**6. Wooden simple bridge**

Wooden bridges have always been built. For this reason, a technology was developed which took into account the characteristics of wood as a material of relatively low strength, but at the same time commonly available and easy to handle. The strength of wooden bridges has always been low. It was good if the serviceability period amounted to 20 years. A wooden bridge is also the result of a skilful application of structural mechanics. For the above reasons, usually simple, mechanically pure solutions were chosen. Let us consider the most common wooden bridge, **Figure 13**. Abutments are meant to transfer two kinds of impacts. The first one refers to vertical reactions caused by loads on the carrying deck. For this reason, a row of poles was constructed under each girder. The other impact refers to active ground pressure behind an abutment. Here, the second row of poles was used—outer poles were put behind so-called planking. As a result, a retaining wall was obtained which was also additionally supported by its anchoring in the ground (deadman). The bridge wings were slanting, of variable heights.

An interesting solution is a saddle which enables reduction of support spans and the values of bending moments under the supports. In the case illustrated in **Figure 13**, the bridge beam is single but wooden bolster composite bridge beams were also used. The composition was not complete—from the modern perspective it should rather be called *partial integration*. The construction of wooden bridges was at least as complex as the modern bridge technology. The dimensions of a saddle are determined on the basis of a simple differential calculus assuming extreme curvatures of the beam and the saddle at the local point of contact. Paradoxically, the best work about wooden bridges is a book [13] written 100 years ago. The technology of wooden bridges is still in use in the case of temporary bridges. Additionally, there is a new option available which is connected with the ecofriendly recycled plastic technology, see [14]. Instead of natural or sawn wood, recycled plastic logs or panels are used which contributes to the minimisation of deforestation.

**Figure 13.** The scheme of a wooden bridge: carrying elements.
