Transcript 1) Varma, A.H. (2006). - College of Engineering, Purdue

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.
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
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 twentynine major recommendations for future work.
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
EXPLORATORY RESEARCH GOALS

We initiated an exploratory study (2002) of the fire behavior of
structural components to:




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


Develop an understanding of the current knowledge base
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.
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
TYPICAL EXPERIMENTAL BEHAVIOR
TYPICAL CFT COLUMNS L > 10 b
Expansion
Reversal
Sustenance
Buckling failure
Circular as well as square CFTs
NRC Researchers in Canada
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
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
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
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)

Cross-Section
Length
Column
fy
f'c
Load
Eccentricity
Fire
(MPa)
(MPa)
(kN)
(mm)
Protection
Re-bar
(mm x mm x mm)
(mm)
1
200x200x6.35
3810
4 x 16 mm
350
47
500
0
-
2
250x250x6.35
3810
4 x 16 mm
350
47
1440
0
-
3
300x300x6.35
3810
4 x 25 mm
350
47
2000
0
-
4
300x300x9
3500
-
357.9
2020
0
-
5
300x300x9
3500
-
357.9
37.5
1350
0
ceramic
6
300x200x7.96
3810
-
341
49
2486
0
spray-type
7
300x150x7.96
3810
-
341
49
1906
0
spray-type
8
219x219x5.30
3810
-
246
18.7
950
0
spray-type
350x350x7.70
3810
-
284
18.7
2700
0
spray-type
350x350x7.70
3810
-
284
18.7
1670
52.5
spray-type
9
10
Results from Step 1
FIRE DYNAMICS ANALYSIS
Symmetry plane
Symmetry plane
Hot air flow direction
Heated wall
Hot air flow direction
Heated wall
rterQuarter
umevolume
FTof CFT
mncolumn
Hot air flow direction
Heated wall
Hot air flow direction
Heated wall
flow direction
ot airHot
flowairdirection
flow direction
Hot airHot
flowairdirection
Heated
wall
ed wall
HeatedHeated
wall wall
Symmetry
Symmetry
plane plane Symmetry plane
Symmetry plane
(b)
(b)
1200
(b)
1000
800
600
400
200
0
0
Figure
1. FDS
model
NRC
Furnace
withFurnace
CFT
column
Figure
1. FDS
model
NRC
Furnace
with
CFT
column
Figure
1.ofFDS
model
of NRC
with CFT
column
Figure
1.ofFDS
model
of NRC
Furnace
with CFT
column
Hot air flow direction
(b)
(a)
Hot air flow direction

of CFT
column
Hot air flow
direction
Temperature (C)

Quarter volum e
of CFT
column
Hot air flow
direction
Quarter
volum e

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
Quarter volum e
of CFT column
Quarter volum e
of CFT column

ASTM E119
FDS
Experiment
FDM
30
60
90
Time (min)
120
150
Results from Step II
Heat 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 !
1200
1200
1000
1000
1000
800
C2, surface, calculated
C2, surface, measured
C2, d=60mm, calculated
C2, d=60mm, measured
600
400
Temperature (C)
Temperature (C)
Temperature (C)
250 mm CFT
1200
300
mm
CFT
200
mm
CFT
800
800
600
600
400
400
C1, surface, calculated
C3, surface, calculated
C1, surface, measured
C3, surface, measured
C1, d=71mm, calculated
C3, d=37mm, calculated
C1, d=71mm, measured
C3, d=37mm, measured
200
200
200
0
0
0
30
60
90
Time (min)
120
150
0
0
0
30
30
60
60
90
90
120
120(min) 150
Time
Time (min)
150
180
180
210
24
MATERIAL PROPERTIES – T Dependent


400
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
Steel s-e-T
500oC
T=500oC
700oC
T=700oC
100
900oC
500oC
30
Stress, MPA
T=300oC
200
Concrete s-e-T
400oC
300oC
300
Stress (MPa)
40
T=100oC
600oC
20
700oC
10
800oC
T=900oC
0
0
0.002
0.004
0.006
Strain (mm/mm)
0.008
0.01
0
0.000
0.010
0.020
Strain (mm/mm)
0.030
0.040
Results from Step III
Nonlinear Stress Analysis

Column failure mode at elevated temperatures


global buckling and local buckling mixed
similar to experiments
Results from Step III
Nonlinear Stress Analysis

The results from the nonlinear stress analysis seem to have some
variation from the experimental results.
NRC Column Tests
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%
?X
?
Results from Step III
Nonlinear 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%
30
30
Axial Displacement (mm)
Axial Displacement (mm)

