Synthesis of National and International Methodologies Used for Bridge Health Indices

Chapter 4. Worst-Conditioned Component Approaches

COMPUTATIONAL APPROACH OVERVIEW

The worst-conditioned approach is driven by bridge component condition data that capture the severity and extent of identified forms of deterioration. The approach captures information about the critical defects in bridge components. Not all damage is factored into the calculation of overall BCI. The condition rating of the whole structure corresponds to the state of the worst conditioned components. The component in the worst condition is related to the individual damage with the worst rating based on its severity and frequency of occurrence among other components of the structure. The number of components contributing to the index and the type of rating system adopted may be different from one country to the other.

GERMANY’S BCI

The German BCI uses a hierarchical approach to assess the overall health of a structure. At the lowest level, an index is assigned to each individual damage identified. The next level involves calculating a condition index for predefined groups of structural components (i.e., piers, bearings, etc.) followed by a final level that computes the overall BCI.

Each instance of damage detected during inspection is rated on a five-level scale in terms of its effect on the bridge’s structural stability (table 15), traffic safety (table 16), and the bridge’s durability (table 17). The extent of damage is not quantified by measured length or area. It is described qualitatively as either small, medium, or large. From this information, a decimal condition index (table 18) ranging from 1.0 (very good condition) to 4.0 (insufficient condition) is assigned for each damage.

Calculating German BCI

The overall condition of the bridge corresponds to the rating of the worst component rather than the aggregate component conditions.

Damage Index

Each component is surveyed for damage or deterioration. For each individual occurrence ofdamage, an index (Z_i) is calculated based on its effect on traffic safety, stability, and durability. The condition index is supplemented with the extent of the identified damage( ∆₁₎ and assigned a value (table 19).

Each component group (CG) consists of damage ratings for each individual occurrence (figure 20).

Figure 20. Equation. Component group.

Next, a component group condition index is calculated.

Component Group-Level Condition Index

The index at the component group level is equivalent to the maximum ratings assigned to damage at the subcomponent level. The number of occurrences of the damage identified within the component group (∆₂) is accounted for in calculating the component group condition index(Z_{CG_i}) (figure 21).

Figure 21. Equation. Component group condition index.

For a substructure component group, ∆₂ is assigned a value according to table 20.

For all other components groups, of ∆₂ is assigned a value according to table 21.

Structure-Level Index

The overall bridge condition index (Z_ges) (figure 22) corresponds to the maximum rating at the component group level, taking into consideration the extent of damage to other component groups. The extent of damage to other component groups (Δ₃) is assigned a value based on the number of damaged component groups (table 22).

Figure 22. Equation. German BCI.

JAPAN’S BCI

The Japan BMS uses visual inspection to assess the condition of bridge components at the element level.⁽¹⁶⁾ Each instance of damage is described based on the type and severity of deterioration alone. A deficiency (or condition) rating is established for each identified instance of damage. During inspection, each element is divided into units, and the condition of the structure is assessed by aggregation of units.

Japan’s BCI is slightly different from that of Germany. It calculates the overall BCI by aggregating worst defects (in terms of severity) detected for all components, whereas the German BCI selects the worst component as the condition of the overall bridge, with no aggregation required. Also, Japan’s BCI calculation does not directly incorporate the extent of damage.

Calculating Japanese BCI

Step 1

Assign deficiency ratings (table 23) for each defect within each structural component of the bridge.

Step 2

Calculate a demerit rating (d) corresponding to the deficiency rating for each type of defect (figure 23). Demerit rating for distress with worst deficiency ratings (d_I) is assigned and not calculated. The remaining demerit ratings are calculated as follows:

Figure 23. Equation. Demerit ratings.

Where:

∝ = Reducing ratio (table 24) corresponding to each deficiency rating.

Step 3

Determine the value of the demerit rating for each structural component by taking the maximum demerit rating for all defects with that component group.

Step 4

Calculate the overall bridge condition rating by adding all defective ratings for the structural groups and subtracting it from 100.

STRENGTHS AND LIMITATIONS

Strengths

Worst-conditioned component approaches are useful for assessing the vulnerability of a bridge in case of disasters or extreme events. At the network level, the approach can be used for identifying high-risk bridges. This is possible because the approach correlates the condition of the bridge to the weakest link in the structure.

Limitations

This approach does not give a full picture of how deterioration is spread over the bridge. The total amount of defects (not the worst defect) is required for planning bridge maintenance repair and rehabilitation projects. Using this approach with weighted averaging methods is more helpful.