U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-15-081 Date: May 2016 |
Publication Number: FHWA-HRT-15-081 Date: May 2016 |
There are other BCIs that cannot be categorized into any one of the methods described in chapter5. These indices use multiple approaches to calculate the overall BHI.
The bridge SR was previously used in the United States for evaluating factors, indicating a bridge’s sufficiency to remain in service, but was superseded as a result of the recent Moving Ahead for Progress in the 21st Century Act (MAP-21) legislation. The inspector rated each of the key components of the bridge by selecting a deterioration that best described the general condition of the component being inspected. The overall bridge rating was obtained by a weighted combination of the structural condition information and functional information. The SR approach was therefore an extension of the weighted averaging approach.
Bridge SRs were calculated by using data from NBI, which is a comprehensive database compiled by FHWA of all bridges with span lengths greater than or equal to 6.1 m on public roads. The database contains a collection of bridge information data items such as bridge identification or location, bridge type and specifications, operational conditions (i.e., age, average daily traffic, bypass, and detour length), bridge geometric or functional data (i.e., structure lanes, shoulder width, etc.), and bridge structural condition data (i.e., rating of deck, superstructure, and substructure). The SR was calculated using bridge functional, operational, and condition information.
Functional data provided an assessment of the level of service provided by the bridge. Examples of functional data in NBI include number of lanes, shoulder width, vertical under clearance (height of the bridge), etc. These data were also used by FHWA to rate or classify the service provided by a bridge. Bridges not meeting minimum clearance values (i.e., narrow lanes, narrow shoulders, inadequate vertical under clearance, etc.) were classified as functionally obsolete. A functionally obsolete bridge contained features below established limits. However, they may have been structurally sound and perfectly safe. In calculating SR, the bridge’s serviceability and functional obsolescence contributed a maximum of 30 percent to the SR rating.
The operational conditions of a bridge were based on the evaluation of factors such as average daily traffic, bypass, detour length, and highway designation. Operational conditions contributed a maximum of 15 percent to the overall SR.
The NBI database describes the structural condition of only key components of a bridge: the deck, the superstructure, and the substructure.
The bridge deck, which directly carries traffic, is inspected for defects such as cracking, scaling, spalling (for concrete decks), broken welds, broken girds, and section loss (for steel grid decks). Timber decks are inspected for splitting, crushing, fastener failure, etc. The condition of the bridge deck is rated based on the defects identified and in accordance with the general condition ratings in table 27. The superstructure that supports the deck and connects one substructure element to another is inspected for signs of distress, which may include cracking, deterioration, section loss, malfunction, and misalignment of bearings. The substructure supports the superstructure and distributes all bridge loads to bridge foundations. The substructure is inspected for visible signs of distress, including evidence of cracking, section loss, settlement, misalignment, scour, collision damage, and corrosion.
| Rating | Description |
|---|---|
9 |
Excellent condition. |
8 |
Very good condition. No problems noted. |
7 |
Good condition. Some minor problems. |
6 |
Satisfactory condition. Structural elements show some minor deterioration. |
5 |
Fair fondition. All primary structural elements are sound but may have minor section loss, cracking, spalling, or scour. |
4 |
Poor condition. Advanced section loss, deterioration, spalling, or scour. |
3 |
Serious condition. Loss of section and/or deterioration of primary structural elements. Fatigue cracks in steel or shear cracks in concrete may be present. |
2 |
Critical condition. Advanced deterioration of primary structural elements. Fatigue cracks in steel shear cracks in concrete may be present or scour may have removed substructure support. Unless monitored, it may be necessary to close the bridge until corrective action is taken. |
1 |
“Imminent” failure condition. Major deterioration or section loss present in critical structural components or obvious vertical or horizontal movement affecting structure stability. Bridge is closed to traffic, but corrective action may put it back in light service. |
0 |
Failed condition. Out of service and beyond corrective action. |
The structural condition of a bridge’s key components (deck, superstructure, and substructure) is used to assess whether it is structurally deficient (NBI rating of 4 or less for deck, superstructure, or substructure). FHWA classifies a bridge as structurally deficient to indicate that the physical conditions of the bridge’s primary load-carrying elements have deteriorated. A “structurally deficient” bridge is not necessarily unsafe, but the owner may need to spend significant amounts on repair and maintenance to the keep the bridge in service, and the bridge would eventually require major rehabilitation or replacement to address the underlying deficiency.