20
10
0
-10
Calculated
-20
Measured
-30
25
20
15
10
Calculated
5
Measured
0
0
10
20
Time (min)
30
40
0
40
80
120
Time (min)
160
200
Results from Step III
Nonlinear 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%
350x 350x 7.7mm CFT
Fy=284; f’c= 19
L=3.5 m; P/Po=56%
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.
Results from Step III
Nonlinear Stress Analysis

Authors claim pin end conditions were achieved in the furnace
column tests, and then provide the following picture of the buckled
specimen
PIN
FIX
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
Column behavior at elevated temperatures is too sensitive to end conditions
NRC Column 1
Inter
Pin
Fix
Pin
NRC Column 2
Inter
Fix
SENSITIVITY ANALYSIS

Column behavior at elevated temperatures is too sensitive to the
applied axial load. Fluctuations in axial load can cause variation
P-0.10Po
P-0.10Po
P-0.05Po
P
P +0.05Po

P-0.05Po
P
P+
0.05Po
The sensitivity of column behavior to elevated temperature material
s-e-T models is currently ongoing
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 --!
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 forcemoment-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.
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.
P
0
120
-2
100
-4
Moment (KN-m)
Axial Displacement (mm)
-6
-8
-10
-12
60
40
20
-14
-16
0
0
P
80
50
100
150
0
20
40
Time (min)
M=P d
60
80
Time (min)
6000
5000
Axial Force (KN)

4000
3000
2000
1000
P
P
0
0
100
200
300
Moment (KN-m)
400
500
100
120
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
experimental data and realistic P-M-f-T behavior
calibrated)
using
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)
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.
CFT WITHOUT INSULATION – THERMAL BEHAVIOR
Step 2 – Results from heat transfer analysis
Step 1 – Results from FDS Analysis for
ASTM E119 T-t curve
1200
Temperature
1000
800
600
400
200
0
0
30
60
90
120
150
Time (minutes)
ASTM E119 T-t
No Insulation
180
oC
Surface
Temperature
=600
oC
Surface
Temperature
=300
Time
= 14.2
Time
= 5.6mts.
mts.
Structural Response – CFT without ins.
Step 3 – P-M-f-T curves for CFT without insulation
150,000.00
1.2
P/Po=0%, T=300oC
90,000.00
(M/M 20 )
o
P=20%, No Insul
P/Po=0%, T=20oC
P/Po=20%,
C
P=0,T=300
No oInsul
120,000.00
0.9
Moment/Moment @ 20 C
Moment (N-m)
P/Po=20%, T=20oC
0.6
P/Po=20%, T=600oC
60,000.00
P/Po=0%, T=600oC
0.3
30,000.00
P/Po=20%, T=900oC
0.0
0
0.00
200
0
2.5E-5
0.005
400
P/P800
C
o=0%, T=9001000
600
Temperature (T)
7.5E-5
10.0E-5
0.01
0.015
0.02
5.0E-5
o
Curvature (1/mm)
1200
12.5E-5
0.025
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
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.
1200
1000
Temperature

800
600
Steel surface
w/o insulation
Steel surface
with insulation
400
200
0
Insulation thick = 13.0 mm
Insulation
thick = 6.5 mm
0Time=180
30mts
60
90
120
150
180
Time=180
mts
Time (minutes)
The heating
of the composite CFTNo
section
becomes more
ASTM E119 T-t
Insulation
(not broiling)
Insulation = uniform
13 mm
Insulation=6.5 mm
Structural Response of CFT with Insulation
P-M-f-T
curves
forfor
CFT
with
b/t=32
P-M-f-T
curves
CFT
with
b/t=48
Normalized
Strength
P-M
Interaction
150,000
150,000
Moment
(N-m) P/P %
AxialMoment
Load Level
o
(N-m)
50
Ins=6.5 mm
120,000
120,000
40
T=20
C
P/P
o=20% Ambient
P/Po=0
o
Ins=13 mm
P/Po=40%
T=20 C
P/Po=20%
P/Po=0
Ins.Ambient
Thick
P/Po=20%
= 13
mmoC
T=20
30
90,000
90,000
b=200 mm, b/t=32
20
o
60,000
60,000
b=200 mm, b/t=48
10
b=300 mm, b/t=32
P/Po=40%
P/PoP/P
=40%
o=0
Ins. Thick
P/Po=20%
P/Po=20%
P/Po=0
13 mm
Ins.= Thick
P/P =0
o
P/Po=40%
P/Po=20% =
P/Po=40%
P/Po=0
P/Po=40%
6.5 mm
Ins. Thick
= 6.5 mm
30,000
30,000
0
00
0
0
0.000
0.2
2.5E-5
0.005
2.5E-5
0.005
0.4
0.6
0.8
0.02
0.015
0.01
10.0E-5
5.0E-5
7.5E-5
0.010
0.015
0.020
5.0E-5
7.5E-5
Moment
/Moment
@ 10.0E-5
P=0
Rotation
Rotation(rad.)
(Rad.)
Curvature
(1/mm)
Curvature
(1/mm)
1
12.5E-5
0.025
12.5E-5
0.025
1.2
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
FAILURE MODE

