Fire performance of concrete filled steel tubular (CFST) column to RC beam joints

Abstract

The performance of concrete filled steel tubular—(CFST) column to reinforced concrete (RC) beam joints under fire is investigated in this paper. A three-dimensional finite element analysis (FEA) model is developed for sequentially coupled heat transfer and structural analysis. Four tests on circular CFST column to RC beam joints subjected to the ISO 834 standard fire performed earlier by the authors' research group are reviewed and used to verify the model. The FEA model is then used to construct the model of a typical full-scale CFST column to RC beam joint and perform analysis to the behavior of this joint in fire with respect to effects of parameters such as beam load ratio, column load ratio and beam to column linear stiffness ratio. Failure criteria in the International Standard ISO 834-1 are adopted to investigate the fire resistance and the failure modes. Extensive studies are also performed to the internal force redistribution, the joint stiffness degradation and typical failure modes in the joint. The results indicate that three typical failure modes of beam failure, column failure, and simultaneous beam and column failure should be considered in the design of composite joints under fire loading.

Introduction

Composite constructions with the combination of concrete filled steel tube (CFST) columns and reinforced concrete (RC) beam and slab system are widely used in various types of constructions such as high-rise buildings and underground public buildings, etc. A typical example of a composite joint using CFST columns and RC beams with the enlarged concrete cover in the joint zone is shown in Fig. 1. In the joint zone, the corner longitudinal bars of RC beam are designed to enclose the column but not to pass through it while the other longitudinal bars are cut down and welded with an I-shaped bracket [1].

The fire resistances of typical members such as CFST columns and RC beams of these buildings have been investigated before. Extensive researches have been previously performed on CFST columns under fire conditions, such as fire resistance tests, numerical models and different simplified methods for the fire performance of CFST columns. Detailed reviews of those are presented elsewhere [2], [3].

RC construction is a traditional structural system that has been frequently used in engineering structures, and for which a large amount of study has been conducted for its fire resistance. Experimental and analytical studies can be found from references [4], [5], [6], [7].

From the literature reviews [2], [3], [4], [5], [6], [7], it can be seen that a fairly good understanding of individual members has been obtained for the CFST columns and RC beams. Various design tools are also readily available for them. A limited amount of work has also been conducted on the fire performance of CFST columns and RC beams within a structural frame. Ding and Wang [8] carried out 10 tests on the structural fire behavior of steel beams to CFST column assemblies with different types of joints subjected to the ISO 834 standard fire [9]. Han et al. [10] carried out six tests on CFST column to RC beam planar frames exposed to the ISO 834 standard fire [9]. Each composite frame consists of two CFST columns and a RC beam with RC slab.

However, as the critical element that connects the beam and the column, the CFST column to RC beam joints have seldom been researched. Set against this background, the fire research group in Tsinghua University was recently engaged in a research project to investigate the fire performance of the CFST column to RC beam joints under fire. Four tests on this type of joints, subjected to the ISO 834 standard fire [9], were carried out in 2007 and reported in [11]. The tests were meant to approximately reproduce the scenario of a joint above a column inside a fire compartment in multi-storey composite buildings so that all the space below the beam was engulfed in fire, as shown in Fig. 2(a), where P is the distributive load applied on the beam. A schematic view of the moment distribution of the composite frame at room temperature was shown in Fig. 2(b). In order to investigate the fire performance of the composite joint under fire, a simplified test module was isolated from the global structure, as shown in Fig. 2(c), where the beams were cut at the location of contraflexure so that the beams can be treated as cantilever in the test and the distributive load was set to an equivalent concentrated load P_F at the far end. N_F was applied to the top of the column to consider the loads coming from the upper stories. As the tests cannot reproduce the real behavior of the joint within a structure, the tested frames were small-scaled and its boundary conditions were revised as well as the loading patterns to fit to the testing equipments. Finite element analysis (FEA) modeling techniques are used to assist the investigation of the fire behavior of the composite joints following the test. In the FEA study, full-scale joints are used and the beams are extended to their mid-span so that the entire behavior of the beams can be captured. The corresponding sub-structure with its loading patterns and boundary conditions is shown in Fig. 2(d).

To qualify the FEA study technique, FEA models are built and verified against individual fire tests on RC beams, CFST columns, and lastly the composite joint tests. Then, a full-size joint that is more representative of real constructions is designed and FEA model is also built for it. Several primary parameters such as beam to column linear stiffness ratio, beam load ratio and column load ratio, are varied to investigate their influence on the fire resistance and failure mechanism of the composite joints under fire.