FUNDAMENTAL BEHAVIOR OF CFT BEAM-COLUMNS UNDER FIRE

LOADING

Amit H. Varma, Sandgo Hong

Purdue University

2005 ASCE Structures Congress

New York City, NY

Findings for 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

EXPERIMENTAL INVESTIGATIONS

Experimental investigations will focus on measuring the P-M--T response of CFT segments. Parameters included in the experimental studies are:

Tube width (b) and b/t ratio Concrete strength f’c

Axial load level Heating (surface temperature) Insulation thickness and type

Experimental test matrix is currently being finalized using results of preliminary investigations

TEST SETUP

The test-setup will be similar to those used for measuring P-M- 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.

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

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.

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

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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0 50 100 150 200Time (min)

Te

mp

era

ture

(C

)

Input tempmeasured 1measured 2measured 3measured 4

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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Time (min)

Te

mp

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ture

(C

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ASTM E119

FDS1

steel 1

steel 2

steel 3

steel 4

Measured surface T-t curves

Specified surface T-t curve

ASTM E119 T-t curve

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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Time (min)

Th

erm

al E

xpan

sio

n (

mm

)

Predicted Thermal Expansion at final 1.7mm

TEMPERATURE MEASUREMENTS

Thermocouples bonded to steel and embedded in concrete to measure temperatures

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Time (min)

Tem

per

atu

re (

C)

computed measured

measured computed

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