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

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 twenty-nine 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 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.

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

Expansion

Reversal

Sustenance

Buckling failure

TYPICAL CFT COLUMNS L > 10 b

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 -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)

Column

Cross-Section Length fy f'c Load Eccentricity Fire

(mm x mm x mm) (mm)Re-bar (MPa

) (MPa) (kN) (mm) Protection

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

9 10

350x350x7.70 3810 - 284 18.7 2700 0 spray-type

350x350x7.70 3810 - 284 18.7 1670 52.5 spray-type

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

(b)

Quarter volum

e of CFT column

(a)

Quarter volum

e of CFT column

Hot air flow direction

(a)

Symmetry plane

Symmetry plane

Heated wall

Heated w

all


Hot air

flow direction

Quarter volume of CFT column

(b)

Figure 1. FDS model of NRC Furnace with CFT column

(a)

Symmetry plane

Symmetry plane

Heated wall

Heated w

all


Hot air flow

direction

Quarter volume of CFT column

0

200

400

600

800

1000

1200

0 30 60 90 120 150

Time (min)

Te

mp

era

ture

(C

)

ASTM E119

FDS

ExperimentFDM

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 !

0

200

400

600

800

1000

1200

0 30 60 90 120 150Time (min)

Te

mpe

ratu

re (

C)

C2, surface, calculatedC2, surface, measuredC2, d=60mm, calculatedC2, d=60mm, measured

250 mm CFT

0

200

400

600

800

1000

1200

0 30 60 90 120 150 180 210 240

Time (min)

Tem

pera

ture

(C)

C3, surface, calculated

C3, surface, measured

C3, d=37mm, calculated

C3, d=37mm, measured

300 mm CFT

0

200

400

600

800

1000

1200

0 30 60 90 120 150 180

Time (min)

Te

mp

era

ture

(C

)

C1, surface, calculatedC1, surface, measuredC1, d=71mm, calculatedC1, d=71mm, measured

200 mm CFT

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.).

900oC

700oC

500oC

300oC

100oC

0

100

200

300

400

0 0.002 0.004 0.006 0.008 0.01

Strain (mm/mm)

Str

ess

(MP

a)

Steel --T

0

10

20

30

40

0.000 0.010 0.020 0.030 0.040

Strain (mm/mm)

Str

ess,

MP

A

500o

C600o

C

700o

C

800o

C

400o

C

Concrete --T

T=100oC

T=300oC

T=500oC

T=700oC

T=900oC

Results from Step IIINonlinear Stress Analysis

Column failure mode at elevated temperatures global buckling and local buckling mixed similar to experiments


The results from the nonlinear stress analysis seem to have

some variation from the experimental results.

200x 200x 6.35mm CFT

Fy=350; f’c=47 MPaL=3.8 m; P/Po=15%

250x 250x 6.35mm CFT

Fy=350; f’c=47 MPaL=3.8 m; P/Po=30%

300x 300x 6.35mm CFT

Fy=350; f’c=47 MPaL=3.8 m; P/Po=33%

NRC Column Tests

??

?

X

Comparisons with experimental results are somewhat

reasonable!

0

5

10

15

20

25

30

0 40 80 120 160 200

Time (min)

Ax

ial

Dis

pla

cem

ent

(mm

)

Calculated

Measured

-30

-20

-10

0

10

20

30

0 10 20 30 40

Time (min)

Axi

al D

ispl

acem

ent (

mm

)

Calculated

Measured

300x 300x 9mm CFT

Fy=358; f’c= ---L=3.5 m; P/Po=80%

FR Steel Japanese Column Tests

300x 300x 9mm CFT

Fy=358; f’c=37 MPa

L=3.5 m; P/Po=25%



Tests done by Han et al. in China. Again comparisons have

issues.300x 200x 8mm CFTFy=341; f’c= 49

L=3.8 m; P/Po=50%

300x 150x 8mm CFTFy=341; f’c= 49

L=3.5 m; P/Po=45%

219x 219x 5.3mm CFTFy=246; f’c= 19

L=3.5 m; P/Po=41%

350x 350x 7.7mm CFTFy=284; f’c= 19

L=3.5 m; P/Po=56%

350x 350x 7.7mm CFTFy=284; f’c= 19

L=3.5 m; P/Po=34% ecc.


