The following areas are addressed in this section of the chapter:

• Additional detail on Interstates, other arterials, collectors and local bridges.
• Characterization of the superstructure material types used in the bridge network.
• Examination of the age distribution, deficiency percentages, and deficiency trends for each superstructure material (concrete, steel, prestressed concrete, timber, and other).

Year of Construction by Functional Classification

The year of construction distribution was presented for all structures in the National Bridge Inventory. Distributions were created for Interstates (see Exhibit 11-13), other arterials (see Exhibit 11-14), collectors (see Exhibit 11-15), and local (see Exhibit 11-16) bridges. There is a distinct peak in the distribution of Interstate bridges with the average year of construction in the mid 1960's. Other functional classifications have much greater dispersion in the year of construction.

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Superstructure Material Types

Predominant materials used for bridge superstructures are steel, concrete, prestressed concrete, and timber. Other materials, such as aluminum, iron, and composite materials, are utilized on less than 1 percent of the structures. The percentage of superstructure materials utilized is shown in Exhibit 11-17 weighting bridges equally (by numbers), weighting by the traffic carried (ADT), and weighting by the size of the structure (by deck area). Steel bridges tend to be utilized for longer than average structures carrying higher volumes of traffic than average. Timber bridges, which constitute 5.7 percent of the inventory by numbers, carry small volumes of traffic and are smaller than average in terms of deck area. Material percentages are shown for Interstates, other arterials, collectors and local functional classifications in Exhibit 11-18.

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The number and percentage of bridges by superstructure material, owner, and functional classification are shown in Exhibit 11-19. Figures include both rural and urban designations. Exhibit 11-20 shows the percentages of material type used for the varying functional classifications and owners for rural bridges. Exhibit 11-21 shows the same information for urban bridges.

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Exhibits 11-20 and 11-21 present the superstructure material percentages for rural and urban designations respectively. Notable differences can be seen in the Interstate bridge population with significantly higher percentages of urban Interstates constructed with steel. Prestressed superstructure bridges also constitute a higher percentage of the inventory in urban environments. Concrete (excluding prestressed concrete) is the dominant material for rural bridges. Timber superstructure bridges are prevalent in rural areas and not common in urban environments.

Concrete Superstructure Bridges (Excluding Prestressed Concrete)

The average age of concrete bridges in the NBI is approximately 40 years with an average year of construction of 1961. The average age of bridges for each combination of ownership and functional classification may be determined in Exhibit 11-22. The year of construction distribution and cumulative ADT are shown in Exhibit 11-23 for all concrete superstructure bridges (exclusive of prestressed concrete). Deficiencies and deficiency trends are shown in Exhibits 11-24 and 11-25 respectively for reinforced concrete superstructure bridges.

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Steel Superstructure Bridges

The average age of steel bridges in the NBI is approximately 44 years with an average year of construction of 1958. The average age of bridges for all combinations of functional classification and ownership may be determined through examination of Exhibit 11-26. The year of construction distribution and cumulative ADT for all steel superstructure bridges are shown in Exhibit 11-27. Deficiencies and deficiency trends are shown in Exhibits 11-28 and 11-29 respectively for steel superstructure bridges.

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Prestressed Concrete

Prestressed concrete was introduced in the middle of the 20th Century, and today the majority of bridges are constructed using prestressed concrete designs. The average age of prestressed concrete bridges in the NBI is approximately 24 years with an average year of construction of 1978. There are no significant differences in the age of rural versus urban prestressed bridges. The average age of bridges for all combinations of functional classification and ownership is shown in Exhibit 11-30. The year of construction distribution and cumulative ADT are shown in Exhibit 11-31 for all prestressed concrete superstructure bridges. Deficiencies and deficiency trends are shown in Exhibits 11-32 and 11-33 respectively for concrete superstructure bridges.

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Timber Bridges

Timber bridges, as described previously, are primarily used in rural environments for small spans carrying small volumes of traffic. The average age of timber bridges in the NBI is 43 years with an average year of construction of 1959. There is no significant difference between the ages of the rural and the urban timber bridge populations. The average age of timber bridges for all combinations of functional classification and ownership is presented in Exhibit 11-34. The year of construction distribution and cumulative ADT are shown in Exhibit 11-35 for all timber superstructure bridges. Deficiencies and deficiency trends are shown in Exhibits 11-36 and 11-37 respectively for timber superstructure bridges.

