strengthening effect of the shear key on the flexural behavior of concrete …c).pdf ·...

Steel Structures 6 (2006) 183-190 www.kssc.or.kr

Strengthening Effect of the Shear Key on the Flexural Behavior of

Concrete Filled Circular Tube

Young-Ho Kim1, Sung-Kun You1, Jae-Ho Jung2 and Soon-Jong Yoon3,*

1Sinil CNI Co., Ltd., Rm. 701 Baekgung Plaza III, 156-2 Jeongja-dong, Bundang-gu, Seongnam 463-834, Korea2Chun Il Eng. Consultants, 732-23 Yeoksam-dong, Gangnam-gu, Seoul 135-080, Korea

3Department of Civil Eng., Hongik University, 72-1 Sangsu-dong, Mapo-gu, Seoul 121-791, Korea

Abstract

In this study, the effect of slip behavior between steel tube and concrete on the flexural behavior of concrete filled steel tube(CFT) under pure bending was investigated experimentally and the effective method to increase the plastic moment capacityof CFT beam was suggested which is to install the shear key at the inside of steel tube. In order to obtain the strengtheningeffect of shear key installed inside the CFT, four point bending tests were performed. The ultimate strength, ductility, and failuremode of CFT beam with shear key was compared with those of hollow circular steel tube, CFT beam, CFT beam with endplate, and CFT beam strengthened by prestressing tendon. From the experimental study, it was found that the perfect bondingbehavior was essential for the composite action in the CFT beam with the shear key installed inside of the steel tube and theload carrying capacity of CFT beam was improved by installing the shear key as much as that of CFT beam strengthened byprestressing tendon.

Keywords: CFT (concrete filled steel tube) beam, End plate, Shear key, Prestressing tendon, Plastic slenderness ratio (d/t)

1. Introduction

The composite CFT member consisting of steel tube

filled with concrete is widely used, for the structures

which required large moment and ductile deformation,

due to its attractive advantages such as an excellent

seismic resistance in two orthogonal directions, a good

hysteresis under cyclic loading, a prevention of local

bucking of outer steel tube, and an enhancement of

ductility of CFT member up to the ultimate load. Those

advantages are mainly introduced by the structural

interactions between inner concrete and outer steel tube

such as a confinement of concrete by steel tube.

Many research results pertaining to the characteristic

behavior of CFT under axial compression are available.

However, the research on the structural behavior of CFT

under flexural loading is rarely conducted. The practical

application of the CFT as a pure flexural member had

been attempted in Japan for the construction of bridge. It

was a three span continuous railroad bridge using four

CFT girders with diameter of 1.3m (Hostaka et al., 1997,

Nakamura et al., 2002). Before the construction of CFT

girder bridge, 4-point bending test of CFT beam was

conducted to ensure the flexural performance of CFT

beam (Hosaka et al., 1997). Elchallakani et al. (2001)

suggested the limit of slenderness ratio (i.e., diameter to

thickness ratio, d/t) of steel tube to reach the plastic

moment (plastic slenderness ratio) based on the experimental

results. Lu et al. (1994) also examined CFT beams by

changing the slenderness ratio of steel tubes.

In these previous studies, the main considerations of

experiment were the slenderness ratio of steel tube and

the strength of infill concrete. They also reported that the

slip between steel tube and infill concrete did not so much

affect on the flexural capacity of CFT beam. It is true but

the load carrying capacity of CFT beam may increase if

the slip between steel tube and infill concrete is prevented.

In this study, we investigated the effective method to

increase the plastic moment capacity of CFT beam in

which the shear keys are installed in its inside. In order to

evaluate the effect of shear key installed inside, 4-point

bending tests were performed. The ultimate strength,

ductility, and failure mode of CFT beam with shear key

were compared with those of hollow circular steel tube,

CFT beam, CFT beam with end plate, and CFT beam

strengthened by prestressing tendon.

