Abstract
Abstract The Brazilian standard pertinent to the design and execution of foundations, NBR 6122/2019, establishes that the safety inspection of foundations of a given project must be conducted based on compliance with specified safety factors. Safety factor verification is imperative to meet regulatory requirements; however, it is not enough to guarantee the safety of a foundation. As there is variability in the resistance and solicitation of the piles that make up a foundation, each safety factor value reflects a failure probability. Therefore, it is fictitious that the use of an adequate safety factor value implies the absence of failure risk. Thus, a reliability analysis, applied to a real building with an access ramp whose foundations are composed of precast concrete piles, based on the probabilistic moments, mean, and coefficient of variation, associated with the variability of pile resistance and solicitation, is presented in this article. The values obtained for the safety factors (2.14 and 1.98 for the building and the ramp foundations, respectively) and failure probabilities (1:2,244 and 1:3,131 for the building and the ramp foundations, respectively) implies that the project in question has an acceptable safety level. This article allows us to conclude that a small variation in the reliability index results in a large variation in failure probability; that a greater global safety factor does not necessarily lead to a lower failure probability and it becomes evident that this reliability analysis is a practical way, to manage the uncertainties inherent in foundation design, allowing rational decision making regarding performance.
Highlights
The Brazilian standard that deals with the design and execution of foundations, NBR 6122 (ABNT, 2019, p. 1), recognizes that “[...] foundation engineering is not an exact science and that risks are inherent to all and any activity involving phenomena or materials of nature.” Even with such awareness, the regulations do not make explicit the need for a reliability analysis that is appropriate for the quantification of project risk and that is allied to the stipulated safety factors to ensure a certain degree of safety in the design of foundations.Cintra and Aoki (2010) claim that it is outdated to assume that the safety factors prescribed in the standard guarantee the absence of failure risk in a project
Based on the solicitation results obtained for each pile and the mobilized resistance results obtained through dynamic loading tests, it is possible to evaluate the global safety factor obtained for the piling and develop a reliability analysis using the probabilistic moments, the mean and coefficient of variation, associated with the resistance variability and solicitation of these piles
Regarding reliability and safety issues, both the multipurpose building foundation and the access ramp foundation showed results of failure probability considered acceptable for foundation engineering according to technical literature, e.g. Whitman (1984) and Dell'avanzi and Sayão (1998)
Summary
The Brazilian standard that deals with the design and execution of foundations, NBR 6122 (ABNT, 2019, p. 1), recognizes that “[...] foundation engineering is not an exact science and that risks are inherent to all and any activity involving phenomena or materials of nature.” Even with such awareness, the regulations do not make explicit the need for a reliability analysis that is appropriate for the quantification of project risk and that is allied to the stipulated safety factors to ensure a certain degree of safety in the design of foundations. Based on the solicitation results obtained for each pile and the mobilized resistance results obtained through dynamic loading tests, it is possible to evaluate the global safety factor obtained for the piling and develop a reliability analysis using the probabilistic moments, the mean and coefficient of variation, associated with the resistance variability and solicitation of these piles It will become explicit, in a practical and applied manner that even when fulfilling the safety factor stipulated by NBR 6122 (ABNT, 2019), there is an inherent probability of failure that should be evaluated to determine whether it is acceptable or not by the technical design team of the foundations. A methodology for assessing the safety of the foundations of an enterprise is presented, based on deterministic analysis in order to comply with current Brazilian code aspects through safety factors (SF) and based on probabilistic analysis in order to evaluate the failure probability associated with the SF obtained are admissible, assisting in the decision-making process in designs
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