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Status of the Nation's Highways, Bridges, and Transit:
2004 Conditions and Performance
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Chapter 15 (Continued)

Composition of the Bridge Network

An overview of the composition and conditions of the bridge system was presented in Chapter 2. This chapter presents additional detail for the system of bridges as a whole and according to traffic volumes, functional classifications, age, and superstructure materials and designs.

The NBI contains nearly 700,000 records, which describe either the features carried by a bridge, termed as "on" records, or the features crossed by the structure, termed as "under" records. Separating the on records from the under records reveals that there are 591,707 bridges over 6.1 meters (20 feet) in total length located on public roads in the United States. These bridges, on average, carry nearly 4 billion vehicles per day and comprise a total deck area in excess of 300 million square meters.

Q. How do the bridge ownership percentages compare with road ownership percentages?

The majority of bridges (98 percent) and roadways (97 percent) are owned by State and local agencies. The vast majority of roadways, however, are owned by local agencies (77 percent). Bridge ownership is nearly equally divided between State (47 percent) and local agencies (51 percent).

The discussion of bridges in Chapters 2 and 3 primarily considered the number of bridges in different classifications. Using this approach, every bridge in the inventory is counted equally. Thus, large suspension bridges, such as the Golden Gate or the George Washington Bridges, are considered equivalent to small, two-lane bridges carrying low volumes of traffic. In some cases, better insights into the condition or the composition of bridges can be obtained by considering the size of the structure and/or the traffic carried. Considerations of size of the structure can be incorporated through presentation of information using the deck area of the bridge. Considerations of the volume of traffic served by the structure can be incorporated through presentation of information using average daily traffic (ADT).

Bridges by ADT

Approximately 27 percent of structures in terms of numbers have an ADT of 100 or less. In excess of 50 percent of these structures have an ADT lower than 700. 96.5 percent of structures have an ADT of 40,000 or below and 97.5 percent have an ADT of 50,000 or below.

In terms of numbers of bridges, low-volume roadways are predominant. However, the high-volume structures have a significant impact on the user population. There are approximately 21,000 structures with ADT values in excess of 40,000 vehicle crossings daily. These structures are predominantly in urban environments (approximately 90 percent in terms of numbers, nearly 95 percent in terms of deck area). Over 95 percent of such bridges are located on Interstates or other principal arterials.

Weighting the number of bridges by ADT values provides a mechanism for evaluating the impacts of the composition and conditions of bridges in terms of their impact on the highway user. Exhibit 15-2 shows that the distribution is significantly skewed to lower values of ADT.

Exhibit 15-2, bridges by ADT values, distribution and cumulative percentages. Bar and line chart. Bars plot the number of bridges in thousands against the ADT range in thousands; the line chart plots cumulative percent. The value for the range 0 to 1 is 421.5 thousand. The value for the range 1 to 2 is 31.3. As the ranges increment by one, the trend is downward steadily from a value of 14.9 for range 2 to 3 to a value of 1.2 for range 9 to 10. The value for ADT range greater than 10 is 5.0. The line has an initial value of just under 85% at range 0.1, and rises in a long arc to reach 100% at range greater than 10.

Bridges by Functional Classification

Exhibit 15-3 shows the percentage of bridges by functional classification with bridges equally weighted by numbers, weighted by ADT, and weighted by deck area. Rural bridges are predominant when the percentages are determined by numbers, as 77.1 percent of all structures are located in a rural environment. Urban bridges, which comprise 22.9 percent of the inventory, carry over 73 percent of all daily traffic. Not surprisingly, urban structures are generally larger in terms of deck area as additional lanes are required to carry larger volumes of traffic. Urban structures constitute 52.6 percent of all total deck area on bridges in the inventory.

The disparity between urban and rural structures in terms of traffic carried and size is readily evident on the national level by comparing the percentages. Further examination of Exhibit 15-3 reveals similar trends across functional classification. Whereas bridges on Interstate and other arterial routes comprise approximately one-third of the inventory by numbers, they carry close to 90 percent of all daily traffic and approximately 70 percent of the deck area. Likewise, the local and collector roads constitute two-thirds of the inventory by numbers, but carry only 10 percent of total daily traffic volume.

Exhibit 15-3 Bridges by Functional Class Weighted by Numbers, ADT, and Deck Area
Functional Class Total % by Nos.
(% of All)
% of ADT % of Deck
Interstate 27,316 4.6% 10.2% 8.0%
Other Principal Arterials 35,227 6.0% 6.5% 9.0%
Minor Arterial 39,587 6.7% 3.7% 6.6%
Major Collector 94,781 16.0% 3.8% 10.0%
Minor Collector 49,320 8.3% 0.9% 3.6%
Local 209,722 35.4% 1.6% 10.2%
Rural Total 455,953 77.1% 26.6% 47.3%
Interstate 27,929 4.7% 35.2% 19.2%
Other Expressways 16,844 2.8% 14.3% 9.2%
Other Principal Arterials 24,307 4.1% 12.1% 10.7%
Minor Arterial 24,516 4.1% 7.1% 7.0%
Collectors 15,171 2.6% 2.3% 2.8%
Local 26,609 4.5% 2.3% 3.5%
Urban Total 135,376 22.9% 73.3% 52.6%
Unclassified 378 0.1% 0.0% 0.1%
Total    591,707
Source: National Bridge Inventory.