In calculating SR, the bridge’s structural adequacy and safety together with the inventory loading contribute a maximum of 55 percent to the total rating.
The calculated SR (figure 24) is a function of four factors: structural adequacy and safety (A), serviceability and functional obsolescence (B), essentiality for public use (C), and special reductions (D), which is a maximum of 13 percent of the total rating. Elements considered include the detour length, traffic safety features, and main structure type.
Figure 24. Equation. SR.
Previously, FHWA used SRs with a status flag, indicating whether a bridge is structurally deficient or functionally obsolete to decide on its eligibility for funding. A structurally deficient (or functionally obsolete) bridge with an SR less than 50 qualified for replacement, whereas a structurally deficient (or functionally obsolete) bridge with an SR greater than 50 but less than 80 qualified for rehabilitation.
Moon et al. proposed a risk-based method for prioritization of bridge repair and replacement projects in a network. This method provides the basis for a bridge prioritization tool being tested by NJDOT. Although this is a risk-based framework, because risk and resilience are critical components of bridge health and performance, this method is included in this synthesis to address that aspect of bridge health.(19)
The objective of this proposed method is to provide a risk-based approach that transportation authorities can use as a more transparent and objective approach to bridge evaluation and project prioritization. While the method appears qualitative in nature, it has distinct advantages over many current approaches. This approach defines risk as a product of hazards, vulnerabilities, and exposures and therefore explicitly recognizes key performance limit states. In addition, it incorporates the uncertainties associated with various assessment techniques, provides flexibility for their implementation, and provides a means to capture (in a useable format) expert knowledge and heuristics from top bridge engineers.
The proposed bridge assessment methodology is based on the concept of relative risk, which extends the reliability-based assessment approach to explicitly consider the consequences of not performing (in this definition called exposure). The inclusion of consequences is a necessary consideration for rational decisionmaking, and it is therefore imperative that consequences be included within the assessment procedure. The proposed framework takes into consideration a more partitioned definition for perceived relative risk (referred to as “risk” in this report) as a combination of hazard, vulnerability, exposure, and an uncertainty premium (figure 25).
Figure 25. Equation. Perceived relative risk.
Where:
Hazard = Probability of a hazard occurring.
Vulnerability = Probability of failure (to perform adequately) given hazard.
Exposure = Consequences associated with a failure to perform adequately.
Uncertainty Premium = A factor to account for the level of uncertainty associated with the selected assessment approach, including the quality control measures employed.
Table 28 outlines some proposed hazards, vulnerabilities, and exposures for the four performance limit states to be considered by the proposed risk-based assessment approach.
| Performance Limit States | Hazards | Vulnerabilities | Exposures |
|---|---|---|---|
| Safety— geotechnical/ hydraulic |
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| Safety— structural |
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Loss of life. |
| Serviceability and durability |
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Maintenance costs. |
| Operations |
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History of congestion. |
ADTT = Average daily truck traffic.
The framework scales the calculated risk using the Department of Homeland Security’s five-level risk scale (figure 26) in which “I” represents low risk level, and “V” represents severe risk level. This scale is easy to understand for both engineers and the general public.
Figure 26. Chart. Risk scale for risk-based prioritization framework.