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Other Superstructure Materials

There are a small number of bridges, in terms of percentage of the population, composed of other materials, which includes aluminum, wrought and cast iron, masonry, and other uncategorized materials. The average age of these bridges is 67 years with an average year of construction of 1935. Urban bridges are, on average, older than rural bridges constructed of these other materials. The average age of these structures is shown for all combinations of functional classification and ownership in Exhibit 11-38. The year of construction distribution and cumulative ADT are shown in Exhibit 11-39 for all structures constructed of these other materials. Deficiencies and deficiency trends are shown in Exhibits 11-40 and 11-41 respectively.

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Culverts

In addition to examining the bridge infrastructure in terms of functional classification and ownership, it is important to examine the types of design utilized, the age of the structures, and other factors. Considering the types of design utilized, the records in the NBI describe either traditional bridge designs (80 percent— approximately 474,000 records), culverts (20 percent—approximately 117,000 records), or tunnels (104 records). The inventory is composed almost entirely of traditional bridge and culvert designs. Both of these structures provide the same purpose of providing network connectivity. However, the design and engineering properties of bridges and culverts differ dramatically. Consider the definitions of these structures as defined in the National Bridge Inspection Standards (23 CFR 650.3):

• Bridges are defined as “supports erected over a depression or an obstruction, such as water, highway, or railway, and having a track or passageway for carrying traffic or other moving loads, and having an opening measured along the center of the roadway of more than 20 feet [6.1 meters] between undercopings of abutments or spring lines of arches.” Traditional bridges will have distinct decks, superstructures, and substructures.
  
  
• Culverts are structures “designed hydraulically to take advantage of submergence to increase hydraulic capacity. Culverts, as distinguished from bridges, are usually covered with embankment and are composed of structural material around the entire perimeter, although some are supported on spread footings with the streambed serving as the bottom of the culvert. Culverts may qualify to be considered “bridge” length.”

For safety assurance and funding purposes, culverts and bridges are equivalent; however, since the design characteristics are significantly different, it is expected that differences in deterioration patterns will occur between the populations. Thus, it is useful to examine differences between bridge and culvert designs. The number of records describing bridge, culvert, and tunnel design is tabulated together with the traffic carried (total ADT) and the percentage of total deck area in Exhibit 11-42.

Differences in bridge ownership and functional classification versus culvert ownership and functional classification are examined in the following figures. Examination reveals that there are only minor deviations from the overall percentages when examining alternative combinations of functional classification and ownership of bridges versus culverts. The design-type used for a particular situation is thus dependent on the conditions of the crossing and not the functional classification or jurisdictional issues.

The average age of structures in the National Bridge Inventory is approximately 40 years with an average year of construction of 1963. The age distribution of traditional bridge designs and culvert designs is examined and compared in Exhibit 11-43. Culverts tend to be younger than bridges with an average age of approximately 35 years, compared to an average age of approximately 40 years for traditional bridge designs. The average year of construction and standard deviation for traditional bridge designs and culvert designs are shown in Exhibit 11-44 and Exhibit 11-46 for all combinations of ownership and functional classification. Year of construction distributions and cumulative ADT percentages are shown in Exhibit 11-45 for traditional bridge designs and Exhibit 11-47 for culvert designs.

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Conclusion

Bridges, as critical components of the highway system, must be maintained and preserved to ensure safety to the traveling public, support commerce and mobility within the Nation, and retain the significant accumulated asset value of the inventory. The Nation's bridges and culverts are aging and traffic demands are increasing. At the same time, funds for capital construction are becoming scarcer. Asset management principles through management systems and transportation system preservation techniques are becoming more important as the States, locals and the Federal Government struggle to maintain the safe condition of the Nation's bridges and culverts, while at the same time providing for increased demands on the highway network. Improved bridge and culvert inspection techniques, through the use of new and innovative equipment, are needed to better insure the safety of the motoring public. Longer design life structures, using the latest material and design technologies, are needed so that the Nation can maintain a functional transportation network, provide longer service life, and improve the safety of the highway network. Emphasis is needed on research so that we can continually improve the condition of the Nation's bridges and culverts.