Fire behaviour of self-consolidating concrete-filled double skin steel tubular columns

2017-05-31T06:09:29Z (GMT) by Lu, Hui
The concrete-filled double skin tubular column (CFDST) is an innovative type of steel-concrete element which has potential for use as piers in bridges and columns in buildings. The use of self-compacting concrete (SCC) in CFDST is seen as a solution to resolve the challenge of concrete compaction in the columns. The behaviour of CFDST under static and cyclic loading at ambient temperature has been extensively studied. The fire performance of CFDST is crucial due to the direct fire exposure of the outer steel tube. Nevertheless, there is at present little information about the fire performance of CFDST. This thesis reports on research into the fire performance of SCC-filled CFDST columns. The research aims to understand the fundamental behaviour of SCC-filled CFDST columns under standard fire exposure by means of standard fire tests and finite element modelling and to develop guidance for fire resistance design of the columns. Given the complexity in the responses of SCC-filled CFDST in fires, SCC-filled steel tubular stub columns (CFST), a specific type of CFDST without an inner steel tube, were selected for study in the first phase of the research program because of their simple configuration. Six SCC-filled CFST stub columns were prepared for standard fire tests and a finite element model to simulate the thermal and structural responses was developed. The main purpose of this phase of research was to develop a methodology for the further study of fire behaviour of CDFST columns by investigating how SCC affects the fire behaviour of composite columns and how steel tube affords confinement of the SCC to alter its behaviour at elevated temperature. A series of standard fire tests for CFDST columns was conducted in the second phase of the research. Six CFDST columns and sixteen CFDST stub columns were prepared for standard fire tests. A further two CFDST stub columns were prepared as reference specimens to test the axial capacity at ambient temperature. The columns and stub columns in the tests represent slender and stub columns in authentic engineering practice in which the failure modes are buckling and compression failure respectively. Data obtained from the fire tests, i.e. temperatures, axial deformation, failure modes and fire resistance, were used to investigate the fire behaviour of the columns. Interaction among the three components in the columns during fire exposure was clearly shown in the tests. Such interaction is beneficial to the fire performance of CFDST columns. Confinement of the tubes on the SCC prevents its spalling. Components which help each other through a load transfer mechanism are typical of the composite interaction in CFDST columns. Methods to enhance the fire resistance performance of CFDST, i.e. fire protection and steel fibre reinforced concrete, were also validated in the tests. In addition, the fire tests accumulated data to verify the numerical model in the subsequent research. A finite element model was developed and used to study in detail the fire behaviour of CFDST columns and for the parametric study in the third phase of the research. An advanced finite element model which accounts for material and geometric non-linearity and the interaction of tubes and concrete was proposed to simulate the thermal and structural responses of CFDST columns under standard fire exposure. A concrete material model for confined concrete in CFDST at elevated temperature was developed accordingly. The model was then used to analyse the fire behaviour of the columns in terms of stress, strain and load share in components which cannot be obtained directly from fire tests. Yield of the inner steel tube was found to be a major cause of the final failure of the columns. The influence of a number of parameters on the fire resistance performance of the columns was investigated. The parameters which have significant influence on the fire resistance performance of the columns were identified. Finally, guidelines for the fire resistance design of CFDST columns are proposed. These guidelines deal with how to appropriately select parameters for CFDST columns to ensure the columns achieve the anticipated fire resistance. Several practical design tables are also presented to illustrate how to use the guidelines to design CFDST columns to achieve a certain level of fire resistance.