- The National Bridge Inspection Program and the Highway Bridge Replacement and Rehabilitation Program
- Overview and Evolution of the Bridge Programs
- Information Collected Through the Bridge Inspection Program
- Composition of the Bridge Network
- Bridges by ADT
- Bridges by Functional Classification
- Bridges by Age of Construction
- Bridges by Type of Superstructure Material
- Conditions of Bridges
- Actions Taken to Remove Deficiencies
The National Bridge Inspection Program and the Highway Bridge Replacement and Rehabilitation Program
Bridges are critical elements within the highway transportation network supporting commerce, economic vitality, and personal mobility. Every day, close to 4 billion vehicles cross bridges in the United States. The public expects these structures to be safe and to have the capability to support their transportation. The safety of the bridge network came into question in the late 1960s when, on December 15, 1967, the Silver Bridge spanning the Ohio River between West Virginia and Ohio collapsed during rush-hour traffic. This catastrophic event resulted in 46 fatalities and numerous injuries, prompting national concern about bridge conditions and safety. Following this disastrous event, programs were established to ensure periodic safety inspection of bridges and provide mechanisms for funding of bridge replacement and rehabilitation needs. The primary bridge programs include the National Bridge Inspection Program (NBIP) and the associated Highway Bridge Replacement and Rehabilitation Program (HBRRP).
General information on the composition and conditions of bridges has been presented in Chapter 2 and Chapter 3. These chapters provide an overview of bridge composition and performance with a focus on ownership and functional classification. As bridges are vital elements within the system, additional detail is provided on the conditions, composition, and performance of the U.S. highway bridge network in this chapter. Additional information concerning bridges on the National Highway System (NHS) can be found in Chapter 17.
As shown in the tables and discussions that follow, the Nation's highway bridges have remained safe as a result of the bridge programs, and progress has been made toward the Federal Highway Administration (FHWA) strategic goals of reducing deficiencies. However, with an ever-aging population of highway structures, increasing traffic demands, and limited budgets, it is important to examine transportation system preservation strategies, such as 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.
Overview and Evolution of the Bridge Programs
For the last 30 years, bridges in excess of 6.1 meters in total length located on public roads have received periodic inspections to ensure safety to the traveling public. Inspections are guided by Federally defined minimum data collection requirements. Every year, bridge information collected for that year is submitted from the States and Federal Agencies to FHWA. Information collected and maintained by FHWA forms the basis for determining the condition of the Nation's bridges and for the apportionment of bridge replacement and rehabilitation funds to the States. Since initiation of the legislation guiding the development of the National Bridge Inspection Standards (NBIS) and associated funding programs, over $60 billion in HBRRP funding alone has been allocated and utilized to improve the condition of the Nation's bridges. Other sources of funding from Federal and State programs are also utilized for bridge activities.
Bridges are critical elements within the highway transportation network. Deterioration of structures must be periodically mitigated through proactive interventions to ensure safety of the traveling public, ensure connectivity of the network, and retain the significant intrinsic asset value of the bridge stock. These preservative actions cost significantly more than highway pavement activity on a unit cost basis. In addition, bridges may become functionally obsolete due to changing traffic demands. Actions must be taken to avoid adverse economic impacts to the traveling public, which may result from this functional obsolescence of the structures.
Programs have been developed and legislated to ensure bridge safety and provide funding for rehabilitation, improvement, and replacement of the structures. These programs are summarized in this section. The information collected through the bridge inspection process, which represents the most comprehensive source of bridge condition and composition data at the national level, is summarized to give a background for the in-depth examination presented later in this chapter.
On December 15, 1967, the Silver Bridge carrying U.S. 35 between Point Pleasant, West Virginia, and Gallipolis (Kanauga), Ohio, collapsed during rush-hour traffic. Thirty-one vehicles fell into the Ohio River or onto the Ohio shore, killing 46 people and injuring nine. The collapse, which was the first major failure of a structure since the wind-induced failure of the Tacoma Narrows Bridge in 1940, prompted national concern about bridge conditions and safety.
Congressional hearings on the failure resulted in mandates requiring the U.S. Secretary of Transportation to develop and implement the NBIS. The NBIS, developed by FHWA in cooperation with the American Association of State Highway and Transportation Officials, was enacted as part of the Federal-Aid Highway Act of 1970. This landmark legislation was enacted on December 31, 1970, and established, for the first time in U.S. history, uniform, national-level standards for bridge inspection and safety evaluation. The Act also designated funding for the replacement of deficient bridges on the Federal-aid highway system. Through the legislation:
- All States were required to perform periodic inspection of bridges in excess of 6.1 meters (20 feet) located on Federal-aid highway systems.
