Study for a Quantitative Risk Assessment on Accidental Actions

Abstract

Lately structural safety is an often discussed subject in The Netherlands. Recent collapses have made people question if our buildings are actually safe enough. Research shows that the individual probability of death due to structural failure is below the limit of 10-5 per year. However, it is unknown if every single building is safe enough as well. The main cause of structural failures are human errors (90% of the collapses). However, the Dutch Building Decree does not mention the unidentified accidental actions (the type the human errors belong to) as those an engineer has to take into account when designing the building structure. The Eurocode does mention unidentified accidental actions and states that for consequence class 3 buildings a risk assessment has to be done to check which accidental actions are hazardous and if the total risk to a building is acceptable. However, the Eurocode does not give a maximum value of an acceptable risk in their risk assessment method. So currently when engineers perform a risk assessment, they cannot verify whether the risks of accidental actions on their designed structure are acceptable according to the regulations. This thesis conducts research for a new method for performing a risk assessment in a quantitative way. First it investigates what the advantages and disadvantages of existing methods for performing a risk assessment are. It appears that the main step missing in existing methods is the verification, based on a quantified limit, whether the risks of accidental actions are acceptable. Based on the known methods, a new protocol for performing a risk assessment is proposed. This protocol includes a determined limit for the probability an accidental action occurs and causes a certain failure mechanism and consequence. This limit is based on the maximum probability the Eurocode gives for failures with a consequence indicated as consequence class 1, 2 and 3 (CC1, CC2 and CC3). By dividing the accidental actions that can occur and their failure mechanisms into the same consequence classes, the estimated probability and the limit of the Eurocode can be compared. In this way the known consequence classes from the Eurocode are used in another way than engineers are used to. Furthermore research on past structural failures has been investigated to establish whether it can be used to predict the probability a certain accidental action causes a CC1, CC2 or CC3 consequence. Not enough research is done to fully trust on these outcomes; in the protocol proposed in this thesis the engineer will have to estimate a part of the probabilities himself, with the help of a classification table. The principal advantage of performing a risk assessment on accidental actions in a quantitative way with a prescribed maximum probability per consequence class, is that it is made clear whether a building fulfills the requirements or not. This can be helpful for communication with other involved parties. However, it will never be 100% clear what the limit should be, because estimating the risks is based on the engineer’s estimations and outcomes may differ as they depend on the person performing the risk assessment. This means a hard line, below which the risks are acceptable and above the risks are considered too high, cannot be drawn. An area in between is needed in which it is up to the engineer and client to decide whether or not they find the risks acceptable.