*3.1.2.3 Reinforced, pre-stressed and post-tensioned concrete bridges*

With the introduction of Portland cement on the market and the development of concrete construction techniques, design theories for reinforced concrete were developed for structures on the early 20th century.

For long span bridges, reinforced concrete bridges based on arch below main deck are used. It basically consists of an arch in the lower section and piers to support the main deck. All the elements of the bridge are working under compression, with the exception of the main deck that works at flexure. These types of bridge take advantage of the material capacity on compression and avoid tension elements.

**Figure 7.** *Bridge of pre-stressed concrete Girders el Zacatal, México [7].*

Most of the existing bridges cover small and medium spans, which are very useful for roads and highways connecting cities. Girder-based structures with simple or continuous supports are widely used for these cases.

Girder-based bridges can be inefficient due the bending behavior of the girder if we compare it with trusses. However, the girders are relatively easy to build and the relationship between cost and benefit makes these types of structures economically competitive. **Figure 7** shows El Zacatal Bridge, located on Mexico and based on prestressed concrete girders.
