**4. Bridges in the middle ages**

The Pons Fabricius marked the beginning of a new way of looking at bridges that, aesthetically, are relatively light and this trend has survived in Rome till this day. With the exception of the Ponte Pietro Nenni that carries an underground line, all the bridges in Rome have arches. Among them, one finds the first reinforced concrete bridge in the world, the Risorgimento of 100 m span, constructed in 1912 by F. Hennebique. Also, in 2011, a beautiful pedestrian steel

In the Roman period, bridges known as aqueducts were also built, slender and beautiful in their monumentality. The only load they carried was the dead load with an insignificant

**Figure 6.** Roman aqueducts, first century A.D.: (a) Los Milagros (the miracles); (b) Segovia aqueduct.

It should be mentioned that during the Roman Empire, but 200 years later, tower aqueducts were constructed. Water flew in a leak-proof pipe located on relatively short arcades. The level differences were solved on the basis of the principle of communicating vessels according to,

footbridge with shallow arches was built—the Ponte dela Musica.

**Figure 5.** Puente Romano de Mérida, built in first century A.D., Guadiana River.

addition of flowing water, **Figure 6**.

6 Structural Bridge Engineering

surprisingly, Pascal's theorem, see **Figure 7**.

During the European Middle Ages, stone arch bridges were continued to be built, although their variety was limited. Also, the road development slowed down. Bridges usually had a defensive character. **Figure 8** shows two bridges leading to the medieval capital of Spain, Toledo, which are a good example of the role and technique of the medieval bridge construction.

**Figure 8.** The Toledo bridges over the Tajo River: (a) St Martin's Bridge, fourteenth century; (b) Gothic Alcántara Bridge, thirteenth century.

The Toledo Alcántara Bridge should not be mistaken for another Roman bridge of the same name located in the vicinity of the town of Cáceres and constructed in 105 A.D.

In this period in Turkey, a bridge construction canon was developed. Nowadays, such structures are sometimes called Turkish. The bridges in question were still arch bridges, usually made of light colour stones. They have a main span and, possibly, additional spans that serve as flood relief channels in case of high water. It should be born in mind that in the case of the arch bridge, only the arch is the carrying element. The extended walls in arch bridges act only as a façade, very often hiding empty spaces. These spaces were frequently used as utility rooms by bridge guards and sometimes even as guest rooms for travellers. This was the case of the bridge on the river Batman in Turkey, twelfth century, **Figure 9**.

**Figure 9.** Malabadi Bridge near Sivan: (a) side view; (b) entrance to the bridge room.

A twin object, built by Mimar Hayruddin in sixteenth century, can be found in Mostara on the river Neretva. The stem of the name of the town, which is an adjective, comes from the word *most*, i.e., *bridge*.

**Figure 10.** Stone arch bridges in the vicinity of Sille Village near Konya: (a) double parabolic arch structure; (b) parabolic arch with the deck partially ruined.

The popularity of arch bridges results from the arch mechanics. To offer an insight into the issue, we can refer to an exercise solved by students in the structural mechanics class. It is about a parabolic three-hinged arch, evenly loaded. Performing basic transform calculations we find that the bending moment at any point of the arch equals nought. It means that, in fact, the arch is not three-hinged but it is a sequence of hinges. Similarly, in the case of a shearing force we find that it equals naught at all the points. It is not a classical approach, but consequently reasoning we can conclude that the arch is a sequence of hinges functioning of which is based on a shear force. Another conclusion is that the only non-zero internal force in the arch is the force normal with regard to its cross-section, and, what is more, it is a compressive force. In practice, we can shape the sides of stone blocks in such a way that, geometrically, they form a parabolic arch. This arch is going to be a durable and efficient structure—on one condition. In arches, the outward-directed horizontal reaction, called *thrust*, is of primary importance. It has a significant impact and, more often than not, the lack of the proper ground resistance results in the destruction of the arch as a whole or its substantial weakening as a superstructure—at best. The discussed case of a parabolic arch equally loaded is a theoretical one, however, in the case of a real structure we can imagine a set of material points of the highest bearing capacity and that will be an illustration of such an arch. **Figure 10** shows two viaducts on a mountain path near Konya, made with processed stone blocks assembled without mortar.

as flood relief channels in case of high water. It should be born in mind that in the case of the arch bridge, only the arch is the carrying element. The extended walls in arch bridges act only as a façade, very often hiding empty spaces. These spaces were frequently used as utility rooms by bridge guards and sometimes even as guest rooms for travellers. This was the case of the

A twin object, built by Mimar Hayruddin in sixteenth century, can be found in Mostara on the river Neretva. The stem of the name of the town, which is an adjective, comes from the word

**Figure 10.** Stone arch bridges in the vicinity of Sille Village near Konya: (a) double parabolic arch structure; (b) para-

The popularity of arch bridges results from the arch mechanics. To offer an insight into the issue, we can refer to an exercise solved by students in the structural mechanics class. It is about a parabolic three-hinged arch, evenly loaded. Performing basic transform calculations we find that the bending moment at any point of the arch equals nought. It means that, in fact, the arch is not three-hinged but it is a sequence of hinges. Similarly, in the case of a shearing force we find that it equals naught at all the points. It is not a classical approach, but consequently reasoning we can conclude that the arch is a sequence of hinges functioning of which is based on a shear force. Another conclusion is that the only non-zero internal force in the arch is the force normal with regard to its cross-section, and, what is more, it is a compressive force. In

bridge on the river Batman in Turkey, twelfth century, **Figure 9**.

**Figure 9.** Malabadi Bridge near Sivan: (a) side view; (b) entrance to the bridge room.

*most*, i.e., *bridge*.

8 Structural Bridge Engineering

bolic arch with the deck partially ruined.

The advantages of arches in bridge structures make this solution commonly used until this day. The photograph below shows an arch bridge destroyed to such a degree that the arch structure is plainly visible, **Figure 11**.

**Figure 11.** Structure of stone bridge, Samaria Gorge, Crete: (a) side view; (b) longitudinal view.

Numerous antique bridges were built without mortar. Among the objects shown here there are the Roman Bridge in Mérida and the aqueduct in Segovia. Nevertheless, the columns of the aqueduct of *Los Milagros* are composite, according to modern standards. The external cladding was used as permanent formwork, filled with pozzolana cement concrete.
