- Highway Conditions
- Bridge Conditions
- Transit Conditions
- Road Conditions
- Pavement Terminology and Measurements
- Overall Pavement Condition
- Rural and Urban Pavement Conditions
- Pavement Condition by Functional Classification
- Roadway Alignment
- Lane Width
- Bridge System Conditions
- Classification of Bridge Deficiencies
- Condition Rating Structural Deficiencies
- Structural Appraisal Ratings
- Appraisal Rating Functional Obsolescence
- Number of Deficient Bridges
- Deficient Bridges by Owner
- Deficient Bridges by Functional Classification
- Classification of Bridge Deficiencies
- Transit System Conditions
- Bus Conditions
- Urban Bus Maintenance Facilities
- Rail Vehicle Conditions
- Urban Rail Maintenance Facilities
- Other Rail Urban Infrastructure
- The Value of U.S. Transit Assets
- Rural Transit Vehicles and Facilities
- Special Service Vehicles
Bridge System Conditions
The National Bridge Inspection Standards (NBIS), in place since the early 1970s, requires biennial safety inspections for bridges in excess of 6.1 meters in total length located on public roads. Information is collected documenting the conditions and composition of the structures. Baseline composition information is collected describing the functional characteristics, descriptions and location information, geometric data, ownership and maintenance responsibilities, etc. This information permits characterization of the system of bridges on a national level and permits analysis on the composition of the bridges. Safety, the primary purpose of the program, is ensured through periodic hands-on inspections and rating of the primary components of the bridge, such as the deck, superstructure, and substructure. This composition and condition information is maintained in the National Bridge Inventory (NBI) database maintained by FHWA. This database represents the most comprehensive source of information on bridges throughout the United States.
|Q.||How often are the bridges inspected?|
Most bridges in the US Highway Bridge inventory are inspected once every two years. These inspections are performed by qualified inspectors. Where structures have advanced deterioration or other conditions warranting closer monitoring, inspections can be performed more frequently. Certain types of structures in very good condition may receive an exemption from the two-year inspection cycle. Inspections can be performed on these structures once every 4 years. Qualification for this extended inspection cycle is reevaluated depending on the conditions of the bridge. Eighty three percent (490,000 bridges) are inspected once every 2 years, twelve percent (71,000 bridges) are inspected annually, and five percent (28,000 bridges) are inspected on a 4-year cycle.
Classification of Bridge Deficiencies
From the information collected through the inspection process, assessments are performed to determine the adequacy of the structure to service the current demands for structural and functional purposes. Factors considered include the load-carrying capacity, clearances, waterway adequacy, and approach roadway alignment. Structural assessments together with condition ratings determine whether a bridge should be classified as structurally deficient. Functional adequacy is assessed by comparing the existing geometric configurations to current standards and demands. Disparities between the actual and desired configurations are used to determine whether a bridge should be classified as functionally obsolete. Structural deficiencies take precedence in the classification of deficiencies, so that a bridge suffering from a structural deficiency and functional obsolescence would be classified as structurally deficient.
Condition Rating Structural Deficiencies
The primary considerations in classifying structural deficiencies are the bridge component condition ratings. The NBI database contains ratings on the three primary components of a bridge: the deck, superstructure, and substructure. A bridge deck is the primary surface used for transportation. The deck is supported by the superstructure. This transfers the load of the deck and the traffic carried to the supports. Within the superstructure are the girders, stringers, and other structural elements. The substructure is the foundation of the bridge and transfers the loads of the structure to the ground. The superstructure is supported by the substructure elements, such as the abutments and piers.
Condition ratings are assigned for these primary components during periodic safety inspections. Condition ratings are also assigned for the channel and channel protective systems and for culvert designs. These structures do not have distinct deck, superstructure, or substructure elements. The ratings do not translate directly into an overall rating of a bridge's condition, but are good indicators of the quality of specific components. Condition ratings are either assigned directly by the bridge inspector or translated from more detailed element-level models employed in bridge management systems, such as Pontis, using the FHWA-provided translator.
