The structural behaviour of the composite slim floor frame as a whole in fire conditions has been investigated. Both the deformation behaviour of the structural members and the mechanical interaction between the members were studied. The additional lateral deformation of the side-column caused by the thermal expansion and the catenary action in the beam in the different fire phase was highlighted. The moment variation in the head of the columns during fire and the variation of the axial force in the heated beam were also investigated. A comparison between the deformation behaviour of the heated beam in the plane frame and the spatial frame indicated the excellent effects of the composite floor slab on the stability of the frame structures in fire.
When subjected to fire, the steel and composite structures will lose their loading capacity and stiffness. To ensure the safety of the life and properties of the public, the indispensable fire resistance of the building is required by the authority [1]. Traditionally, the fire resistance ability of the structural members was tested using the isolated element heated by the ISO standard fire. In this methodology, the buildings were treated as a series of individual members, and the continuities and interaction between these members were assumed to be negligible. Consequently, most of the structural steel members need to be protected by the insulation materials, such as intumescent paints and fire protection boards, in order to achieve the required fire resistance.
Throughout the 1990s, following the investigation of the fire event in Broadgate (1991, UK), fire tests in William Street (1992, Australia), and full-scale fire tests on a 8-storey composite steel-framed building in Cardington (1995, 1996, UK) 2. and 3., it was found that the structural member in the frame had a significantly better behavior in fire than that in the standard fire resistance test. The standard fire test was very conservative by disregarding the interaction between members. The fire event and tests also highlighted that the current Codes, although conservative, were not addressing the true behavior of building structure in fire, since the building was not acting as a series of individual members 4., 5., 6. and 7..
In recent years, increasing interest has also shown throughout Europe in developing and designing shallow floor systems in steel-framed buildings 8., 9., 10. and 11.. In the shallow floor system, the steel beam is contained within the depth of the pre-cast concrete floor or composite slab with profiled steel decks. Recently, interest has been concentrated on the asymmetric hot-rolled steel in UK [9] and on the asymmetric welded steel beam in Finland 10. and 11..
The analysed steel-framed building in the present study was designed to resemble a part of the typical office or apartment, constructed with the typical composite slim floor frame. The layout of the frame building is shown in Fig. 1. The building has 4 storeys with an overall height of 12 m. There are 5 equally spaced bays along the length of the building. Across the width there are 2 bays spaced 6 m, connected by the slim floor beam. The steel beam was designed as simply-supported, acting compositely with the floor slab. Between rows of the single frame, the tie members are employed to link them together. The applied uniform load on the floor is 2.5 kN/m2, and four equally spaced concentrated loads (110 kN for each) was applied on the beam. The fire region is also shown in Fig. 1.The Finnish asymmetric slim floor beam was employed and the section shape is shown in Fig. 2. The steel column was filled with the autoclaved aerated concrete blocks between the flanges. The width and thickness of the flange are 300 mm and 20 mm, respectively. The height of the column section is 300 mm and the web thickness is 20 mm.The aim of this research was to investigate the structural behaviour of the composite slim floor frame as a whole in fire conditions. The deformation behaviour of the structural members and the mechanical interaction between the members were studied. Both the beneficial and the detrimental effects of the frame continuities on the structural members were explored. The influence of the concrete floor slab on the fire resistance of the whole frame building was also addressed.
The structural behaviour of the composite slim floor frame as a whole in fire conditions has been investigated using numerical methods. Both the plane frames and the three-dimensional frames were analysed. The deformation behaviour and the mechanical interaction between structural members have been studied.
The study has shown that two kinds of restraints in the plane frame have significant influences on the frame behaviour in fire—rotational and axial restraints on the heated beam. The rotational restraints could be efficiently quantified by using the modified load ratio according to Eq. (1). The effect of the axial restraint on the whole frame is embodied by the thermal expansion and the catenary action of the heated beam. Usually, at the earlier heating the thermal expansion takes a leading role. With the increasing fire exposure, the materials deteriorate and the deflection of the heated beam continues to increase, which causes the catenary action to have the leading role. The occurrence of the catenary action depends on the fire duration. As far as natural fires are concerned, the catenary action may not occur if the fire intensity and the fire duration are limited.
The axial force in the heated beam causes the additional moments and lateral deformations in the columns. The columns were pushed by the beam expansion at the earlier heating phase and then pulled due to the increasing catenary action in the later heating phase.
The numerical analysis of the three-dimensional frame also shows that the heated beam has better deformation behaviour than that in the plane frame. Apparently the floor slab contributes to this point. A bigger axial restraint is applied to the heated beam in the three-dimensional frame than in the plane frame, since the floor slab connects the other columns together.
