Safety format for the flexural design of tunnel fibre reinforced concrete precast segmental linings

Abstract

Fibre reinforced concrete (FRC) is increasingly used in tunnel segmental linings constructed with tunnel boring machines. Several codes include the FRC as a structural material and specific guidelines cover the design of FRC precast segmental linings. All of these consider the traditional limit state format, applying partial safety factors to the loads (γL) and material strengths (γM). Often, the γM applied to the residual tensile strength of FRC is assumed the same as for concrete in compression. However, the variability in the determination of the residual tensile strength may be several times bigger than that of the compressive strength. In this context, the application of the same γM should be revised as it could lead to reliability indexes lower than the established for traditional reinforced concrete structures. The objective of this paper is to estimate safety factors for the residual flexural response of FRC used in tunnels segments, considering the variability of the material in both the material characterisation test and in the real-scale structure. After a study about the influence of the element size on the variability, an evaluation of the structural reliability of the segmental linings made with FRC is performed using the FORM method and a database of real-scale test results compiled from existing literature. Based on that, values of γM are proposed according to different failure consequences levels and relative economic costs required to increase the structural safety.