A tensile structure solution

The McGrath Amphitheater is a PTFE tensile canopy that helped rejuvenate a downtown riverfront park.

By Jake Kulju

Place plays an important role in the development of a community, and the redevelopment of one. In 2008, Cedar Rapids, Iowa, experienced some of the most intense flooding in the city’s history. More than 10 square miles of the city suffered severe flood damage when the Cedar River crested over 31 feet, impacting upwards of 18,000 people and destroying more than 10,000 homes.

The city has been diligently renovating flood-damaged areas, including the construction of a new convention center and hotel, an historic theater, and a library. At the center of the redevelopment project in a popular and important riverfront area is a new park that serves as a levee and as the site of the new McGrath Amphitheater. Situated on the banks of the once swelling Cedar River, the amphitheater is regarded as a symbol of the city’s healing process.

A tensile structure solution

The city envisioned an amphitheater that would provide a venue for outdoor entertainment and visual appeal, but the project had a tight budget. Due to the structural framing system that was specified, however, initial quotes for the construction came in far over the city’s budget. The winning bid and design came from Kansas City, Mo.-based Structurflex. Their design team bid for the work on the initial specifications, but also offered a less expensive approach. The project owner appreciated this new input, and Structurflex was ultimately awarded the project.

“This project was bid on two separate occasions: the initial bid was approximately one year prior to our award of the contract,” says Bart Dreiling, Structurflex’s president of American operations. “The initial bid was significantly over budget due to structural details specified by the architect and engineer. They had designed the structure to be made up of plate welded into a box section as opposed to utilizing standard steel sections readily available in the marketplace. Additionally, a ‘weathering steel’ alloy was originally specified, which added significantly to material and fabrication costs. Structurflex offered voluntary alternates that were considered during the rebid. All of our recommendations were taken into consideration. When the project was awarded to Structurflex, we then became the engineer of record for the structure and had the liberty to design the steel structure and specify steel grades in a manner that was most efficient for the structure’s geometry.”

The Structurflex design team worked from the architect’s original concept to develop an affordable alternative structural solution that met the original design intent. Clean, attractive and smart connection details allowed a seamless erection process in the field, while adjustable tensioning and clamping systems simplified the challenging process of installing PTFE fiberglass during cold weather.

PTFE fiberglass material was chosen for its durable and aesthetic properties. “The architect’s design intent was to have a subtly curved structure rising up from the earth, much like a bridge spanning over the nearby river,” Dreiling explains. “Translucency in the fabric was important for nighttime lighting effects during concerts. The lifespan of the fabric is always of utmost importance. The PTFE-coated fiberglass has a proven history to last well beyond 25 years.”

The Structurflex team then determined that Sheerfill® II, manufactured by Saint-Gobain, Courbevoie, ÃŽle-de-France, was the best material for the project based on engineering requirements and quality standards.

Sheerfill II is a composite material of fiberglass and polytetrafluoroethylene (PTFE). The textile is manufactured in a range of strengths and light transmission levels, making it an extremely versatile fabric, especially for outdoor enclosures. Additionally, Sheerfill membrane fabric conforms to rigid fire and building codes for permanent buildings and structures.

Installation challenges

Installation of the membrane was tricky for two reasons. The slope of the structure meant the installation crew had to work against gravity. “The material was packaged and bundled in a manner that allowed for the material to be rolled out onto the structure in a manner that reduced the movement of the material,” Dreiling says. “The primary challenge was sliding the material into a keder rail track that was pre-welded to the structural steel. This required the material to move uphill from one side to the other. Typical boom lifts were used by a crew of four to five installers.”

The weather also created installation challenges. Delays in foundation construction forced the contractor to reschedule work and added time to the PTFE installation. “Installation of PTFE fiberglass in cold weather conditions requires greater care, more attention to avoid folding and creasing,” Dreiling says. “It also significantly increases the pre-stressing time during installation. Added risk comes with this the extended pre-stress time as the longer period can expose the membrane to high winds and heavy precipitation before the membrane has reached its service state.”

After its completion and grand opening in August 2013, the McGrath Amphitheater quickly became the jewel in the crown of the Cedar Rapids riverfront redevelopment plan. The amphitheater is regularly used for outdoor entertainment and provides a beautiful backlit visual after sundown.