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

Conditions of Bridges

In Chapter 3, an overview of the condition of the highway bridge network was presented. Chapter 17 presented information on structural deficiencies and functional obsolescence for high and low-volume NHS and non-NHS mobility measure categories.

Structural deficiencies and functional obsolescence are not mutually exclusive, and a bridge may have both types of deficiencies. When deficiency percentages are presented, however, bridges are indicated as being structurally deficient, functionally obsolete, or nondeficient. As structural deficiencies may imply safety problems, they are considered more critical; thus, a bridge that is both structurally deficient and functionally obsolete is identified only as structurally deficient. Approximately 50 percent of the structurally deficient population also will have functional issues that must be addressed. Bridges that are indicated as functionally obsolete do not have structural deficiencies.

Q. What makes a bridge structurally deficient, and are structural deficient bridges unsafe?
A.

Bridges are considered structurally deficient if significant load carrying elements are found to be in poor or worse condition due to deterioration and/or damage or, the adequacy of the waterway opening provided by the bridge is determined to be extremely insufficient to point of causing intolerable traffic interruptions. The fact that a bridge is "deficient" does not immediately imply that it is likely to collapse or that it is unsafe. With hands-on inspection, unsafe conditions may be identified and, if the bridge is determined to be unsafe, the structure must be closed. A "deficient" bridge, when left open to traffic, typically requires significant maintenance and repair to remain in service and eventual rehabilitation or replacement to address deficiencies. In order to remain in service, structurally deficient bridges are often posted with weight limits to restrict the gross weight of vehicles using the bridges to less than the maximum weight typically allowed by statute.

Overall, there are 162,869 bridges that are deficient within the highway bridge network. This represents 27.5 percent of the total inventory of highway bridges when bridges are weighted equally. The overall percentage of deficiencies is roughly the same when considering traffic carried (27 percent deficient) and deck area (27.5 percent deficient). Over 1 billion vehicles cross deficient bridges daily, and close to 90 million square meters of deck area are on deficient bridges.

Exhibit 15-12 shows the percentage of structurally deficient (SD) and functionally obsolete (FO) bridges by functional classification and owner. The overall percentage of structurally deficient bridges is approximately equal to the percentage of functionally obsolete bridges. There are nearly twice as many functionally obsolete bridges across all functional classifications for State and Federal owners. For bridges owned by local agencies, private entities, and others, the number of structural deficiencies outweigh the number of functionally obsolete bridges.

Deficiencies can be examined by functional classification irrespective of ownership. With bridges carrying higher functional classifications, such as the Interstates and arterials, the percentages of structural deficiencies is significantly lower than the percentages of functionally obsolete bridges. For bridges carrying collector roadways, the percentage of structurally deficient bridges is roughly equal to the percentage of functionally obsolete bridges. For bridges carrying local roadways, 20 percent are structurally deficient, outweighing the 12.7 percent functionally obsolete.

