Expanded Reliability-based Design for Spread Foundations

Abstract eng:
Design is a process of determining a set of parameters that describe and specify a geo-structure (e.g., spread foundation) that satisfies a series of performance requirements, including the ultimate limit state (ULS), serviceability limit state (SLS), reliability and economic requirements. This paper presents a general reliability-based design (RBD) approach for foundations that addresses, explicitly and simultaneously, the ULS, SLS, reliability and economic requirements (Wang et al. 2011 and Wang 2011). The design process is formulated as an expanded reliability problem (Au 2005) that is evaluated by a single run of Monte Carlo simulation (MCS). The expanded reliability-based design (i.e., expanded RBD or RBD^E) approach with MCS is illustrated through a design example of a spread foundation with a fixed depth that has been used for the evaluation of Eurocode 7 (Orr and Breysse 2008). The results are consistent with those from previous reliability analysis reported in literature. In addition, the expanded RBD approach with MCS addresses the Eurocode 7's reliability requirement explicitly and simultaneously with the ULS and SLS requirements, and it leads to a design that satisfies both the ULS design target failure probability PT^ULS = 7.2X10^-5 and SLS design target failure probability PT^SLS = 6.7X10^-2 in Eurocode 7. A design calculation using the expanded RBD approach with MCS is equivalent to a sensitivity study on failure probability P_f versus the design parameters. These results provide additional insights into how the expected performance level of spread foundations changes as design parameters change. They also allow design engineers to adjust target failure probability P_T, without additional computational efforts, to accommodate specific needs of particular projects, such as failure consequences. Such adjustment is not possible for current RBD approaches, without re-calibration of partial factors, although it is common in traditional allowable stress design (ASD) practice where the design factor of safety (FS) can be adjusted, where and when warranted.

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