fighters. “This gave us a robust position with which to bring inherent fire fighting concerns to the table of the developer, the architect and their subject matter experts,” says Roden.
The variance committee had even broader concerns, including whether a major blaze would undermine the building’s structural integrity. Other major safety issues concerned wood’s vulnerability to fire during construction, means-of-egress protection for stairwells and public hallways, and reliable water supply to the building’s fire protection systems.
Consequently, the city is requiring a temporary standpipe system, installed up the elevator core, in place from the beginning of the timber installation through final occupancy. Also, the permanent fire protection system will be operational prior to occupancy.
There will be two water supplies for standpipes and fire sprinklers so that if one supply fails, the other will still work. And there will be enhanced smoke detection within each living unit.
Further, Ascent will have cast-in-place concrete stairwells and more robust fire protection in the public hallways, in the form of gypsum concrete wall coverings.
“This alleviated the majority of operational concerns and proved New Land’s willingness to design and build the safest timber building in the world,” says Roden.
The results of the first-ever 3.5-hour controlled burn on unprotected glue-laminated timber column specimens, conducted a year ago, also went a long way toward convincing committee members to advise Roberts to approve Ascent.
Half of the mass timber in the building will be architecturally expressed, rather than concealed. “We knew we needed a burn test” to determine the exposed glulam columns’ fire-resistance rating, which would help show equivalent fire performance to structures made from noncombustible material, says John Peronto, a senior principal with Ascent’s structural engineer, Thornton Tomasetti, which set the wheels in motion for the fire tests.
When a wood column burns, it develops surface char that insulates and protects the member’s core. The results of the fire tests on unloaded glulam specimens, underwritten by and conducted last year at the U. S. Dept. of Agriculture’s Forest Products Laboratory in Madison, are significant. There had never been testing to validate char rates beyond two hours, says Laura Hasburgh, FPL’s materials research engineer.
The tests demonstrated that exposed glulam columns properly oversized to have an outer sacrificial char layer qualify for a three-hour fire-resistance rating. “The tests also validated that the rate of burn from two hours to three hours was less than the rate of burn from one hour to two hours,” says Peronto, who adds that exposed columns in Ascent will have a 4-in.-thick sacrificial layer.
A report on the study, which also created a design model to properly oversize exposed columns, will likely be published early next year, after a presentation in mid-January at the World Conference on Timber Engineering 2021 in Santiago, Chile.
“The published data will benefit the whole country if not beyond,” says Korb.
For Jim Daul, a DNS plan examiner and variance coordinator, the results of the fire tests were important because they demonstrated the efficacy of Thornton Tomasetti’s structural design parameters for exposed columns.
Milwaukee’s developer-friendly variance-request process also was important to get Ascent across the approval line, says Roberts. Her advice to developers seeking permission for tall mass-timber buildings: Start the variance process before the design is set, be transparent and remain responsive to questions and requests.
Her advice to other building departments: Create a collaboration between building and fire officials and the building team. Open communication along the way avoids surprises and ensures the team doesn’t “spend time going down a rabbit hole,” says Roberts.
Leaning on Framework
For approval, the Ascent team leaned on the experience of Framework, a 148-ft-tall timber building permitted but never constructed because of development issues, in Portland, Ore. The project, designed by LEVER Architecture, “forged the path” for Ascent, says Korb.
For example, the Ascent team used the results of two-hour CLT fire tests done for Framework, funded by a $1.5-million U.S. Tall Wood Building Prize. And Framework’s fire protection engineer, David Barber, a principal of Arup, became Ascent’s subject matter expert.
A difference between the 12-story pure-timber Framework and Ascent, aside from Framework’s shorter height, absence of concrete cores and location in a high seismic zone, is the starting point for the path to approval.
Framework was approved under Chapter One of the model International Building Code (IBC), which allows an alternative material, design or method of construction when the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of the code. The Framework team used performance-based seismic and fire engineering to satisfy the city that the building’s performance would be equivalent to a 12-story concrete structure, called Type I.
The alternative path was not open to the Ascent team, says Michael Mazmanian, a DNS operations director who oversees plan review and construction trades. Milwaukee uses the state’s building code. And though Wisconsin adopts most of the IBC, it does not adopt Chapter One.
As a first-class city in Wisconsin, “we have the authority to review and approve variances, but we cannot modify the code through adoption,” says Mazmanian.
Instead, DNS recommended the Ascent team apply for variances to the prescriptive requirements for wood buildings, called Type IV construction in the city’s code.
Ascent’s timber-framed tower, which will offer views of the nearby Lake Michigan, will contain 259 one, two and three-bedroom rental apartments and amenity spaces.
To resist gravity loads, the tower, a pudgy L in plan, will have glulam columns and beams supporting CLT floor diaphragms. Like glulam, CLT is engineered wood produced by gluing small beams of lumber together under pressure to make layers or plies. Unlike glulam’s parallel plies, CLT’s plies are perpendicular to each other.