Moon et al. stress that the proposed framework is very rudimentary and needs to be refined based on expert elicitation and input from the many relevant professional organizations and committees.(19) While this framework has subjective components (due to the current lack of quantitative and objective data), as bridge performance research programs such as the LTBP Program expand their field data collection efforts, it is expected that this framework will become increasingly objective in nature by using data-driven inputs.(19)
Figure 27 shows a flowchart for the proposed risk-based prioritization. In this approach, the level of risk assessment is defined first, which identifies the acceptable uncertainty premium. After this definition, the estimation of relative risk is done by determining the hazard, vulnerability, and exposure of the bridge. The risk level is then calculated, which helps informed decisionmaking and budget allocation.
Figure 27. Flowchart. Proposed risk-based assessment framework.(19)
Uncertainty premiums associated with different levels of risk assessment are listed in table 29. The major deciding factor in the uncertainty premium is the level at which the risk is computed, whether at an aggregate level or divided up into individual risks. Although computing the risk in an aggregate level is more conservative and time efficient, it sometimes overestimates the actual risk drastically. In these cases, calculating a more realistic risk based on individual hazards as an accurate risk assessment can be worthwhile. The assessment levels reflect the specific approaches and technologies employed. More advanced analytical and experimental technologies are becoming available that can help users better understand the conditions of a structure and reduce the uncertainty premium. Also, a wide range of successful quality assurance programs have been developed. To recognize their influence and benefits, assessment levels that take advantage of these developments will have a lower uncertainty premium associated with them.
Table 30 through table 32 show how hazard, vulnerability, and exposure may be quantified for levels1 and 2 assessments. In this case, the risks are groups in four categories: safety—geotechnical/hydraulic; safety—structural; serviceability, durability, and maintenance; and operational and functional. For each of these categories, the hazard, vulnerability, and exposure are assigned a value of 1 through 3 based on location, structural and operational attributes, age, etc. The risk levels are then be calculated as discussed earlier in this section. Table 33 lists the preliminary risk levels.
| Level | Example Approaches | Resolution | Quality Assurance | Uncertainty Premium |
|---|---|---|---|---|
1 |
Visual Inspection, Document Review |
Aggregate Risks | Minimum Standards | 2.5 |
2 |
Visual Inspection, Document Review |
Aggregate Risks | Best Practices | 2.0 |
3 |
Visual Inspection, Document Review, Analytical Techniques |
Individual Risks | Minimum Standards | 1.5 |
4 |
Visual Inspection, Document Review, Analytical Techniques |
Individual Risks | Best Practices | 1.25 |
5 |
Visual Inspection, Document Review, Analytical and NDE Techniques |
Individual Risks | Best Practices | 1.0 |
NDE = Nondestructive evaluation.
| Hazards Considered |
Hazard Values | |||
|---|---|---|---|---|
| 1 | 2 | 3 | ||
| Safety—geo/hydraulic | Scour, debris and ice, vessel collision, seismic— liquefaction, settlement, flood |
Outside of a 500-year flood plain | Outside of a 100-year flood plain | Within of a 100-year flood plain |
| Seismic design category A |
Seismic design categories B and C | Seismic design categories D, E, and F | ||
| Over a non-navigable channel | Navigable channel for mid-sized vessels |
Navigable channel for large vessels | ||
| Located more than 804 km from coast |
Located more than 80.4 km from coast |
Located within 80.4 km from coast | ||
| No potential for scour | A rating of NBI item 113 (scour) of 7, 5, or 4 | Not applicable | ||
| No records of significant earthquake, floods, or storm surge |
Records of moderate earthquake, floods, or storm surge | Observed drift and debris at piers/abutment history of iceflows in waterway | ||
| Safety—structural | Seismic, fatigue, vehicle collision, overload, fire | Seismic design category A | Seismic design categories B and C | Seismic design categories D, E, and F |
| ADTT less than 500 | ADT less than 10,000 | ADT more than 10,000 | ||