Condition ratings are used to describe the existing, in-place status of a component and not its as-built state. Rather, the existing condition is compared with an as-new condition. Bridge inspectors assign condition ratings by evaluating the severity of the deterioration or disrepair and the extent it has spread through the component being rated. They provide an overall characterization of the general condition of the entire component being rated and not an indication of localized conditions. Exhibit 3-22 describes the bridge condition ratings in more detail.
|7||Good||No problems noted.|
|6||Satisfactory||Some minor problems.|
|5||Fair||All primary structural elements are sound but may have minor section loss, cracking, spalling, or scour.|
|4||Poor||Advanced section loss, deterioration, spalling, or scour.|
|3||Serious||Loss of section, deterioration, spalling, or scour have seriously affected the primary structural components. Local failures are possible. Fatigue cracks in steel or shear cracks in concrete may be present.|
|2||Critical||Advanced deterioration of primary structural elements. Fatigue cracks in steel or shear cracks in concrete may be present or scour may be removed substructure support. Unless closely monitored, it may be necessary to close the bridge until corrective action is taken.|
|1||Imminent Failure||Major deterioration or section loss present in critical structural components, or obvious loss present in critical structural components, or obvious vertical or horizontal movement affecting structural stability. Bridge is closed to traffic, but corrective action may put back in light service.|
|0||Failed||Out of service; beyond corrective action.|
Condition rating distributions are shown in Exhibit 3-23 for the deck, superstructure, and substructure. Condition ratings of 4 and below indicate poor or worse conditions and result in structural deficiencies. Approximately 7 percent of all bridge decks are deficient based on condition rating, and 7 percent of all superstructures and 9% of all substructures are deficient. These classifications are not mutually exclusive, and an individual structure may have one or more than one deficient component.
There are 118,394 culverts in the bridge inventory. These structures do not have a deck, superstructure, or substructure, but rather are self-contained units under roadway fill. Culverts are typically constructed of concrete or corrugated steel. Multiple pipes or boxes placed side-by-side are considered given that together they span a total length in excess of 6.1 meters and carry a public roadway. As these structures lack decks, superstructures, and substructures, individual ratings are provided to indicate the condition of the culvert as a whole. The distribution of culvert condition ratings is shown in Exhibit 3-24. Of all 118,394 culverts in the inventory, approximately 2 percent are classified as structurally deficient based on condition ratings less than or equal to 4 (poor conditions).
Structural Appraisal Ratings
Condition ratings are the primary criteria used in the classification of structural deficiencies; 80 percent of all structurally deficient bridges have condition rating deficiencies in their decks, superstructures, substructures, or culvert ratings. The remaining 20 percent of structural deficiencies are classified based on inadequate structural appraisal ratings and/or inadequate waterway adequacy ratings. These appraisal ratings evaluate a bridge in relation to the level of service it provides on the highway system on which it is located. The appraisal ratings compare the existing conditions with the current standards used for highway bridge design. Exhibit 3-25 describes appraisal rating codes in more detail.
|9||Superior to present desirable criteria.|
|8||Equal to present desirable criteria.|
|7||Better than present minimum criteria.|
|6||Equal to present minimum criteria.|
|5||Somewhat better than minimum adequacy to tolerate being left in place as is.|
|4||Meets minimum tolerable limits to be left in place as is.|
|3||Basically intolerable requiring a high priority of corrective action.|
|2||Basically intolerable requiring a high priority of replacement.|
|1||This value of rating code is not used.|
Load-carrying capacity does not influence the assignment of the condition ratings, but it does factor into the structural evaluation appraisal rating. This is calculated according to the capacity ratings for various categories of traffic in terms of average daily traffic (ADT). A rating of 2 or less indicates the carrying capacity is too low and the structure should be replaced. In this case, the bridge is classified as structurally deficient.
The waterway adequacy appraisal rating assesses the opening of the structure with respect to the passage of flow through the bridge. This factor, which considers the potential for overtopping of the structure during a flood event and the potential inconvenience to the traveling public, is assigned based on criteria assigned by functional classification. Waterway adequacy appraisal ratings of 2 or less categorize a bridge as structurally deficient.
The distribution of structural evaluation appraisal and waterway adequacy ratings is shown in Exhibit 3-26. Roughly 6 percent of bridges are structurally deficient based on inadequate structural evaluation appraisal ratings, indicating the existing deficiencies require replacement of the structure. Waterway adequacy impacts a much smaller percentage of structures, with 0.3 percent of the bridges in the network classified as structurally deficient resulting from ratings of 2 or below.
Appraisal Rating Functional Obsolescence
The primary considerations for functional obsolescence focus on functional- and geometric-based appraisal ratings. Ratings considered are the deck geometry appraisal rating, the underclearance appraisal rating, and/or the approach roadway alignment appraisal rating. For each of these appraisals, ratings are assigned based on the descriptions provided in Exhibit 3-25.
