seismic performance of rc structures strengthened with ... · elastic modulus （gpa） d6 360 515...

Fédération Internationale du Béton Proceedings of the 2nd International Congress June 5-8, 2006 – Naples, Italy ID 9-36 Session 9 – Seismic evaluation of concrete structures

Seismic Performance of RC Structures Strengthened with Precast Prestressed Concrete Braces Kono, S., Watanabe, F. Department of Architecture and Architectural Engineering, Kyoto University, Nishikyo, Kyoto, 6158540, Japan INTRODUCTION Many buildings designed using the pre-1981 Japanese building standards experienced serious damage during the Kobe Earthquake in 1995 and it was recognized that such buildings were in need of prompt seismic retrofits. During the same year, the national organization promoting the evaluation of seismic susceptibility of buildings was founded and a law was enacted to promote seismic upgrades to deficient buildings. Since that time, a few public schools and government offices started upgrading their buildings, but the Ministry of Land Infrastructure and Transport recently announced that only 6% of 90,000 public schools and government offices had finished seismic upgrading and 1.2 million private buildings need seismic upgrading. Many excellent seismic upgrading schemes enhancing strength and/or ductility already are available. However, seismic upgrading has made little progress due to reasons such as: suspension of building services during long construction periods, noisy and taxing construction process, and high cost. The probability of major earthquakes in the Japanese urban area between Tokyo and Osaka is reported as 80% in next thirty years. Before the existing old buildings are damaged by earthquakes, some measures need to be taken promptly. The purpose of this research is to develop a simple seismic retrofit method satisfying the following criteria.

a) No re-bar connection or bolt anchorage between the brace and existing frames. b) A short construction period. c) Low construction costs, including the out-of-service loss.

The authors have already proposed a precast, prestressed concrete brace system [1] that needs no bolt anchorage, resulting in a quiet, quick, and economical seismic upgrading scheme. The system was originally devised as a strengthening scheme and the shear deformation mode was assumed to dominate in design procedures as shown in Fig. 1(a). However, the system will be used widely if it exhibits ductility. Large ductility can be achieved by allowing the multiple-braced frame to undergo flexure-dominant deformation mode and fail due to the tensile yielding of the first story column as shown in Fig. 1(b). In this paper, experiments on two types of test specimens are explained. They were designed to show either shear-dominant or flexure- dominant deformation modes in order to demonstrate the efficiency of the system in both strength and ductility enhancing retrofit schemes as shown in Fig. 1(c). It was confirmed that local failure modes such as direct shear failure near the beam-column joint or bearing failure at the brace interface did not take place before the braced frame reached the required lateral load capacity or deformation capacity. Attention was also paid that excessive moment did not act on braces so that the brace section is designed with the minimum dimension as a concentrically compressed member. After summarizing the experimental results, some major design issues on the proposed system are discussed. Keywords: seismic retrofit, precast prestressed concrete brace, strength enhancement, ductility enhancement

Proceedings of the 2nd Congress Session 9

June 5-8, 2006 – Naples, Italy Seismic evaluation of concrete structures

2

RETROFIT USING PRECAST PRESTRESSED CONCRETE BRACES Watanabe [1] previously presented the fundamental concept of the proposed retrofit scheme. A precast prestressed concrete brace consists of multiple precast units. Figure 2 shows a five-unit assemblage with four legs and one central unit. These units are assembled at a construction site and prestressing force is introduced to two diagonal directions using external cables. Gaps between brace ends and frame corners are filled with high strength no-shrinkage mortar. After grout mortar hardens, the prestressing force is released so that the brace extends to fix to the existing frame by itself. When the frame with the brace is subjected to lateral seismic load, only one of diagonal members works effectively in compression. Without the prestressing force, the tension diagonal member would lose contact with the frame. In order to prevent this, a device made of flat springs and steel pipe (FSSP) in Fig. 2(b) is installed at the bottom end. This device maintains a minimal compressive force in the diagonal member while it undergoes elongation. When a sufficient compressive force is applied to the FSSP device, a top bearing steel plate touches a steel pipe. This steel pipe section was designed to have enough strength for the axial compression even if a diagonal member fails in axial compression. It also maintains compressive force in the diagonal member when the diagonal member undergoes elongation up to a certain limit.

