The Tower of Voices National Memorial in Stoystown, Pa., is as much a musical instrument as it is a monument. The 93-ft-tall precast concrete tower houses 40 custom-made aluminum wind chimes. Each chime was cast to produce a distinct musical note, so that together they create a set of 40 “voices” to memorialize the 40 passengers and crew who lost their lives when United Airlines Flight 93 was hijacked by terrorists on September 11, 2001.
Precast concrete was selected for the tower frame because of the precision required to accommodate the complex and unusual shapes of each component, and the strength needed to support the tall, slender frame, says Paul Murdoch of Paul Murdoch Architects. “The customizable nature of precast concrete allowed for a unique design, featuring faceted columns and diagonal, curved beams.”
BIM accommodates zero tolerance
The 93-ft-tall tower features a cross section in a “C” shape that allows sound to reflect outwardly from the open side in a fan-shaped pattern. The chimes are suspended at variable heights within the tower, starting from 20 ft above the main plaza and ascending to the top.
The elegant design is a beautiful memorial to the people aboard Flight 93; however, it created some construction challenges. The structure lacked a clean geometric starting point due to its curves, and this made form building and checking the pieces extremely difficult, says Greg Gorman, senior vice president and chief operating officer for PennStress.
The design also required an extremely high degree of accuracy. “The use of Building Information Modeling (BIM) software was imperative to ensuring that the detailed geometry of the pieces was exact, and that no interferences for the reinforcing steel, splice sleeves, lifters, electrical conduit, or various other internal components could occur,” Gorman says.
His team used 3D printing technology to create prototypes to better understand the structural relationships between the tower columns and beams. “The pieces were designed in a circular and skewed layout, adding complexity that required a BIM program to ensure accuracy,” Gorman says. Several scale mockups were also created and collaboratively discussed to ensure the best possible connections. This ensured not only that the structure looked as it should but also that the structure would maintain its appearance after the precast concrete was coated with a water-repellent coating and anti-graffiti protection.
During the design phase, the precast concrete producer determined that the slenderness of the full-length columns would make them too vulnerable to damage during transportation and erection. Therefore, the precast concrete producer used diagonal column splices that allowed the joints to blend in with the diagonal beams. At the joints, pockets and splines aligned the column pairs vertically, creating symmetry in a structure with various angles, curves, and heights. “BIM using TEKLA (model-based construction software) was integral to the precast concrete design and production process to achieve near zero tolerances,” Gorman says.
Using precast concrete also provided speed and performance benefits, says Russell Dickson, vice president of engineering for PennStress. “It allowed for a much higher level of accuracy due to using custom-fabricated steel forms, and easier access for checking to ensure the highest standards were maintained.” Also, because the production facility was enclosed, the team was able to produce the pieces during winter months to maximize the schedule.
The customizable features included in the precast concrete will enable the chimes to “sing” even at low wind speeds, creating a monument that will give voice for decades to come to those individuals who were lost on Flight 93.