While modeling and testing large-scale wind chimes for what amounts to a 93-ft-tall precast concrete musical instrument, designers broke every taboo about avoiding emotions in engineering. “That word [emotions] has been used many times,” says Arup’s Elizabeth Valmont, the project’s lead acoustical consultant for the Tower of Voices at the Flight 93 National Memorial near Shanksville, Pa. Engineering a complex internal striking mechanism for what the team believes is the largest grouping of tuned wind chimes in the world, Valmont’s team filled nights and weekends with “tears, laughter and exhaustion. … It’s been a ride,” she says, pausing to collect herself.

The tower’s 40 chimes represent each passenger and crew member aboard the hijacked United Airlines plane that crashed on Sept. 11, 2001. Each 8-in.-dia cylindrical chime installed inside the C-shaped structure has a distinct tone. Collectively, the chimes produce an original song every time the wind blows. The stirring melodies projected across the 2,200-acre national park will serve as an “ever-changing living memorial through sound,” says architect Paul Murdoch of Beverly Hills, Calif.-based Paul Murdoch Architects.

Murdoch, a Pennsylvania native who grew up camping and canoeing in the area, won an international competition in 2005 to design the phased memorial. The chimes’ echoes evoke the final phone calls and voicemails those aboard made to loved ones. Before crashing, passengers and crew fought off terrorists believed to be targeting the U.S. Capitol.

 

 

The nearly $6-million tower is the final feature of the national park’s $46-million permanent memorial, which includes a wall of names, visitor and education centers, and a flight-path walkway to the crash site. Sited at the memorial entrance, the tower’s orientation and openings “contribute to behavior of the wind within the tower where the chimes are suspended,” says Paul Rothgery, the National Park Service’s project manager. “Every change made during the design of the tower structure impacted the conveyance of wind to the sails of the chimes.”

The project team—which included a musician and tuning theorist, a chimes artist, acoustical and structural engineers and wind consultants—created tones using musical tuning theory. They tested tower shapes with computational fluid dynamic modeling, simulated variable wind conditions with wind tunnel testing and performed 3D audio simulations of chime configurations at Arup SoundLab in Los Angeles.

In addition to testing physical chime prototypes in the SoundLab, the team also conducted mock-up sessions in quiet, remote locations such as Tuzigoot National Monument in Arizona. Murdoch says the chime mock-ups helped to mitigate the fact that the NPS contract didn’t allow the chime fabricator to serve in a design-build capacity. Also, Murdoch says, the “fixed budget and fixed schedule in what should otherwise be an open-ended process of testing and analysis” made the project challenging.

The team worked on the chime design for more than a year before the fabricator, Illinois-based Fugate, cut, tuned and assemble the chimes. Varying in length from five ft to 10 ft, the size of each chime depends on the musical note and frequency it is designed to play.

The chimes’ internal strikers proved to be the biggest design challenge because they don’t have much room to move inside the chimes to generate the necessary force needed to amplify sound. Each striker and its sail is connected to a suspension rod running through the tubes. The team determined the striker and sail specs that created the desired decibels and pitches during the second mock-up session last year. But the session also revealed that the original “light and buoyant” cable system that facilitated those results also frayed easily, Murdoch says. He says a more “heavy and rigid” cable didn’t perform well during the third mock-up session earlier this year.

The team eventually replaced the cable with a linked system using a combination of stainless steel rods and universal joints “to allow a certain amount of rotation and sway,” Murdoch says. Additionally, the team replaced the originally designed “flat sail” with “an S-shape so that there’s a certain amount of drag that allows the sail to move with the wind in a direction to help create strikes,” Murdoch says.

As of Sept. 5, the team expected to have eight of the 40 chimes installed before a Sept. 9 dedication ceremony. The remaining chimes are expected to be delivered and installed later this fall. 

Tasked with tethering the chimes to corbels jutting from the structure’s interior columns is contractor Leonard S. Fiore. Earlier this summer, the Altoona, Pa.-based firm erected the tower’s 53 precast concrete segments. Beams connecting the columns were installed at a 20° angle to represent the branches of the park’s hemlock trees and to optimize airflow. PennStress fabricated the pieces, ranging from 25,000 lb to 31,000 lb.

The 274-ton tower’s foundation is supported by 25 micropiles on a reclaimed strip mine. The piles comprise continuous, high-strength 3-in.-dia steel rods grouted inside a 7-in.-dia steel casing driven 15 ft into bedrock that lies 83 ft below the tower plaza’s surface. Michael L. Fiore, project management director, is eager to show off the “finished product.”

Though Murdoch quipped that the dedication’s success “depends on the wind,” he is all business “until we’re finished on Sept. 9 and listening to those chimes.”

This story was updated with new information on Sept. 6