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FUNDAMENTAL BEHAVIOR OF COMPOSITE MEMBERS UNDER FIRE LOADING

FUNDAMENTAL BEHAVIOR OF COMPOSITE MEMBERS UNDER FIRE LOADING. Amit H. Varma Assistant Professor School of Civil Engineering, Purdue University Contributors: Victor Hong, Ph.D. Student at Purdue University Guillermo Cedeno, Ph.D. Student at Purdue University

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FUNDAMENTAL BEHAVIOR OF COMPOSITE MEMBERS UNDER FIRE LOADING

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  1. FUNDAMENTAL BEHAVIOR OF COMPOSITE MEMBERS UNDER FIRE LOADING Amit H. Varma Assistant Professor School of Civil Engineering, Purdue University Contributors: Victor Hong, Ph.D. Student at Purdue University Guillermo Cedeno, Ph.D. Student at Purdue University Jarupat Srisa-Ard, M.S. Student at Michigan St. Univ.

  2. PRESENTATION OUTLINE • Current Knowledge Base and Issues • Research Goals and Objectives • Behavior of Composite CFT Columns Under Fire Loading • Analytical models, investigations, and findings • Need for fundamental measures of behavior under fire loading • Analytical investigations of fundamental behavior • Experimental investigations of fundamental behavior • Conclusions so far • Future research needs and capabilities

  3. CURRENT KNOWLEDGE BASE • Current building codes emphasize prescriptive fire resistant design provisions that are rooted firmly in the standard ASTM E119 fire test of building structure components. • The standard fire test determines the fire resistance rating FRR of structural components for comparative purposes. • It does not provide knowledge or data of the fundamental behavior of structural components that can be used to calibrate analytical models. • This design paradigm has been challenged by several engineers and researchers over the years. • More recently, NIST BFRL researchers have conducted an exhaustive investigation of the 9/11 WTC collapse. They have developed twenty-nine major recommendations for future work.

  4. ISSUES • Three of these recommendations R5, R8, and R9 are extremely important for building design (structural) engineers. • R5 - The technical basis for the ASTM E119 standard fire test should be improved. • R8 - The fire resistance of structures should be enhanced by requiring a performance objective that uncontrolled building fires result in burnout without local or global failure /collapse. • R9.1 – Develop and validate analytical tools, guidelines, and test methods necessary to evaluate the fire performance of the structure as a whole system. • R9.2 – Develop performance-based standards and code provisions, as an alternative to current prescriptive design methods, to enable the design and retrofit of structures to resist real building fire conditions. • Our current research focuses on R9.1 – because it is my area of expertise as a structural engineer

  5. EXPLORATORY RESEARCH GOALS • We initiated an exploratory study (2002) of the fire behavior of structural components to: • Develop an understanding of the current knowledge base • Develop analytical approaches for predicting and investigating behavior • Determine the type of knowledge or data needed to develop and validate analytical models that can be used to investigate the behavior of complete structural systems • We selected a structural component to explore these questions • Composite concrete filled steel tube (CFT) columns • Why? • Combines both steel and concrete materials – of interest to industry. • Area of significant expertise for the researcher (seismic behavior of CFTs) • CFT columns are considered to have good fire resistance due to the presence of concrete • Lots of data from various sources.

  6. PRIOR EXPERIMENTAL RESEARCH • Standard ASTM E119 fire behavior of CFT columns investigated by researchers in Canada (NRC), China, and Japan • Experiments conducted in expensive and specially-built column furnaces in these countries • Column placed in the furnace. • Fix-end boundary conditions • Subjected to axial force • Furnace air follows the ASTM E119 T-t curve • Columns expand, then contract, and eventually fail mostly by columns buckling • No fire protection material needed • Lack of clarity regarding loads and boundary conditions achieved in the experiments • Experimental results are limited to the overall displacement-time response and temperatures through the section

  7. TYPICAL EXPERIMENTAL BEHAVIOR Expansion Reversal Sustenance Buckling failure TYPICAL CFT COLUMNS L > 10 b Circular as well as square CFTs NRC Researchers in Canada

  8. PRIOR ANALYTICAL MODELS • Heat transfer analysis: Finite difference method (FDM) simulations of heat transfer from furnace air to column surfaces, and then from column surfaces through the sections, using temperature dependent thermal properties (Lie and Irwin 1995) • Structural analysis: Fiber model simulation of the column buckling behavior. Cross-section modeled using elements with uniaxial s-e-T behavior. Assumptions include: • plane sections remain plane, • linear curvature variation along column length, • no slip, and no transverse interaction between the steel and concrete. • No basis presented for making these simplifying assumptions • Such models do not provide knowledge of fundamental behavior or complex stress and strain states at elevated temperatures

