School of Civil Engineering, Shenyang Jianzhu University, Shenyang , China.

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Preliminary Analysis of Concrete Filled Steel Tube Reinforced Concrete Columns under Axial Compression after Exposure to Fire Wanqing Yu, Qingxin Ren*, Lianguang Jia School of Civil Engineering, Shenyang Jianzhu University, Shenyang , China Key words: load-bearing Capacity; concrete filled steel tube reinforced concrete column; finite element analysis; axial compression; after fire Abstract. In this paper, a further research has been carried on mechanical properties of concrete filled steel tube reinforced concrete columns after exposure to fire. A finite element analysis (FEA) model for concrete filled steel tube reinforced concrete columns after exposure to fire under axial compression is developed by ABAQUS. The temperature of cross-section element after exposure to fire has been obtained. The FEA model of temperature field is then used to investigate the mechanism of such composite columns further. Influences of parameters on Load-bearing Capacity such as fire duration time and steel ratio were analyzed. The work in this paper provides a basis for further theoretical study on concrete filled steel tube reinforced concrete columns after exposure to fire. Introduction In china, Concrete filled steel tube reinforced concrete (CFSTRC) column has become an innovative type of composite column and it has been extensively used in some high-rise buildings recently. In the past, Han et al. (2009) [1] carried out a series of test on CFSTRC columns under cyclic loading. However, there is still a lack of information on the composite members under axial compression after exposure to fire. It indicates a need for further research in this area. A theoretical model that calculates axial compression ultimate strength of the column after exposure to fire is described in this paper and influences of the changing parameters of the column on the axial compression performance are analyzed. Finite element modeling 2.1 Temperature field The column temperatures are calculated using the finite element method. The columns are heated followed ISO-834 standard fire curve. The method for deriving the heat transfer equations and calculating temperature, taking account of thermal properties is described in detail in reference [2]. The temperature of an element in cross-section is assumed to be equal to the temperature at its center. 2.2 Stress-strain relations of the steel and concrete after exposure to fire (1) Concrete The stress-strain relation for confined concrete at ambient condition is shown in the (Han et al.2001) [3] and the residual compressive strength of concrete heated to a maximum temperature T and having cooled down to the ambient temperature of 20 may be found (Song et al.) [4]. Fracture energy versus displacement cross crack relation is used to describe the tensile behavior of concrete. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,23:06:02)

2 Applied Mechanics and Materials Vols (2) Steel A typical stress strain curve for steel can consist of two stages. Detailed expressions are given in Han et al. (2002) [5]. Elastic modulus (E s ) and Poisson s ratio for steel after exposure to fire are taken as (N/mm 2 ) and 0.3, respectively. 2.3 Finite element model analyses The calculation of the residual strength of the column involves the calculation of the highest element temperature during exposure to fire and the strength after fire to which the column is exposed. Fig. 1 shows the model of the column after exposure to fire where L is the effective length of the column. L Y Z O X Fig. 1 Model of the column A numerical model was presented out for the analysis of the residual strength of the CFSTRC after exposure to ISO-834 fire standard. The steel tube of CFSTRC column is modeled by reduced-integration shell element (S4R), and the concrete core, outer concrete as well as the end plates, is modeled by 8-node brick elements (C3D8R). The uniform loading in the Y direction is applied to the top surface of the end plate. The end plate connects with the steel tube by SHELL TO SOLID (an interface model in ABAQUS), which ensures the displacements and rotational angles of the contact elements keep the same in the whole loading process. The tie relation is selected for the end plate and the concrete as well as inner concrete, outer concrete and steel. 2.4 Temperature of Cross-section Element after Exposure to Fire As the time goes, the temperature of elements across the model section is on the rise and the farther the distance by the side exposure to fire the lower the temperature of elements also the slower the rising of temperature. Temperature versus time curves is shown in Fig. 2. D is the distance by the face exposure to fire. With the increasing of the steel rate, the temperature of the inner concrete becomes lower while the concrete out of the steel tube becomes higher. Because of distance between steel tube and the fire face at the rate of 0.25 is nearer than that of 0.16, so the temperature of steel tube is higher as the steel rate rises. It is mainly due to the steel tube well stop the heat transfer from outer concrete and making a good protection for the inner concrete, so lots of heat storage in the outer concrete contribute to the higher temperature of the outer concrete and the lower temperature of the inner concrete moderately with the steel tube ratio (a s ) increased from 0.16 to Analysis of the load strain relation The basic parameters used in the calculations are: D=200mm, d=80mm, t=3mm, H=2000mm, f cu,in =60MPa, f cu,out =40MPa, f sy,stl =345MPa, f sy,stp =335MPa, T=0min, 60min and 120min. Where, D, H and T are the diameter, height and time exposure to fire, respectively; t and f sy are the wall

