Synthesis of National and International Methodologies Used for Bridge Health Indices

Chapter 1. Review of Methodologies Used for Bridge Health Indices (BHIs)

Introduction

Bridge performance measures are an important component of any successful bridge management system (BMS). They can be used as a tool for communicating with legislatures, bridge managers, and, most importantly, the public on issues such as traffic safety and structural vulnerability of bridges to disasters such as earthquakes, scour, etc.^(1,2) Different types of performance measures have been developed for various purposes. The type of performance measure is usually a reflection of the agency goals. A bridge health or condition index is used as a performance measure by agencies interested in preserving the condition of bridge structures or prioritizing the maintenance or replacement projects within their bridge inventory. Other performance measures, such as geometric and inventory ratings, are used to improve traffic safety of a bridge. Vulnerability and/or resiliency ratings are examples of performance measures used to show how vulnerable bridge structures are to structural or operational hazards such as hurricanes, earthquakes, or over-load trucks and how well they perform in these situations.

The bridge condition or health index is a useful tool for assessing the structural or functional health of a bridge. The index is calculated based on the condition of the bridge’s structural elements and the service provided by the bridge. For the purposes of bridge management, the most important use of a BHI is to identify which structures in the inventory are the most deteriorated and are most urgently in need of repair work. Most BMSs also use a bridge condition index (BCI) to help track the general system condition over time, evaluate the benefits of an agency’s bridge maintenance and rehabilitation programs, and serve as a basis for allocating resources to bridges within a network.

The increased availability of element-level inspection information influenced the redevelopment of BHIs used around the globe. Currently, most BMSs rely on element-level information for calculating BHIs.^(2,3) Based on the computational approach used, current methods for developing condition or health indices can be grouped into the following four approaches:

• Ratio-based methods assign a BCI or bridge condition number(BCN) based on the ratio of the current condition to the condition of the structure when it was new. The objective for this method is to calculate the remaining value of the bridge. The California BHI and the health index method used by AASHTOWARE^TM Bridge Management software, BrM (formerly Pontis),^[1] are the examples for ratio-based methods discussed in this report.
  
  
• The weighted averaging approach is suitable for planning bridge maintenance and rehabilitation activities. The approach estimates the condition of the whole structure by combining condition ratings of all individual bridge elements weighted by their significance or contribution to the structural integrity of the bridge. This approach is common in systems that rely on element-level inspection data. BCIs used in Australia (BCN), the United Kingdom (BCI), South Africa (BCI), and Austria (BCI) are the examples of weighted combination approaches discussed in this report.
  
  
• The worst-conditioned component approach is common in systems that carry out inspections on key bridge components. This method is used to extract the critical defects in bridge components. In this approach, the BCI is approximated to the rating of the component in the worst condition. Some States also use the worst (lowest) National Bridge Inventory (NBI) rating to report bridge conditions at performance dashboards. The Michigan Department of Transportation uses the lowest NBI rating in its Bridge Condition Forecasting System (BCFS). BCFS helps Michigan with bridge project selection decisions. The German and Japanese BCIs are the examples of this approach and are discussed in this report.
  
  
• Qualitative methods do not report the condition of the bridge on a numerical scale. They describe a structure as either “Poor,” “Fair,” or “Good,” based on the condition state and importance of the elements under investigation. Washington, Florida, and other States use NBI condition ratings to classify bridges as “Good,” “Fair,” or “Poor.” The Bridge Health Indicator used by Roads and Maritime Services (merger of Roads and Traffic Authority and New South Wales Maritime) in Sydney, Australia, is discussed in this report as an example to highlight the use of qualitative methods in the assessment of overall bridge health.

There are other BHIs that were developed by combining some of the above listed methods. One example, no longer used in the United States, is sufficiency ratings (SRs), which combine the weighted averaging and the worst condition component approaches. The SR was used in funding decisions. Additionally, a risk-based prioritization method currently being tested by the New Jersey Department of Transportation (NJDOT) is also discussed in this report. This approach combines different performance limit states to calculate the perceived relative risk for each bridge.

Although resilience is a very important aspect for management of the bridge network, consensus metrics do not currently exist for it. Research programs are actively working on defining metrics and acceptable thresholds to address bridge resilience aspects. Some examples of possible resilience metrics include the following:

• Regional conditions such as natural hazard zones.
  
  
• Geology, seismicity, and geotechnical features of the site (e.g., faulting, landslides, or liquefaction).
  
  
• Toughness and resilience in the event of damage or element failure (e.g., loss of a girder or fatigue-induced fracture not leading to collapse).

Although some of these metrics do not currently exist, they may be quantifiable through expert elicitation and risk assessment.

This report reviews state-of-the-art BCIs being used as one metric to assess performance of bridges in the United States and other countries. Table 1 summarizes these indices and their calculation approaches.

To facilitate comparison between different bridge condition or health indices, each system is reviewed based on the following:

• Computational Approach—The general approach used in calculating the health index (e.g., ratio-based, weighted average, worst conditioned component, qualitative, or other approach).
  
  
• Data Inventory and Condition Rating—Relevant data input for computing the condition index (e.g., types of damage observed, severity of damage, extent of damage, urgency of damage, etc.).
  
  
• Condition Index—Steps used to aggregate the condition information and calculate the overall BHI.
  
  
• Strengths and Limitations—Discussion of the key strengths and limitations of the health index.