FUNDAMENTAL BEHAVIOR OF CFT BEAM-COLUMNS UNDER FIRE LOADING

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FUNDAMENTAL BEHAVIOR OF CFT BEAM-COLUMNS UNDER
FIRE LOADING

• Amit H. Varma, Sandgo Hong
• Purdue University
• 2005 ASCE Structures Congress
• New York City, NY

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INTRODUCTION

• Significant research has been conducted on the
  fire resistance of composite CFT columns under
  standard fire loading.
• Researchers at NRC-Canada (Lie, Kodur, Irwin,
  etc.), China (Lin-Hai Han), and Japan (Sakumoto
  using FR steel)
• Focus on the behavior of columns subjected to
  constant axial loads, end conditions, and ASTM
  E119 fire time-temperature loading
• The results provide fire resistance rating (FRR)
  values and have been used to develop standard
  fire resistant design
• They do not provide knowledge of the fundamental
  force-deformation-temperature behavior of the CFT
  column or the critical failure segments
• Limited research has been conducted on the
  fundamental force-deformation-temperature
  behavior of composite CFT beam-columns under
  combined axial and flexural loads and elevated
  temperatures from fire loading

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MOTIVATION

• Why is it relevant? Four reasons
• The section force-deformation-temperature
  (P-M-f-T) represents the fundamental behavior of
  CFT beam-columns and it can be used to
  investigate the effects of various geometric,
  material, and insulation parameters on fire
  resistance.
• These P-M-f-T responses can be used to calibrate
  beam-column finite element models used to conduct
  structural analysis under fire loading
• The behavior and stability of moment resisting
  frames under fire loading depends on the strength
  interaction P-M-T curve for and the fire
  resistance of the connections
• The stability of columns under fire loading also
  depends eventually on the P-M-f-T response of the
  critical failure segment at mid-span.

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RESEARCH OBJECTIVES

• The objectives of this research project are
• To analytically and experimentally investigate
  the fundamental force-deformation-temperature
  (P-M-f-T) behavior of CFT beam-columns under
  elevated temperatures from fire loading.
• To evaluate the effects of various material
  (concrete strength, steel yield stress),
  geometric (column size, width-to-thickness
  ratio), and insulation (thickness, thermal
  conductivity) on the fundamental P-M-f-T behavior
  of CFT beam-columns.
• To develop (or calibrate) fiber-based finite
  element models for modeling CFT columns and
  beam-columns while investigating the fire
  behavior of CFT structures.

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RESEARCH APPROACH

• Development and validation of analytical approach
  for simulating the thermal and structural
  behavior of CFT members under structural loads
  and fire loading.
• Preliminary analytical investigations of the
  fundamental P-M-f-T behavior of CFT beam-columns
  under elevated temperatures from fire loading.
  Evaluate the effects of geometric, material, and
  insulation parameters.
• Experimental investigations to measure the
  fundamental P-M-f-T behavior of CFT beam-columns
  under combined axial and flexural loads and
  elevated temperatures from fire loading.
• Analytical model calibration. The experimental
  results will be used to validate (or calibrate)
  the preliminary analytical models. The
  experimental results and calibrated models will
  be used to develop beam-column finite element
  models .

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ANALYTICAL APPROACH

• The analytical approach was developed and
  validated using existing experimental data for
  CFT columns tested under fire loading by
  researchers at NRC, China, and Japan.
• Development and validation of the analytical
  approach was presented in detail at the 2004 ASCE
  Structures Congress
• The approach consists of three sequentially
  coupled analysis steps, where the results from
  each step are required to continue the analysis
  in the subsequent step
• Step I Fire dynamics analysis
• Step II Nonlinear heat transfer analysis
• Step III Nonlinear stress analysis

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Step 1 - FIRE DYNAMICS ANALYSIS

• Fire dynamics analysis is conducted to simulate
  the convection and radiation heat transfer from
  the fire source (or furnace walls) to the
  structural component by solving the simplified
  Navier-Stokes equations numerically.
• It is conducted using the NIST-BFRL developed
  software FDS, and the results include the heat
  flux incident upon the surfaces of the component
  or the surface T-t curves.