  9. ANALYTICAL APPROACH • Need a more general and more robust analytical approach to model the fire behavior of structural members. • We use a three step sequentially coupled analytical approach, where the results from each step are required to continue the analysis in the subsequent step. • Step I - Fire Dynamics Analysis is conducted to simulate the convection and radiation heat transfer from the fire source to the surfaces of the structural component. It is conducted using FDS, which is a program developed by NIST BFRL researchers. • Step II – Nonlinear Heat Transfer Analysis is conducted to simulate the heat transfer through the section and along the length. It is conducted using 3D finite element models and nonlinear temperature-dependent thermal properties

  10. ANALYTICAL APPROACH • Step II – Nonlinear Heat Transfer Analysis (continued) • The results from Step I (surface T-t curves) serve as thermal loads • The results from Step II include the temperature histories (T-t) for all nodes of the finite element model • Step III – Nonlinear Stress Analysis is conducted to determine the structural response of the component for the applied structural and calculated thermal loads. • It is conducted using 3D finite element meshes that are identical or similar to the heat transfer analysis meshes, and nonlinear temperature -dependent material models • The nodal temperature histories from Step II define the thermal loads for this analysis

  11. ANALYTICAL MODELING • CFT columns tested by researchers from different parts of the world • NRC Canada (1-3), • Sakumoto et al. from Japan using FR steel (4, 5) • Han et al. from China (6-10)

  12. ASTM E119 FDS Experiment FDM Results from Step 1 FIRE DYNAMICS ANALYSIS • FDS model of the furnace. Used to predict the surface T-t curves for 200, 250, and 300 mm CFT columns that were tested at NRC. • The FDS predictions compare well with the experimentally measured and FDM predicted T-t curves. FDM is less conservative • Surface T-t curves are slightly lower than the ASTM E119 T-t curves • The column size (200-300 mm) seems have small influence

  13. 300 mm CFT 250 mm CFT 200 mm CFT Results from Step IIHeat Transfer Analysis • The heat transfer analysis models were developed and analyzed using ABAQUS. The steel and concrete temperature dependent thermal properties  Lie and Irwin (1995) • The latent heat of water was included in the model • The results from the heat transfer analysis were found to compare well with the experimental results !

  14. 400oC 500oC 600oC 700oC 800oC MATERIAL PROPERTIES – T Dependent • Temperature dependent thermal and structural material properties were used along with the 3D finite element models • These material properties were based on values generally reported in the literature (Lie and Irwin 1995 etc.). 100oC Concrete s-e-T Steel s-e-T T=100oC 300oC T=300oC 500oC T=500oC 700oC T=700oC 900oC T=900oC

  15. Results from Step IIINonlinear Stress Analysis • Column failure mode at elevated temperatures • global buckling and local buckling mixed • similar to experiments

  16. 200x 200x 6.35mm CFT Fy=350; f’c=47 MPa L=3.8 m; P/Po=15% 250x 250x 6.35mm CFT Fy=350; f’c=47 MPa L=3.8 m; P/Po=30% 300x 300x 6.35mm CFT Fy=350; f’c=47 MPa L=3.8 m; P/Po=33% NRC Column Tests ? ? ? Results from Step IIINonlinear Stress Analysis • The results from the nonlinear stress analysis seem to have some variation from the experimental results. X

  17. Results from Step IIINonlinear Stress Analysis • Comparisons with experimental results are somewhat reasonable! FR Steel Japanese Column Tests 300x 300x 9mm CFT Fy=358; f’c=37 MPa L=3.5 m; P/Po=25% 300x 300x 9mm CFT Fy=358; f’c= --- L=3.5 m; P/Po=80%

  18. Results from Step IIINonlinear Stress Analysis • Tests done by Han et al. in China. Again comparisons have issues. 300x 200x 8mm CFT Fy=341; f’c= 49 L=3.8 m; P/Po=50% 300x 150x 8mm CFT Fy=341; f’c= 49 L=3.5 m; P/Po=45% 219x 219x 5.3mm CFT Fy=246; f’c= 19 L=3.5 m; P/Po=41% 350x 350x 7.7mm CFT Fy=284; f’c= 19 L=3.5 m; P/Po=34% ecc. 350x 350x 7.7mm CFT Fy=284; f’c= 19 L=3.5 m; P/Po=56%

