CAMBRIDGE INSTITUTE OF TECNOLOGY TATISILWAI , RANCHI.pptx

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1. CAMBRIDGE INSTITUTE OF TECNOLOGY TATISILWAI , RANCHI GUIDE D BY :- PROF. VIMAL KUMAR PROF. BINOD KUMAR MAHTO CIT , RANCHI PERFORMED BY : - JYOTI KUMARI REG. NO - 22050730205 EFFECT OF THERMAL LOADING ON CFST ELEMENT USING ABAQUS

2. OUTLINE - INTRODUCTION OBJECTIVE LITERATURE REVIEW RESEARCH WORK BY SU-HEE PARK, KYUNG-SOO CHUNG AND SUNG-MO CHOI WORK DONE SO FAR FUTURE WORK REFERENCES

3. INTRODUCTION Concrete filled steel tube (CFST) members utilize the advantages of both steel and concrete. They comprise of a steel hollow section of circular or rectangular shape filled with plain or reinforced concrete. Steel members have high tensile strength and ductility, while concrete members have high compressive strength and stiffness, so composite members utilise the behaviour of both steel and concrete to prevent local buckling of steel, thus increases the compressive strength of the column . They are widely used in high-rise and multi storey buildings as columns and beam-columns, and as beams in low-rise industrial buildings because of high load carrying capacity and reduced size width

4. The Eurocode 4 and ABNT NBR show simple methods for the design of the composite columns under fire situations. The temperature distribution and was determined by simulations using the computer package ABAQUS .

5. Thermal behavior of typical CFT columns During the initial stages of fire exposure, the steel tube expands faster than the concrete core, in such a way that the steel section carries higher load than the concrete core.This happen due to the high thermal conductivity of steel w.r.t concrete. The temperature rise in concrete is very slow and after a period of around 20-30 minutes the strength of steel starts to decrease due to its high temperature .

6. OBJECTIVE To develop analytical models and using the 3D finite element method implemented in the software ABAQUS . Determine the fundamental F-d-T behaviour of CFT members under standard thermal loading . To evaluate the effects of various material, geometric, and loading parameters on the fundamental F-d-T behavior of CFT members. The parameters included will be the concrete strength, the steel tube b/t ratio, the axial load level, and the fire protection thickness . To develop analytical models that can be used to: ( i ) predict the fundamental F- d- T behavior of CFT members, (ii) to investigate the behavior and stability of CFT columns under realistic fire loading.

7. LITERATURE REVIEW Research work by Su- Hee Park, Kyung-Soo Chung and Sung-Mo Choi Objective : A Study on Failure Prediction and Design Equation of Concrete Filled Square Steel Tube Columns under Fire Condition. The temperatures in square CFT columns exposed to fire on four sides could was established by using a two dimensional non linear thermal analyzer for structural cross-sections subjected to fire environment with the assumption that no heat was flowing along the longitudinal axis. The two-dimensional heat flow problem was modeled mathematically by the Fourier law of heat transfer.

8. 1 . A statement of the heat balance equation is given as follows 2.Based on parametric studies , the design equation for fire resistance time in the square CFT Dc : Width of in-filled concrete in mm N: Applied axial load in kN fck : Specified 28 day concrete strength in MPa Ac: Cross-sectional area in mm2 fckAc (= Nc ): Compressive resistance of concrete core of the column in kN where a = the material diffusivity, c = the material specific heat, ρ = the, and λ = material thermal conductivity

9. 4.Structural behavior of CFST element under temperature loading We have to take help from Design Equations on CFT columns exposed to temperature loading or fire . Design equations from Eurocode (1994-1-2) 4.1Thermal properties of concrete at elevated temperatures 4.1.1.Thermal elongation Siliceous aggregates:

10. 4.1.2 Specific heat 4.1.3 Thermal conductivity

11. Stress-Strain of concrete at elevated temperature( Eurocode ) Plastic stress strain of strength 40 MPa at elevated temperature

12. 4.2Thermal properties of Carbon Steel at elevated temperatures 4.2.1Thermal elongation

13. 4.2.2 Specific heat

14. 4.2.3 Thermal conductivity

15. Stress strain curve for steel at elevated temperatures ( Eurocode )

16. Yield stress (293MPa) of carbon steel at increased temperature 4.3Thermal properties of stainless steel at elevated temperature Thermal elongation (∆l/l) = ( 16 + 4.79 * 10 -3  a - 1.243 * 10 -6  a 2 ) * ( a - 20 ) * 10 -6 Where: l is the lenth at 20˚C Δl is the temperature induced expansion  a is the steel temperature.

