‘Tower of Voices’ provides everlasting memorial to Flight 93 victims

PennStress, a division of the MacInnis Group, LLC, provided more than 50 precast concrete segments for a 93-ft. tower located on the grounds of the Flight 93 National Memorial near Shanksville, Pa. Commemorated just days ahead of the 17th anniversary of the September 11 terrorist attacks, the Tower of Voices is a musical instrument with 40 uniquely pitched chimes to represent the voices of each passenger and crew member who died aboard the hijacked United Airlines flight.

The tower’s chime system is designed using music theory to identify a mathematically developed range of frequencies needed to produce a distinct musical note associated with each chime. PHOTO: Tami Heilemann – Department of the Interior

Designed by Beverly Hills, Calif.-based Paul Murdoch Architects, the “C” shaped, 274-ton structure is oriented to optimize air flow to the interior chamber. Beams connecting the columns rest at a 20-degree angle to represent the branches of the park’s hemlock trees and augment airflow. Each wind-activated chime ranges between 8 and 16 inches in diameter and varies in length from 5-10 feet, depending on the musical note and frequency it is designed to play.

According to PennStress Vice President of Engineering Russell Dickson, PE, representatives from Paul Murdoch Architects, contractor Leonard S. Fiore and National Park Service visited the company’s Roaring Spring, Pa., plant for a mock-up review prior to the start of production and then again near the end of casting. The company produced 16 columns, 35 beams, 40 corbels, two large cap pieces, and eight benches at the plant, which is located 44 miles from the project site. Custom fabricated steel forms—three corbel, two beam, a 52-ft. column and a 17- x 17-ft. half-pipe shaped cap—were manufactured by Helser Industries of Tualatin, Ore. Since the chimes would be tethered to the corbels jutting from the tower’s eight interior columns, each corbel had to be cast individually with bars projecting out of them, and then cast into the columns with the bars embedded.


Contractor Leonard S. Fiore of Altoona, Pa., erected the tower’s precast concrete segments earlier this summer. PHOTO: National Park Service

The precast segments were produced from a 7,000-psi, self-consolidating mix, and ranged from 25,000-31,000 lbs. All pieces are of PCI Grade A architectural finish with a light sandblast. “The color was gray, which may sound simple, but it has many shades and makes it very difficult to match,” says Dickson. “On this project, we did not have to match any sample so the color selection was simple but the light sandblast finish was difficult and took dozens of practice pieces to get right.” In fact, one of the architect’s requirements was to use the same concrete in the field as was used in the precast to provide the best color match between the two. PennStress rented a mixer truck to transport the concrete to the site.

The nearly $6 million Tower of Voices, which was funded by a National Park Foundation grant, marks the final phase of major construction at the 2,200-acre national park. The $46 million permanent memorial includes a visitor’s center that opened three years ago and a plaza that opened in 2011.