Design of Crossrail’s Precast Tunnel Linings for Fire

Summary

This paper describes the approach taken to design Crossrail's precast concrete running tunnel linings for structural stability during a severe fire incident. In recent decades there have been a number of fire incidents in road and rail tunnels which have caused severe damage to the lining structures. These incidents have demonstrated that fires in tunnels can heat up much more rapidly than assumed by the standard cellulose design fire curve and that concrete in tunnel linings is prone to explosive spalling. As these issues are not as prevalent for the design of other types of structures they are not (yet) explicitly covered by prescriptive design rules in design codes and standards. The approach taken for the design of Crossrail's precast concrete tunnel linings involved an initial design phase assessment followed by pre-construction phase verification testing. The design phase assessment used thermal material properties obtained from historic testing of the Channel Tunnel Rail Link (CTRL) precast lining segments. These properties were used to model the temperature distribution in the Crossrail lining segments when exposed to a RABT-ZTV fire design curve. A simplified calculation method was then used to estimate the capacity of a reduced structural cross-section, which had been estimated from the temperature distribution and included a nominal allowance for spalling. This was compared against an accidental loading case. Prior to construction, prototype Crossrail lining segments were subjected to full scale fire testing under load. Crossrail's main running tunnels are split into three separate construction contracts, each of which is independently manufacturing precast concrete segments using different concrete mix designs. Therefore three separate test programmes were undertaken. The results from these test programmes are compared with the design phase predictions and show good agreement.

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