17. Specific heat Ca = 450 + 0.280 *  a - 2.91 * 10 -4  a 2 + 1.34 * 10 -7  a 3 J/ KgK Thermal conductivity  a = 14.6 + 1.27 * 10 -2  a W/ mK Plastic Stress strain curve for stainless steel at elevated temperatures ( Eurocode )

18. S tress-strain curve of stainless steel of strength (293 MPa )

19. 5.Characteristics of the Numerical Model A three-dimensional numerical model for simulating the fire behavior of CFT column was developed. The main parameters of the model were the length (L), steel tube thickness (t), external diameter (D), end conditions, and the material properties. 5.1 Scheme of analysis condition Circular column loading

20. 5.2Analytical approach A three-step sequentially coupled analytical approach was used to predict the thermal and structural behavior of CFT columns subjected to standard fire tests. The approach consists of three sequential analysis of steps. S tep 1- Axial Loading Before application of thermal loading initially column specimen must have some compression loading . Step 2- Thermal load The thermal load ( i.e steel temperature) was given at the outer surface of the steel tube which follows ISO 834 standard fire curve

21. Step 3- Heat transfer analysis The surface T-t curves determined in the step 2 were used as the thermal loading for simulating the heat transfer through the CFT cross-section and along the height. The couple thermal displacement analysis was conducted using ABAQUS. The material thermal properties required for conducting the heat transfer analysis include thermal conductivity, specific heat for both the steel and concrete which are temperature dependent. The thermal material properties were taken from Eurocode (1994-1-2). Stress analysis In the first step axial load was applied at the top. During this step the column undergoes some deformation. In the second step, the heat (T-t curves) was introduced into the model while the axial load is maintained. The heating period is kept equal to the couple thermal displacement analysis period. During the analysis the strength of the column decreases gradually and reaches a point where it cannot sustain any load. The time required to reach such condition where column fails by compression or buckling is called as the Fire Resistance time

22. 6.Finite Element Modelling For couple thermal displacement analysis, conduction ,convection and radiation part of the heat transfer was modeled. Steel tube and concrete was modeled using couple thermal displacement elements C3D8T an 8-node thermally coupled brick trilinear displacement and temperature. The interface between the concrete and steel parts was modeled as a hard contact with friction factor of 0.25 . 7 .Validation of Numerical Models Validation for the columns exposed to thermal load Specimen no Sectional dimention(mm) Boundary condition fy (MPa) Es (*10 5 ) ( MPa ) Fck (MPa) Test Load (KN) R Test (min) R predicted (min) CS01 200*1830*3 F-F 292.9 2.01 40.3 500 200 178 CS02 200*1830*3 F-F 292.9 2.01 40.3 770 120 90

23. In the first stage of fire exposure, the stainless steel hollow section has higher expansion than concrete due to direct exposure to fire and larger thermal expansion coefficient. As a result, the stainless steel hollow section undertakes most of the load during this stage , which results in early yielding of the stainless steel and tension developed in the inner part of concrete . Temperature distribution in Column specimen CS02 Temperature distribution in Column specimen CS01

24. The yielding of the stainless steel have led to large axial deformation but core concrete prevents such deformation by carrying the load transferred from the stainless steel hollow section and by providing lateral support for the tube to maintain its bearing capacity to some extent after local buckling occurs. The stainless steel deteriorates gradually under elevated temperatures, the core concrete bears most of the load. Due to the non-uniform temperature distribution in the core concrete, the outer part of the concrete has higher temperature and compressive stress, which is therefore crushed first .