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Step 2 - NONLINEAR HEAT TRANSFER ANALYSIS

• Nonlinear heat transfer analysis is conducted to
  simulate the heat transfer through the
  cross-section of the component and along its
  length (and the associated convection and
  radiation losses).
• The surface heat flux or T-t curves from the fire
  dynamics analysis serve as thermal loading for
  conducting the heat transfer analysis.
• The heat transfer analysis can be conducted using
  the FDM or FEM. It is assumed uncoupled from the
  stress analysis, which is adequate for structural
  materials.
• We used FEM because it links more easily with
  step 3. The results from the heat transfer
  analysis include the T-t curves for the nodes of
  the FEM mesh and thermal contours.

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Step 3 NONLINEAR STRESS ANALYSIS

• Nonlinear stress analysis is conducted to
  determine the structural response of the
  component under applied structural and thermal
  loads.
• The nodal time-temperature (T-t) curves obtained
  from the heat transfer analysis of step 2 define
  the thermal loads for the nonlinear stress
  analysis
• The stress analysis can be conducted using the
  finite element method while using identical
  meshes for both steps 2 and 3.
• The results from the analysis include the
  complete structural response deflections,
  strains, stresses, load-displacement-temperature
  relationships.

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Step 3 NONLINEAR STRESS ANALYSIS

• For example, the behavior of CFT columns tested
  according to the ASTM E119 was investigated using
  the 3-step approach
• The sequentially coupled heat transfer and
  structural analysis were conducted using the FEM
  and option in ABAQUS.
• The analytical approach was validated for an
  assortment of CFT columns with different
  material, geometric, insulation parameters tested
  independently by researchers in Canada, China,
  and Japan

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GENERAL FINDINGS

• The analytical approach was developed and
  validated, but the behavior of CFT columns under
  fire loading were found to be very sensitive with
  respect to
• Temperature dependent steel and concrete material
  structural properties, which are not measured or
  reported explicitly for most CFT specimens.
• Column end conditions (rotational and axial
  restraint). End conditions obtained in the
  experiment may vary from those assumed in the
  analysis.
• Variations in axial load level (P/Po). Axial load
  level can vary due to changes in axial load P due
  to restraint, or due to Po which depends on steel
  and concrete strength variation.
• Relative motion (slip) between steel tube and
  concrete infill at ends. This slip occurs for
  some columns that were tested individually and
  the slip was allowed to occur. This may not be
  realistic.

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PRELIMINARY ANALYTICAL INVESTIGATIONS

• Preliminary investigations were conducted using
  the (developed and validated) analytical approach
  to determine the more fundamental
  force-deformation (P-M-f) behavior of CFT
  sections under elevated temperatures from fire
  loading.
• These P-M-f-T responses and the effects of
  various material, geometric, and insulations
  parameters are the focus of the research for
  reasons explained earlier.
• CFT parameters
• Width b 200 or 300 mm.
• Width-to-thickness ratio 32 or 48
• Steel tube A500 Gr. B (300 MPa)
• Concrete strength (fc35 MPa)
• Axial load levels (P0, 20, 40)
• Thermal insulation thickness (0, 7.5, 13 mm
  thick)

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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.

Step 1 FDS analysis to simulate heat transfer
to the surface of the segment Step 2 Nonlinear
heat transfer analysis to simulate transfer
through section and along length Step 3
Nonlinear stress analysis for constant axial
load, monotonic flexural loading moment), and
nodal thermal loading (T-t) from step 2 Steps 2
and 3 conducted using the finite element method
and ABAQUS
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MATERIAL PROPERTIES T Dependent

• Temperature dependent thermal and structural
  material properties were used along with the 3D
  finite element models of the CFT failure segment.
  
• These material properties were based on values
  generally reported in the literature (Lie and
  Irwin 1995 etc.).
• T-Thermal properties are given in a table in the
  paper

100oC
Concrete s-e-T
Steel s-e-T
T100oC
300oC
T300oC
500oC
T500oC
700oC
T700oC
900oC
T900oC
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Thermal response- CFT without insulation
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Structural Response CFT without ins.
Step 3 P-M-f-T curves for CFT without insulation
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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 Tlt 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

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Thermal response of CFT with insulation

• Consider similar CFTs with some insulation.
  Assume commonly used insulation materials with
  properties given in the paper.
• The presence of thermal insulation results in a
  slow increase in the steel surface temperature.

