**7. Iron bridges**

m. The bridge was not built, however. In 2001 in Norway, creator Vebjorn Sand constructed a footbridge in the town of Ås [12], which was a reference to Leonardo da Vinci's concept. However—in the author's subjective view—it is only partially successful, both in the aesthetic sense and as an incorporation of the Renaissance genius' idea. Looking at the drawing in the manuscript, it is difficult to decide about the bridge structure. Sand's footbridge seems to be

**Figure 12.** Golden Horn Bridge project, 1502. Paris Manuscript *L*: (a) 66th page of the manuscript; (b) bridge enlarged.

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

one of many possible interpretations.

10 Structural Bridge Engineering

**6. Wooden simple bridge**

The industrial revolution in England was a turning point in bridge construction. In 1779 an arch bridge, known as the Coalbrookdale Bridge or Iron Bridge [15], was constructed by Abraham Darby III with cast iron from his ironworks located in Shrewsbury, **Figure 14a**.

One hundred years later, bridges of spans reaching 160 m were built with wrought iron. **Figure 14b** shows a photograph of the bridge on the river Duro in Porto. It was constructed by T. Seyrig, Gustave Eiffel's associate. At present, the bridge has no utilitarian function, but it is one of the highlights of the city of Porto.

**Figure 14.** Cast iron bridges: (a) Iron Bridge, 1779 (*photo by Jason Smith* [16]); (b) Dona Maria-Pia Bridge, 1877 (*photo by Dr.Eng. W. Nurek*).