2. Test Specimens and Experimental Set-up

2.1. Test specimens

It is known that the plastic slenderness ratio, d/t, is one

of the most important factors affecting the structural

*Corresponding authorTel: +82-2-320-1479, Fax: +82-2-3141-0774E-mail: [email protected]

184 Young-Ho Kim et al.

behavior of CFT beam. According to the experimental

result of CFT performed by Chung (2003), the specimens

with slenderness ratio, d/t, from 29.6 to 56.4 reached the

plastic moment. By consideration of this result, we

prepared the steel tube specimen with slenderness ratio of

50.7. For the material of steel tube, SS400 was used

according to the Korean Specification for Highway

Bridges (2003).

In this study, we planed to investigate the effect of

shear key installed at the inner surface of steel tube on the

plastic moment capacity of CFT beam. Therefore, the

specimens were prepared by changing the resisting factors

of slip between steel tube and infill concrete. Table 1

shows the description and dimensions of each specimen.

In Table 1, each specimen designation was identified

by the resisting factor and composed of three terms. In

the first term, the letters BOO and BCF indicate the

hollow steel tube and CFT beam specimen, respectively.

The third term represents the end bearing plate of steel

tube; the letter E indicates that the end plate is installed

and welded at the edge of steel tube, and the letter N

indicates that the end plate is not installed or it is installed

but not welded at the edge of steel tube. The additional

property is presented in the second term as N, R, or P.

The letter N indicates no additional property, and R and

P represent that the shear key is installed at the inside of

steel tube and that the prestressing force is applied to the

infill concrete after it hardened, respectively. For

example, BOO-N-N is the hollow steel tube specimen

without concrete filling and any additional resisting

factors, and BCF-R-N is the CFT specimen with shear

key but no end plate.

For the shear key, small rectangular steel plates are

used to give an effect of mechanical shear resistance and

the space of each shear key is 150 mm. Fig. 1 shows the

specimen BCF-R-N and installation of shear key. For the

specimen BCF-P-N, the prestressing force of 582.42 kN

was established that the compressive stress of concrete

should not exceed the allowable compressive stress of

concrete (e.g., 75% of the allowable compressive stress)

according to the Korean specification for concrete and the

force was applied to introduce the initial compressive

Table 1. Designation and dimensions of specimens

Name ofspecimen

Diameter(mm)

Thickness(mm)

d/tratio

Concretefilling

Shear key End plate Prestressing Remarks

BOO-N-N 355.6 7 50.7 No No No No

BCF-N-N 355.6 7 50.7 Yes No No No

BCF-N-E 355.6 7 50.7 Yes No Yes No

BCF-R-N 355.6 7 50.7 Yes Yes No NoSize of shear key50 × 25 × 4.5 mm

BCF-P-N 355.6 7 50.7 Yes No Yes YesEnd bearing

plate is not welded

Steel tube and shear key: yield strength Fy = 235 MPa, ultimate strength Fu = 400 MPaInfill concrete: design compressive strength Fck = 21.4 MPaWire strand: yield strength Fy = 1,598 MPa, ultimate strength Fu = 1,881 MPa

Figure 1. Specimen BCF-R-N.

Figure 2. Specimen BCF-P-N.

Strengthening Effect of the Shear Key on the Flexural Behavior of Concrete Filled Circular Tube 185

stress to the steel tube using three 7-wire strands

(φ15.2 mm). Fig. 2 shows the specimen BCF-P-N. When

the specimen BCF-P-N was prepared, the end bearing

plate was also used to apply the prestressing force to the

specimen but the edge of plate was not welded to the end

of steel tube in order to eliminate the effect of end plate.

Fig. 3 shows the specimens prepared.

2.2. Experimental Set-up

A 4-point bending test was prepared as shown in Fig.

4. According to experimental study by Kilpatrik (1997),

he recommended the shear span ratio of 2.7 to observe

the full load transfer without slip in the range of shear

span. Since the purpose of this experimental investigation

is to obtain the flexural load carrying capacity of CFT, we

designed the experimental set-up with shear span ratio of

4.5 as shown in Fig. 4. To minimize the local crippling of

steel tube, the loading and bearing devices were specially

fabricated for the two loading and reaction points as

shown in Fig. 5.

Steel strain gauges were attached on the outer surface

of steel tube in the circumferential direction to measure

strains at the center of test specimen. From the strain

measurement the location of neutral axis according to

each loading stage was traced. The displacement transducer

was also installed to measure the deflection at the center

of specimen. The location of strain gauges in the BCF

specimen is schematically shown in Fig. 6.