The tower’s six-level podium will have post-tensioned floor slabs. Two full-height concrete cores resist lateral loads in the podium and tower. Foundations consist of 162 piles that are 125 ft long, either supporting mats under the core walls or pile caps under the columns.
During the design phase, some of which was concurrent with the approval process, the building grew a few times, “to make the project pencil,” says New Land’s Gokhman. “We didn’t start out with the idea of building the world’s tallest timber tower,” he adds.
In Milwaukee, the current cost premium for a timber frame is roughly 30%, compared with New Land’s customary reinforced concrete residential buildings with post-tensioned concrete slabs. The timber contract is about 10% of the total development cost because the premium is just one component of the cost of the overall structure, says Gokhman.
For Ascent, there also are “huge” premiums for insurance, at least for now, he adds, declining to provide financial specifics other than that the development cost is just under $125 million.
Timber does offer some cost offsets. Piles are smaller and fewer because wood weighs less than concrete. There also is a shorter erection schedule than for cast-in-place concrete because timber components are prefabricated.
The team predicts 22 months for construction—four months shorter than for cast-in-place concrete. That translates to about $500,000 less for general conditions, says Korb.
Fretting Over Financing
While the architect was concentrating on getting the variance, the developer was fretting over financing. “It was incredibly difficult to get investors and financing for a luxury high-rise in a pandemic in a secondary market,” even with only one other luxury residential building downtown, says Gokhman.
The difficulty had more to do with market factors than the mass timber, though the wood did have some effect, he adds.
The project’s $87.65-million construction loan package from Bank OZK and Hines Realty Income Fund didn’t close until last month. “We started construction prior to closing the loan,” says Gokhman. “We had to take the risk to keep the construction schedule.”
Though no stick of wood is expected on site before May, much of the “heavy lifting” was finished before construction started. To assist with cost estimates and planning, New Land brought Ascent’s contractors in early, during a long preconstruction phase concurrent with design and approvals.
“We worked to get pricing to a place where the project would be marketable and affordable,” says Rich Severson, a vice president of C.D. Smith Construction, the core and shell construction manager and concrete contractor, which is in a partnership with Catalyst Construction, a longtime New Land advisor, responsible for Ascent’s interior fit-out.
Getting a handle on the timber system’s pricing, components and assembly took more than six months of research. “We were like sponges,” investigating every type of mass timber product, especially “to come to grips with cost,” says Adam M. Arndt, Catalyst’s president.
To get up to speed, Korb, Thornton Tomasetti, Catalyst and others boned up on modern timber construction everywhere. They investigated more than 20 suppliers the world over, says Arndt. And there was a visit to timber buildings in Portland, Ore.
As with many prefabricated systems, the mass timber work, including the design of the steel connectors and all the locations of penetrations in both the wood and concrete elements, is front-loaded. This allows the glulam and CLT suppliers time to fabricate the pieces, including prefixing the steel hardware, encased in wood for fire protection. To allow prefixing, Thornton Tomasetti had to design all the steel connectors in advance of fabrication.
Also, to predetermine penetrations, coordination of all the building systems in the tower had to be done before crews cast the concrete cores, says Severson.
Timber fabrication begins next month. Wiehag Timber Construction in Altheim, Austria, is supplying all the glulam columns and beams, made from European spruce. “The good news about working with Wiehag is that they have been fabricating glulam since the 1970s,” says Taylor Cabot, project manager for Swinerton Mass Timber, part of Swinerton.
KLH Massivholz in Teufenbach-Katsch, Austria, is fabricating all the CLT for the floors, also made from European spruce.
Swinerton won’t begin timber erection until mid-May, after concrete crews complete the cores. There is no timber-concrete overlap because there is only one tower crane.
The wood framing sequence is similar to the sequence for structural steel erection, says Severson. All the wood members will be shipped in containers for just-in-time delivery and installation. Crews will finish one level each week, as if erecting 19 single-story buildings.
Acoustic tests are pending for the finished CLT floors. “No one has measured how sound travels through CLT,” says Arndt. “Each wood type is different.”
Thanks to Ascent and recent model code changes, there may be a more promising future for tall wood in the U.S. IBC 2021, published this fall, allows timber buildings up to 18 stories.
But in Milwaukee, that won’t change permitting, at least for several years. The state allows the use of newer code versions, says DNS’ Mazmanian. But the hitch is that because the new timber provisions deviate from the current code, applicants would still have to apply for a variance. And any deviation from the new provisions, such as a timber building taller than 18 stories, also would need a variance.
Still, fire official Roden views Milwaukee as a role model for other jurisdictions interested in tall timber construction. “Having the good narrative coming out of Milwaukee will help,” he says.
Meanwhile, Secretary Cole, still a big booster of both Milwaukee and timber, is watching with interest as Ascent ascends. “It’s exciting for us,” he says. “I may have to move there.”
Editor's note: This article was updated on Jan. 6, 2021 to reflect new information.