Simplified approach for the evaluation of the bearing capacity of deep R/C tunnels exposed to fire

The structural performance of R/C tunnels when exposed to fire can represent a critical aspect in the design phase, due to some specific issues concerning this kind of infrastructures such as (a) the geometry of the fire compartment (favouring the development of severe fire scenarios) and (b) the inherent structural redundancy (due to the almost axisymmetry and to the presence of the surrounding soil). Both aspects foster the development of sizable indirect actions which can definitely modify the initial state of stress in the lining. It follows the need for a reliable evaluation of the fire performance of tunnels exposed to high temperature, considering the decay of the material properties due to the rise of the temperature, while taking into account the evolution of the internal actions due to the restrained thermal dilation. This task often entails the implementation of complex thermo-mechanical non-linear analyses, which can be properly implemented via advanced finite elements codes capable of multi-physics simulations (as for example Abaqus or Safir). In this regard, in the present study, a simplified approach is described for the non-linear analysis of deep R/C tunnels exposed to fire, based on the main assumption of (I) plane section and (II) axysimmetrically loaded lining. These assumptions allow to reduce the problem into a sectional analysis which can be even implemented in common worksheets. In the paper, the overall method is described, showing sensitivity analyses regarding some parameters involved in the assessment of the structural performance of tunnels exposed to fire.