A series of analytical studies were conducted to investigate the behaviour of masonry walls supporting a reinforced concrete slab under a number of fire scenarios. The main purpose is to use various types of sub-assembly in predicting the structural behaviour in fire. The analysis of sub-frames rather than complete structures is attractive since they have the potential to reduce computation time dramatically, which could be of significance if large numbers of results need to be generated for design documents. The analyses are all performed using a finite element program, called MasSET (Masonry Subject to Elevated Temperatures), whose most recent development has been the capability to take into account the interface element between the masonry wall and the slab. Separation, sliding, and opening and closing of initial gaps at the interface between the masonry wall and the reinforced concrete slab are accounted for by adjusting the properties of interface elements.