Abstract
Behaviour of cut-and-cover tunnels exposed to fire should be analysed by using a realistic structural model that takes account of mechanical and thermal effects on the structure. This has been performed with the aid of Finite Element (FE) software package called SOFiSTiK in parallel, for two types of elements as a scope of research project financed by the German Bundesanstalt für Straßenwesen BAST. Since the stiffness of the structure at elevated temperatures is highly affected, a realistic model of structural behaviour of the tunnel could be only achieved by considering the nonlinear analysis of the structure. This has been performed for a 2–cell cut and cover tunnel by taking account of simultaneous reduction of stiffness and strength and the time-dependent increasing indirect effects due to axial constraints and temperature gradients induced by elevated temperatures. The thermal analyses have been performed and the effects were implemented into the structural model by the multi-layered strain model. The stress–strain model proposed by EN 1992-1-2 is implemented for the elevated temperature. Since there was sufficient amount of Polypropylene fibres in the concrete mixtures, modelling of spalling was excluded from the analysis. The critical corresponding stresses and material behaviour are compared and interpreted at different time stages. The main parameters affecting the accuracy and convergence of the results of structural analysis for the used model are identified: defining a realistic fire action, using concrete material model fulfilling the requirements of fire situation in tunnels, defining appropriate time intervals for load implementations. These parameters along with other parameters, which influence the results to a lesser degree, are identified and investigated in this paper.
Highlights
In the event of fire, structural behaviour of road tunnels are affected by imposed thermal effects to high extent and this might, result in failure of the structure
The stress– strain curve proposed by EN 1992-1-2 [1] for concrete exposed to elevated temperatures is for the concrete without PP fibres and the heating rate between 2 to 50 K · min−1 which occurs at the fire growth stage for building structures
To be able to analyse a 2–cell cut-and-cover tunnel exposed to fire, an advanced analysis according to [1] has been performed by taking into account a model with a simplified boundary condition and nominal ZTV-ING Fire Exposure Curve [2]
Summary
In the event of fire, structural behaviour of road tunnels are affected by imposed thermal effects to high extent and this might, result in failure of the structure. For verification of Ultimate Limit States (ULS) criteria, structural calculations are performed for fire as an accidental design situation In this case, the indirect thermal effects result from forces due to restrained thermal elongation or rotation dependent on the temperature distribution in the concrete cross section. The superposition of thermal effects in different time steps with the most unfavourable effects from standard load combinations is performed as so called thermo–mechanical analysis This analysis is based on an iterative nonlinear calculation to determine the internal forces by considering the simultaneous decrease of stiffness of each cross section exposed to elevated temperatures, and considering the indirect thermally– induced effects. To be able to analyse a 2–cell cut-and-cover tunnel exposed to fire, an advanced analysis according to [1] has been performed by taking into account a model with a simplified boundary condition and nominal ZTV-ING Fire Exposure Curve [2]
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