3 774 Advances in Civil Structures IV thickness and yield strength of the steel tube, respectively; and f cu is the compression strength of concrete. The calculated curve of axial load versus longitudinal strain is shown in Fig. 3. T( ) α=0.16 α=0.25 D=20mm steel tube D=100mm Time(min) Fig. 2 Temperature versus time curve min 1500 N(kN) min min ε(µε) Fig. 3 Calculated curve of axial load versus longitudinal strain It is clear from Fig. 3 that structural response of the CFSTRC stub column after exposure to fire under axial load can be generally divided into three obvious stages: elastic, plastic and descending stage. The CFSTRC columns after exposure to fire of different times came into plastic stage nearly in the same time, whereas in the elastic stage, the load of the CFSTRC column without firing increases sharper than the others. In the plastic stage, the longer time it exposure to fire the faster axial deformation of CFSTRC column accelerates, meanwhile has a smooth descending stage relatively. 2.7 Stress distribution of the steel tube, concrete and stirrup The distributions of stress at the ultimate load for every part instances are shown in Fig. 4. It can be found that with the increasing of time at which the columns exposure to fire, the maximal Misses stress values of the steel tube, concrete and stirrup are becoming smaller and smaller when the ultimate strength of the CFSTRC is reached. It is also seen from Fig. 4 that, the column overall buckling to one side gradually at ultimate strength when the fire time is increased and the stress at buckling side is higher than the other side. Conclusions Based on the analytical results of this study, the following conclusion can be drawn. (1) Fire exposure increases the deflections and decreases the strength of CTSTRC columns. The load-bearing capacity decreased 41% and 56% after exposure to fire of 60min and 120 min, respectively.

4 Applied Mechanics and Materials Vols (2) Core concrete has been well protected by steel tube and the ultimate strength of CFSTRC has increased 28% and 40% when the steel tube ratio increased to 0.25 after exposure to fire of 60min and 120min, respectively. Outer concrete Inner concrete Steel tube Stirrup 0 min 60 min 120min Fig. 4 Stress contours of columns at ultimate load after exposure to different fire times Acknowledgments The research reported in the paper is part of Project supported by the National Natural Science Foundation of China (NSFC) and the Science and Technology Project of the Shenyang Urban and Rural Construction Committee of China. The financial support is highly appreciated. References [1] Linhai Han, Feiyu Liao, Zhong Tao, Zhe Hong. Performance of concrete filled steel tube reinforced concrete columns subjected to cyclic bending. Journal of Constructional Steel Research, 65(2009) [2] Linhai Han. Fire resistance of concrete filled steel tubular columns. Advance in Structural Engineering, 2(1998) [3] Linhai Han. Fire performance of concrete filled steel tubular beam-columns. Journal of Constructional Steel Research, 57(2001) [4] Tianyi Song, Linhai Han, Hongxia Yu. Concrete filled steel tube stub columns under combined temperature and loading. Journal of Constructional Steel Research, 66(2010) [5] Linhai Han, Hua Yang, Shuliang Cheng. Residual strength of concrete filled RHS stub columns after exposure to high temperatures. Advance in Structural Engineering, 5(2002)

5 Advances in Civil Structures IV / Preliminary Analysis of Concrete Filled Steel Tube Reinforced Concrete Columns under Axial Compression after Exposure to Fire /