  19. PIN FIX Results from Step IIINonlinear Stress Analysis • Authors claim pin end conditions were achieved in the furnace column tests, and then provide the following picture of the buckled specimen

  20. Column behavior at elevated temperatures is too sensitive to end conditions NRC Column 1 NRC Column 2 SENSITIVITY ANALYSIS • Parametric studies were conducted to determine the sensitivity of column behavior with respect to various parameters: • (1) Boundary Conditions • (2) Steel and concrete material properties as functions of T • (3) Axial load level • (4) Geometric imperfections Inter Fix Pin Pin Inter Fix

  21. P-0.10Po P-0.10Po P-0.05Po P P-0.05Po P P +0.05Po P + 0.05Po SENSITIVITY ANALYSIS • Column behavior at elevated temperatures is too sensitive to the applied axial load. Fluctuations in axial load can cause variation • The sensitivity of column behavior to elevated temperature material s-e-T models is currently ongoing

  22. FINDINGS FROM EXPLORATORY PROGRAM • The three step analytical approach with FDS and 3D finite element models for heat transfer and stress analysis can be used to predict the behavior of members under fire loading. • The results from FDS and heat transfer analysis compare favorably with experimental data. The results from stress analysis, however have significant variations. • The behavior of columns at elevated temperatures is extremely sensitive to the loading and boundary conditions achieved in the experiments. • The experimental results of fire resistance rating  must be considered carefully before any general conclusions are made. • The ASTM E119 gets around this situation by saying that the members should be tested with the same boundary conditions as those achieved in a real structure --!

  23. FUNDAMENTAL BEHAVIOR • The experimental results from a standard fire test do not provide knowledge of the fundamental behavior of structural members independent of boundary conditions and other issues. • We need a more fundamental measure, for e.g., the axial force-moment-curvature P-M-f-T behavior of the composite member at elevated temperatures from fire loading. • This P-M-f-T behavior defines the fundamental behavior of the member (sort of like a material s-e-T behavior) and can be used in a variety of ways to: • (a) conduct analytical parametric studies • (a) develop and calibrate analytical models, e.g., fiber models • (c) predict actual member behavior and failure • (d) and to design fire proofing.

  24. P P M=P d P P FUNDAMENTAL BEHAVIOR – Why? • For example, the behavior and failure of columns under constant axial load and elevated temperatures from fire loading also depends on the section P-M-f-T response of the failure segment.

  25. FUNDAMENTAL BEHAVIOR – Why? • Researchers around the world have developed finite element method based computer programs to conduct structural analysis under fire loading. • For example, researchers at Liege Univ. (SAFIR), Sheffield Univ. (FEMFAN), Univ. of Manchester, Nat. Univ. of Singapore (SINTEF) • Most of these programs use fiber-based or concentrated hinge based beam-column finite elements for modeling the behavior of columns and beam-columns under fire loading • These finite elements must be validated (or calibrated) using experimental data and realistic P-M-f-T behavior

  26. ANALYTICAL INVESTIGATIONS • The three-step analytical approach was used to investigate the fundamental P-M-f-T behavior of CFT beam-columns subjected to standard fire loading. • The effects of various geometric (width b and width-to-thickness b/t) parameters and insulation parameters on the behavior were also evaluated analytically. • CFT beam-columns with parameters: • Width b = 200 or 300 mm. • Width-to-thickness ratio = 32 or 48 • Steel tube A500 Gr. B (300 MPa) • Concrete strength (f’c=35 MPa) • Axial load levels (P=0, 20%, 40%) • Thermal insulation thickness (0, 7.5, 13 mm thick)

  27. PRELIMINARY ANALYTICAL INVESTIGATIONS • The analytical investigations were conducted on a segment of the CFT beam-columns. The length of the segment was equal to the cross-section width b. • It represents the critical segment of CFT column or beam-column subjected to axial and flexural loads and elevated temperatures from fire loading.

