|PATENTED Innovative, patented bridge design avoids wetlands.|
The designer of two hybrid bridges that currently traverse a fragile wetland at an Alaskan golf course recently received a U.S. patent and has formed a company with the contractor to market the technology. While Freespan Inc. has not sold any more bridges yet, it has proposals out for sites around the country.
Without large abutments and supports needed for a conventional footbridge, the "FreeSpan" bridges utilize epoxy-coated or galvanized prestressed tendons in a configuration of horizontal strands connected to steel and concrete abutments at both ends. Structural support is realized by strand tension. The design can accommodate vehicle weights up to 10,000 lb and spans of 400 ft.
Two bridges were built in 1998 at the Moose Run Golf Course, owned by the U.S. Army at Fort Richardson in Alaska. According to designer Dennis Nottingham of Peratrovich, Nottingham & Drage Inc., Anchorage, the Alaskan design avoids the high cost of conventional trail bridges for lengths greater than 100 ft and fulfills the need to maintain environmentally sensitive areas. The trail crossed a protected wetland and salmon-spawning stream.
The two bridges, 140 and 250 ft long and 12 and 8 ft high respectively, were built for a total cost of $360,000, says Mike Swalling, president of Swalling Construction, the Anchorage firm that installed the bridges.
"At 100 feet and above, this is where the cost savings really show up," Swalling says. He estimates a per linear foot total package cost of about $1,500 to $2,000 for the FreeSpan concept, compared to conventional bridge costs of up to $3,000 per linear foot for spans greater than 100 feet.
Swalling, who formed Freespan Inc. with Nottingham, says part of the cost savings is due to ease of construction. Once abutments are installed, superstructure erection proceeds from one side using temporary horizontal strand-supported rollers to launch the deck sections. Decking material can be wood, aluminum, steel or fiberglass and no heavy equipment is required.
"Depending on the site, construction takes four to six weeks. You don't need a big crew; we did the golf course bridges with only four people and a forklift," says Swalling.