Wood has been used as far back as the Neolithic era to cross rivers. It is estimated that 17,000 years ago, covered logs laid flat made up the first wooden bridges. The first documented accounts of wooden structures are available from the Egyptians, Phoenicians, Greeks and Celts, particularly relating to structures crossing the Euphrates and certain tributaries of the Nile from 2,000 to 3,000 years ago.
Later, the soldiers of the continental empires (Persian, Roman, etc.) used floating wooden barges as intermediate piers to create simple timber bridges to quickly cross huge obstacles. Records document armies crossing the Rhine, Bosphorus and Dardenelle Straits using these boat bridges.
However, it wasn’t until the height of the Roman Empire (circa 100 BC) that new more advanced timber structures, in particular bridges with beams, strut frames and arches, developed. Caesar’s bridge across the Rhine, where the width of the river was 140 metres, was built in only 10 days and is an excellent example of timber bridges at that time. They remained simple, generally temporary in nature and were frequently washed away by flood waters.
Throughout medieval times, timber bridges became very common and had become more and more ornate. The timber deck bridges were then generally built with stone piers on timber pile foundations. Bridges became a symbol of power and affluence and this led to a demand for more permanent structures.
Constructors soon became aware that rot was wood’s major enemy and that it could be avoided by keeping the material dry. Some of the most respected engineers of the time recommended that all timber bridges be covered and, where this advice was taken, some structures built during this period have displayed remarkable longevity (the Kapelbrücke in Lucerne built in 1333 was only destroyed by fire in 1993). However, in general maintenance was expensive and was seldom addressed for any prolonged period of time leading to a number of high profile bridge collapses and an average lifespan for timber bridges of approximately 25 years.
For this reason by the end of the fourteenth century, timber bridges went out of favour and were generally only built as temporary structures or where economic conditions would not allow for more permanent solutions in stone.
Timber bridges received a new lease of life in the eighteenth and nineteenth centuries, particularly in the USA where it is estimated that approximately 10,000 timber bridges were built between 1805 and 1885. This revival can be attributed to a number of factors including; the development of creosote; the transfer of knowledge from the new European settlers and; the fact that timber as a lightweight material was ideally suited for use in the newly developed suspension bridges. Many covered bridges and bridges with spans up to 85m were built during this period, many of which remain in service today. However, for longer spans, timber simply did not provide sufficient strength for the heavier loads to which bridges were being designed during the railway boom years.
More recently during the twentieth century, there has been a renewed interest in timber as a bridge building material particularly for footbridges. Developments in the use of timber in bridge construction include; the design of decks from pre-stressed wood and; the use of glued-laminated wood. Nevertheless although the problems relating to durability (rot, insect attack, fungus) remain, some very impressive timber structures have been built utilising orthotropic steel decks as well as polymethacrylate roofing to protect the timber structure below.