Material inelasticity combined with local buckling produce failure
Longitudinal Strain
Longitudinal Stress (Pa)
Steel tube longitudinal strain
Steel Tube
Steel tube longitudinal stress
Steel(Pa)
Tube
Longitudinal Strain
Concrete longitudinal
strain
Concrete Infill
Longitudinal Stress (Pa)
Concrete
Infill
Concrete longitudinal
stress
(Pa)
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)
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)



The experimental investigations are being conducted in two phases:




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
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.
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.
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
2”
CFT stub
2”
2”
Heating
equipment
3ft
Heated
Area
6”
1”
2”
6”
Concrete
pedestal
6”
2”
4”
3”
2”
EXPERIMENTAL RESULTS
Experimental results indicate that the heating system does an
excellent job of subjecting the gypsum surface to the T-t curve
CFT 12 x 12 x 3/8 in. A500 Gr.-B, f’c=5 ksi,
Gypsum thickness = 0.5 in.
1100
Gypsum surface
specified
1000
900
Gypsum surface
measured
800
Temperature (C)

700
600
500
Steel surfaces
measured
400
300
200
100
0
0
50
100
Time (min)
150
200
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
COMPARISON
OFOF
EXPERIMENTAL
COMPARISON
EXPERIMENTALRESULT
RESULT
WITHWITH
RESULTS
FROM
HEAT
TRANSFER
RESULTS
FROM
HEAT
TRANSFER MODEL
MODEL
450
450
400
400
TEMPERATURE
Temperature (c) (C)

350
350
300
300
250
250
200
200
150
150
100
100
50
50
00
0
0.00
25
25.00
50
50.00
75
75.00
ABAQUS 4 steel
in
conc.
exp.
1 4in S2
conc. 2 in S3
conc.
steel exp.
4 2 in S4
100
100.00
125
125.00
TIME
(MIN)
Time (min)
ABAQUS
steel exp3 2in
150
150.00
175
175.00
conc. 3 in steel
S1 exp.conc.
3 2 in S1
ABAQUS
2in temp
conc. 1 in S3
ABAQUS 1 in
abaqus steel
200
200.00
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
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
EXPERIMENTAL INVESTIGATIONS

TEST SETUP
AXIAL
LOADING
Hollow Core Jack
P
Axial Loading Beam
Axial Tension Rod
LATERAL
LOADING
H
CFT
Hydraulic Ram Direction
C
F
T
Clevis
Column BASE
Steel Base Plate
Concrete Block
EXPERIMENTAL INVESTIGATIONS
PID Controllers
HEATERS
Heater in Action
EXPERIMENTAL INVESTIGATIONS

STATIC PUSHOVER
Local buckling
failure
Local buckling failure
End of test. Lateral displacement = 8 in.
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 subpixelation 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
SENSOR DISTRIBUTION
Vertical
Displacement
1ft
Camera Sensor
Locations
1ft
1ft
Lateral
Displacement
1ft
* *
* *
1ft
Rotationmeter
location
Digital Cameras = Sensors for Measuring
Deformation (f)
Sensor Locations
Measuring movement
Digital Camera Sensor Calibration and Validation
Displacement at 13.5 inch high
DT : 0.007in
45
40
35
Force (kips)
Accuracy: min.
of 0.0015 inch
and it is much
higher than
0.0015 inch due
to sub-pixel
analysis
50
30
cam0
25
cam2
20
[email protected]
15
10
5
0
0
0.2
0.4
0.6
0.8
Displacement @13.5 inch high (inch)
1
1.2
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)
Lateral Force vs. Displacement at Top
60
Ambient
50
40
Force (kip)

Heated
30
20
10
0
0
1
2
3
4
5
Displacement (inch)
6
7
8
EXPERIMENTAL RESULTS
Comparing M-f-T behavior of the 10 in. CFTs. Curvature obtained
from photogrammetric measurements
Moment - Curvature Relationship
350
y = 472762x
R2 = 0.8529
300
250
Moment (kip-ft)