3. Comparison of Results

3.1. Failure modes

The hollow steel tube specimen B00-N-N was failed by

local crippling of steel tube in compression zone as

shown in Fig. 7(a). After the local crippling occurred, the

applied load was immediately decreased. In the specimens

BCF-N-N, BCF-N-E, and BCF-P-E, the compression

zone between loading points was expanded and the local

buckling occurred as shown in Figs. 7(b) and 7(c) but the

load was steadily increased after the local buckling

occurred. In the specimen BCF-R-N, the crack of steel

tube in tension zone was observed after the local buckling

occurred and finally the local crippling occurred as

shown in Fig. 7(d).

3.2. Load-deflection Behavior

Fig. 8 shows the load-deflection relation of test specimens.

First of all, the hollow steel tube specimen and CFT

specimen were compared in Fig. 8(a). In the figure, it is

clearly shown that the load carrying capacity of hollow

tube specimen is immediately decreasing after reaching

the ultimate strength of steel tube due to the local

buckling of steel tube in compression zone. On the other

hand, the descending part is not shown in the load-

deflection curve of the CFT specimen after yielding of

steel tube and the ductility and initial stiffness of CFT is

greater than those of hollow steel tube. Those results are

Figure 3. Test specimens.

Figure 4. Test set-up.

Figure 5. Details of bearing and loading devices.


well known phenomena in CFT beams.

In order to obtain the effect of end bearing plate, the

results of specimens BCF-N-N and BCF-N-E are

compared as shown in Fig. 8(b). The end bearing plate

was installed to prevent or reduce the slip deformation

between the concrete and steel tube and it was expected

that specimen BCF-N-E has more ductility and high

strength than the specimen BCF-N-N. However, the

discrepancy of flexural behaviors of those two specimens

was negligibly small as shown in Fig. 8(b).

Figure 6. Location of strain gage.

Figure 7. Tested Specimens.


Fig. 8(c) shows the effect of shear key on the flexural

behavior of CFT. In the figure, the solid line without

symbol is the load-deflection curve of BCF-R-N which is

the CFT specimen with shear key and the solid line with

triangular symbol is the result of CFT specimen without

shear key. As shown in the figure, the stiffness and yield

load of specimen BCF-R-N are greater than those of

specimen BCF-N-N. Such higher yield strength and

stiffness may be caused by increasing the bonding

capacity between concrete and steel tube.

Fig. 8(d) shows the comparison of load-deflection curves

of specimens BCF-N-N and BCF-P-N. The stiffness and

ultimate strength of BCF-P-N specimen are higher than

those of specimen BCF-N-N since the prestressing force

applied to specimen BCF-P-N reduces the tensile stress

of steel tube and slip between steel tube and concrete.

From the above results, it is found that the load

carrying capacity and the stiffness of specimen BCF-R-N

are improved and the increase of ultimate strength and

stiffness is similar to those of specimen BCF-P-N. This

increase is caused by the inter-rocking action of shear key

and concrete, that is, the shear keys restrain the slip of

concrete and those restricting forces transfer to steel tube.

Then, the tensile strain of steel tube may be reduced due

to similar action as prestressing force. This phenomenon

is illustrated in next section.

3.3. Load-slip behavior

In order to investigate the slip behavior between steel

tube and concrete, the slip at the ends of each specimen

was measured. Fig. 8. illustrates the load-slip behavior of

specimens BCF-N-N and BCF-R-N. As shown in the

figure, the maximum slip measured in the specimen BCF-

N-N during experiment was 4.2 mm, however, negligibly

small amount of slip was observed in the specimen BCF-

R-N. From the result, it is found that the shear key is

effectively restricting the slip between steel tube and

concrete.

To investigate the contact surface between steel tube

and concrete, we cut the steel tube and observed the

surface of concrete of specimen BCF-R-N after failure.

As shown in Fig. 10, the crushing trace of concrete can

be observed at the location of shear key toward the

support and this trace illustrates the mechanical resisting

effect of shear key on the slip between steel tube and

concrete.

