ABSTRACT
Tunnel structural collars are complex structures often connecting different tunnel elements such as mined tunnels with bored tunnels or shaft openings. The Bromford Tunnel is a twin bored tunnel part of the High Speed 2-Phase 1 project in the UK where over 30 permanent reinforced concrete collars need to be cast. This paper presents the design methodology followed for the assessment of the bored tunnel to cross passage collars under the EUREKA curve fire load.The main innovation introduced in this assessment is the use of a non-linear staged analysis considering a Mohr-Coulomb concrete material model. This is used to represent the plastic behaviour and captures both the flexural and shear failure of the structure. Such an analysis proved to better represent the fire event and the behaviour and stress redistribution on the structure during the fire. This methodology was able to allow an optimised reinforcement intent design where the full structural capacity of the structure was used.
