Innovative UBRC Piers for High Seismic Performance

H. Iemura, Y. Takahashi & N. Sogabe School of Civil Engineering, Kyoto University, Kyoto, 606-8501, Japan

ABSTRACT: In this research, at first the rational performance design of bridge piers under the two-level seismic design methods is investigated. Next, in order to realize the concept, new type of RC pier with additional unbonded longitudinal bars is proposed1). These are called unbonded reinforced concrete (UBRC) piers. The unbonded bars can control the post-yield stiffness, which provide high performance of piers under extreme earthquake motions. The ultimate strength in plastic region be-comes much higher than the yielding strength, and residual deformation after the earthquake motion becomes much smaller. In order to clarify the fundamental characteristics, many types of experimental models are tested under cyclic loading and hybrid earthquake loadings. As the results, it is clearly verified that proposed structure has a higher performance than conventional RC structure under strong earthquake motion.

KEYWORDS: UBRC piers, post-yielding stiffness, two-level seismic design, high seismic perform-ance

1 INTRODUCTION

Hyogo-ken Nanbu earthquake caused severe damages to many RC piers. From there lessens, re-searches for flexural and shear reinforcement of RC piers have actively been carried out. The innova-tive reinforcement of RC piers is developed and it is realistic to construct new type of piers with high seismic performance.

After the Hyogo-ken Nanbu earthquake, the two-level seismic design method was proposed in the Seismic Design Specification of Japan Roadway2), 3). The design takes into account two levels of de-sign ground motion. For moderate ground motion (Level 1), bridges should behave in an elastic man-ner without essential structural damage. For extreme ground motion (Level 2), inelastic response is al-lowed, however, standard bridges should not go over the critical failure limit state, and important bridges shall be repaired with a few days work to be operated for restoration of damaged areas.

In this paper, at first, the rational performance of bridge piers under the two-level seismic design methods is investigated. Next, in order to realize the concept, the new RC structure with high seismic performance is proposed. And the results of cyclic loading, hybrid earthquake loadings tests and analysis are reported.

2 RATIONAL SEISMIC DESIGN USING POST YIELDING STIFFNESS

RC piers are modeled as elasto-plastic load-displacement relationship with zero post-yielding stiffness by Highway Design Specifications in Japan. This model is set as the conservative rule, although RC piers have small post-yield stiffness due to the strain hardening of reinforcements etc.

After the Kobe earth-quake, the two-level seismic design method was proposed. The design takes into account two types of design ground motion. For moderate ground motion (Level 1), bridges should be-have in an elastic manner without es-sential structural damage. For extreme ground motion (Level 2), standard bridges should prevent criti-cal failure, while im-

portant bridges should perform with limited damage. The required strength for Level 2 design is usu-ally much higher than that for Level 1. Although the pier satisfies the standard for Level 1, its section must be enlarged and/or the quantity of reinforcement must be increased for the Level 2 design. If the post-yield stiffness can be used effectively, the design for Level 2 becomes possible without increas-ing the section nor reinforcement from those determined by the Level 1 design. The load-displacement relationships with and without post yielding stiffness are schematically illustrated in Figure 1.

Figure 1. Load-displacement relationship, according to two-level design

Displ.

Load

Strength demandedLevel Ⅱ earthquakemotion

Strength demandedLevel Ⅰ earthquakemotion

Performance by rationalseismic design usingtwo-level design

Design change

Small post-yield stiffness achieved by conventional design, also causes large residual displacement response by Level 2 earthquakes, which makes the repair works after earthquakes difficult. Therefore, the specification recommend the residual displacement after earthquakes to be evaluated from the fol-lowing equation and it should not be larger then 1% of the height of piers4).

yRRR rc δµδ )1)(1( −−=

in which residual displacement of a pier after earthquake, c modification factor, re-sponse ductility factor of pier,

=Rδ =R =Rµ=r ratio between the pre and post-yield stiffness, yield dis-

placement. It is easily understood from this equation that the larger the ratio =yδ

r is, the smaller the re-sidual displacement becomes. That is, the piers with high value of r become to have higher seismic performance. From the above discussions, it becomes clear that the seismic performance of RC piers can be the most effectively improved by increasing the post-yield stiffness.

3 RC PIERS WITH UNBONDED BARS

Figure 2 shows the proposed structure of RC piers in this study. This structure consists of conven-tional RC pier and unbonded high strength bars. The bars can behave in an elastic manner even under large deformation, because they are unbonded to concrete and are located inside of the section. Fur-thermore, if the gap is installed at the bottom end of bars, the active elastic range of bars can be shifted to larger deformation range.

