FIRE PERFORMANCE OF R/C AND FRC TUNNELS EXPOSED TO FIRE: COMPARISON BETWEEN LINEAR AND NON LINEAR ANALYSES VIA A SIMPLIFIED 1D APPROACH

The mechanical response of reinforced concrete tunnels (R/C) exposed to fire can represent a critical issue in the design phase of this kind of infrastructures, due to the combination of a few negative aspects such as the development of severe fire scenarios, the development of sizable indirect actions and the severe compression state which can foster spalling phenomenon. 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 and the evolution of the internal actions. This task is however often complex, since it can need the implementation of non-linear analyses which consider the diffusion of heat in the structural elements, the variation of the mechanical properties of the materials and the interaction between lining and surrounding soil. Such analyses are often performed by means of advanced finite element codes which can perform multi-physics simulations (as for example Abaqus or Safir). On the other hand, in the present study, a simplified approach is described for linear and non-linear analyses of deep R/C tunnels exposed to fire, based on the main assumption of axisymmetric loading and heating. This assumption is generally kept for deep tunnels, in which the ratio between vertical and horizontal pressure is usually not too far from the unit value. The assumption of axisymmetry makes it possible to describe the behavior of the lining via a sectional analysis (1D approach), in which the plane section assumption is kept. Such algorithm can be rather easily implemented in any work sheet or programming code (as for example Matlab, Fortran, or similar). This simplified approach allows to rapidly perform parametric analyses necessary for understanding the role played by different key parameters, and to understand the advantages of performing a non-linear analysis allowing the plasticization of the section.