The Age of Timber

Wood: The Universal Building Material

Wood dominated bridge construction for thousands of years due to its availability, workability, and reasonable strength-to-weight ratio. Unlike stone, wood could handle both compression and tension forces reasonably well, opening up new possibilities for bridge design that went beyond the limitations of arch construction.

The material properties of wood that made it attractive for bridge builders included its relatively high strength in both tension and compression, its light weight compared to stone, and its ability to be shaped with simple tools. Different types of wood offered different characteristics—oak for its durability and strength, pine for its availability and workability, and tropical hardwoods for their resistance to decay and insects.

Wood's greatest advantage in bridge construction was its versatility. Timber could be used to create simple beam bridges for short spans, complex truss structures for longer spans, and even arch forms that competed with stone construction. The development of sophisticated joinery techniques allowed builders to create strong connections between wooden members without relying entirely on metal fasteners.

However, wood also presented significant challenges as a bridge material. Its susceptibility to decay, fire, and insect damage meant that wooden bridges required constant maintenance and periodic replacement. Exposure to weather, particularly cycles of wetting and drying, caused wood to crack, warp, and lose strength over time.

The great wooden bridges of the 18th and 19th centuries represented the pinnacle of timber bridge engineering. Builders like Theodore Burr and Ithiel Town in America developed standardized truss designs that could span impressive distances while using relatively small timber members efficiently. These covered bridges, protected from weather by roof and siding systems, could last for decades with proper maintenance.

Engineering Properties of Timber

Understanding wood as an engineering material requires recognizing its anisotropic nature—its properties vary dramatically depending on the direction of loading relative to the wood grain. Wood is strongest when loaded parallel to the grain (along the length of the tree trunk) and weakest when loaded perpendicular to the grain.

This directional dependency shaped how bridge builders used timber. Main load-carrying members were oriented so that their grain ran parallel to the primary load direction, maximizing the wood's strength. Connection details had to account for wood's weakness across the grain, leading to the development of specialized joints and reinforcement techniques.

Moisture content dramatically affects wood's properties and behavior. Green timber, freshly cut with high moisture content, is weaker but more flexible than seasoned timber that has been dried. However, as wood dries, it shrinks and can develop cracks that reduce its strength. Bridge builders had to understand these effects and design accordingly, often using green timber for construction but accounting for shrinkage and movement as the structure dried in service.

The development of laminated timber construction in the 20th century allowed engineers to overcome some of wood's natural limitations. By gluing multiple layers of wood together with controlled grain orientations, laminated timber beams could achieve greater strength, consistency, and size than solid timber members. This technology has enabled a renaissance in timber bridge construction, with modern wooden bridges spanning distances that would have been impossible with traditional construction methods.