Bridges by Age of Construction

For each bridge in the NBI, the year of construction is recorded and a year of construction distribution may be generated. This is shown in Exhibit 15-4 where the number of bridges constructed by year is presented for all owners and for all functional classifications. Note that some of the annual "spikes" seen in the number of bridges constructed before 1970 are artificial, as some localities have recorded year of construction information using 5-year increments for older bridges. Peak periods of construction are seen mainly before World War II and during the Interstate construction era.

Exhibit 15-5 shows the average year of bridge construction by functional classification and owner. Standard deviations are provided with the mean values in order to give additional information on the distributions. Bridges in the inventory are, on average, 40 years old with an average year of construction of 1964. Urban structures are slightly younger than rural structures, with an average year of construction of 1968. Comparing rural bridges across ownership classifications shows that State, local, and Federal owners have values within a few years of the mean for all rural bridges. Rural bridges owned by other owners, which are primarily private owners and railroads, are on average 10 years older than the general population. With urban bridges, State and locally owned bridges are slightly younger or slightly older than average, respectively. Federally owned urban bridges and urban structures owned by others are 5 to 10 years older than State and local counterparts on average. It is important to note, however, that the number of bridges owned by Federal and other agencies is much smaller.

Exhibit 15-4, bridges:  year of construction distribution. Bar chart plotting number of bridges constructed for each year from 1910 to 2000. Distinct peaks in construction occur in 1910, 1920, 1930, 1935, 1940, 1950, 1960, 1965, 1970, and 1975. 1942 through 1946 had much less construction activity than years prior or after. The recent trend is a drop in construction.  Source: National Bridge Inventory.

Exhibit 15-5 Average Year of Bridge Construction by Owner and Functional Classification
Functional ClassAverage Year of Construction and Standard Deviation
StateLocalFederalOtherAll Owners
Interstate1968  (11)1959  (28)1963  (6)1965  (11)1968  (11)
Other Principal Arterial1965  (22)1968  (24)1967  (18)1973  (19)1966  (22)
Minor Arterial 1958  (23)1972  (28)1968  (21)1966  (27)1959  (23)
Major Collector 1960  (21)1963  (22)1968  (18)1949  (32)1962  (22)
Minor Collector 1962  (20)1964  (24)1962  (19)1950  (32)1963  (23)
Local1966  (22)1963  (28)1965  (20)1946  (33)1963  (27)
All Rural Bridges1963  (21)1963  (26)1965  (20)1952  (32)1963  (24)
Interstate 1970  (12)1963  (17)1956  (8)1975  (19)1970  (12)
Other Freeways and Expressways1973  (16)1971  (20)1945  (37)1980  (15)1973  (16)
Other Principal Arterial 1964  (22)1962  (25)1961  (30)1960  (32)1964  (23)
Minor Arterial 1964  (22)1964  (25)1965  (21)1946  (33)1964  (24)
Collector 1966  (22)1965  (25)1959  (18)1953  (34)1965  (24)
Local 1969  (20)1966  (25)1958  (21)1949  (36)1966  (25)
All Urban Bridges1968  (18)1965  (25)1959  (22)1960  (33)1967  (21)
Rural and Urban
Interstate 1969  (11)1963  (17)1962  (6)1973  (18)1969  (11)
Other Principal Arterials1967  (21)1964  (24)1965  (22)1974  (22)1967  (21)
Minor Arterials 1960  (23)1965  (26)1968  (21)1951  (32)1961  (24)
Collectors 1961  (21)1964  (23)1963  (19)1951  (33)1963  (22)
Local 1966  (22)1963  (28)1965  (20)1947  (34)1964  (27)
All: Rural and Urban1964  (20)1964  (26)1965  (20)1957  (33)1964  (24)
Source: National Bridge Inventory.

The cumulative distributions shown in Exhibit 15-6 depict the increased rate of construction during the Interstate era. Cumulative distribution curves are presented for the numbers, ADT, and deck area. The mean year of construction occurs where the curves pass through the 50 percent value and is roughly equivalent when bridges are weighted equally (numbers) or when bridges are weighted by traffic carried (ADT). Half of all the bridges in the country were built before 1964, and 50 percent of all daily traffic carried by the system travels over these structures. The mean year of construction is approximately 1971 where structures are weighted by deck area. This indicates that recent structures tend to be larger than their older counterparts. This conforms with conventional wisdom as standards have changed over time.

