Full length articlePerformance of concrete-filled steel tubular column-wall structure subjected to ISO-834 standard fire: Experimental study and FEA modelling
Introduction
Concrete-steel composite structures have been widely used in long-span and high-rise buildings owing to their high capacity, ductility and relatively good fire performance [1]. Recent years, more and more applications of concrete-filled steel tubular (CFST) structures can be seen in multi-story residential buildings. Among them, an evolutional type of concrete-steel composite structure called concrete-filled steel tubular column-wall has brought about a lot of benefits in both constructional and economical aspects. Concrete-filled thin-walled steel tubular column-wall is an assembly of several single U-shaped steel tube welded together, with concrete poured inside, functioning as both load-bearing column and shear wall in a structure. A typical floor plan using this kind of composite structure is shown in Fig. 1(a). Fig. 1(b) and (c) show a real project using CFST column-wall in Hang Zhou and a cross-shaped CFST column-wall inside a building. Fig. 2 shows the schematic views of CFST column-walls with different shapes.
Apart from the traditional advantages of composite structures, a significant feature of CFST column-walls is that CFST column-walls can be quite flexible in shapes, so that they can be easily fit into all kinds of designs. This also contributes to the pre-fabrication process of the entire building, which brings about many benefits such as short construction period, relatively low cost and little pollution to the environment. Furthermore, unlike traditional circular or rectangular columns, which may take up more clear space when used at corners or in the middle of a room, CFST column-walls can make full use of the room space, thus bring about lots of convenience to both designers and residents.
However, the lack of systematic and proper design approaches for this kind of structure restricts its application in different areas, especially when fire design is a concern. Studies of the temperature distribution and mechanical behavior in CFST columns have been carried out by previous researchers e.g. [3], [4], [5], and the research on the fire behaviors of walls was mainly focused on steel or concrete wall systems e.g. [6], [7], [8]. The studies on steel-concrete composite walls under fire are rare [9]. Although CFST column-wall is also one kind of CFST structure, things may be different for this case in that, functioning as both load bearing column and shear wall. When investigating the fire performance of CFST column-wall, not only structural stabilization should be taken into consideration, but also thermal insulation and physical integrity. The temperature transition path, concrete-steel interface property and failure modes can be complicated and different from those in traditional columns or walls because of a high aspect (D/B) ratio and a unique steel rib system. This paper fills the knowledge gap in the fire behavior of CFST column-wall.
In the current program, a set of experiment on the fire performance of eight full-scale CFST column-walls was conducted. The specimens were exposed to both axial loading and ISO-834 standard fire with different insulation scenarios. A FEA model was developed to predict the temperature field distribution and structural behavior of CFST column-wall, followed by a sensitivity study taking into consideration the effect of water vapor, thermal resistance, and the spalling of the insulation layer. Interface property between concrete and steel, as well as the confinement factor were also discussed. The FEA model was verified against the test data and proved to have reasonable accuracy in predicting the fire behavior of CFST column-wall.
Section snippets
Experimental program
The basic parameters of the specimens are based on a 90-meter tall, 33-storey residential building constructed by Hangxiao Steel Structure Corporation in China. Considering the complexity of T-shaped, L-shaped, cross-shaped and other shaped CFST column-walls, the most elementary chain-shaped one is chosen as the typical cross-sectional type of the specimens. All specimens were subjected to axial compressive load with both spherical hinges on each end of the furnace, two long sides of the
Temperature
Fig. 6 shows the temperature (T) versus time (t) curves captured by the four thermocouples inside each specimen. The curves for the furnace air temperature are also given in each figure for comparison.
Among all the measure points, measure points 1 and 2 were meant to record the temperature rise of the outer and inner surface of the steel tube respectively. As can be seen in the test results, the temperature difference between these two measure points can be as high as 400 °C, especially for
FEA modelling and discussion
This paper referred to the modelling methods adopted by previous researchers [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] to investigate the temperature distribution and mechanical behavior of traditional CFST columns, since CFST column-walls can be treated as several rectangular CFST columns welded together or a rectangular CFST column with ribs. A 3D finite element model with sequentially coupled thermal-stress analysis procedure in the ABAUQS software was adopted for thermal
Thermal properties for steel and concrete
Lie [14], [15], [16] proposed a set of formulas for the thermal capacity ρccc (ρscs) and conductivity kc (ks) of concrete and steel under elevated temperatures, which had been used by Han [3], Yang [4], [17], and Song [18], [19], [20] to reasonably predict the temperatures in CFST columns under both uniform and non-uniform fire. Additionally, Han [12] suggested a way to consider the water evaporation effect of the inner concrete by assuming that all water would vaporize at 100 °C. Corresponding
Conclusions
Based on the limited experimental testing and finite element analysis, the following conclusions can be drawn:
- (1)
The temperature distribution in CFST column-wall is non-uniform when exposed to fire with two sides. The temperature fields within every U-shaped steel are very much the same with the steel ribs serving as thermal bridges inside the structure, transforming heat directly from the fire-exposed outer steel tube. Vent holes are essential in CFST members, which help evacuate the water vapor
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