Keywords: fuzzy failure rate, fuzzy possibility score, construction operation, fault tree analysis, safety assessment, fault event.

Safety is a major requirement in all types of construction engineering systems and is necessary to determine the reliability of the system. Safety can be modelled in terms of enabling events, triggering events and safety nets. Once a progressive failure is triggered many possible sequences are possible. In order to evaluate the system reliability, all of these sequences must be considered. Fault Tree Analysis (FTA) is an ideal tool for evaluating alternative paths to failure initiated by a triggering event.

FTA is widely used to analyze the reliability of complex systems such as construction engineering systems. It is a logic block diagram approach for systematically quantifying the probability of the undesirable event in a system. However, the ambiguous and imprecise events such as human errors tend not to be handled effectively by using FTA. Moreover, the degree of uncertainty, where equipment failure or human error leads to a system accident, cannot be quantitatively expressed with precision. However the degree of uncertainty can be measured by the theory of possibility, which has been developed with the explicit intention of imitating human cognitive process.

Assessing a system safety is often performed by experts based on their engineering judgment. It is essential to express system safety qualitatively in practice. Fuzzy set theory is a tool that can be used to represent incomplete and imprecise information. In fuzzy sets as opposed to crisp sets there exists a degree of membership represented by a membership function. Such a membership function can be used to represent judgements.

To overcome the problems inherent in engineering judgment, this paper presents an approach for converting Fuzzy Possibility Scores (FPS) to Fuzzy Failure Rates (FFR) so that the difficulty of estimating probabilities for abnormal system events is solved. The Fuzzy Fault Tree Analysis (FFTA) method is incorporated into the model. Finally, an example is illustrated to demonstrate the effectiveness of this process in analyzing the safety of a human-equipment construction system.

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