Exhibit 15-6, cumulative percentage of numbers, ADT, and deck area by year of bridge construction. Line chart. The trend for all three categories is a flat plot just above 0% to about 1920, then a gradual rise to 1955, followed by a steeper rise to 1975, and a less steep rise to 100% at the year 2000. Source: National Bridge Inventory.

Chapter 17 provides information on the composition and conditions of high and low-volume bridges on and off the NHS. The majority of traffic is carried on NHS structures, which include the Interstate System. High and low-volume NHS structures are defined using a threshold of 50,000 vehicle crossings daily. NHS structures include the majority of higher functional classifications and are typically owned by State agencies. The threshold value for distinguishing between high and low-volume NHS structures is 10,000. Local ownership tends to focus on low-volume non-NHS structures. Exhibits 15-7 and 15-8 show the year of construction distributions for high and low-volume NHS and non-NHS bridges.

Exhibit 15-7, year of construction distribution for high- and low-volume NHS bridges. Stacked bar chart plotting counts of high-volume and low-volume bridges over the years from 1900 to 2005 in five-year periods. For the time period up to 1920, the bars trend very close to zero. The trend starts upward in the period 1921 to 1925, and hovers just above 5,000 in the period 1931 to 1935 and 1936 to 1940. In the period 1941 to 1945, the bar is above 1,000, then climbs quickly, reaching more than 20,000 for the period 1956 to 1960, and more than 20,000 for the period 1956 to 1960. The values peak above 30,000 for the period 1961 to 1965 and 1966 to 1970. Values drop steadily to just above 10,000 for the period 1981 to 1985. A slight increase to about 12,000 follows in the period 1986 to 1990. Values remain above 10,000 through 2000, and drop to about 1,000 in the period 2001 to 2005. The ratio of high volume to low volume bridges is roughly equal across the years.  Source: National Bridge Inventory.

Bridges by Type of Superstructure Material

Superstructure material types are maintained in the database for the main span and for the approach spans. 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 15-9 weighting bridges equally by numbers, weighting by ADT, and weighting by deck area. While only 33.0 percent of bridges have steel superstructures, these bridges carry 35.7 percent of bridge traffic, and represent 46.3 percent of total deck area on all bridges. From these percentages, it may be inferred that steel bridges tend to be utilized for longer-than-average structures carrying higher-than-average volumes of traffic. Timber bridges, which constitute 5.5 percent of the inventory by numbers, carry small volumes of traffic and are smaller than average in terms of deck area.

Exhibit 15-8, year of construction distribution for high- and low-volume non-NHS bridges. Stacked bar chart plotting counts of high-volume and low-volume bridges over the years from 1900 to 2005 in five-year periods. From a value of about 10,000 bridges in the period before 1900, the value drops below 5,000 for the period 1901 to 1905 and for 1906 to 1910. In the following ten years, the value is under 10,000. The values increase to between 20,000 and more than 25,000 for the periods spanning 1926 to 1940. A steep drop to below 10,000 is shown for 1941 to 1945. In the period 1946 to 1950, values climb from over 20,000 to nearly 40,000 by 1956 to 1960. By 1980, the values drop to just above 30,000, climb to above 35,000 for the period 1986 to 1990, drop slightly to 30,000 by 1996 to 2000, and fall sharply to below 5,000 for the period 2001 to 2005. In all cases, the share of high volume is a very small fraction of the total number of non-NHS bridges constructed. Source: National Bridge Inventory.

The number of bridges by type, superstructure material, functional classification, and ownership are shown in Exhibit 15-10. The average year of construction and the standard deviation are shown for these combinations in Exhibit 15-11. Bridges carrying Interstate, other principal arterial, and minor arterial routes are predominantly constructed of reinforced concrete, steel and prestressed concrete. Timber superstructures and other materials become more significant within the population of bridges carrying collectors and local roadways.

Concrete and steel superstructure bridges on the Interstate are, on average, 35 to 40 years old. Prestressed designs were introduced more recently and have become the predominant superstructure material employed today, with over 50 percent of new structures employing prestressed concrete. Today, there are over 45,000 prestressed superstructure bridges carrying Interstates, other principal arterials, and minor arterials in the United States. There are also sizable numbers of prestressed concrete bridges carrying collector and local roadways. Bridges constructed of this material are, on average, 25 years old. The average age of timber superstructure bridges is approximately 45 years, while the average age of other materials is in excess of 65 years. Other materials are used on many older designs that used iron and masonry or on newer structures employing composites or other new materials.

