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Status
of the Nation's Highways, Bridges, and Transit:
2002 Conditions and Performance Report |
| Chapter 11: Federal Bridge Program Status of the Nation's Bridges | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 Index Introduction Highlights Executive Summary Part I: Description of Current System
Part II: Investment Performance Analyses
Part III: Bridges
Part IV: Special Topics Part V: Supplemental Analyses of System Components
Appendices |
Specific Bridge TypesThe following areas are addressed in this section of the chapter:
Year of Construction by Functional ClassificationThe 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. Superstructure Material TypesPredominant 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. 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.
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.
Steel Superstructure BridgesThe 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.
Prestressed ConcretePrestressed 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.
Timber BridgesTimber 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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