| Not spanning over a roadway | Spanning over a roadway with ADTT less than 1,000 | Spanning over a roadway with ADTT more than 1,000/spanning a rail line | ||
| Located more than 16 km from heavy industry |
Located more than 16 km from heavy industry |
Located less than 16 km from heavy industry | ||
| No history of overloads, collision, or earthquake | Limited number of overloads or collision or minor earthquakes | History of overloads, collision, or severe earthquake | ||
Serviceability, durability, and maintenance |
No routine use of deicing salts | Moderate usage of deicing salts | High usage of deicing salts | |
| Located more than 100mi from the coast |
Located more than 25mi from the coast |
Located less than 25mi from the coast | ||
| Low number of freeze-thaw cycles | Moderate number of freeze-thaw cycles | Moderate number of freeze-thaw cycles | ||
| No history of overloads | History of isolated overloads | History of repeated overloads and permits | ||
Operational and functional |
ADTT less than 1,000 and ADT less than 10,000 | ADTT less than 10,000 and ADT less than 50,000 | ADTT more than 10,000 and ADT more than 50,000 | |
| No history of fatal accidents | History of isolated fatal accidents | History of repeated fatal accidents | ||
| No history of congestion | History of moderate congestion | History of high congestion | ||
| Vulnerabilities Considered | Vulnerability Values | ||
|---|---|---|---|
| 1 | 2 | 3 | |
| Safety—geo/hydraulic | Founded on deep foundations or bedrock | Founded on shallow foundations on cohesive soil | Founded on shallow foundations or noncohesive soil |
| No history and no evidence of scour or settlement | Evidence of minor scour/ undermining during past/present underwater inspections | Evidence of moderate to significant scour/ undermining during past/present underwater inspections | |
| Meets current pier impact and scour protection standards | Pier protection system in good condition | Pier protection system missing or in poor condition | |
| Superstructure above 100-year flood level |
Superstructure above 100‑year flood level | Superstructure below 100‑year flood level | |
| No tilt of substructure elements | Minor tilt of substructure elements | Significant tilt of substructure elements | |
| Safety—structural | Meets all current design specs | Does not meet all current design specs, but most of them | Noncomposite construction |
| Structure displays bi‑directional redundancy | Simply supported constructed with transverse distribution capabilities | Simply supported construction with minimal transverse distribution capabilities | |
| 20 years or less since construction or major renewal | 50 years or less since construction or major renewal | 50 years or more since construction or major renewal | |
| A and B fatigue details | C and D fatigue details | E and E' fatigue details | |
| Elastomeric bearings | Steel bearings | Rocker bearings, intrinsic force dependency, exposed prestressing strands, and pin and hanger details | |
| No evidence of structural damage | Minor evidence of structural damage within the critical loadpath | Evidence of structural damage within the critical load path | |
| Clearance more than 15.2cm of current standard | Clearance within 15.2cm of current standard | Clearance below current standards | |
| No history of excessive displacements or vibrations | History of significant displacements or vibrations | History of excessive displacements or vibrations | |
| Substructure elements plumb | Substructure elements within 10 percent of plumb | Substructure elements more than 10 percent of plumb | |
| Serviceability, durability, and maintenance | No visible cracks | Minor local cracking | Extensive cracking and spalling |
| No evidence of reinforcement corrosion | Some evidence of reinforcement and structural steel corrosion | Evidence of widespread reinforcement and structural steel corrosion | |
| Paint in good condition | Paint in moderate condition | Paint in poor condition | |
| Elastomeric bearing | Steel bearing | Frozen bearings and exposed prestressing strands | |
| Joints in good operating condition | Joints with minor evidence of leaking | Failed expansion joints | |
| Approach does not display rutting | Approach displays minor rutting | Approach displays significant rutting | |
| Scuppers are less than 10percent clogged | Scuppers are between 10 and 50 percent clogged | Scuppers are between 50 and 100 percent clogged | |
| Operational and functional |