3.4. Neutral axis

Fig. 11 shows the transition of neutral axis in tested

specimens at each loading stage. The steel strains measured

in the composite beam are significantly different from the

steel strains measured in the hollow steel beam. For the

hollow section beam, the experimental measurement on

Figure 8. Load-deflection Relations.


the location of neutral axis shows that the neutral axis

coincides with central axis of the section as increase of

the applied load but the tensile strain is increased after

local buckling occurs at the compression zone of steel

tube. In the CFT beam specimen, the location of neutral

axis is unchanged for small moments, but just after the

crack of concrete initiates, the location of neutral axis

shifts rapidly upward as the length of crack increase.

Especially, the neutral axis moves upward as much as

132.6 mm from its original position in the specimen

BCF-R-N. Tensile strains of concrete infilled steel tube

are much greater simply because of the fact that the

tensile force in the steel must balance the compressive

force in combined steel and concrete.

Figure 9. Slip measurement and view of the slip.

Figure 10. Concrete surface of specimen BCF-R-N after Failure.


In addition, the restraining effect of shear key is also

evaluated in those measured strain. As shown in Fig.

11(b), (c), and (d), the strains of steel tube of specimen

BCF-R-N are much smaller than those of other specimens

before reaching its maximum load. This phenomenon

also illustrates the restraining effect of shear key on the

slip between steel tube and concrete.

4. Conclusions and Discussions

In this study, the experimental investigations on the

flexural behavior of CFT beams were performed. To

increase the load carrying capacity of CFT beam, the

method of installing shear key elements at the inside of

steel tube was suggested and its performance was

evaluated experimentally. From the experiments, it was

found that the perfect bonding is essential for the

composite action in CFT beam by installing the shear key

at the inside of the steel tube and the load carrying

capacity of CFT beam with shear key improved as much

as CFT beam strengthened by prestressing tendon. It was

also found that those improvements are achieved by

enhancing the bonding capacity between steel tube and

concrete.

In the previously published works (Lu et al., 1994;

Kilpatrick et al., 1997), it was reported that the slip

between steel tube and concrete does not seriously affect

the load carrying capacity of CFT beams. It is true when

the high strength concrete is used. In addition, to reduce

the self-weight of CFT beam, it is necessary to use the

light weight concrete. In this experimental investigation,

the use of shear key at the inside of steel tube is examined

to improve the flexural load carrying capacity.

In this experimental study, only one type of shear keys

was used, and the composite action between deck slab

and CFT beam was not considered. Therefore, further

experimental researches on effective types of shear key

are necessary to develop the rational design guideline for

the CFT under flexural loading.

References

Chung, C.H., Kim, Y.J., and Kim, S.U. (2003). “A Study on

Analysis and Design of Composite Bridge with Concrete

Filled Tubes,” Research Report, Daewoo Technical

Research Center, Suwon.

Elchallakani, M., Zhao X.L., and Grzebieta, R.H. (2001).

“Concrete Filled Circular Steel Tubes Subjected to Pure

Bending,” Journal of Constructional Steel Research, Vol.

57, p.1141-1168.

Hosaka, T., Umehara, T., Nakamura, S., and Nishiumi, K.

(1997). “Design and Experiments on a New Railway

Bridge System Using Concrete Filled Steel Pipes,”

Figure 11. Neutral axis of BOO-N-N, BCF-N-N, BCF-N-E, and BCF-R-N.


ASCCS Seminar, Concrete Filled Steel Tubes, A

Comparison of International Codes and Practices,

Innsbruck, Austria, p.367-372.

Kilpatrik, A.E. and Rangan, B.V. (1997). “Tests on High-

strength Composite Concrete Column,” Research Report,

No. 1, Curtin University of Technology, Australia.

Korean Specification for Highway Bridge (2003). Korean

Society of Civil Engineers.

Lu, Y.Q., Kennedy, D.J.L. (1994). “The Flexural Behavior of

Concrete-Filled Hollow Structural Section,” Canadian

Journal of Civil Engineering, Vol. 21, No. 1, p. 111-130.

Nakamura, S., Momiyama, Y., Hosaka, T., and Homma, K.

(2002). “New Technologies of Steel/Concrete Composite

Bridges,” Journal of Constructional Steel Research, Vol.

58, p. 99-130.

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