　

　

　

Existing building hasinsufficient load and deformation capacity.

Drift

Load

Strengthenhancement

Ductility enhancement

Intermediate upgrade

(a) Shear deformation (b) Cantilever deformation (c) Schematic representation of retrofit procedures. Fig. 1. Dominant deformation mode for the braced frame.

Steel plate

Element A

Element B

Element C

Rebars extending fromElement B are inserted in ducts embedded inElements A or C. The ducts are grouted later.

Prestressing force is introduced with external cable system.

Element C

FSSP device

Flat spring

Steel pipe

A set of flat springs

Bearing plate Bearing plate

Min

imum

leng

th

Free

leng

th

(a) Assembled brace (b) Flat spring and steel pipe (FSSP) device Fig. 2. Precast prestressed concrete brace.



3

EXPERIMENT Setups Specimen configurations Two one-story specimens (No.1 and No.2) and one two-story specimen (E1) were constructed as shown in Fig. 3. Reinforcing bars used are listed in Tab. 1. The boundary frames of three specimens modeled a four-story reinforced concrete building that was designed following the pre-1980 Japanese Building Standard. Mechanical properties of materials are shown in Tab. 2. Test variables for three specimens are listed in Tab. 3. No. 1 was designed to fail due to the buckling of a brace, No. 2 due to the direct shear failure at the column top, and E1 due to tensile yielding of the first story column.

300300 2100 300 300Prestressing bar- 32 to simulatethe long term axial loading.

φ

Prestressing bar- 13 2 toprevent beam yielding in tension

×φ

φ4@100

φ4@70

φ4@50

2075

1425

325

325P1

P2Beam section 275x325

Column sect ion300x300

Brace sect ion100x120 for No. 1120x150 for No. 2

(a) No. 1 and No. 2 (b) E1 Fig. 3. Test specimen and reinforcement (unit: mm).

Tab. 1. Reinforcing bars used in specimens. (a) No. 1 and No. 2 (b) E1

Rebar used Amount ofrebar （%）

Longitudinal 16-D10 1.268

Shear φ4@70 0.120

Longitudinal Upper　4-D10Lower 4-D10

0.3510.351

Shear φ4@100 0.0914


Shear φ3@50 0.268


Shear φ3@50 0.214


0.1210.121

Shear φ4@50 0.0503

Foundationbeam

Location

Column

Beam

Brace forNo. 2

120x150mm

Brace forNo. 1

100x120mm

Rebar used Amount ofrebar （%）


Shear φ4@70 0.119


0.6490.649

Shear φ4@100 0.0909

Longitudinal Upper　2-D6Lower　2-D6

0.3560.356

Shear φ3@50 0.188


0.1210.121

Shear φ4@50 0.050

Foundationbeam

Location

Column

Beam

Brace



4

Tab. 2. Mechanical properties of materials. (a) Concrete (b) Reinforcing bars

Specimen MemberCompressive

strength（MPa）

Tensilestrength（MPa）

Elasticmodulus（GPa）

Originalframe 27.5 2.53 26.1

Brace 24.9 2.58 23.3Originalframe 25.0 2.51 24.3

Brace 64.1 4.52 33.3Originalframe 24.9 2.35 22.5

Brace 82.7 5.58 38.0

No. 1

No. 2

E1

Specimen Bartype

Yieldstrength（MPa）

Tensilestrength（MPa）

Elasticmodulus（GPa）

D6 360 515 171D8 576 645 215D10 378 555 184φ3 576 618 230φ4 626 662 211φ6 351 484 190

D10 370 508 189D13 392 558 196

E1

No.1 andNo. 2

Tab. 3. Test variables Specimen Expected failure mode

No1 Buckling of a brace. Brace section was 100mm by 120mm.No2 Direct shear at a column top. Brace section was 120mm by 150mm.E1 Tensile yielding of the first story column.

