The effects of protected beams and their connections on the fire resistance of composite buildings

• Model a large scale, 45 m×45 m, composite frame under different fire conditions.
• Systematically investigate the impact of the connections for protected beams on the tensile membrane actions of supported floor slabs.
• Understand the influence of the vertical deflections of protected beams on the tensile membrane action of the floor slabs.
• Analyse the effects of different reinforcement details of the floor slabs on the performance of composite floor under different fire conditions.

According to full-scale fire tests, it is noticed that tensile membrane action within the concrete floor slabs plays an important role in affecting the fire resistance of composite buildings. It is well known that the development of tensile membrane actions relies on the vertical support along the edges of the slab panel. However, there is at present a lack of research into the influence of vertical supports on the tensile membrane actions of the floor slabs. In this paper, the performances of a generic three dimensional 45 m×45 m composite floor subjected to ISO834 Fire and Natural Fire are investigated. Different vertical support conditions and three steel meshes are applied in order to assess the impact of vertical supports on tensile membrane action of floor slabs. Unlike other existing large scale modelling which assumes the connections behave as pinned or rigid for simplicity, two robust 2-node connection element models developed by the authors are used to model the behaviour of end-plate and partial end-plate connections of composite structures under fire conditions. The impact of connections on the 3D behaviour of composite floor is taken into consideration. The load-transfer mechanisms of composite floor when connections fail due to axial tension, vertical shear and bending are investigated. Based on the results obtained, some design recommendations are proposed to enhance the fire resistance of composite buildings.