Comparison with Current Rating Methods

Numerous rating methods are used in Australia and different countries as discussed in Chapter 2. Taking into account that the current rating systems are very different and subjective, makes the comparison of this method with other rating methods through quantifying their reliability and practicality impossible. For instance, in the current existing rating systems that can be applied to a network of bridges, the current practices consider different ranges of numbers (e.g. 1-5, 1-10, 1-4 etc.) or even descriptive information to describe the condition of the components. When the existing rating methods assess the condition of bridges, they do not conduct structural analysis or use measurable tools such as an appropriate SHM system, hence, their results are not scientifically and reliably measurable. For those methods that can only be applied to one important bridge, although their results are reliable, they cannot be applied to a network of bridges as they are very costly. Therefore, in this research efforts were made to elaborate the advantages of the developed rating method over other existing rating methods through discussion in different parts of developing the method and explaining its philosophy.

The discussions will focus on the sources of subjectivity, how reliable tools such as structural analysis and appropriate SHM systems are selected among others and how they can be incorporated into the newly developed rating methods to reduce the subjectivity. In this method, contrary to current existing methods that can be applied to a network of bridges, the main decisions for conducting the repair and maintenance of components will be made based on the results of structural analysis and design, hence, this method is scientifically measurable.

In comparison to the reliable methods that can only be applied to one important single bridge, the proposed method only avoids conducting costly structural analysis methods such as alternative load path, or performing more detailed structural assessment by adopting SHM systems. Therefore, considering that for the majority of the railway bridges that have simple structures, the above detailed structural evaluations are not necessary, the above strategy will help to enhance the practicality of the method to be applied to a network of bridges, and the results of the proposed method will be similar to a sophisticated method. As previously mentioned, for important railway bridges with complex structures, their assessments were outside of the scope of this research as their numbers in a network of bridges were minimal and more sophisticated methods might

be required. Therefore, their condition assessments and ratings, and allocation of their budgets can be separately conducted.

The practicality of the developed method in this research was also evaluated based on assessing the capabilities of current existing BMSs in Australia (the industry partners of the main project of this research) and other countries. The common sources of subjectivity were identified in this research by criticality reviewing the literature and interviewing the experts of the industry partners of the main project (LCMRB) in charge of repair and maintenance of more than a thousand railway bridges in Australia. The available knowledge and appropriate tools, such as Analytic Hierarchy Process (AHP), structural analysis, and available risk assessment in design standards were also selected to be used in this method by conducting a comprehensive literature review and comparing the advantages and disadvantages of these selected tools in respect to others.

As an example, to briefly illustrate the advantages of the proposed synthetic rating method, it will be compared with some rating methods which are used in Australia (e.g. VicRoads, DMR Qld and RMS NSW) (Austroads, 2004). According to VicRoads (Austroads, 2004), the criticality rating (CR) for each component is calculated based on the material and the structural group that the component belongs to. CR varies from 1 to 12, and higher values of CR show the higher criticality of the components. CR in VicRoads is equivalent to the weighting factor term that is used in this research. Table 5-19 shows the weighing factors (CR as in VicRoads) that are calculated for each component of Bridge 1. The CR values are the same for all components of the same type and same material for all bridges with different structural geometries and ages. CR values also do not change when the structure is subjected to different loads. As can be observed, compared to the weighting factors shown in columns v to ix of Table 5-6 that were calculated based on the synthetic rating method, the VicRoads weighting factors do not take into account,

1) The contribution of different critical factors to bridge deterioration,

2) The D/C ratios of the components when they are subjected to different loads, 3) The vulnerability of the components to different critical factors, or

4) The section properties of different components, and the geometry of different structures.

Therefore, rating of components and identifying the components in most need of repair cannot be conducted in a reliable way and appropriate remedial actions associated

with different critical factors cannot be determined. According to VicRoads, the condition of the bridge is calculated based on the above weighting factors (CR), hence, the condition of the bridge will be predicted in a very subjective way. Other BMSs such as DMR Qld and RMS NSW (Austroads, 2004), have the same problems as mentioned above.

Similar comparisons between the rating method developed in this research and other rating methods used in Australia show the incomparable reliability of the present synthetic rating method to others. The synthetic rating method involves structural analysis and SHM systems in the condition assessment process in a practical way, and uses data in BMS to evaluate the contribution of different critical factors towards bridge deterioration. As mentioned previously, based on the comprehensive review of the literature conducted in this research, the same gaps as mentioned above are common among available rating systems used in practice throughout the world, and the proposed method can fill these gaps in a practical and reliable way.

Table 5-19 Criticality Rating (CR) (as weighting factors in this thesis) of components of Bridge 1

Components CInsp

(i)

Structural Group Material

CR

Substructure Superstructure

Steel Cast in-situ

Concrete C1 1 3 3 9 C11 2 3 3 9 C12 2 3 3 9 C13 1 3 3 9 C2 2 3 3 9 C21 2 3 3 9 P1 1 3 1 3 P11 3 3 1 3 P2 2 3 1 3 P21 2 3 1 3 P3 1 3 1 3 P4 2 3 1 3 P41 1 3 1 3 P42 3 3 1 3 P43 2 3 1 3 P5 1 3 1 3 P51 1 3 1 3 P52 2 3 1 3 P53 1 3 1 3 P6 2 3 1 3 P61 2 3 1 3 Diaphragms Mid span 2 3 1 3 5.5 CONCLUDING REMARKS

This chapter illustrated the application of the SRP on a network of two bridges. The number of bridges and their components in the network could be any and there is no limit on this. For the above two bridges, the importance of critical factors was identified, the weighting factors of the components calculated, and the criticality and vulnerability of the components evaluated. The ratings of the components at bridge and network level were shown in tables and figures. The required actions for components and bridges

based on the results of SRP were identified. The results were discussed for further illustration of the method.

According to the results obtained in this section, it can be concluded that, by using the synthetic rating method and its procedures, and contrary to current practical rating methods, engineers can identify the effect of any damage in any component of the bridge on the whole structure by performing structural analyses. For important bridges, the condition of the components and their performance can be monitored using SHM sensors, and the results can be used as weighting factors of components in the synthetic rating method. In addition, the method can reliably determine the most critical and vulnerable components and most damaged structures at the network level and deadlines for taking action. This information about the current and future conditions of the components and bridges and deadlines for taking action is extremely important for identifying the best time for intervention. The best time for intervention before the identified deadlines based on SRP will be calculated after taking into account other non- structural factors such as cost, human and social factors and through prioritization and optimization processes in BMS.