Deck geometry ratings consider the width of the bridge, the ADT, the number of lanes carried by the structure, whether two-way or one-way traffic is serviced, and functional classifications. The minimum desired width for the roadways is compared with the actual widths and used as a basis for appraisal rating assignment. Minimum vertical clearances are also considered by functional classification. Underclearance appraisals consider both the vertical and horizontal underclearances as measured from the through roadway to the nearest bridge component. The functional classification, federal-aid designation, and defense categorization are all considered for the underpassing route. Approach alignment ratings differ from the deck geometry and underclearance appraisal rating philosophy. Instead of comparing the approach alignment with current standards, the alignment of the approach roadway is compared with the alignment of the bridge spans. Deficiencies are identified where the bridge route does not function adequately because of alignment disparities.
|Q.||How does a bridge become functionally obsolete?|
Functional obsolescence is a function of the geometrics of the bridge in relation to the geometrics required by current design standards. While structural deficiencies are generally the result of deterioration of the conditions of the bridge components, functional obsolescence results from changing traffic demands on the structure. Facilities, including bridges, are designed to conform to the design standards in place at the time they are designed. Over time, improvements are made to the design requirements. As an example, a bridge designed in the 1930s would have shoulder widths in conformance with the design standards of the 1930s. However, the design standards have changed since the 1930s. Therefore, current design standards are based on different criteria and require wider bridge shoulders to meet current safety standards. The difference between the required, current-day shoulder width and the 1930s designed shoulder width represents a deficiency. The magnitude of these types of deficiencies determines whether the existing conditions cause the bridge to be classified as functionally obsolete.
The structural evaluation appraisal ratings, as mentioned, are used as a factor for determining whether a bridge has a structural deficiency. Descriptions of the ratings are given in Exhibit 3-25. A rating of 3 indicates the load-carrying capacity is too low; however, the situation can be mitigated through corrective action. In this case, the bridge is classified as functionally obsolete. Likewise, waterway adequacy appraisal ratings of 3 result in functional obsolescence. Ratings of 2 or below for either the structural evaluation or waterway adequacy appraisals result in structural deficiencies as these ratings typically are not correctable without replacement.
The distribution of structural evaluation appraisal and waterway adequacy ratings is shown in Exhibit 3-26. Approximately 3 percent of bridges are classified as functionally obsolete based on structural evaluation appraisal ratings. Waterway adequacy impacts a much smaller percentage of structures, with 0.7 percent of bridges classified as functionally obsolete resulting from a rating of 3, indicating corrective actions are required to mitigate the inadequate waterway capacities.
Functional obsolescence occurs primarily because of the deck geometry, underclearance, and approach alignment appraisals. Distributions of the number of structures classified as functionally obsolete by appraisal ratings are given for these factors in Exhibit 3-27.
Number of Deficient Bridges
The most commonly cited indicator of bridge condition is the number of deficient bridges. Of the 591,707 bridges in the inventory, 162,869 are classified as deficient (27.5 percent), either for structural or functional causes. Of these, 81,304 are classified as structurally deficient and 81,565 are classified as functionally obsolete. Thus, roughly half of the deficiencies are structural and half are functional.
Exhibit 3-28 shows the trend of deficiency percentages from 1994 through 2002. Bridge deficiencies have been reduced primarily through reduction in the numbers of structurally deficient bridges. The percentage of functionally obsolete bridges has remained static over this time period.
As indicated earlier, structural deficiencies and functional obsolescence are considered mutually exclusive, with structural deficiencies taking precedence where ratings classify a given bridge as both structurally deficient and functionally obsolete. Roughly half of the 81,304 structurally deficient bridges have no functional obsolescence issues and are deficient solely on the basis of structural safety and deteriorated bridge component conditions. The remaining structurally deficient bridges also have some type of functional obsolescence.
Deficient Bridges by Owner
Bridge deficiencies by ownership are examined in Exhibit 3-29. For Federally owned bridges, the number of bridges classified as functionally obsolete outweighs the number classified as structurally deficient by a 2 to 1 ratio. Similar percentages are seen for State-owned bridges. These bridges constitute a much more significant proportion of the overall inventory of structures, since State agencies own 47 percent of all bridges. Locally owned and private bridges have opposite trends, with the number of structurally deficient bridges outweighing the number of functionally obsolete bridges. These percentages have not changed significantly from those reported in the 2002 edition of the C&P report, based on year 2000 data.
|% of Total Inventory for Owner||2%||47%||51%||0%||100.0%|
|% Structurally Deficient||8%||9%||18%||25%||13.7%|
|% Functionally Obsolete||16%||16%||12%||19%||13.8%|
Examination of ownership percentages for structurally deficient and functionally obsolete bridges reveals the majority of structurally deficient bridges are owned by local agencies, while the majority of functionally obsolete bridges are owned by State agencies. These percentages can be contrasted with the ownership percentages for all bridges in Exhibit 3-30. The percentages are dominated by State and local ownership, with only small percentages of the total population of all structures attributable to Federal, private, and other owners.