The load-displacement model of conventional RC piers is modeled as elasto-plastic. Figure 3 shows load-displacement relation when elastic members are in-stalled in RC piers. Installing unbonded bars gives two effects to the load-displacement relation. One is to add the positive post-yielding stiffness on the load-displacement relation. The other is that the yield strength becomes lager than that of conventional RC piers. And hysteretic response becomes stable with positive post-yield stiffness under strong earthquakes, consequently residual deformation is decreased.

Structural reinforcements

StirrupConcrete

Inelastic hinge

Unbonded rangeAnchor plate

A A

B B

A-A section B-B section

Figure 2. Proposed RC piers

In recent years, the research of prestressed concrete piers (PC piers) is being conducted actively5). The hys-teresis loop is the origin-oriented type; therefore the re-sidual deformation is small. However, because of the prestress, it becomes difficult to achieve high ductility of piers. The proposed pier is similar to a PC pier, but it is not necessary to apply prestressed load. And bars are installed around the plastic hinge region and are un-bonded to a pier. By this unbonding treatment, the strain becomes low and uniform along the longitudinal direction and they remain elastic even for large defor-mation of piers. The proposed RC piers have two reinforcements for seismic performance, one for absorbing energy and the other for adding post-yield stiffness.

4 CYCLIC LOADING TESTS AND ANALYSIS

4.1 Outline

In order to assess the fundamental effects of unbonded bars, four different pier specimens are pre-pared. All specimens have the same dimensions, which is 320 mm square cross section, and is about 1500 mm height. Figure 4 shows the arrangement. The high strength PC tendons are used as un-bonded bars, of which yielding strain is 5102µ . And in order to make unbonding treatment, the bars installed in tubes. Their both ends are anchored to piers. The No.1 specimen is the conventional RC model. And No.2 is the specimen with unbonded bars. The No.3 specimen has fewer structural rein-forcements than No.1 to investigate roles of conventional structural reinforcements, on strength and absorbing energy. In order to obtain effects of location of the unbonded bars, No.4 specimen has as

Figure 3. Concept of proposed RC piers

δ y δu

PyPu=

(a) Force-displ. relation for normal RC piers

Load

Dspl.

Crack

Yielding of reinforcing barsCollapse of concrete

(b) Force-displ. relation for unbonded bars

Load

Dspl.

Elastic behavior

δ y δ u

PyPu=

(c) Force-displ. relation for proposed RC piers = (a) + (b)

Post yield stiffness

Increasing yield strengthLoad

Dspl.

same amount of bars as in No.2, but at the different location.

Figure 5 shows the loading system for experiment. In this system, the computer controls horizontal and vertical actuator. In the cyclic loading tests, axial stress was set to 1.46MPa. The horizontal dis-placement is applied at quasi-static rate in cycles with ductility factor of µ=±1,±2, etc (δy=5.0mm).

4.2 Result of cyclic loading tests

Figure 6 shows load-displacement rela-tionships. No.1 shows almost perfectly elasto-plastic behavior. On the contrary,

the others exhibit positive post-yield stiffness.

300

1807

900

640

Unit(mm)

D10

D6

290

290

320

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290

290

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320

140

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320 140

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290

320

320

200

200

（１） Section of No.1

（２） Section of No.2

（３） Section of No.3

（４） Section of No.4

High strength bars

Figure 4. Specimens piers for cyclic loading tests

Figure 5. Loading system for experiment

Computerfo

loaddispl.

load

r Controll

The post-yielding stiffness of No.2 is almost the same as that of No.3, and No.4 shows larger than others. This result suggests that the post-yield stiffness is defined by the location of bars in the section, not by the amount of conventional structural reinforcement. But in spite of unbonding treatment, the No.4 yields earlier than others. Therefore it is considered that there is the optimal location of un-bonded bars for the additional stiffness and elastic behavior.

Figure 7 shows the result of residual displacement, which is the displacement of zero-crossing at unloading on hysteresis loop from the maximum displacement. Those of No.2, No.3 and No.4 speci-mens are smaller than that of No.1 specimen. In other word, even if piers is not prestressed such as PC piers, residual displacement can be decreased only by installing elastic members in RC piers. The decrease of residual displacement depends on amount of unbonded bars. This trend is the same as PC piers.