Exhibit 15-12 Bridge Deficiency Percentages by Functional Class and Owner
DescriptionState
# of Bridges
%SD / %FO
Local
# of Bridges
%SD / %FO
Federal
# of Bridges
%SD / %FO
Other
# of Bridges
%SD / %FO
All
# of Bridges
%SD / %FO
Rural
Interstate27283
4.0% / 11.7%
10
10.0% / 30.0%
18
0.0% / 5.6%
5
0.0% / 40.0%
27316
4.0% / 11.8%
Other Principal Arterial 34686
5.4% / 9.5%
300
7.0% / 13.7%
55
5.5% / 25.5%
186
2.2% / 4.3%
35227
5.4% / 9.6%
Minor Arterial 36682
8.5% / 11.1%
2414
9.0% / 12.2%
402
17.2% / 15.2%
89
22.5% / 25.8%
39587
8.6% / 11.2%
Major Collector 52737
11.2% / 13.5%
41742
13.1% / 7.3%
179
13.4% / 10.6%
123
34.2% / 19.5%
94781
12.1% / 10.8%
Minor Collector 16602
12.4% / 13.9%
31423
14.7% / 9.7%
1178
5.7% / 17.4%
117
38.5% / 12.0%
49320
13.8% / 11.3%
Local 28177
14.3% / 16.9%
173578
22.6% / 11.0%
7255
7.5% / 15.0%
712
39.8% / 22.3%
209722
21.1% / 11.9%
All Classes196167
9.2% / 12.6%
249467
19.9% / 10.2%
9087
7.8% / 15.2%
1232
32.0% / 18.7%
455953
15.1% / 11.4%
Urban
Interstate 27601
6.0% / 20.0%
307
21.2% / 30.6%
2
50.0% / 0.0%
19
0.0% / 26.3%
27929
6.1% / 20.1%
Other Freeways and Expressways 15429
6.1% / 20.3%
970
8.9% / 26.3%
2
0.0% / 0.0%
443
0.5% / 10.8%
16844
6.1% / 20.4%
Other Principal Arterial 18785
8.9% / 20.9%
5317
10.3% / 27.5%
17
11.8% / 29.4%
188
22.3% / 16.5%
24307
9.4% / 22.3%
Minor Arterial 11939
10.8% / 27.7%
12288
10.0% / 24.6%
42
26.2% / 11.9%
247
28.7% / 27.5%
24516
10.6% / 26.1%
Collector 5086
11.6% / 30.7%
9850
11.1% / 21.9%
20
20.0% / 30.0%
215
23.3% / 27.4%
15171
11.5% / 24.9%
Local 4956
10.4% / 29.6%
21096
11.8% / 16.1%
195
10.8% / 34.4%
362
31.2% / 23.2%
26609
11.8% / 18.8%
All Classes 83796
8.0% / 22.6%
49828
11.1% / 20.8%
278
14.0% / 29.9%
1474
18.9% / 20.0%
135376
9.2% / 21.9%
All: Rural and Urban
Interstate 54884
5.0% / 15.9%
317
20.8% / 30.6%
20
5.0% / 5.0%
24
0.0% / 29.2%
55245
5.1% / 16.0%
Other Principal Arterials 68900
6.5% / 15.0%
6587
10.0% / 26.7%
74
6.8% / 25.7%
817
5.9% / 10.6%
76378
6.8% / 16.0%
Minor Arterials 48621
9.0% / 15.2%
14702
9.9% / 22.5%
444
18.0% / 14.9%
336
27.1% / 27.1%
64103
9.4% / 16.9%
Collectors 74425
11.5% / 14.8%
83015
13.5% / 9.9%
1377
6.9% / 16.7%
455
30.1% / 21.3%
159272
12.5% / 12.3%
Local 33133
13.7% / 18.8%
194674
21.5% / 11.5%
7450
7.6% / 15.5%
1074
36.9% / 22.6%
236331
20.0% / 12.7%
All Classes279963
8.8% / 15.6%
299295
18.4% / 12.0%
9365
8.0% / 15.7%
2706
24.8% / 19.4%
591329
13.7% / 13.8%
SD = Structurally Deficient
FO = Functionally Obsolete
Source: National Bridge Inventory.

Rural functional classifications and ownership percentages follow the same general trend as the overall population. With bridges carrying higher functional classifications, such as Interstates and principal arterials, functional obsolescence percentages exceed the structural deficiency percentages. The reverse is true for bridges carrying lower functional classification roadways in rural environments where the structural deficiencies outweigh the functional issues. In the urban environment, functional obsolescence percentages were higher than structural deficiency percentages for all functional classifications and for all owners.

Exhibit 15-13 shows the percent of structural deficiencies and functional obsolescence where bridges are weighted using different methods. Percentages determined by equal weighting through counting of the number of bridges are compared with percentages where bridges are weighted by ADT and deck area. In general, if the percent deficiencies by ADT are higher than those determined using number of bridges, it may be inferred that the deficiencies are occurring on bridges with higher-than-average traffic volumes. Likewise, where the deck area percentages exceed the percentages determined by numbers, it may be inferred that the deficiencies are occurring on bridges with higher-than-average deck areas. For both cases, the converse is also true; with lower percentages, it may be inferred that the deficiencies are occurring on bridges with lower-than-average traffic or area.