Considering functional classifications, only small variations are seen in the average age of construction between the owners. For all functional classifications and for all material types, the average year of construction (1964/1965) are effectively equivalent for State, local, and Federal owners. There is also minimal variation between the functional classifications with average ages for all functional classifications for State, local, and Federal owners in the 1960s.

Exhibit 15-9, percentage of superstructure material types: bridges weighted by numbers, ADT, and deck area. Three pie charts in five segments. By numbers, steel accounts for 33%, timber accounts for 5.5%, concrete accounts for 40.6%, prestressed accounts for 20.4%, and other accounts for 0.6%. By ADT, steel accounts for 35.7%, timber accounts for 0.3%, concrete accounts for 39.1%, prestressed accounts for 24.5%, and other accounts for 0.4%. By deck area, steel accounts for 46.3%, timber accounts for 1.2%, concrete accounts for 20.9%, prestressed accounts for 31.1%, and other accounts for 0.5%. Source: National Bridge Inventory.

Exhibit 15-10 Number of Bridges by Superstructure Material, Functional Classification, and Owenership
Material & Functional ClassOwnership
StateLocalFederalOtherAll Owners
Timber43  7
Other541  55
Other Principal Arterial
Timber27528  303
Minor Arterial
All Bridges
* Note: Records with unknown or incorrectly coded materials, functional
classifications, or ownership codes were not included.
Source: National Bridge Inventory.
Exhibit 15-11 Average Year of Construction and Standard Deviation for Superstructure, Functional Classification, and Ownership Combinations
Material & Functional ClassOwnership
StateLocalFederalOtherAll Owners
Concrete 1966  (10)1969  (23)1963  (2)1974  (19)1966  (10)
Steel 1968  (11)1958  (11)1958  (9)1966  (17)1968  (11)
Prestressed1975  (11)1989  (12)1968  (5)1993  (5)1975  (12)
Timber 1971  (13)1969  (15)0  (0)0  (0)1970  (13)
Other 1987  (16)1979  (0)0  (0)0  (0)1986  (16)
All Materials 1969  (11)1963  (17)1962  (6)1973  (18)1969  (11)
Other Principal Arterial
Concrete 1959  (21)1960  (23)1965  (17)1963  (24)1960  (21)
Steel1965  (19)1959  (24)1954  (20)1964  (30)1965  (19)
Prestressed 1981  (15)1979  (18)1985  (9)1981  (14)1981  (15)
Timber 1942  (12)1957  (22)0  (0)0  (0)1944  (14)
Other1943  (51)1913  (36)1918  (0)1910  (19)1933  (49)
All Materials1967  (21)1964  (24)1965  (22)1974  (22)1967  (21)
Minor Arterial
Concrete 1954  (22)1965  (24)1961  (23)1953  (33)1957  (23)
Steel 1959  (20)1956  (27)1968  (20)1943  (30)1958  (22)
Prestressed1979  (18)1977  (19)1978  (12)1970  (29)1978  (18)
Timber1945  (14)1968  (29)1958  (14)1947  (29)1953  (23)
Other 1916  (41)1910  (41)1983  (13)1909  (9)1914  (42)
All Materials 1960  (23)1965  (26)1968  (21)1951  (32)1961  (24)
Concrete1957  (20)1963  (22)1959  (17)1951  (33)1960  (21)
Steel 1959  (20)1956  (24)1962  (20)1943  (31)1958  (22)
Prestressed1979  (17)1978  (17)1977  (12)1977  (25)1978  (17)
Timber 1952  (16)1959  (22)1955  (15)1931  (23)1957  (21)
Other1933  (41)1938  (40)1954  (43)1925  (24)1936  (41)
All Materials1961  (21)1964  (23)1963  (19)1951  (33)1963  (22)
Concrete1961  (22)1966  (26)1960  (19)1957  (36)1965  (26)
Steel 1964  (20)1954  (29)1963  (22)1940  (32)1956  (28)
Prestressed1979  (15)1980  (18)1977  (14)1974  (26)1980  (17)
Timber 1959  (22)1960  (23)1964  (19)1937  (26)1960  (23)
Other1953  (52)1936  (43)1951  (35)1906  (27)1939  (44)
All Materials 1966  (22)1963  (28)1965  (20)1947  (34)1964  (27)
All Classes
Concrete 1959  (20)1965  (25)1960  (18)1957  (32)1962  (22)
Steel1964  (18)1955  (28)1963  (22)1945  (32)1959  (24)
Prestressed 1978  (15)1980  (18)1977  (13)1978  (20)1979  (16)
Timber 1953  (19)1960  (23)1964  (19)1937  (26)1959  (23)
Other1940  (48)1934  (43)1954  (37)1913  (23)1936  (44)
All Materials 1964  (20)1964  (26)1965  (20)1957  (33)1964  (24)
Source: National Bridge Inventory.
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