The design and construction of the first solid wood commercial office building in the Philadelphia metropolitan area required intense coordination to combine solid wood with multiple structural systems.
Located in Newtown Square, Pa., the $44.3 million, 105,000-square-foot building is a five-story building complex that will be finally completed on schedule and on budget this month about 18 months after the start of construction.
The use of solid wood for the building above the level sequesters 720 metric tons of carbon dioxide emissions, equivalent to taking 201 cars off the road, conventional white steel and concrete were deployed for elements below the level The project also includes a precast concrete garage. This variety of building materials required O’Donnell & Naccarato, the project engineer of record for design, to research and develop connections and design requirements for the structure to be supported by the foundation and a structure of ‘composite steel that transfers the loads from the ground. floor through the five floors. The team also had to incorporate the two supports into the two-level prefabricated parking structure below. The team used expansion joints “to limit any differential movement between the part of the structure that supported the solid wood building and the parking structure itself,” says Dennis Mordan, president of O&N.
The helical piles helped avoid a costly shoring excavation for two full stories in an area of the building that extends beyond the parking lot.
Courtesy of O’Donnell & Naccarato
To incorporate a conventional lateral system into the massive timber structure, the team chose steel braces as the 2018 International Building Code limited shear walls and CLT lateral members, explains Mordan.
Modeling CLT floors as semi-rigid diaphragms helped to properly distribute lateral loads. “A load path with steel bars for lateral resistance in a timber frame is not standard,” notes Mordan. The team also coordinated the details of connecting steel braces to the solid wood columns with solid wood subcontractor Nordic Structures.
The team also came up with innovative sequencing to ensure they had enough time to complete the garage.
“We had to finish the east and north facades before erecting the prefab parking structure because we couldn’t drive the big construction equipment into the garage to avoid damaging it,” says Alex Byard, senior project manager for IMC Construction, general contractor for the project. .
Crews installed steel bar braces to the solid wood to incorporate a conventional side system into the solid wood structure
Courtesy of O’Donnell & Naccarato
Barriers of the Foundation
In early designs, O&N “wanted features that required a cantilevered structure and approached the design aesthetic as if it were a traditional steel building,” says Mordan.
Initially, this did not address the “structural complexities that would be present in a massive timber structure,” he notes.
When the O&N structural team was designing a support system for the exterior screen wall and part of the east wall where the wall overhangs the third floor, they realized that “there was no easy way to a purely rigid moment connection of a timber structure,” Mordan recalls.
But they came up with two solutions. On the east wall, glulam beams ran continuously, cantilevered over dropped columns to create the overhang.
“This required some special detailing at the intersection of columns for the stability of the stacked structure,” he says.
Workers connecting a laminated beam to a laminated column
Courtesy of IMC Construction
In the screen wall areas, the team chose steel outriggers that allowed the use of traditional rigid connections, which minimized the size of the cantilever members.
After a year-long design phase, five months of foundation work finished this spring. The helical piles were deployed in the building built over a parking garage built into a hillside. “Due to site constraints and to eliminate the need for expensive shoring excavation for two full stories for an area of the building that spans [35 ft] beyond the parking lot, the team chose to use helical piles,” says Mordan.
In late May, “due to unforeseen inadequate soil conditions, most piles had to be installed five to seven times deeper to meet strength requirements,” Byard says.
The team lost about two weeks on schedule, but was able to “quickly pick up what we lost on the helical piles during the wood erection,” says Byard. The wooden erection was completed in a shorter duration: 13 weeks instead of 15 weeks, he notes.
After a year of pre-planning and coordination, the wooden construction took 13 weeks to complete
Courtesy of O’Donnell & Naccarato
Fire classification
D2 Groups designated the office building as Type III under the 2018 IBC, which required two-hour fire-rated exterior walls and two-hour fire-rated shafts, with all other structural frames requiring a one hour rating. To calculate the fire rating of a solid wood member based on the rate of charring (a predictable 1.5 inches per hour), the team had to consider the number of sides of each member that will be exposed to a fire “When drop beams are used to accommodate mechanical ducts that run between the beams and pass over the drop beams, there is more fire exposure for the drop member which may require a larger member size for the fire,” says Mordan.
“For this project, wood-to-wood connections for each condition had to be addressed individually from a fire rating perspective,” adds Byard. “The details were not rule-of-thumb solutions.”
The permitting challenge, which took four months to complete, “was educating stakeholders about the science of solid wood and the program process,” says Spaeder. “It required local municipalities to understand something new to overcome their initial apprehension.”
During the inspection process, officials “have spent more time making sure fire systems are properly sized and located,” he says.
Exposed solid wood such as heavy lumber is inherently fire resistant and can be left exposed and still achieve a fire rating, according to the Washington, DC-based nonprofit Wood Works Wood Products Council.
The exposed mass timber seen in this interior view of Ellis Mass Timber is inherently fire resistant like heavy mass timber and can be left exposed and still achieve a fire rating.
By Jessica Hind
Massive wood inspiration
The project is the “capella” of a master-planned mixed-use development started 20 years ago at Ellis Preserve, a 218-acre complex with 75,000 square feet of shops and restaurants, a hotel and a conference facility and banquets of 18,000 square meters. , says Steve Spaeder, president and CEO of Equs Capital Partners, which owns the project. The national real estate investment manager is investing in massive timber projects in the residential, medical, industrial and hospitality sectors.
“As solid wood becomes more widely adopted, we will have more suppliers and more assemblers of the product that will bring the cost curve back into line more competitively with steel and concrete,” he says.
Although the Ellis Mass Timber project has been more expensive than traditional steel and concrete, “the quality of construction, desirable aesthetics and environmental benefits have provided a positive rental/cost ratio compared to construction traditional,” says Spaeder.