Roadway approach alignment and bridge geometry up to current standards | Lane width within 0.3 m of current standards | Lane width more than 0.3mless than current standards |
| Guard rail and road paint in good condition | Guard rail and road paint in fair condition | Guard rail and road paint in poor condition | |
| Not posted | Posted for more than 90percent of legal truck weight | Posted for less than 90percent of legal truck load | |
| Good ride quality of deck | Moderate ride quality of deck | Poor ride quality of deck | |
| Breakdown lane/shoulders | Breakdown lane/ shoulders not present | Breakdown lane/shoulders not present | |
| No rutting of pavement | Minor rutting of pavement | Significant rutting of pavement | |
| Exposure Considered | Exposure Values | ||
|---|---|---|---|
| 1 | 2 | 3 | |
| Safety—geo/hydraulic Safety—structural | ADT less than 10,000 | ADT less than 50,000 | ADT more than 50,000 |
| Replacement cost less than $2 million |
Replacement cost less than $10 million |
Replacement cost more than $10 million | |
| Not on a critical route (lifeline,evacuation route, etc.) |
Not on a critical, nonredundant route (life line, evacuation route, etc.) |
On a critical, nonredundant route (life line, evacuation route, etc.) | |
| Detour route less than 8km | Detour route less than 16km | Detour route more than 16km | |
| Serviceability, durability, and maintenance | Low maintenance costs | High maintenance and repair costs |
N/A |
| ADT less than 50,000 | ADT more than 50,000 | ||
| Operational and functional |
No history of congestion | Average peak hour delays of more than 10min | N/A |
| ADT less than 25,000 | ADT more than 25,000 | ||
| ADTT less than 10,000 | ADTT more than 10,000 | ||
| Risk Level | Threshold Risk Values |
|---|---|
| Level V: Severe risk bridges | > 40 |
| Level IV: High risk bridges | 30–40 |
| Level III: Significant risk bridges | 20–30 |
| Level II: General risk bridges | 10–20 |
| Level I: Low risk bridges | < 10 |
In order to translate risk levels into appropriate actions, assessment techniques, and required intervals for assessments, a set of minimum requirements and optional assessment programs is needed. A preliminary estimate of this relationship is shown in table 34.
| Risk Level | Mandatory | Option 1 | Option 2 |
|---|---|---|---|
| Severe | Level 3 / 1 Year |
Level 4 / 18 months |
Level 5 / 2 years |
| High | Level 2 / 1 Year |
Level 4 / 2 years |
Level 5 / 3 years |
| Elevated | Level 2 / 2 years |
Level 4 / 3 years |
Level 5 / 4 years |
| Guarded | Level 1 / 2 years |
Level 4 / 4 years |
Level 5 / 6 years |
| Low | Level 1 / 2 years |
Level 4 / 4 years |
Level 5 / 6 years |
Note that the acceptable risk level that triggers more refined risk assessment and also relative quantitative values for uncertainty need to be calibrated based on case studies and expertise of experienced engineers.
The proposed approach recognizes the diverse set of performance limit states relevant to management decisions and can readily be incorporated within risk-based decision-support tools. While this framework remains highly qualitative and subjective in nature, it has the advantage of requiring very limited changes on the actual practice of bridge inspections, and it can be implemented for most bridges using current inspection data and other publicly available data sources.
This approach not only provides decisionmakers with a more complete picture of the uncertainty associated with various assessment procedures, but it also promotes the use of more reliable approaches while still providing States some freedom regarding implementation depending upon their individual priorities and concerns.
Although calculating actual risks assocated with bridges is ideal, it is not possible in practice. For this reason, performance-based risk methods yield a perceived risk, which is valuable in a relative sense.
The proposed framework adopts key performance limit states (safety; durability, serviceability, and maintenance; and operations and functionality), including State or regional costs associated with operation, evaluation, maintenance, and repair. Bridge performance is a more complex concept, and performance of a bridge may cover other limit states that are not fully known. It is expected that as this assessment procedure matures and the findings of the LTBP Program are released, additional performance limit states may be included, and some of these performance limit states may be subdivided to allow for a higher resolution assessment.
