Assessing Risk for use in Bridge Management Systems
Transportation asset management uses data and objective analysis to improve decision making, with the objective of providing the required level of service in the most cost effective manner. Asset management systems are tools to quantify relevant performance measures, and forecast future outcomes as they are affected by agency policies, resource levels, and exogenous factors.
The “Moving Ahead for Progress in the 21st Century Act’’ or ‘‘MAP-21’’ requires the development by each state of a “risk-based asset management plan” and declares that “it is in the vital interest of the United States … to use a data-driven, risk-based approach and cost-effective strategy for systematic preventative maintenance, replacement, and rehabilitation of highway bridges and tunnels to ensure safety and extended service life.” The legislation does not specify a method, nor performance measures, for considering risk. However, the language makes clear the desirability of incorporating risk in a way that can reasonably be supported in a data-driven, cost-effective process.
For the present study, the risks of interest include those where an exogenous uncommon hazard, with random timing and location, acts on one bridge or a small subset of bridges, causing a loss of access, seriously degraded functionality for road users, unexpectedly rapid deterioration, or unexpectedly high costs. For these types of risks, it may be possible to estimate hazard likelihood at the level of geographic zones or structure categories, and vulnerability or resilience at the level of individual assets. These approximate, probabilistic inputs may enable an agency with suitable tools to quantify and manage risks in a data-driven cost-effective manner even if the exact risk for each bridge can never be measured.
The hazards satisfying these criteria may include, but are not limited to, earthquakes and other types of earth movement, hurricanes and tornadoes, floods and scour, fires, vehicular or vessel collisions, fatigue, and advanced deterioration. These hazards have consequences that are beyond the normal deterioration and functional deficiencies already assessed in bridge management systems. The probability of each hazard, and the ability of each structure to resist the hazard, is not always consistently and quantitatively assessed at the current state of the practice.
The objective of this research project is to develop a methodology for data-driven assessment of risks at the bridge level, suitable for use in a bridge management system. The method should consider, to the extent feasible:
· The likelihood of a hazard and/or extreme event at the asset level or for groups of assets where the data may be available;
· The typical consequences of the hazard or event, to the bridge itself and to the public, including the network effects of detours and delays of traffic;
· The probabilities of different levels of severity of the event and consequences.
The investigation is not limited to existing data sources, but the cost of new data collection must be carefully weighed and minimized where possible. The method must be customizable so each agency can weigh its own costs and benefits when deciding how much of the methodology to implement. Recommendations for future research may be appropriate for aspects of risk believed to be quantifiable by research.
Transportation agencies currently lack objective, quantitative methods to assess risks for bridge management systems. The research will provide a procedure agencies can implement to objectively prioritize risk mitigation actions, and a method for incorporating risk into larger questions of bridge replacement and rehabilitation. The method should consider risk aversion, but will apply it in a deliberate, consistent manner to all types of risks across all assets.
With these tools, agencies will be able to allocate resources efficiently with bridge-level risk fully considered, and will have a means of compiling and maintaining a detailed risk register. The resilience of structures will be able to be measured and tracked at the network level, to show that the agency is measuring and managing its risk at the most responsible level given resource constraints. The public and stakeholders will be in a better position to understand the tradeoffs among risk, funding, and other aspects of transportation system performance.
In the absence of this kind of objective methodology, major risk management decisions are often made based on fear caused by unexpected events. Such decisions are unlikely to allocate resources efficiently. It is unknown at this time how much money can be saved or reallocated, but unless risk is measured there is no way to measure the effectiveness of risk management practices.
The National Bridge Inventory provides minimal opportunities for risk assessment. AASHTO provides more relevant data items in its Element Inspection Manual and in the AASHTOWare Bridge Management software. However, many of the available items are largely unused due to lack of guidance on how to use them in asset management analysis and decision making.
A very comprehensive vulnerability assessment process is documented in a series of manuals in New York. Florida DOT has completed research on estimating likelihood of a variety of natural and man-made hazards, and has demonstrated one method of incorporating this information in a bridge management system. Minnesota’s Bridge Replacement and Improvement System demonstrates a process where risk assessment is used for prioritizing projects for inclusion in the STIP. NCHRP Report 590 documents some alternative methods for combining risk with other performance measures in a multi-objective bridge management analysis. Transportation Asset Management Plans in all 50 states are starting to develop concepts for incorporating risk into the highest levels of decision making in each agency.
The emerging field of geotechnical asset management, where risk is a primary performance concern, is developing methods for routinely assessing the geological and hydraulic risks to each section of road, and quantifying the effects of slopes, embankments, retaining walls, culverts, and other constructed facilities in resisting these hazards. The methods often adopt a level of service perspective, where the “service” is the reduction of event likelihood or the increase in road section resilience. This experience may provide some useful ideas for bridge management.
It is envisioned that the research will involve at least the following tasks:
Gather and synthesize the literature on risk assessment and risk management. This effort is expected to reach far beyond the field of bridge engineering to include considerations of climate change, driver behavior, geological and hydraulic studies, fatigue design methods, behavior of deteriorated structures, historical records of various classes of extreme events (especially national sources such as FEMA and NOAA), the considerable resiliency literature that arose after the events of 2001, and the established practices of the private sector.
Develop a preliminary framework for considering risk in asset management, encompassing all the typical TAM functions such as performance assessment at the asset, corridor, and network levels; tracking of past performance over time; forecasting of future performance; identification of feasible actions; prioritization of projects; resource allocation; development of performance targets; maintenance and capital budgeting; STIP development; policy formation; and communications with stakeholders and the public, including performance dashboards and TAM Plans. The framework should identify the types of quantitative information needed for each use case, and should sketch the possible methods for computing the information including actual or desired data sources.
Identify a set of hazards which are to be addressed using the methods to be developed in the study. For each hazard, identify the types and severity of harm which might typically arise, data sources, the likely range of risk magnitude, and the prognosis for quantifying the likelihood and consequences. For each hazard, sketch a set of alternative roadmaps for obtaining data and estimating risk at the bridge level. The alternative roadmaps would represent different levels of investment in new data collection and different means of imputing missing data in the analysis.
Develop an outline for a risk assessment guidebook documenting the definitions, levels of service, data sources, inventory and inspection methods, supplementary data collection, and computational methods necessary to implement the framework.
Prepare a guidebook following the approved outline. The guidebook will include a synthesis of state-of-the-practice methods, linkages to available data, data collection and assessment methods for each category of risk, guidelines for computing the various metrics required for TAM, example applications, and guidance for customizing the methodology for any given agency. Accompanying the guidebook will be a series of webinar slide presentations, each presentation designed to last 90 minutes and be focused on one type of risk or one area of application to asset management.
Prepare a Final Report which documents the work completed in the study and provides supplementary detail on each risk assessment method.
This research is envisioned to feed directly into existing efforts to develop multi-objective optimization models in bridge management systems. It will provide default models and methods each agency can use as it customizes BMS models for its own use. Continued FHWA and AASHTO support for bridge management systems, and in general for asset management and performance management, will help to ensure successful and widespread implementation.
The results will be highly suitable for a webinar series where one type of risk or one area of application is covered in each 90-minute presentation. The webinar format allows each agency to be selective in which topics to cover and which personnel to send to each session.
|Sponsoring Committee:||AKT50, Bridge and Structures Management
|Research Period:||24 - 36 months|
|Index Terms:||Asset management, Bridge management systems, Service life, Hazards, Cost effectiveness, Deterioration, State of the practice, Risk assessment, Moving Ahead for Progress in the 21st Century Act, |
Maintenance and Preservation
Planning and Forecasting
Bridges and other structures