A finite element insight on reinforced concrete tunnels under fire conditions

Paolo Cucino, Donato Fanti, Alberto Omizzolo
SWS Engineering

Daniele Schiavazzi

Fire performance evaluation is becoming an important part of the overall structural performance assessment and is assuming an increasing role within international standards. Simple approaches, based on single section performance, fail often to give a realistic description of the phenomenon especially for statically indeterminate structures, where thermo-mechanical coupling and non linear dependency of stress, moduli and expansion need to be accounted for. In the present work, the concrete tunnel section is discretized using longitudinal and shear truss fibers, together with stiff coupling elements where a transient heat fire analysis takes place. A single model can be thus employed to perform a full quasi static one-way-coupled thermo-mechanical analysis, where first temperatures and then internal forces are computed. Due to the number of temperature dependent constitutive relationships adopted within a single model, many simplified tuning exercises where investigated to match known results for situations where some of the simulation variables were kept constant. The computation of the internal forces by integration over groups of fibers is obtained by an automatic API procedure. Finally, a case study is presented where the structural performance is evaluated after 120 minutes of fire exposure for the railway tunnel located in Florence and denominated “Imbocco Nord Riffredi – Opera di Scavalco – Tratto E-M” part of the Italian Milano-Napoli Railway.