Train fire spalling risk assessment

Structural fire protection is provided to structures where necesssary to ensure the resilience of the structure in the event of a fire. Loss of material strength at high temperature and spalling of concrete can lead to structural damage and collapse if not adequately mitigated. However, provision of structural fire protection can add significant cost and program time and is frequently over-specifed because location-specific requirements cannot be assessed. This paper describes a methodology to determine appropriate design guidance for element structural fire protection.

The methodology includes analysis of the gas phase heat flux to structures from a train fire using different modelling approaches. The predicted gas phase heat flux is used to assess the risk of explosive spalling within individual structural elements, based on 1D modelling of conduction within concrete combined with a literature review of the key factors that influence spalling.

The detailed analysis presented in this paper is for Victoria Road Crossover Box (VRCB). This fire risk assessment is also used to develop line-wide guidance for all open structures and for all other open infrastructure assets in the HS2 Phase One Lots S1 and S2 contracts which cover the Euston Tunnels and Approaches, and the Northolt Tunnels.

The purpose of the fire risk assessment is to avoid over-design by providing appropriate temperature criteria to design structural fire resistance. This guidance also provides information to avoid the provision of measures such as synthetic polypropylene (PP) fibres where not required. It is particularly beneficial where PP fibres cannot easily be included, such as in the concrete diaphragm walls of VRCB, avoiding the need for alternative costly measures.