Civil Engineering Magazine | Catherine A. Cardno, Ph.D. | December 2014
The design has been completed for a pedestrian bridge in Tallahassee, Florida, that will be lit by energy captured from suspended fabric solar cells.
December 9, 2014 — The capital city of the Sunshine State will soon have its very own solar-powered pedestrian bridge. The $2.5-million Capital Cascades Connector Bridge in Tallahassee, Florida, has been completed by a design team led by the Tallahassee-based bridge engineering firm FIGG. Angled posts that will hold the deck of the bridge will also support the flexible fabric canopies that appear to flit across the top of the bridge, offering shade to those who pass beneath and generating power above via integrated solar panels. The panels are expected to gather enough energy to power the bridge's nighttime lighting, while excess energy will be passed back to the energy grid.
"This new signature bridge is an example that shows that aesthetic, form, and function can be achieved simultaneously," says Linda Figg, M.ASCE, the president and chief executive officer of FIGG. Figg served as the visual quality and sustainability manager for the bridge and led a charette process through which the community selected the bridge design from various options prepared by FIGG.
The bridge is part of the Capital Cascades Trail, a 4 mi project that will provide a multiuse recreation trail and park within the city, according to Figg. "The Connector Bridge is a modern concrete bridge with organic shaping based on a theme of canopies and ecofriendly design inspired by the iconic canopy roads that are part of Tallahassee, Florida's, unique landscape," she notes. "The bridge deck creates a slender, uniform ribbon from end to end, supported by a series of circular posts positioned at angles."
A 72 ft 6 in. main span will extend across four lanes of traffic, while two side spans — one measuring 13 ft 9in. in length and the other 30 ft — will connect the bridge to the park on either side of the main span. The bridge is approximately 13 ft 10 in. wide with a full 12 ft available curb to curb so that pedestrians and bicyclists can cross comfortably.
The bridge is founded on rectangular concrete footings that are supported by 24 in. steel pipe piles, according to Figg. The piles are mechanically connected to the main footings, which support the bridge's angled substructure posts, which hold both the deck and the canopies.
The 24 in. diameter angled, steel substructure posts will be finished with a white powder coating for aesthetic value, and will vary in length between approximately 25 and 42 ft. They will be filled with reinforced concrete to increase their stiffness, according to Figg. "The canopies will connect at the top of each post, which are oriented in different angles in the longitudinal direction of the bridge and have a constant inclination angle in the transverse direction," she says.
New York City-based Pvilion, a vertically integrated solar design and manufacturing company, created the fabric canopies that will top the bridge. The firm has been involved in the project from the design engineering phase and will continue to be involved all the way through manufacturing and installation, according to Colin Touhey, the chief executive officer of Pvilion.
"For this project, FIGG has designed the idea of solar tensile structures on a pedestrian bridge and came to us early on in their design process so that we could help incorporate our technology into the project," notes Todd Dalland, the president of Pvilion.
"This isn't something that flaps in the wind like a shade canopy you might see in the backyard of someone's house," Touhey notes. "These are really engineered like buildings." The canopies are engineered to protect against the same wind loads called for in building codes as the permanent structures in the area, according to Dalland.
Overall, flexible solar fabric canopies are a lighter-weight material than traditional construction materials and are quicker to install, according to Touhey. "All of our projects come sort of plug-and-play….We do all the work, and then you basically put this thing up and plug it in. It doesn't require tremendous amounts of wiring on-site," he notes. In addition to the ease of installing solar such canopies , which can be used on the facades of buildings, as well as rooftops, the cost is comparable to other construction material, Touhey notes.
The solar fabric used in the pedestrian bridge will be coated with a polyvinyl fluoride film so that it can be largely self-cleaning in the rain, according to Pvilion.
The bridge is expected to be built within 12 months of the notice to proceed being issued, Figg says.
To create the canopies, a three-dimensional, digital, structural model was developed to include all of the forces acting on the canopies, according to Figg. This included the finished installation, as well as the forces that would be present on the fabric during the installation and stressing of the cables. "Moment, shear, and stresses were computed for all the structural members and connections," she says.
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