200
150
y = 147691x - 65.599
R2 = 0.9534
100
50
0
0
0.001
0.002
0.003
0.004
0.005
Curvature (1/inch)
0.006
0.007
0.008
0.009
TEST MATRIX


Test matrix includes fourteen total CFT specimens that will be tested
at ambient and heated temperatures
Parameters being considered:








Size of column (b) = 250 and 300 mm.
Tube width-to-thickness (b/t) ratio = 32, 48
Axial load level (P=15% or 30% Po)
Fire protection thickness (0.25 and 0.50 in.)
Some repeat specimens
Experiments are ongoing – Complete by Summer 06.
Validation of analytical models using experimental results – Complete
by Fall 06.
Determined the fundamental force-deformation-temperature behavior
of composite beam-columns
Test matrix

Parameters included: Axial load levels, Concrete
strength, Width to wall thickness ratio, Crosssectional size effects
Specimen Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Size
10" x 10" x 5/16"
10" x 10" x 5/16"
10" x 10" x 5/16"
10" x 10" x 1/4"
12"x12"x3/8"
12"x12"x5/16"
10" x 10" x 5/16"
10" x 10" x 5/16"
10" x 10" x 5/16"
10" x 10" x 5/16"
10" x 10" x 1/4"
10" x 10" x 1/4"
12"x12"x3/8"
12"x12"x5/16"
Fy (ksi)
46
46
46
46
46
46
46
46
46
46
46
46
46
46
F'c (ksi)
5
5
10
5
5
5
5
10
5
10
5
5
5
5
b/t
34
34
34
42
34
41
34
34
34
34
42
42
34
41
P/Po
0.15
0.3
0.15
0.15
0.15
0.15
0.15
0.15
0.3
0.3
0.15
0.3
0.15
0.15
Ambient
Ambient
Ambient
Ambient
Ambient
Ambient
Elevated Temperature
Elevated Temperature
Elevated Temperature
Elevated Temperature
Elevated Temperature
Elevated Temperature
Elevated Temperature
Elevated Temperature
EXPERIMENTAL RESULTS
Comparing M-f-T behavior of the 10 in. CFTs. filled with 7 ksi concrete.
b/t = 32, Axial Load = 15% Po
Curvature obtained from photogrammetric measurements
Moment Curvature Response: 10x5/16 7 ksi
300
Moment (kip-ft)
250
200
150
100
Exp. ambient 15%P
Exp. heated 15%P
50
0
0
0.002
0.004
0.006
Curvature (1/inch)
0.008
0.01
EXPERIMENTAL RESULTS
Comparing M-f-T behavior of the 10 in. CFTs. filled with 7 ksi concrete.
b/t = 48, Axial Load = 15% Po
Curvature obtained from photogrammetric measurements
Moment Curvature Response: 10x10x1/4 7 ksi
300
Moment (kip-ft)
250
200
150
100
Exp. Ambient
ABAQUS
Exp. Heated
ABAQUS Heated
50
0
0
0.001
0.002
0.003
0.004
0.005
Curvature (1/inch)
0.006
0.007
0.008
EXPERIMENTAL RESULTS
Comparing M-f-T behavior of the 10 in. CFTs. filled with 10 ksi concrete.
b/t = 32, Axial Load = 15% Po
Curvature obtained from photogrammetric measurements
Moment Curvature Response: 10x5/16 10ksi
300
Moment (kip-ft)
250
200
150
100
50
Exp. 15%P ambient
ABAQUS 15% ambient
Exp. 15%P heated
ABAQUS 15%P heated
0
0
0.001
0.002
0.003
0.004
0.005
Curvature (1/inch)
0.006
0.007
0.008
0.009
EXPERIMENTAL RESULTS

Capacity Reduction:
 About 30 to 40%


As expected from the analysis
Stiffness Reduction:
 About 40 to 50%

As expected from the analysis
FINDINGS





A unique experimental approach was developed to
determine the fundamental behavior of composite CFT
beam-columns
This approach builds upon years of experimental
research, the PIs expertise, and the requirements of the
problem
The heating approach works well for the application we
tested
The close-range photogrammetry measurements work
well for measuring deformations at elevated temperatures
The experimental data needs to be improved with higher
rates of cycling.
Where do we go from here?



R9.1 – Develop and validate analytical tools, guidelines, and test
methods necessary to evaluate the fire performance of the structure
as a whole system.
In this research we focused on developing an analytical approach and
unique experimental approach that can be used to predict the
fundamental force-deformation-temperature behavior of members.
Analytical fiber beam-column elements can be calibrated to the
experimental and analytical data developed using the approach
outlined. Then, the validated beam-column elements can be used
while predicting the fire performance of structures as whole system