Last month, my column discussed the need for risk analysis and pointed to a few guidance documents that are available and applicable. This month, I will dig a little deeper into some of the details of how one might go about doing a basic risk analysis.
In order to get into the practicalities, I’ll discuss the methodology used in the ASCE Manuals and Reports on Engineering Practice No. 128, “Building Security Rating System.” The Manual of Practice (MOP) is intended to allow for determination of the overall security of a building and, maybe, more importantly, to ensure the security measures put into place are appropriate given the risks and consequences of an adverse security event. Although this document is used to evaluate security measures, the overall methodology illustrates the thought process that must be followed when designing for increased resilience for all hazards.
The MOP is divided into three sections: (1) building classification, (2) security countermeasures, and (3) building security rating. I will be concentrating on the building classification procedure this month, since it is a direct expression of risk. The concept of dividing buildings into different classifications is a natural one, since the design decisions to be made can vary greatly depending on the building. It makes no sense to design the same security measures into two different building classifications, such as a suburban apartment building or federal courthouse in the downtown area of a major city. Likewise, when designing for enhanced resilience, the type of building and its intended use during and after an event will dictate which design decisions are appropriate.
Determining the building classification in the MOP is based on two primary parameters: threat (hazards) and consequences (impact). Taken together, these parameters form a basic risk analysis since they are exactly what you consider — what should I be worried about, and what happens to the building if it occurs? In order to determine the threat and consequences, the user answers a series of questions for each parameter, and the questions are further divided into basic and secondary parameters. For instance, parameters such as building occupancy category and history of hazards/threats are commonly considered as basic parameters of the threat. For the consequence, the basic parameters include the local economic significance and if the building is tied into the infrastructure or transportation network. Each of these parameters has a group weight that is further modified by the answer to the question, leading to a total weight for each parameter. Secondary parameters, which affect the need for security but are less important, also have group weights that are multiplied by the weights of the individual responses. Secondary parameters include items such as whether there is a child care facility on-site, how many public visitors are received on a typical day, etc.
Based on the primary and secondary parameters, both threat and consequence are given total weights. The precise mathematical formula for calculating the total weight isn’t important for this discussion, but the fact that many different factors are considered in determining both the threat to the building and the potential consequences of any event is what I want to emphasize. While many of the parameters will likely be the same for a risk analysis for any hazard — for example, occupancy category — the other parameters that should be considered might be quite different. The desire to use a building to provide temporary shelter, or to serve as a distribution point for supplies, might be a primary factor in determining what level of resilience is appropriate when designing for an earthquake or hurricane.
Once the hazard and impact total weights are determined, they are combined to arrive at a final risk weight. The MOP requires a three-step process: First, the primary hazard and impact weights are combined using an SRSS method in order to emphasize the interrelationship between the two categories when determining risk. Then, the secondary hazard and impact weights are combined in a similar manner. Finally, the primary and secondary weights are combined for a total risk score, which determines the building classification.
Other formulas could easily be chosen for determining risk, and, for many situations, there may be no need to develop any kind of mathematical score. Additional factors that may be considered are potential changes in occupancy or building use. How much these potential changes should factor into the risk analysis is very case-specific and may be dictated by the owner or jurisdiction. However, the point remains that, by asking pertinent questions regarding the threats to buildings and the potential consequences, it is possible to perform a relatively simple risk analysis to help with decisions regarding the appropriate design parameters.
In summary, performing a risk analysis can help to inform design decisions and ensure the needs of the owner and community are being met, whether they are resilience, sustainability, or security-related. The example risk analysis methodology discussed in this column emphasizes the need to ask questions regarding the threats to a building and the consequences of adverse events, be they natural hazards, climate change-related, or security concerns.