Hysteretic absorbed energy (Area of hysteresis loop) is shown in Figure 8. From this figure, it is clear that the ab-sorbed energy depends on the amount of structural rein-forcement. That is, only No.3 has poor performance of ab-sorbing energy, and the others have the same performance in spite of the difference of bar arrangement. The figure also exhibits that unbonded bar has no contribution forwards en-ergy absorption.

Figure 9 shows strain distribution along the unbonded bar of No.2 at each peak of input waves. This figure shows that strains are almost the same along the height, while the fail-ure of the pier concentrated into the bottom. This result in-dicates that the unbonding treatment is effective in prevent-ing bars from yielding.

4.3 Analytical results for cyclic loading

The UBRC pier consists of the RC pier and the unbonded bars. Therefore the analytical model is also represented as the combination of the RC pier model (Fiber model) and the unbonded bar model. The UBRC element cannot be satis-fied with the Bernoulli's assumption because the behavior of

Figure 6. Load-displacement rela-tionship

-80

0

0

40

80

-0.08 -0.04 0 0.04 0.08

No.1

Displacement (m)

Loa

d (k

N)

-4

-80

-40

-4

Ene

rgy

Dis

sipa

tion(

kNm

)R

esid

ual D

isp(

m)

.08 -0.04 0 0.04 0.08

Displacement (m)

-80

0

0

40

-0.08 -0.04 0 0.04 0.08

Loa

d (k

N)

Displacement (m)

40

80

No.4

N)

80

No.3

-80

-40

0

-0.08 -0.04 0 0.04 0.08

Loa

d (k

Displacement (m)

0

40

80

-0

No.2

Loa

d (k

N)

Figure 7. Residual Displacement

Figure 8. Amount of absorbed energy

-0.06

-0.03

0

0.03

0.06

-0.08 -0.04 0 0.04 0.08

No.1No.2No.3No.4

Disp of Loading Point(m)

0

20

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100

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No.1No.2No.3No.4

Disp of Loading Point(m)

the bars is independent from that of the RC pier between the anchors. The bars are deformed following the deformation of the RC pier satisfying the compatibility condition. Therefore, additional axial force and moment due to the unbonded bars are calculated by compatibility condition of deformed RC fiber element. And, these forces due to the unbonded bars are loaded to anchor point of analytical model of UBRC pier.

In this study, numerical analyses for cyclic loading are carried out in order to assess adequacy of analytical model for UBRC pier. The dimensions of UBRC piers for analysis are same as experimental specimens.

The analytical results of specimens shown in Figure 6 are shown in Figure 10 respectively. Comparing with the experi-mental results, it is found that the analyses can predict the ef-fect of the unbonded bars very well. From these results, it veri-fies that fiber model together with considered effect of unbonded bars can be used for accurate numerical of UBRC pier.

5 HYBRID EARTHQUAKE LOADING TESTS

5.1 Outline

In order to verify the seismic performance of the UBRC pier, the hybrid earthquake loading tests were carried out. In these tests, the inelastic restoring force characteristics are input into a computer to calculate earthquake response, which is simultane-ously applied to the specimen. So, accurate earthquake response and performance of specimen can be evaluated with this test.

In the hybrid earthquake loading tests, we constructed three 1/7.5 scale models of existing highway bridges6). The speci-mens had a 320mm square cross section and the loading height of 1.28m (Figure 11). In case of the UBRC pier, PC rods with the unbonding treatment were installed at 110mm from the cen-

Figure 9. Strain distribution of unbonded bars for No.2 specimen

igh

mm

)

0 1000 2000 3000 4000 50000

20

40

60

80

Strain

He

t(

（μ ）

0

40

80

-0.08 -0.04 0 0.04 0.08

No.1

Loa

d (k

N)

Displacement (m)

-40

0

40

80

No.2

Loa

d (k

N)

Figure 10. Analytical results for cyclic loading analysis

-80

-40

-80

-40

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No.3

Displacement (m)

Loa

d (k

N)

-80

-40

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-0.08 -0.04 0 0.04 0.08

No.4

Displacement (m)

Loa

d (k

N)

-80-0.08 -0.04 0 0.04 0.08

Displacement (m)

ter of the cross section. Both ends of the bars were anchored in the concrete mechanically.

The Kobe JMA record (NS dir.) (Figure 12) was used as input ground motion. By considering the similarity law, using the scaled model, the tests carried out seismic response calculation for the actual-size structure in the computer7). The total weight of superstructure is set as 5000kN assuming it is the steel I-shape girder. In the tests, the axial stress was set to 0.88MPa.