The stacked bars in Exhibit 15-13 allow evaluation and comparison of deficiencies. For both rural and urban structures, percentages of deficiencies increase for the lower functional classifications. Bridges carrying principal arterials clearly have lower deficiency percentages than bridges carrying local roadways. Percentages of functionally obsolete bridges remain relatively constant across the functional classifications, and the increases shown are primarily attributable to structural deficiencies.

Actions Taken to Remove Deficiencies

Over $60 billion in HBRRP funding alone has been allocated and utilized to ensure safety and continuing functionality of the bridge network. Historically, HBRRP funds have been utilized only for repair, rehabilitation, or replacement of deficient bridges. An examination of bridge construction and bridge rehabilitation activity with Federal fund participation, including HBRRP and other funding programs through 1998 reveals the following:

In 1990, 17 percent of activity with Federal fund participation involved new bridge construction. This percentage has decreased from 1990 to 1998, and today approximately 90 percent of all projects receiving Federal fund participation involve reconstruction or rehabilitation.

Exhibit 15-14 shows the number and percent of deficient bridges reconstructed, as indicated in the NBI database. The information is presented by functional classification, rural/urban designation, and owner. The average number of years before the reconstruction was undertaken is also indicated.

Exhibit 15-13, Percent of bridge deficiencies by numbers, ADT, and deck area. Bar chart and data table. Bar chart plots numbers as percentages for two categories of deficiency in rural and urban bridges. Among rural bridges, the category of functionally obsolete ranges from 9.5% to about 11.9% across all bridge types. The category of structurally deficient is lowest for the interstate type at 4%, increases to 5.4% for other principal arterial, to 8.6% for minor arterial, to 12.1% for major collector, and 13.8% for minor collector. It extends to 20.1% for the local type. Among urban bridges, the category for functionally obsolete extends from 18.8% to 26.1%. The category of structurally deficient ranges from about 6.1% to 11.8%. Source: National Bridge Inventory.
Functional Classification % by Numbers % by ADT % by Area
SDFOSDFOSDFO
Rural
Interstate 4.0%11.8%4.5%10.8%4.9%9.7%
Other Principal Arterials 5.4%9.5%5.4%10.3%6.8%10.4%
Minor Arterial 8.6%11.2%8.8%15.2%9.8%12.9%
Major Collector12.1%10.8%10.4%18.8%11.4%11.1%
Minor Collector 13.8%11.3%11.7%20.0%10.9%10.6%
Local 21.1%11.9%11.8%26.3%14.7%13.0%
Urban
Interstate 6.1%20.1%6.9%19.3%9.0%22.0%
Other Freeways & Expressways 6.1%20.4%7.4%19.6%8.8%20.1%
Other Principal Arterial 9.4%22.3%9.8%23.1%12.1%23.7%
Minor Arterial 10.6%26.1%9.7%27.9%11.8%25.5%
Collector 11.5%24.9%10.2%28.8%10.8%25.6%
Local 11.8%18.8%8.5%31.8%10.1%26.8%
Source: National Bridge Inventory.

Historically, Interstate bridges undergo rehabilitation approximately 22 years after they are placed in service. The time to rehabilitation is longer for other functional classifications. Bridges carrying higher functional classifications, such as Interstates and principal arterials, are rehabilitated sooner than bridges carrying lower functional classifications, such as collectors and local routes. This trend is seen for rural and urban functional classifications for all owners and does not necessarily apply for all owner/functional classification combinations.