- Bridge inspection data collection requirements were established.
- Qualifications for key bridge inspection personnel were defined.
- Training programs for bridge inspectors were developed and implemented.
- The Special Bridge Replacement Program (SBRP) was established to provide funding for the replacement of bridges located on the Federal-aid system.
Over time, the NBIS has been fine-tuned, additional inspection requirements have been added, and funding programs have been updated. It quickly became evident that safety assurance was required for all structures located on public roadways. The requirement to inventory and inspect bridges on Federal-aid highways was extended to all bridges in excess of 6.1 meters (20 feet) located on public roads. Data collection requirements were enhanced, and training programs continued to be developed and expanded as more knowledge became available through research and experience. Funding programs were expanded to permit the use of Federal funds for replacement of both Federal-aid and non-Federal-aid bridges.
Despite efforts to continually enhance the process of bridge inspection, unforeseen events periodically necessitated expansion. The scene was Interstate 95, the primary highway on the Atlantic seaboard that connects Florida and Maine, approximately 30 miles east of New York City, near Greenwich, Connecticut. On June 28, 1983, a section of the Mianus River Bridge catastrophically failed because of instantaneous fracture of a pin and hanger detail. This failure resulted in several fatalities and disrupted commerce in the northeastern United States for several months. Following this event, significant research into fatigue of steel connections was performed, and tremendous insight into the behavior of steel connections was obtained. The program was enhanced to incorporate more rigorous inspection procedures for fracture critical structures. Training programs were developed, putting the research results and accumulated experience and understanding of fatigue and fracture into practice.
On April 5, 1987, disaster struck again with the collapse of a bridge carrying the New York State Thruway (Interstate 90) across the Schoharie Creek. With rising water levels from localized flooding, the soil around the pier was simply washed or scoured away. The loss of soil around the pier resulted in the subsequent loss of bearing capacity for the foundation of the center pier, which collapsed. Several fatalities resulted from this collapse. A failure due to the washing or scouring of supporting soil from a major pier or abutment of a structure is termed a scour-induced failure. Other notable scour-induced failures occurred throughout the country, including the collapse of the Hatchie River Bridge in Tennessee on April 1, 1989. These bridges indicated the potential problem, given that more than 80 percent of the bridges on public roads cross over waterways. With approximately 475,000 structures crossing waterways, program enhancement was required. The FHWA acted quickly, providing guidance for scour assessment and requiring periodic underwater inspection of all structures at risk and susceptible to scour damage.
The combination of research, experience, and technology transfer of knowledge acquired has been used to train professionals performing inspections of fatigue and scour-susceptible structures. Catastrophic failures due to scour and fatigue, such as the Mianus River and the Schoharie Creek bridges, have been avoided. Additional knowledge is required on these and other extreme events, such as earthquakes and collisions, to avoid such calamities in the future. Research efforts performed by FHWA and transfer of results to experienced engineers practicing in the field continue to proactively mitigate potential failures.
Catastrophic events highlighted the need to replace bridges before they collapse. The SBRP, created by the Federal-Aid Highway Act that provided funds to help States replace bridges, required expansion to permit rehabilitative activities. Again, action was taken and, in 1978, the Surface Transportation Assistance Act replaced the SBRP with the HBRRP.
The program initiated through the Federal-Aid Highway Act has been incrementally enhanced so that, today, all structures in excess of 6.1 meters on public roads receive, in general, biennial safety inspections. Notable changes in legislation can be seen in Exhibit 15-1. "Best practices" for routine, fracture-critical, and underwater inspections have been defined and published. Qualifications of inspection personnel have been established and training programs implemented to ensure completeness of engineering reviews and consistency of inspection condition assessments.
|Act and Date||Requirements|
|Federal-Aid Highway Act of 1970: (P.L. 91-605)||Inventory requirement for all bridges on the Federal-aid system
Established minimum data collection requirements
Established minimum qualifications and inspector training programs
Established Special Bridge Replacement Program
|Surface Transportation Assistance Act of 1978 (P.L. 95-599)||Provided $4.2 billion for the HBRRP over 4 years
Extended inventory requirement to all bridges on public roads in excess of 6.1 meters
Established Highway Bridge Rehabilitation and Replacement Program (extending funding to Rehab) to replace Special Bridge Replacement Program
|Highway Improvement Act of 1982||Provided $7.1 billion for the HBRRP over 4 years|
|Surface Transportation and Uniform Relocation Assistance Act of 1987||Provided $8.2 billion for the HBRRP over 5 years
Added requirements for underwater inspections and fracture-critical inspections
Allowed increased inspection intervals for certain types of bridges
|Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA)||Provided $16.1 billion for the HBRRP over 6 years
Mandated State implementation of bridge management systems
Increased funding in HBRRP
|National Highway System Designation Act of 1995||Repealed mandate for management system implementation|
|Transportation Equity Act for the 21st Century (TEA-21, 1998)||Provided $20.4 billion in HBRRP funding over 6 years|
Information Collected Through the Bridge Inspection Program
As part of the NBIS, qualifications of key personnel have been identified, training programs developed and offered to bridge owning agencies, assistance with bridge program development provided, and minimum data collection requirements defined. The information that is obtained through the process defined by the NBIS is discussed below. This information forms the basis for the subsequent examinations of the conditions and performance later in this chapter.
