Institute for Transportation

According to the most recent National Bridge Inspection Standards (NBIS) listing, there are approximately 25,000 bridges across the State of Iowa, and about 10% of them are timber bridges.

“And of those approximately 2,500 timber bridges, a significant portion are either in disrepair or don’t meet current standards. Furthermore, there are an additional 46,000 timber bridges nationwide, many that are over 60 years old,” says Travis Hosteng, director of the National Center for Wood Transportation Structures (NCWTS).

Meeting the needs of aging and deteriorating bridges is one of the primary research focal points of the NCWTS. In fact, its work in developing wood—a naturally sustainable forest resource—as an efficient bridge material can be traced back to 1988, when Congress first passed the Timber Bridge Initiative.

Since then, they conduct approximately $200,000 to $300,000 worth of research annually, often in cooperation with government agencies, universities, and private industry. With the Federal Highway Administration, Forest Products Laboratory, and the National Park Service as partners, their work impacts bridges both in Iowa and across the country.

Their research includes an emphasis on the development of smart timber bridge systems and standardized plans and resources for timber bridge designers and engineers.

During the summer of 2016, a smart timber demonstration bridge was built in Buchanan County, Iowa, consisting of a glulam girder superstructure supported on geosynthetic-reinforced soil foundations with an extensive structural health monitoring system.

According to Hosteng, monitoring systems are especially important, since they continuously monitor both the substructure and superstructure remotely.

“The sensor array has somewhere around 80 to 100 sensors. It collects a range of data—from strain, tilt, and moisture content to temperature and bearing loads.”

Data is sent a NCWTS server, where it is processed and analyzed 24 hours a day, 7 days a week, and 365 days a year.

Demonstration projects like this one have helped bring to light the potential success of glued-laminated timber bridge technologies. In fact, combined with accelerated bridge construction (ABC) techniques, NCWTS research speaks to the sustainability of timber bridges.

“Ask anyone about timber bridges, and they’ll either describe the bridges of Madison County, Iowa, or a solid-sawn structure they once drove across,” says Hosteng. “But, ask a county engineer the same question, and deteriorating timber piles will likely be one of their biggest issues. These perceptions—that timber is outdated, rots over time, and is stick built—don’t accurately describe current timber products and timber bridges being built today in 2017.”

With many high-profile state projects across the nation already using ABC technology, Hosteng sees it as only a matter of time before obsolete bridges are not only replaced with timber, but local agencies begin regularly adopting ABC as a cost-effective alternative to traditional bridge construction. This type of sustainable research will remain an emphasis in work conducted by the NCWTS now and into the future.