*3.1.2.1 Steel truss bridges*

Since the increase of traffic flow and weight of vehicles started, the wooden bridges were already insufficient to support these vehicle loads and the structures began to use steel materials. With the structural steel available on construction market, these bridges had a significant improvement, including the increase of spans length covered and supporting higher loads. Steel trusses bridges replaced wooden trusses and began to build bigger structures. **Figure 5** shows an example of this bridge's type.

disadvantages of this type of structures is the aerodynamic stability, product of the slenderness relationship between main slab, span clearage and the action of thrust

With the introduction of Portland cement on the market and the development of

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

Most of the existing bridges cover small and medium spans, which are very useful for roads and highways connecting cities. Girder-based structures with sim-

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 pre-

Depending on the span to be covered, the traffic flow, the availability of materials and labor, the designer will define the geometry of the bridge. Most of the bridges have short spans and use girders of reinforced concrete, pre-stressed concrete or structural steel. Bridges with intermediate spans use trusses and arches. For

a. Cover the vehicular flow demand, with enough lanes and/or spaces.

ple or continuous supports are widely used for these cases.

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

very large spans, suspended bridges are the best option. All types of bridges must have the following qualities:

b. Support dead, live and accidental loads.

c. The structure is economically viable.

stressed concrete girders.

**3.2 Structures and functions**

concrete construction techniques, design theories for reinforced concrete were

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

developed for structures on the early 20th century.

*Bridges: Structures and Materials, Ancient and Modern DOI: http://dx.doi.org/10.5772/intechopen.90718*

forces produced by the wind loads.

elements.

**Figure 7.**

**99**