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 conditionsNRC Column 1

NRC Column 2

Pin

Inter

Fix Pin

Inter Fix

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 --T models is currently ongoing

P

P +0.05Po

P-0.05Po

P-0.10Po

P

P + 0.05Po

P-0.05Po

P-0.10Po

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

force-moment-curvature P-M--T behavior of the

composite member at elevated temperatures from fire

loading.

This P-M--T behavior defines the fundamental behavior of

the member (sort of like a material -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--T response of the failure segment.

-16

-14

-12

-10

-8

-6

-4

-2

0

0 50 100 150Time (min)

Axi

al D

isp

lace

me

nt (

mm

)

0

20

40

60

80

100

120

0 20 40 60 80 100 120Time (min)

Mom

ent (

KN

-m)

P

P

P

M=P

P0

1000

2000

3000

4000

5000

6000

0 100 200 300 400 500Moment (KN-m)

Axi

al F

orce

(KN

)

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--T behavior

ANALYTICAL INVESTIGATIONS

The three-step analytical approach was used to investigate

the fundamental P-M--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

0

200

400

600

800

1000

1200

0 30 60 90 120 150 180Time (minutes)

Tem

per

atu

re

ASTM E119 T-t No Insulation

Step 1 – Results from FDS Analysis for ASTM E119 T-t curve

Step 2 – Results from heat transfer analysis

Surface Temperature =300oCTime = 5.6 mts.

Surface Temperature =600oCTime = 14.2 mts.

Structural Response – CFT without ins.

0.00

30,000.00

60,000.00

90,000.00

120,000.00

150,000.00

0 0.005 0.01 0.015 0.02 0.025

Curvature (1/mm)

Mom

ent (N

-m)

2.5E-5 5.0E-5 7.5E-5 10.0E-5 12.5E-5

P/Po=20%, T=20oC

P/Po=20%, T=300oC

P/Po=20%, T=600oC

P/Po=20%, T=900oC

P/Po=0%, T=20oC

P/Po=0%, T=300oC

P/Po=0%, T=600oC

P/Po=0%, T=900oC

Step 3 – P-M--T curves for CFT without insulation

0.0

0.3

0.6

0.9

1.2

0 200 400 600 800 1000 1200Temperature (T)

Mom

ent/

Mom

ent

@ 2

0o C

(M/M

20)

P=20%, No Insul

P=0, No Insul

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.

0

200

400

600

800

1000

1200

0 30 60 90 120 150 180Time (minutes)

Te

mpe

ratu

re

ASTM E119 T-t No InsulationInsulation = 13 mm Insulation=6.5 mm

Steel surface w/o insulation

Steel surface with insulation

Insulation thick = 6.5 mmTime=180 mts

Insulation thick = 13.0 mmTime=180 mts

The heating of the composite CFT section becomes more uniform (not broiling)

Structural Response of CFT with Insulation

0

30,000

60,000

90,000

120,000

150,000

0 0.005 0.01 0.015 0.02 0.025

Rotation (rad.)

Mo

me

nt

(N-m

)

P-M--T curves for CFT with b/t=32

Ins. Thick = 13 mm

Ins. Thick = 6.5 mm

Ambient T=20oCP/Po=0

P/Po=20%

P/Po=40%

P/Po=0P/Po=20%

P/Po=40%

P/Po=0

P/Po=20%

P/Po=40%

2.5E-5 5.0E-5 7.5E-5 10.0E-5 12.5E-5

Curvature (1/mm)

0

30,000

60,000

90,000

120,000

150,000

0.000 0.005 0.010 0.015 0.020 0.025

Rotation (Rad.)