For most structures, the NBIS requires visual inspection once every 2 years. For structures with safety concerns, inspections may be performed more frequently. Likewise, for structures with special favorable characteristics, the period of observation may be increased. The bridge owners (States, cities, municipalities, etc.) are responsible for these inspections with oversight by the State department of transportation. Information is collected on the bridge composition and conditions and reported to FHWA where the data are maintained in the National Bridge Inventory (NBI) database. This information forms the basis of the bridge safety assurance efforts and provides the mechanism for the determination of fund requirements and fund apportionments.
The NBI database maintains inventory information characterizing the structure, condition ratings, appraisal ratings, and calculated fields. This information has been collected and maintained in the NBI database for over two decades. The NBI database represents the most comprehensive source of information available on the national level.
Inventory information includes location and description fields, geometric data (lengths, clearances, lane widths), functional descriptions (classification, NHS designation, service carried and crossed, etc.), and design characteristics (superstructure designs and materials, deck types, design load, etc.). This information permits classification of structures according to serviceability and essentiality for public use. The composition of structures in the network can be ascertained through examination of the inventory data.
Through periodic safety inspections, data are collected on the condition of primary components of a structure. Condition ratings are collected for the following components of a bridge:
- The bridge deck, including the wearing surface
- The superstructure, including all primary load-carrying members and connections
- The substructure, considering the abutments and all piers
- Culverts, recorded only for culvert designs
- Channel/channel protective systems, for all structures crossing waterways.
In general, each traditional bridge design has distinct deck, superstructure, and substructure components that are each rated independently. Culvert designs are also included in the bridge inventory, if they are located on a public road and have a total length in excess of 6.1 meters. As culverts are considered as "bridges" under the NBIS for funding purposes, they are inspected biennially. Culverts have different design properties, behave differently under subject loads, and have different considerations than traditional bridge designs. Culvert designs are typically used for short-span, low-volume channel flow situations. Since culverts do not have distinct deck, superstructure, and substructure components, an individual culvert condition rating is assigned during the inspection process. These culvert ratings are used to guide deficiency status determination and eligibility of the structure for Federal fund participation.
Condition ratings are also developed for the channel and the channel protection system during the bridge inspection process. The channel/channel protective system rating describes the physical conditions of slopes and the channel for water flow through a bridge. Condition evaluation of these elements is increasingly important for structures susceptible to scour, which can occur and increase in situations due to channel degradation or failed channel protection.
Condition ratings are assigned by bridge inspectors utilizing a 10-point rating system, as described in Chapter 3 [see Exhibit 3-22]. Code 9 indicates excellent, as-new condition, and code 0 indicates a failed condition. Codes 7 through 9 indicate satisfactory to excellent conditions. Codes 5 and 6 indicate either fair or satisfactory conditions of the components. Codes 4 through 0 indicate poor, serious, critical conditions, and conditions representing imminent failure of the component or failed conditions. Inspectors assess the ratings in a visual fashion based on engineering expertise and experience. Extensive training for inspectors is provided, and references are available to guide assignment of the ratings. These ratings form the basis for assessing the structural condition of a bridge.
Functional adequacy is also a concern in the bridge population. Following collection of the inventory information and condition ratings, appraisal ratings are calculated to assess the adequacy of a structure to provide the required service. Appraisal ratings are quantified for
- Structural evaluations (load-carrying capacities);
- Deck geometry (indicating constrictions that affect safety);
- Underclearances (which, if insufficient, result in detours); and
- Waterway adequacy (the ability of the opening to handle the flow rates).
A bridge may be structurally deficient and/or functionally obsolete. These determinations are assessed based on the condition and appraisal ratings. Structural deficiencies result from poor condition ratings or from low load ratings. Functional obsolescence results from low appraisal ratings or from low design-load capacities. Inadequate waterway adequacy can be a contributing factor for either structural deficiencies or functional obsolescence.