Loading Loading system for No. 1 and No. 2 is shown in Fig. 4(a). Same amount of horizontal loads was applied at both ends with two hydraulic jacks. Load was applied twice at the prescribed drift and the drift was incremented by 1 mm. Axial load of 300 kN was kept constant for each column throughout the test to simulate long-term gravity load. Loading setup for E1 is shown in Fig. 4(b). Lateral load was applied with a 1MN hydraulic jack. When it extended, force transferred to the north end of the loading beam. When the jack extended, the applied load was transferred through the north end of the loading beam. When it contracted, it pulled on bars that extended to the south side of the loading beam, and the applied force was transferred in compression through the south end of the loading beam. In this way, the loading condition was symmetric in the positive and negative directions. Loading was controlled by the loading point drift. Columns were subjected to axial force variation as follows. 1 2 234 0.73N and N Q= ± (kN) (1) where 1N and 2N are south and north axial forces in kN, respectively, and Q is the lateral force.

Oil jack A Oil jack BSpecimen

NORTH SOUTH

Load cell Load cell

Posit iveloading

Negativeloading

Center hole jackLoad cell

(a) No. 1 and No. 2 (b) E1 Fig. 4. Loading setup.

Hydraulic jack Hydraulic jack



5

Experimental results Lateral load-loading point drift relation The lateral load - total drift ratio relations for No.1 and No.2 are shown in Fig. 5(a) and (b). Both curves show elastic response with small energy dissipation until failure. Concrete of braces started to crush when the drift angle (R) reached 0.45% for No.1 and longitudinal reinforcement yielded when the drift reached 0.55% for No.2. Lateral load-drift relation for E1 in Fig. 5(c) shows different behavior in that it had much larger deformation capacity after the peak. This large deformation was made possible by tensile yielding of the first story columns. Stiffness of the frame decreased first when the flexural cracking started at R= 0.05± %, then again when longitudinal bars of columns started to yield at R= 0.4± %. The maximum lateral load in positive direction, +324 kN, was recorded at R= 1.5+ %. The force in the diagonal brace at the first floor was computed from the mean strain measured with strain gages on all four brace surfaces and the stress-strain relationship obtained from a cylinder compression test. The horizontal component was subtracted from the total lateral load to obtain the contribution due to the two columns of the original frame. The portion of the lateral load carried by the original frame is compared to the total load carried by the braced frame in Fig. 5. The lateral load carrying capacity of braced frames was nearly three times larger than that of the original frames for all three specimens. In addition, the stiffness of the braced frame was much higher than that of the original frame. Three braced specimens showed some slip near the origin. The axial stiffness of a brace changes when the steel tube of FSSP touches/separates from the bearing plate. The amount of slip deformation depends on several parameters such as the initial prestressing force and the stiffness of FSSP device. Although the effect of this slip deformation to the seismic performance has not been examined, a small amount of slip deformation is considered permissible to some extent for elastic responding systems or strength resisting systems. Damage process and failure mode No.1 failed due to crushing of the brace concrete at R=0.45% and No.2 failed due to yielding of longitudinal reinforcement of the beam at R=0.55% as shown in Fig. 6(a) and (b). Damage to the other members was restricted to minor flexure cracks. Observed damage of E1 is shown in Fig. 6(c). Flexural cracking was observed at R= 0.05± %, then tensile cracking penetrating the column sections gradually increased after R= 0.15± %. All longitudinal bars of columns started to yield in tension at R= 0.4± %. At R= 0.75+ %, almost all longitudinal bars of the first story column yielded in tension at its mid-height and base. The final failure was caused by a shear failure of the first story column under compression at R= 4.7+ %. Deformations due to shear and flexure The forces due to each flexure and shear mode are plotted against their respective deformations for E1 in Fig. 7(a) and (b), respectively. Based on these figures, dissipated energy due to each deformation mode was computed as shown in Fig. 8. Energy dissipated in flexure mode became dominant after R=0.4% at which the longitudinal bars of the first story columns yielded. Additional study showed that most of the dissipated energy in flexure mode was due to the tensile behavior of the first story columns and the deformation of the second story was negligible. Flexure-dominant deformation mode is excellent in securing large deformation capacity and dissipating large amount of energy. Behavior of brace A variation of the axial force of diagonal members with respect to drift is shown in Fig. 9 for No. 1 and No. 2. Only one of the diagonal members resisted the external force. Other diagonal member stayed rest although the force remained compressive due to the FSSP device. Kink point in the cyclic curve corresponds to the touching/separation point for the steel tube to the bearing plate. As the drift increased, a free contact zone also increased where both diagonal members did not resist to the external force near R=0%.



