Science Story

Better Bridges: Considering Wood–Concrete Construction

Tom Owens
Forest Product Laboratory
December 18th, 2015 at 7:15PM

The United States is facing an infrastructure crisis. According to the American Road & Transportation Builders Association, more than 61,000 bridges in America are structurally deficient. Although expensive to maintain, particularly in these tough economic times, when these vital pieces of our transportation system fail, the human toll is incalculable.

One needs to look no further for a reminder than the 2007 collapse of the Interstate 35 bridge near Minneapolis, Minnesota. The steel truss arch bridge suddenly failed under the load of rush hour traffic, plunging into the Mississippi river below. The bridge had consistently ranked near the bottom of nationwide federal inspection ratings, and its collapse claimed 13 lives.

A scene from the 2007 collapse of the Interstate 35 bridge near Minneapolis.

As states continue to evaluate and improve their transportation infrastructure, researchers at the Forest Products Laboratory (FPL) are working hard envisioning the future of durable and cost-effective bridges. They believe that the answer may lie in wood, one of humankind’s oldest construction materials, used in conjunction with another time-tested material, concrete.

In cooperation with Iowa State University, Research General Engineer Jim Wacker, from the Engineering Properties of Wood, Wood Based Materials, and Structures unit at FPL, has set out to investigate the state-of-the-practice related to the use of concrete decks supported by glued-laminated (glulam) timber girders for highway bridge applications. Glulam timber bridges have already proven themselves in our nation’s National Forests, but the practice of using them in conjunction with concrete decks is relatively scarce across the highways of America.

A composite timber–concrete bridge consists of a concrete slab rigidly connected to supporting timber sections so that the combination functions as a unit. Composite timber–concrete bridges are of two types: T-beam decks and slab decks.

T-beam decks are constructed by casting a concrete deck, which forms the flange of the T, on a glulam beam, which forms the web of the T. Composite slab decks are constructed by casting a concrete layer on a continuous base of longitudinal nail-laminated sawn lumber.

Recent research has found that performance of timber bridges constructed 50 to 70 years ago is above average, but despite this, only a small percentage of new bridges built every year are built with graded and engineered lumber.

Wacker’s reassessment of concrete–timber bridge construction will arm bridge engineers with a wealth of knowledge on the best practices of the past—so that bridges of the future can be as cost-effective, durable, and safe as possible.