Better Bridges: Considering Wood-Concrete Construction

The United States is facing an infrastructure crisis. According to the American Road & Transportation Builders Association, over 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. In the disaster’s wake, fearing similar incidents, federal and state governments mobilized to assess the condition of their own bridges.

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 mankind’s oldest construction materials, used in conjunction with another time-tested material, concrete.


An example of a composite timber girder–concrete deck bridge

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. The project, which commenced earlier this year, is expected to be finished June 2017.

A composite timber-concrete bridge consists of a concrete slab rigidly connected to supporting timber sections so that the combination functions as a unit. There are two types of composite timber-concrete bridges: 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 on the other hand 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. This project hopes to change that.

Composite slab decks have been used as far back as the 1930s, and 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.

For more information, please see the FPL Research in Progress publication Investigation of Glulam Girder Bridges with Composite Concrete Decks.

Newest Forest Products Journal Features Adhesives: Many FPL Researchers Present


Photomicrograph of an adhesive bond of two pieces of wood. The blue areas show the adhesive penetration into the wood structure.

The latest issue (Volume 54, No. 1/2, 2015) of The Forest Products Journal is all about adhesives. Featuring 10 selected articles addressing a theme of efficient use of wood resources in wood adhesive bonding research presented at the 2013 International Conference on Wood Adhesives in Toronto, Canada, we hear from several FPL scientists.

FPL has played an integral role in developing technical understanding of adhesives and setting product and performance standards by organizations such as the ASTM International (formerly American Society for Testing and Materials), American Institute of Timber Construction (AITC), APA–The Engineered Wood Association (APA), and the American Forest and Paper Association (AF&PA).

The first glue development research at the FPL in 1917 was to improve water resistance of the best glues available for manufacture of WWI aircraft components. At that time, FPL began to develop composites in an attempt to conserve our forests and make use of waste wood. Adhesives for housing, other buildings, timber bridges, and other structures has always been important.

In the Introduction to Special Issue: Wood Adhesives: Past, Present, and Future, Team Leader, Wood Adhesives, Forest Biopolymer Science and Engineering, Charles Frihart provides a comprehensive history and explanation of the important role that adhesives have played in the efficient utilization of wood resources.

Speaking about wood products, Frihart says: “Adhesives will continue to be a growing part of efficient utilization of forest resources. However, acquiring suitable wood resources will continue to be a challenge because of a diminished supply of high-quality wood and competition for wood from wood pellet and biorefinery industries. The challenges involve dealing with species that are not currently being used and with a greater mixture of species. More plantation wood could involve increased porosity and lower strength because of increased proportion of earlywood. The wood may also have increased or more variable moisture content as a result of efforts to reduce drying costs.

Wood products volume should continue to increase especially if engineered wood products replace other building materials for multi-story buildings and if there are sufficient housing starts. One challenge could be in bonding wood to other materials if glulam or laminated veneer lumber start using layers of stronger polymers or composites for greater strength. There also might be markets for bonding to modified wood, such as acetylated wood or heat-treated wood.”

Challenges in our changing forests and in changing construction practices will keep Frihart and his team busy for years to come as they find ways to use their adhesive research to adjust to change and best utilize our natural resources.



Throwback Thursday: Building With Glued Arches


Laboratory utility building of plywood with glued arches, the first structure of this type in the United States.

The first research at FPL on engineering design data for glued laminated arches was undertaken in 1934, when a number of three-hinged arches were fabricated and installed in what was called Building 2, the packaging research building.

It was no ordinary building—it was built using laminated arches and also included other arches made from wood. The purpose was to provide a useful building but also one in which a visitor could observe different types of arches and see the advantages of design to decrease material and improve aesthetics. It included tests of structural units to check such factors as design formulas and working stresses, and the effect on strength of curvature, scarf joints, and knots in the inner laminations.

Results of this research are presented in United States Department of Agriculture Technical Bulletin 691, The Glued Laminated Wooden Arch, which provides the technical data necessary for the use of laminated arches on a sound basis.

The building suffered a major fire in later years, but when the firefighters learned the building was built with wood beams, they were able to save the structure. In 2010, this building was dismantled and the arches were saved for testing.

FPL History: Laminated Construction



FPL’s pioneering work on the engineering design of glued-laminated construction helped launch the laminating industry in the United States. Much of the research on laminated wood originated at the time of the first World War when the Bureau of Aircraft Production approached FPL with a need for lightweight airplane wings.

Shortly after the U.S. entrance into the war, FPL initiated a very elaborate investigation into the mechanical properties of plywood, as no information was available on this subject and its importance in connection with aircraft design was evident. As this investigation proceeded, the possibilities in the structural use of this material became greater and scientists applied the new knowledge as quickly as possible. The photo above demonstrates various products produced from laminated wood.

FPL scientists have been at the forefront of designing laminated arches and beams for construction. FPL researchers have designed and evaluated various beams to determine how to economically fabricate beams to maximize strength, and they determined if underutilized species could be used. The results of research have eliminated the need to cut large trees to produce satisfactory beams for construction. Also, many smaller, less utilized species of wood can now be assembled and used as large beams.


Glulam beams being installed in an FPL building in about 1931.


Historic Glued-Laminated Arches Evaluated for Structural Quality


Building Two on the FPL campus was constructed in 1934 and deconstructed in 2010. Bottom photo credit: Steve Schmeiding, FPL.

The second glued-laminated structure built in the United States was constructed at the USDA Forest Products Laboratory (FPL) in Madison, Wis. “Building Two” was constructed in 1934 to demonstrate the performance of wooden arch buildings. At various times it acted as a supplementary laboratory, lecture hall, and storage facility. Building Two was decommissioned in 2010.

A new General Technical Report (FPL-GTR-226) by FPL engineer Doug Rammer and Jorge Daniel de Melo Moura of the Department of Architecture and Urbanization, Universidade Estadual de Londrina, in Parana, Brazil, details a systematic evaluation of the glued-laminated arches used to construct Building Two. Glued laminated timbers are a manufactured structural timber product composed of layers of dimensional lumber glued together.


Construction of Building Two in 1934 used three different glued laminated arch configurations. Click on the photo to view a larger version on the FPL Flickr site.

Shortly after the construction of Building Two, researchers evaluated the glued-laminated arch structure for uniform loading on the center arch. This structural system evaluation was added to the existing laboratory work on glued-laminated arches to develop the foundation on which the current glued-laminated arch design criteria is based.

After decommissioning, recovered arches were tested in the Engineering Mechanics and Remote Sensing Laboratory at FPL to evaluate the loss of structural performance by comparing original and current deformation. Based on a preliminary visual and structural assessment Rammer and Melo Moura found minimal loss of structural performance in all the arches but one, an arch that was exposed to a significant amount of water resulting from extinguishing a fire in Building Two.