  28. Step 1 – Results from FDS Analysis for ASTM E119 T-t curve Surface Temperature =300oC Time = 5.6 mts. Surface Temperature =600oC Time = 14.2 mts. CFT WITHOUT INSULATION – THERMAL BEHAVIOR Step 2 – Results from heat transfer analysis

  29. P/Po=20%, T=20oC P/Po=0%, T=20oC P/Po=20%, T=300oC P/Po=0%, T=300oC P/Po=20%, T=600oC P/Po=0%, T=600oC P/Po=20%, T=900oC P/Po=0%, T=900oC 7.5E-5 12.5E-5 2.5E-5 10.0E-5 5.0E-5 Structural Response – CFT without ins. Step 3 – P-M-f-T curves for CFT without insulation

  30. Findings for CFTs Without Insulation • For CFTs without insulation: • Fire loading results in quick heating of the steel tube (broiling) while the concrete infill remains relatively cooler. Significant portions remain at T< 100oC till much later • This relative heating causes rapid reduction in flexural stiffness and strength of the CFT section under fire loading effects • This reduction depends primarily on the rise in steel temperature, and is independent of axial load level, width, and other parameters • This by itself, may not be a cause of concern unless the demands placed on the CFT without insulation exceed the reduced stiffness and strength at elevated temperatures

  31. Steel surface w/o insulation Steel surface with insulation Insulation thick = 13.0 mm Time=180 mts Insulation thick = 6.5 mm Time=180 mts The heating of the composite CFT section becomes more uniform (not broiling) CFT WITH INSULATION – THERMAL BEHAVIOR • Consider CFTs with some insulation. Assume commonly used insulation materials – gypsum cement • The presence of thermal insulation results in a slow increase in the steel surface temperature.

  32. P-M-f-T curves for CFT with b/t=32 P-M-f-T curves for CFT with b/t=48 Normalized Strength P-M Interaction Ambient T=20oC T=20oC Ins=6.5 mm Ins=13 mm P/Po=20% P/Po=0 P/Po=40% P/Po=20% Ambient T=20oC Ins. Thick = 13 mm P/Po=0 Ins. Thick = 6.5 mm Ins. Thick = 6.5 mm P/Po=20% b=200 mm, b/t=32 P/Po=0 Ins. Thick = 13 mm P/Po=40% P/Po=20% P/Po=40% P/Po=20% b=200 mm, b/t=48 P/Po=0 P/Po=40% P/Po=0 P/Po=20% b=300 mm, b/t=32 P/Po=40% P/Po=0 P/Po=40% 12.5E-5 12.5E-5 2.5E-5 2.5E-5 10.0E-5 10.0E-5 5.0E-5 5.0E-5 7.5E-5 7.5E-5 Curvature (1/mm) Curvature (1/mm) Structural Response of CFT with Insulation

  33. CFTs with Insulation • The insulation thickness becomes the most important parameter influencing P-M-f-T behavior and strength (P-M) under elevated temperatures from fire loading. • As expected, CFTs with b/t =48 have greater increase in moment capacity with increase in axial load (below the balance point). This continues to be true at elevated temperatures also. • The tube width (b) and width-to-thickness (b/t) ratio do not have significant influence on the P-M-f-T behavior of CFTs at elevated temperatures from fire loading

  34. FAILURE MODE • Material inelasticity combined with local buckling produce failure Stress analysis results for CFT with b/t=32, axial load level = 20%, and insulation thickness=6.5 mm (curvature = 12.5 x 10-5 1/mm)

  35. EXPERIMENTAL INVESTIGATIONS • Real challenge is to determine this fundamental P-M-f-T behavior of a structural member experimentally. This has never been done before (although a group of researchers from U.K. considered it) • Need experimental data to validate the analytical approach and models • Need experimental data to show that the fundamental P-M-f-T behavior can be measured in the laboratory – efficiently • The experimental investigations are being conducted in two phases: • Phase I – focusing on the thermal behavior of CFT beam-columns • Phase II – focusing on the structural behavior of CFT beam-columns • The results from Phase I will be used to validate or calibrate the nonlinear heat transfer analysis models of the CFT (Step II). • The results from Phase II will be used to validate the nonlinear stress analysis models developed in Step III.

  36. HEAT TRANSFER EXPERIMENTS • The heat transfer experiments are being conducted on short (36 in. long) CFT stub columns. The specimens are 12 x 12 in. in cross-section with different b/t ratios (32, or 48). • The parameters considered in the heat transfer experiments are: • Gypsum plaster thickness (0.25 and 0.50 in.) • Concrete strength f’c (5 ksi and high strength 10 ksi), and • Presence of reinforcement bars. • Twelve CFT short stubs were tested by subjecting them to elevated temperatures simulating fire loading. For now, the surface of the gypsum plaster was controlled to follow the ASTM E119 T-t curve. • The heating was applied using ceramic fiber radiant heaters. These heaters integrate high temperature iron-chrome-aluminum (ICA) heating element wire with ceramic fiber insulation, and can provide surface temperatures up to 1200oC when placed close (250 mm) to them. • They can controlled to follow specified T-t or heat flux-time curves using Watlow F4 PID controllers with communications.