6

To investigate the bending moment induced to a brace concrete section, axial strains were measured at four faces of each brace. From strain measurements the brace axial force and the eccentricity of axial force in brace sections were calculated and shown in Fig. 10 for No.1 and No.2. The area inside a lozenge indicates the kern of a section of diagonal members. Calculated eccentricities are very small and all of response points fell inside the kern. This helps to design braces against buckling since the additional moment does not have to be considered. It also simplifies numerical modeling.

- 600

- 400

- 200

0

200

400

- 0. 4 - 0. 2 0 0. 2 0. 4 0. 6

Br aced f r ameOr i gi nal f r ameAnal ysi s

Loadi ng poi nt dr i f t ( %)

- 600

- 400

- 200

0

200

400

600

800

- 0. 4 - 0. 2 0 0. 2 0. 4 0. 6


Loadi ng poi nt dr i f t ( %) (a) No. 1 (b) No. 2

- 400

- 300

- 200

- 100

0

100

200

300

400

- 3 - 2 - 1 0 1 2 3 4 5


Loadi ng poi nt dr i f t ( %) (c) E1 Fig. 5. Lateral load – loading point drift relation.

Concrete crushed.

Longitudinal bars yielded.

Crack occurred atdrift=+9mm. Shear

failure

North South (a) No. 1 (b) No. 2 (c) E1 Fig. 6. Cracking of the first story after testing.



7

- 1500

- 1000

- 500

0

500

1000

1500

- 2 - 1 0 1 2 3 4θ ( %)

- 400

- 300

- 200

- 100

0

100

200

300

400

- 40 - 20 0 20 40δ s ( ㎜)

(a) Overturning moment-flexural displacement relation (b) Shear-displacement relation

Fig. 7. Energy dissipation for flexure and shear deformations for E1.

0

10

20

30

40

50

60

70

80

0 0. 5 1 1. 5 2 2. 5

Fl exur al def or mat i onShear def or mat i on

Loadi ng Poi nt Dr i f t ( %) Fig. 8. Energy dissipation for flexure and shear deformations for E1.

-50

0

50

100

150

200

250

300

-8 -4 0 4 8

No.1

Diagonal 1Diagonal 2

-0.4 -0.2 0 0.2 0.4 0.6

Bra

ce c

ompr

essi

ve fo

rce

(kN

)

Lateral displacement (mm)

Drift angle (%)

-200

0

200

400

600

800

-8 -4 0 4 8

No.2

Diagonal 1Diagonal 2

-0.4 -0.2 0 0.2 0.4 0.6

Bra

ce c

ompr

essi

ve fo

rce

(kN

)

Lateral displacement (mm)

Drift angle (%)

(a) No. 1 (b) No. 2 Fig. 9. Compressive force in diagonal members– lateral displacement relationships

M1

θδ f

M2

δ s

γ

Q



8

-16

-12

-8

-4

0

4

8

12

16

-20 -15 -10 -5 0 5 10 15 20

Section-1Section-2Section-3Section-4

Eccentricity in X direction (mm)

Ecc

entri

city

in Y

dire

ctio

n (m

m)

Kern of a section

No.1 Specimen-20

-15

-10

-5

0

5

10

15

20

-20 -10 0 10 20

Section-1Section-2Section-3Section-4

Ecc

entri

city

in Y

dire

ctio

n (m

m)

Eccentricity in X direction (mm)

No.2 Specimen

Kern of a section

(a) No. 1 (b) No. 2 Fig. 10. Eccentricities of axial force induced to brace sections

0

5

10

15

20

25

0 0. 5 1 1. 5 2 2. 5

E1No. 1

Loadi ng Poi nt Dr i f t ( %)

×Cr ushi ng ofbr ace concr et e

Fig. 11. Variation of equivalent viscous damping.