5.2 Results of hybrid earthquake loading tests

The results of No.1 and No.2 are shown in Figure 13. These results are plot-ted for true prototype struc-ture.

From the load-displacement hysteresis loop of No.2, the positive post-yield stiffness can be observed. Also, the shape of hysteresis loop had the linearity compared with that of No.1. It is found that the effect of the unbonded bars can be recognized as sinuous to that observed during the cyclic loading tests.

Figure 11. Specimens of hybrid earthquake loading tests

Figure 12. Kobe JMA record (NS dir.)

-1000

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0 5 10 15 20 25 30

Acc

eler

atio

n (g

al)

Time(s)

450

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40@16=640

Unit(mm)

D10, SD295

D4, SD345

Loading point

1100

250

320

320

68@424 24

SD345 D4

110@2

High strength bars (PC tendon φ 9.2)

No.1 (RC)

No.2 (UBRC)

From the displacement time histories, the maximum responses in both cases of No.1 and No.2 are about 30 cm, so no significant difference due to the installation of bars was obtained in this case. One of the reasons for the small difference of the maximum response is that the input motion is insensitive to the variation of post-yield stiffness of the structure. But during 6, 7 seconds, No.2 returned to the original position, but No.1 returned only to about 0.12 m. As the result, the residual displacement of No.2 after the earthquake was only 1.5 cm, whereas that of the No.1 was 5 cm. This result shows that by installing unbonded bars, the residual displacement after earthquake can be reduced and the hys-teretic behavior can be stabilized.

6 CONCLUSION

1. From the view of high performance of RC piers against strong ground motion, a structure with post-yielding stiffness has advantages over conventional RC piers with no post-yielding stiffness.

2. By installing the unbonded bars into RC section, it is possible to generate the post-yielding stiff-ness of RC structure, which makes the hysteretic behavior stable From the results of the cyclic loading tests, generation of the post yielding-stiffness is confirmed.

3. This structure can be realized just by installing unbonded bars into RC section with conventional anchors. Therefore, the proposed structure can be constructed with simple and easy construction works and hence with low cost.

Figure 13. Result of hybrid earthquake loading test

-0.4

-0.2

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0.4

0 5 10 15 20 25 30

No.1 (RC)No.2 (UBRC)

Dis

plac

emen

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)

Time(s)

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No.1 (RC)

Loa

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D isplacement (m)

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No.2 (UBRC)

Loa

d (k

N)

D isplacement (m)

4. From good agreement between numerical analyses and cyclic loading test, it is verified that the fiber model with consideration of compatibility condition between RC element and unbonded bars element can accurately predict dynamic behavior of UBRC piers.

5. From the results of the hybrid earthquake loading tests, it is found that the inelastic seismic re-sponse of the UBRC structures is stable. Installation of unbonded bars reduces especially the re-sidual deformation after strong earthquakes.

6. Considering easy construction works and low construction cost, the UBRC structure can be im-plemented for construction of bridge piers with high seismic performance.

REFERENCES

1) H. Iemura, Y Takahashi, N. Sogabe: Innovation of High-Performance RC Structure with Unbonded Bars for Strong Earthquakes Structures (in Japanese ), Journal of JSCE, Vol 1-60, No. 710, pp283-296, 2002

2) Japan Society of Civil Engineers: Proposal on Earthquake Resistance for Civil Engineering Structures (in Japanese ), Magazine of JSCE, Vol 80, No.7, 1995

3) Japan Society of Civil Engineers: Second Proposal on Earthquake Resistance for Civil Engineering Structures (in Japanese ), Magazine of JSCE, Vol 81, No.2, 1996

4) Japan Road Association: Seismic Design Specification for Highway Bridge (in Japanese). Maruzen Ltd., 1996

5) Ikeda, S. :Seismic Behavior of Reinforced Concrete Columns and Improvement by Vertical Prestressing, Proc. of the 13th FIP Congress on Challenges for Concrete in the Next Millennium, Vol.2, pp879-884, 1998

6) Hoshikuma J., Unjo S., Nagaya K.: Cyclic Loading Tests of Real-Size RC Piers (in Japanese), Proc. of the 4th Symposium on Ductility Design Method for Bridges, pp198-194, 1999

7) Saizuka K., Itho Y., Kiso E. Usami T.: A Consideration on Procedures of Hybrid Earthquake Response Test Taking Account of the Scale Factor (in Japanese), Journal of JSCE, Vol.1-30, No.507, pp179-190, 1995