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Structural Response of CFT with Insulation
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Findings for 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

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EXPERIMENTAL INVESTIGATIONS

• Experimental investigations will focus on
  measuring the P-M-f-T response of CFT segments.
  Parameters included in the experimental studies
  are
• Tube width (b) and b/t ratio
• Concrete strength fc
• Axial load level
• Heating (surface temperature)
• Insulation thickness and type
• Experimental test matrix is currently being
  finalized using results of preliminary
  investigations

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TEST SETUP

• The test-setup will be similar to those used for
  measuring P-M-f response of beam-column specimens
  at ambient temperature
• It will be a cantilever column with axial force
  and lateral load applied at the top (free) end
  and the bottom end clamped.

• A custom-built portable furnace will be placed to
  surround the plastic hinge region. It will
  subject the surface to the selected T-t curve.
• Thus, the specimen plastic hinge region will be
  subjected to P, M, and T.
• The deformations of the plastic hinge region will
  be measured using close-range photogrammetry and
  digital image processing techniques.

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FURNACE DESCRIPTION

• The portable furnace consists of ceramic fiber
  radiant heaters. Four such panel heaters are
  assembled to form a box around the hinge region.
• Heaters have wattage density of 2.5 kW/ft2. Can
  provide surface temperature approaching 1200oC.
  They use radiant heating which is efficient and
  economic

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FURNACE T-t CONTROL

• Radiant heaters are fully controllable. Specify
  control surface T-t curve and the heater will
  provide it.
• For example, in the experiment below, we are
  controlling steel surface T-t curve under the
  insulation.
• The insulation surface T-t curve can also be
  directly controlled. This experiment is in
  progress.

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FURNACE T-t CONTROL

• In this experiment we are directly controlling
  the steel surface T-t curve without insulation to
  follow the ASTM E119 gas T-t curve.

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DEFORMATION MEASUREMENTS

• The deformation (or movement) of any point on the
  specimen surface can be measured using
  close-range photogrammetry and digital image
  processing fundamentals.
• For example, the thermal expansion was measured
  as shown below.

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TEMPERATURE MEASUREMENTS

• Thermocouples bonded to steel and embedded in
  concrete to measure temperatures

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EXPERIMENTAL INVESTIGATIONS

• Will be conducted this year. Results to be
  presented at next ASCE Structures Congress.
• Acknowledgments
• National Science Foundation - funding
• Purdue University
• Dr. Jim Bethel photogrammetry
• Jarupat Srisa-Ard - student

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• THE END

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MOTIVATION 2

• 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

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MOTIVATION 3

• Consider a 6-story structure with perimeter
  moment resisting frames for lateral stiffness and
  stability.
• Design for dead, live, wind loads.
• Satisfy building and interstory drift
  requirements.
• Consider state when subjected to
  D0.5LWCompartment Fire load

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MOTIVATION 4

• 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.

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Step 1 - FIRE DYNAMICS ANALYSIS

• During experiments, the furnace gas temperature
  was controlled to follow the ASTM E119 T-t curve.
  The temperatures of the CFT column surfaces were
  measured using thermocouples.
• FDS model of the furnace, and the surface T-t
  curves for 200, 250, and 300 mm CFT columns that
  were tested shown below.

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Step 2 - NONLINEAR HEAT TRANSFER ANALYSIS

• For example, nonlinear heat transfer analyses of
  CFT columns tested by other researchers were
  conducted.
• The surface T-t curves from step 1 were used as
  thermal loading
• 3D finite element models were developed to
  conduct the heat transfer analysis and analyzed
  using ABAQUS.
• The results were compared with experimental
  results.

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Stress analysis results for CFT beam-column with
b/t32, P/Po 20, and insulation thickness6.5
mm (Curvature 12.5 x 10-5 1/mm)