Exhibit 15-14 Rehabilitation Summary by Functional Class and Owner (% Reconstructed/Average Number of Years to Reconstruction)
Functional ClassStateLocalFederalOtherAll
Rural
Interstate 6220
23%  /  21
2
20%  /  49
1
6%  /  25
0
0%  /  0
6223
23%  /  21
Other Principal Arterial 7526
22%  /  30
51
17%  /  34
16
29%  /  33
61
33%  /  29
7654
22%  /  30
Minor Arterial 8012
22%  /  33
287
12%  /  36
36
9%  /  28
8
9%  /  60
8343
21%  /  33
Collector 8189
12%  /  27
7312
10%  /  41
88
6%  /  36
40
17%  /  49
15629
11%  /  34
Local 2335
8%  /  27
18576
11%  /  40
1646
23%  /  17
77
11%  /  43
22634
11%  /  37
Urban
Interstate 6782
25%  /  23
66
21%  /  25
2
100%  /  37
2
11%  /  28
6852
25%  /  23
Other Principal Arterial 7272
21%  /  29
1315
21%  /  33
5
26%  /  38
100
16%  /  32
8692
21%  /  29
Minor Arterial 2115
18%  /  30
2153
18%  /  36
4
10%  /  39
49
20%  /  62
4321
18%  /  33
Collector 713
14%  /  29
1311
13%  /  37
4
20%  /  17
49
23%  /  65
2077
14%  /  35
Local 512
10%  /  27
2140
10%  /  38
56
29%  /  30
65
18%  /  48
2773
10%  /  36
All: Rural and Urban
Interstate 13002
24%  /  22
68
21%  /  26
3
15%  /  33
2
8%  /  28
13075
24%  /  22
Other Principal Arterial 14798
21%  /  29
1366
21%  /  33
21
28%  /  34
161
20%  /  31
16346
21%  /  29
Minor Arterial 10127
21%  /  33
2440
17%  /  36
40
9%  /  29
57
17%  /  61
12664
20%  /  33
Collector 8902
12%  /  27
8623
10%  /  40
92
7%  /  35
89
20%  /  58
17706
11%  /  34
Local 2847
9%  /  27
20716
11%  /  40
1702
23%  /  17
142
13%  /  45
25407
11%  /  37
Source: National Bridge Inventory.

Progress has been made in reducing the deficiencies. More than 85,000 structures (15 percent of the inventory) have been reconstructed or rehabilitated and are in service today. These reconstruction and rehabilitation efforts have contributed to the reduction in deficiencies discussed in Chapter 3.

Exhibit 15-15 shows the relationship between bridge age and the percentage of bridges that are classified as deficient. When a structure is placed in service, the deterioration process begins on the components of the bridge. As bridges age, increasing numbers of structures become deficient and increasing funds are required to address these deficiencies. This is a concern with the increasing age of the large Interstate population and the relatively short period of time for the average reconstruction effort on Interstate bridges. With this ever-aging, continually deteriorating population of highway structures, increasing traffic demands, and limited budgets, the FHWA and the Nation need to take a closer look at transportation system preservation strategies. This includes increased activity in preventative maintenance and improved bridge inspection and management techniques to continue to ensure the safety of the motoring public and effective stewardship of the public trust.

Exhibit 15-15, age and deficiency percentages. Bar chart and line chart. Bar chart plots number of bridges for age ranges in spans of five years. Values start at about 25,000 for bridges aged zero to 5 years, increase to just below 40,000 for bridges aged 5 to 10 years, and trend in this area to about 41,000 for bridges aged 25 to 30 years. The values reach beyond 50,000 for the next three five-year spans, then drop to just under 10,000 for bridges aged 55 to 60 years. The values for the next three five-year spans are around 25,000. The values for the time spans from 75 to 100 years decline from just over 10,000 to about 1,000, then climb above 10,000 for bridges aged more than 100 years. The line chart plots values for percent of deficient bridges, with the trend line beginning at about 2% for bridges aged zero to 5 years, reaching about 78% for bridges over 100 years old.  Source: National Bridge Inventory.

Conclusions

As can be seen from the information presented in this chapter, the Nation's bridges are aging and traffic demands are increasing. Asset management principles through bridge management systems and transportation system preservation techniques are becoming more important as the States, locals, and Federal government struggle to maintain the safe condition of the Nation's bridges, while at the same time providing for increased demands on the highway bridge network. Improved bridge inspection techniques, through the use of new and innovative equipment, are needed to better ensure 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 safe bridge network that provides the life span needed to avoid congestion and improve safety of the highway bridge network. Such goals can be achieved only through an emphasis on fundamental long-term research.

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