  37. 2” 2” CFT stub 2” 3ft Heating equipment Heated Area 6” 1” 2” 2” 6” 4” 2” 3” Concrete pedestal 6” HEAT TRANSFER EXPERIMENTS • Test setup and thermocouple layout. Since this is only a heat transfer experiment, there are no loads acting on the CFT Thermocouple locations

  38. CFT 12 x 12 x 3/8 in. A500 Gr.-B, f’c=5 ksi, Gypsum thickness = 0.5 in. Gypsum surface specified Gypsum surface measured Steel surfaces measured EXPERIMENTAL RESULTS • Experimental results indicate that the heating system does an excellent job of subjecting the gypsum surface to the T-t curve

  39. EXPERIMENTAL RESULTS • The experimental results included T-t curves measured at various locations (steel surfaces, concrete depths) in the section. • A 3D finite element model was built to perform the heat transfer analysis. The results from the heat transfer analysis are compared

  40. EXPERIMENTAL RESULTS • Similar results and comparisons were obtained for the twelve short CFT specimens. The experimental results are being used to calibrate the nonlinear heat transfer analysis models – work in progress. HEATERS IN ACTION HEATERS IN ACTION

  41. EXPERIMENTAL INVESTIGATIONS • In Phase II, the CFT beam-column specimen is tested by: • Applying axial load (15-30% of Po) • The axial load is maintained constant over the remaining of the test. The axial loading and hydraulic setup can accommodate movement while maintaining constant axial force. • The heating is applied to the segment close to the base of the CFT specimens. The heating is applied using four ceramic fiber radiant heaters that are position around the base segment. • The base of the CFT specimens is protected using gypsum plaster that is embedded in metal lath. This is the procedure we used for our experiments • The heaters are controlled to subject the gypsum surface to the ASTM E119 T-t curve for now. • After two hours of heating, the CFT beam-column is pushed laterally at the top. This causes maximum bending and failure of the heated segment of the base

  42. CFT EXPERIMENTAL INVESTIGATIONS • TEST SETUP AXIAL LOADING P LATERAL LOADING H CFT Column BASE

  43. EXPERIMENTAL INVESTIGATIONS PID Controllers Heater in Action HEATERS

  44. EXPERIMENTAL INVESTIGATIONS • STATIC PUSHOVER Local buckling failure Local buckling failure End of test. Lateral displacement = 8 in.

  45. SENSOR DISTRIBUTION • How to measure deformations at very high temperatures? • Close-range photogrammetry combined with digital image processing techniques. This method has been used recently for medical and microstructure investigation type application. • High precision digital camera – looking at a target that is on the specimen. The camera and data acquisition acquire images and used digital image processing to compute the x, y, and z movement of the target point. • Accuracy can be as high as 0.001 in. depending on the view area (1 in.), lighting condition, etc. Much lower resolutions are possible as sub-pixelation is employed by the software. • We are using 8 digital cameras to track and measure the deformations of the heated failure segment at the base of the column. • The average curvature and rotation over the segment is calculated using these measurements

  46. Vertical Displacement 1ft Camera Sensor Locations 1ft 1ft Lateral Displacement 1ft * * 1ft Rotation-meter location * * SENSOR DISTRIBUTION

  47. Digital Cameras = Sensors for Measuring Deformation (f) Sensor Locations Measuring movement

  48. Digital Camera Sensor Calibration and Validation Accuracy: min. of 0.0015 inch and it is much higher than 0.0015 inch due to sub-pixel analysis DT : 0.007in

  49. EXPERIMENTAL RESULTS • CFT 10 x 10 x ¼ in. A500 Gr.-B steel (46 ksi), 5.0 ksi concrete • Fire protection with ¼ in. of gypsum • Axial load = 15% Po • 2 hours of ASTM E119 heating (steel surface T=550oC) Ambient Heated

  50. EXPERIMENTAL RESULTS • Comparing M-f-T behavior of the 10 in. CFTs. Curvature obtained from photogrammetric measurements

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