# Class 3 and Class 4 Risk Assessments
This section describes the recommended use for Class 3 and Class 4 risk assessments. Within the risk assessment classification taxonomy, Class 3 and Class 4 risk assessments are the most detailed. They both require some degree of site-specific hazard analysis, component-based modeling, and Monte Carlo methods. The final results are presented with associated quantified uncertainty and can be deaggregated by component. The results have less uncertainty than Class 1 and Class 2 assessments. Class 3 and Class 4 assessments both require that the analyst has risk modeling expertise.
Class 4 assessments are typically differentiated from Class 3 assessments in the fidelity of the modeling. Hazard modeling can be more site-specific and potentially incorporate field testing. Vulnerability modeling is more explicit, with actual component quantities used (instead of “standard” quantities for Class 3) and the most detailed modeling of engineered components is typically implemented (e.g., nonlinear finite element modeling for earthquake risk assessments) instead of simplified models for Class 3. For Class 3 assessments, the analyst does not necessarily need to be a subject matter expert in the hazard being analyzed, but for Class 4 assessments, the analyst must be a subject matter expert.
The Class 3 section outlines the recommended use for Class 3 risk assessments. The Class 4 section outlines the recommended use for Class 4 risk assessments.
# Recommended use for Class 3 risk assessments
Class 3 risk assessments are most commonly performed for clients for the following purposes:
Cost-benefit analysis of component-specific mitigations, design enhancements, and other operational improvements. For this purpose, risks are assessed for identification of interventions that can reduce risks that are deemed intolerable. Unlike Class 1 and Class 2 assessments, Class 3 assessments provide results that can be deaggregated by component to identify which components are driving risk. In addition to physical components, Class 3 assessments can also deaggregate risk by operational measures (e.g., financial delays) that can identify non-physical mitigation options for reducing risk.
Prioritization of assets for most detailed analysis (i.e., Class 4). For this purpose, risks may be evaluated to determine if more detailed analysis is warranted in order to make final decisions about high-cost physical mitigations. A Class 3 assessment (unlike Class 1 and Class 2 assessments) can more specifically identify assets or components of assets that pose elevated risks and justify more detailed analysis (i.e., Class 4) for a level of confidence high enough to take action on an expensive physical mitigation. Then the most efficient mitigation strategies can by identified based on this more detailed analysis. It should be noted that this use-case may be applicable for Class 2 assessments, but a Class 3 assessment will provide higher confidence in the results and can provide a more refined view of risks on a component-by-component basis so that the Class 4 assessment can focus on the higher-risk components within the asset.
# Recommended use for Class 4 risk assessments
Class 4 risk assessments are most commonly performed for clients for the following purposes:
Cost-benefit analysis of component-specific mitigations, design enhancements, and other operational improvements. For this purpose, risks are assessed for identification of interventions that can reduce risks that are deemed intolerable. It should be noted that this use-case is also applicable for Class 3 assessments, but Class 4 assessments will provide higher confidence in the cost-effectiveness of possible interventions. Furthermore, unlike Class 3 assessments, Class 4 assessments are typically 3D and can identify specific units of components within a building that may be especially vulnerable (e.g., corner columns vs. interior columns of a building).
Resilience-based design of new buildings. For this purpose, risks may be evaluated to determine if new building meets resilience-based design requirements (e.g., REDiTM Rating System). Class 4 assessments are the only types of assessments discussed in this document that are detailed enough to demonstrate compliance of a new building with design requirements.