Mo

men

t (N

-m)

P-M--T curves for CFT with b/t=48

Ins. Thick = 13 mm

Ins. Thick = 6.5 mm

Ambient T=20oCP/Po=0

P/Po=20%

P/Po=40%P/Po=0P/Po=20%

P/Po=40%

P/Po=0

P/Po=20%

P/Po=40%

2.5E-5 5.0E-5 7.5E-5 10.0E-5 12.5E-5

Curvature (1/mm)

0

10

20

30

40

50

0 0.2 0.4 0.6 0.8 1 1.2

Moment /Moment @ P=0

Ax

ial L

oad

Lev

el P

/Po %

Ins=13 mm

Ins=6.5 mm

T=20oC

b=200 mm, b/t=32

b=200 mm, b/t=48

b=300 mm, b/t=32

Normalized Strength P-M Interaction

CFTs with Insulation

The insulation thickness becomes the most important

parameter influencing P-M--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--T behavior of CFTs at

elevated temperatures from fire loading

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)

Steel tube longitudinal strain Steel tube longitudinal stress

Concrete longitudinal strain Concrete longitudinal stress

(Pa)

(Pa)

Longitudinal Strain Longitudinal Stress (Pa)

Longitudinal Stress (Pa)Longitudinal Strain

Steel Tube Steel Tube

Concrete Infill Concrete Infill



(Pa)

(Pa)



(Pa)

(Pa)

Longitudinal Strain Longitudinal Stress (Pa)

Longitudinal Stress (Pa)Longitudinal Strain

Steel Tube Steel Tube

Concrete Infill Concrete Infill

EXPERIMENTAL INVESTIGATIONS

Real challenge is to determine this fundamental P-M--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--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.

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

3ft

6”

6”

6”

2”

2”

2”

2” 1”

2”4”

3” 2”

Heated Area

Concrete pedestal

CFT stub

Heating equipment

EXPERIMENTAL RESULTS

Experimental results indicate that the heating system does

an excellent job of subjecting the gypsum surface to the T-t

curve

0

100

200

300

400

500

600

700

800

900

1000

1100

0 50 100 150 200

Time (min)

Tem

pe

ratu

re (

C)

Gypsum surface specified Gypsum

surface measured

Steel surfaces measured

CFT 12 x 12 x 3/8 in. A500 Gr.-B, f’c=5 ksi, Gypsum thickness = 0.5 in.

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 OF EXPERIMENTAL RESULT WITH RESULTS FROM HEAT TRANSFER MODEL

0

50

100

150

200

250

300

350

400

450

0.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00 200.00Time (min)

Tem

per

atu

re (

c)

ABAQUS 4 in conc. 4in S2 ABAQUS 3 in conc. 3 in S1 conc. 2 in S1

conc. 2 in S3 conc. 2 in S4 ABAQUS 2in conc. 1 in S3 ABAQUS 1 in

COMPARISON OF EXPERIMENTAL RESULT WITH RESULTS FROM HEAT TRANSFER MODEL

0

50

100

150

200

250

300

350

400

450

0 25 50 75 100 125 150 175 200TIME (MIN)

TE

MP

ER

AT

UR

E (

C)

steel exp. 1 steel exp 2 steel exp. 3steel exp. 4 abaqus steel temp


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


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


TEST SETUP

H o l l o w C o r e J a c k

A x i a l L o a d i n g B e a m

A x i a l T e n s i o n R o d

H y d r a u l i c R a m D i r e c t i o n

C l e v i s

S t e e l B a s e P l a t e

C o n c r e t e B l o c k

CFT

CFT

AXIAL LOADING

LATERAL LOADING

P

H

Column BASE


HEATERS

PID Controllers

Heater in Action


STATIC PUSHOVER

End of test. Lateral displacement = 8 in.

Local buckling failure

Local buckling failure

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

* *

* *

1ft

1ft

1ft

1ft

1ft

Lateral Displacement

Camera Sensor Locations

Rotation-meter location

Vertical Displacement

SENSOR DISTRIBUTION


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

0

10

20

30

40

50

60

0 1 2 3 4 5 6 7 8

Displacement (inch)

For

ce (

kip)

Ambient

Heated


Comparing M--T behavior of the 10 in. CFTs. Curvature

obtained from photogrammetric measurementsMoment - Curvature Relationship

y = 472762x

R2 = 0.8529

y = 147691x - 65.599

R2 = 0.9534

0

50

100

150

200

250

300

350

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009

Curvature (1/inch)

Mo

me

nt

(kip

-ft)

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

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

Documents

FUNDAMENTAL BEHAVIOR OF COMPOSITE MEMBERS UNDER FIRE LOADING