25. The buckling of stainless steel hollow section, weakens the confinement on the inner part of concrete, leading to a decrease in the bearing capacity of the inner part of the core concrete. Finally, the column fails when the concrete cannot bear the axial load including the part transferred from the stainless steel hollow section any more. Comparison of measured and predicted axial displacement of column CS01 Comparison of measured and predicted axial displacement of column CS02

26. 8. Parametric studies Parametric studies are performed by using the FE model to investigate the influence of various factors on the fire resistance period (R) of CFSST columns (1) Steel Type: Carbon and Stainless steel. (2) Load Level (4) Concrete strength . 8 .1 . Carbon steel and Stainless steel Specimen no Sectional dimention(mm) Boundary condition fy ( MPa ) Es (*10 5 ) ( MPa ) Fck ( MPa ) Test Load (KN) Steel type R predicted (min) CS03 200*1830*3 F-F 292.9 2.01 40.3 500 Stainless 178 CS04 200*1830*3 F-F 292.9 2.01 40.3 500 Carbon 123

27. U nder the same fire condition, the strength and stiffness of carbon steel decrease much faster than those of stainless steel in the later stage of fire exposure. T he load transfer from the steel hollow section to the concrete is much faster in the CFST column than that in the CFSST column after steel yields. T he higher residual strength of stainless steel make the load transfer occurred more smoothly. D ifference in thermal properties between different steel the temperature distribution of the core concrete rise faster than those in CFSST, which may also contribute to a lower fire resistance for the CFST. Temperature distribution in column CS04 Temperature distribution in column CS03

28. 8.2 Load level Load level is another key factor affecting the fire endurance of normal CFST columns . When load increases from 500 KN to 1000 KN i.e twice , R decreases by around thrice . 8 .3 Concrete strength It can be seen that, the concrete strength has a moderate influence on fire resistance period. (R) when the sectional dimension of columns is relatively small .

29. Summary The failure of the column takes place as soon as the steel temperature reaches a particular value, regardless of the applied fire intensity. Due to this temperature variation in the radial direction, CFT columns are found to be subjected to the hoop stresses, which are not observed in typical steel columns at elevated temperatures. The friction coefficient between the steel and concrete surface doesn’t have any influence on the column fire behavior The predicted column fire resistance are found to be less than the Experimental column capacities. Stainless steel has more fire resistance capacity as compared to carbon steel Strength of concrete in small dimension CFST has neglible or less effect.

30. Conclusions FE Model shows that the fire resistance of CFSST columns is influenced by load level. Analytical results show that the core concrete bears most of the axial load in the later stage of fire exposure. Due to the differences in thermal and mechanical properties between stainless steel and carbon steel, CFST columns and CFSST columns have different fire performance under the same fire condition. Generally, a CFSST column has higher fire resistance when compared with its CFST column.

31. REFERENCE ABAQUS. ABAQUS/Standard version 6.14 user’s manual: Volumes I-III. Pawtucket, Rhode Island: Hibbit , Karlsson & Sorenson, Inc.; 2005. EN 1993–1-2:2005. Design of steel structures-Part 1–2: General rules structural fire design, European committee for standardization, Brussels; 2005. EN 1991–1-2:2002. Actions on structures-Part 1–2: General rules-Actions on structures exposed to fire. European committee for standardization, Brussels; 2002. EN 1992–1-2:2004. Design of concrete structures-Part 1–2: General rules structural fire design. European committee for standardization, Brussels; 2004. ISO 834–1:1999. Fire-resistance tests-elements of building construction-Part General requirements. International organization for standardization, Geneva; 1999. ISO 834, Fire-resistance Tests—Elements of Building Construction—Part 1: General Requirements, ISO 834-1, International Organization for Standardization, Geneva, Switzerland, 1999 .

32. Han LH. Fire performance of concrete filled steel tubular beam-columns, Journal of Constructional Steel Research 2001:57:695-709. Z. Tao et al. / Journal of Constructional Steel Research 118 (2016) 120–134. L.-H. Han et al. / Engineering Structures 56 (2013) 165–181

33. Thank You