**3. Bridges, a general overview**

We recognize that all structures are part of the infrastructure and each one works together. The bridges take a special role, due its function to connect two different points, crossing valleys, rivers, lakes and cliffs.

Bridges are needed on land transportation infrastructure because they connect different points that usually can be inaccessible. If we analyze a single bridge crossing a river, it can have many views, depending on each person's perspective [3]:


Depending on the needs of the society to have a bridge, it would be its importance. A bridge that serves as a quick link to recreational parks with a low traffic flow will have less impact than a bridge crossing a large river and connecting two points of the city with high traffic flow.

Taking into account the sentence above, we can realize that bridges are not built arbitrarily; a whole planning should be performed including design, construction, operation and maintenance of the structures. Therefore, for the transportation system, the bridge is a key element [3] according to the following reasons:

a. Capacity control

e. Building infrastructure

multiple story levels.

nity, energy operations and living places.

*Infrastructure Management and Construction*

electricity and general supplies.

multi-story buildings in the background.

**3. Bridges, a general overview**

*View of Vidalta cable-stayed bridge, Mexico City [2].*

different points, crossing valleys, rivers, lakes and cliffs.

performance.

**Figure 1.**

**94**

Related to all structures intended for industry requirements, business commu-

i. *Industry and business:* Structures used for industry and business operation purposes. Examples include a single or multiple story structures and may be

ii. *Living places:* Intended for population housing, including buildings of single or

iii. *Basic services:* Structures related to operation services as water distribution,

iv. *Primary services and recreational purposes:* Places designed for city assistance, including fire and police stations, hospitals, schools, theaters and stadiums.

According the previous infrastructure classification, all described structures do not work as isolated buildings. Instead, all elements must be connected looking for the harmony in a society, working effectively and should not affect any building

As an example, there is a hospital that provides all types of medical treatment in an urban area. If the city's electrical stations stop working, the hospital will cease to be fully functional due to lack of electrical power, affecting medical equipment and lighting requirements. Now, if the same hospital has no enough roads for access, its

We recognize that all structures are part of the infrastructure and each one works together. The bridges take a special role, due its function to connect two

Bridges are needed on land transportation infrastructure because they connect different points that usually can be inaccessible. If we analyze a single bridge crossing

a river, it can have many views, depending on each person's perspective [3]:

capacity will be very limited, including medical staff and patients.

**Figure 1** shows a general view of Mexico City and the Vidalta cable-stayed bridge as transportation infrastructure, connecting via highways the whole series of

used as offices, machinery, equipment and industrial process.

i. Bridges must comply with traffic flow needs during its life period.

If a bridge has a small number of lanes, narrow sizes or poor spaces, the structure cannot maintain a continuous vehicular flow.

ii. Bridges must comply with required loads during its life period.

This means that the analysis and structural design must take into account all the loads that the bridge must support. For example, if the structure is located on an interstate highway and was not designed to support heavy truck loads, it will have limited vehicle traffic and those trucks will not be able to use the bridge; therefore, these trucks will have to plan an alternate route.

b. High cost for the entire road system

i. Bridges represent a high percentage of road's budget.

If we analyze the construction process of a road and measure the cost per unit distance, the bridges are very expensive compared to the highway.

ii. High cost variability for different bridge geometries.

Depending on the number of lanes required, types of vehicles to be supported, distances and/or clear span to cover, materials and available labor, the cost of the bridges are variable. Proper planning is required to meet the needs and comply with the budget.

c. The bridge as part of the system

i. If the bridge has a failure, the road system fails.

If we analyze the entire road and at specific place, one of the bridges does not work, the vehicular flow will be affected, increasing traffic flow, delays, time lost and the need of alternative route.

ii. If the bridge is not operating, alternate routes will be affected.

When a bridge does not work, people who used the affected road will have the need to use an alternative route, which probably is not designed for a sudden increase of vehicular flow, causing wasted time, greater distances and additional fuel required.

*3.1.1.2 Wooden and steel truss bridges*

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

a. Trusses with straight bars

*3.1.2 Modern bridges*

transportation demands.

*Stone arch bridge crossing a small river.*

**Figure 3.**

**Figure 4.**

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*Geometry types of trusses.*

By the beginning of the 19th century, the structures used wood as common material and truss bridges began to emerge (**Figure 4**). These wooden trusses took advantage of the axial stress capacity of the bar elements, creating bridges with longer spans, low weight and enough stiffness to withstand higher loads. Due the large number of geometries that can be created with trusses, there are possible arrangements which the bar elements can have taking advantage of the tension

In the mid-19th century, with the development industry sector, vehicles and trains entering into circulation using the current transportation system. Therefore, a greater number of roads, railroad and bridges were built to serve the increase of

and compression stress capacities. Some examples can be mentioned:

i. Geometries as Pratt, Warren and Baltimore types.

ii. Geometries using cables for tension elements.

i. Geometries as Wernwag and Burr types.

b. Combination between trusses and arch

For a successful transportation system, a balance should exist between vehicle volume, supported loads and proper budget. **Figure 2** shows a section of Mexico City's transportation system, showing balance between the number of lanes, signs, bridges and several road accesses.