As indicated earlier, the most commonly used criteria for measuring bridge deficiencies is the actual number of deficient structures. However, there are alternative measures available, such as accounting for traffic by weighting structures according to ADT or accounting for size of structures by weighting according to the bridge deck area. Deficiencies for all structures, regardless of owner, are compared using these alternative performance measures in Exhibit 3-31. Deficiency percentages using these alternative performance measures are compared for Federal, State, local, and other owners in Exhibit 3-32.
|Q.||What bridge deficiency criteria is used in the annual FHWA performance plan?|
The FHWA Fiscal Year 2005 Performance Plan includes targets for the deck area on deficient bridges for NHS and non-NHS bridges. These measures are discussed in Chapter 17.
Deficient Bridges by Functional Classification
Functional classifications are maintained for each bridge recorded in the NBI. The functional classification codes designate whether the bridge carries Interstates or other principal arterials, minor arterials, collectors, or local roadways. The number of structurally deficient and functionally obsolete bridges are shown by functional classification in Exhibit 3-33.
The functional classification codes designate whether a structure is located in a rural or urban environment. As noted in Chapter 2 and as shown in Exhibit 3-33, the majority of bridges in terms of numbers are located in rural environments. With rural bridges, the number of structural deficiencies (15 percent) outweighs the number of bridges classified as functionally obsolete (11 percent). Urban roadways carry significantly higher volumes of traffic, as noted in Chapter 2. With urban bridges, the number of structurally deficient bridges (9 percent) is significantly lower than the number of functionally obsolete bridges (22 percent). Overall, a higher percentage of urban structures is classified as deficient (31 percent total); however, the majority of these deficiencies result from functional obsolescence. While the percentage of rural bridges classified as deficient is lower, the population and hence the number of deficiencies is larger. Structural deficiencies are more prevalent, in terms of percentages, in rural environments.
Bridge conditions in rural and urban areas have steadily improved over the past decade. As seen in Exhibit 3-34, overall deficiencies and structural deficiencies have both decreased. Functional obsolescence percentages, however, have not decreased but have remained static in both rural and urban environments. Exhibit 3-34 does not include structure records with unknown functional classification codes for any of the years depicted. Total numbers are thus slightly lower than the population figures presented in previous exhibits.
|Functional Class||Total Number of Structures||Structurally Deficient||Functionally Obsolete||Total Deficiencies|
|Rural Other Principal Arterial||35,227||1,886||3,364||5,250|
|Rural Minor Arterial||39,587||3,407||4,451||7,858|
|Rural Major Collector||94,781||11,426||10,217||21,643|
|Rural Minor Collector||49,320||6,783||5,579||12,362|
|Urban Other Freeways of Expressway||16,844||1,025||3,431||4,456|
|Urban Other Principal Arterial||24,307||2,273||5,428||7,701|
|Urban Minor Arterial||24,516||2,605||6,402||9,007|
|Total Identified by Functional Class||591,329||81,266||81,525||162,791|
|Rural and Urban Interstate||55,245||2,819||8,827||11,646|
|Rural and Urban Other Principal Arterial||64,103||6,012||10,853||16,865|
|Rural and Urban Minor Arterials||76,378||5,184||12,223||17,407|
|Rural and Urban Collectors||159,272||19,948||19,579||39,527|
|Rural and Urban Local||236,331||47,303||30,043||77,346|
|Total, Including Unknown||591,707||81,304||81,565||162,869|
The trends for individual functional classifications can be examined. Exhibits 3-35 through 3-38 show the trends for Interstate, other arterial, collector, and local bridges, respectively. Decreases in the number of structural deficiencies are exhibited for every functional classification, irrespective of the rural and urban designations. For Interstate bridges, decreases are also exhibited in the percentages of functionally obsolete bridges. For other functional classifications, there has been little change in the functionally obsolete percentages.