Equivalent viscous damping In Fig. 11, equivalent viscous damping ratio - drift relations for No.1 and E1 are compared. Shear deformation dominated No. 1 and the equivalent viscous damping was less than 6% throughout the test. However, a flexural deformation dominated E1 and the damping was about 6% at the minimum. It increased after the yielding of longitudinal reinforcement and reached 14% at R= 1.0 %. The flexure dominant deformation exhibited very high damping factor and the design base shear can be reduced significantly.

DESIGN ISSUES RELATED TO THE PROPOSED SYSTEM Fig. 12 shows the lateral force resisting mechanism and design issues at each critical location with a construction example. The major issues are summarized below.

• Joint shear strength needs to be evaluated at the beam-column joint adjacent to the active diagonal member.

• Direct shear strength needs to be evaluated at columns and beams adjacent to the active diagonal member.

• Tensile yielding of a beam needs to be evaluated as it occurred for No. 2. • The FSSP device should be designed for its strength and deformation capacity.



9

• Initial prestressing force of braces needs to be large enough for diagonal members to work effectively under lateral loads even if prestressing loss occurs due to creep and shrinkage. However, excessive prestressing force decreases the compressive strength of braces.

• Buckling strength of diagonal member needs to be evaluated by taking into account the effect of moment although the braces of No.1 and No.2 experienced fairy small moment.

• Shear capacity of columns and beams needs to be evaluated.

Free length of a set of flat springs

Compression forcedue to lateral seismic force

Small compression force due to spring reaction

Design forjoint shear

Design for direct shear

Design for concrete bearing

Design for beam tension(beam and slab reinforcement)

Design for axial force considering buckling

Non-shrink high strength mortar

Design for additional column tension

Design lateral seismic forceat left corner

Design lateral seismic forceat right corner

Estimation of loss of brace prestressdue to creep and shrinkage under

service load condition

(a) Design issues of the proposed system (b) Retrofit using the proposed system. Fig. 12. Practical design issues and an example. CONCLUSIONS Two one-story specimens and one two-story specimen were constructed to see the seismic performance of proposed precast prestressed concrete brace retrofit system. The boundary frames of three specimens modeled a four-story reinforced concrete building that was designed following the pre-1980 Japanese Building Standard. The proposed system showed good seismic performance from the strength resisting mode to the ductility enhancement mode. In any case, the braced system showed three times as large lateral load carrying capacity as that of the original frame. Unexpected local failure modes did not happen and the system can be used widely for the existing reinforced concrete buildings susceptible to earthquake damage.

• Two specimens (No.1 and No.2), in which shear deformation was dominant, showed the final failure caused by the crushing of braces at the drift of 0.45% or tensile yielding of longitudinal reinforcement at the drift of 0.55%. The damage before the failure was minimal.

• One specimen (E1), in which flexural deformation was dominant, showed the tensile yielding of the first story column at drift of 0.4% and then very ductile deformation without degrading the lateral load carrying capacity until the final shear failure of a column at drift of 4.7%. Equivalent viscous damping for E1 is much larger than No.1 and No.2 and the design base shear for this frame can be reduced significantly.

ACKNOWLEDGMENTS The authors thank the Ministry of Land Infrastructure and Transport，the Ministry of Education, Culture,

Sports, and Technology，and the Kajima Foundation for their financial support. The authors express their sincere thanks to Neturen and Daiwa for donating experimental materials and Takenaka Co. for valuable technical advice. Mr. S. Miyazaki, Mr. Y. Okuno, Ms. Y. Watanabe, Mr. K. Takao, Mr. S. Shibata, and Mr. T. Matsuda made great contributions conducting the experiment and processing data.



10

REFERENCES 1. Watanabe F.，Miyazaki S.，Tani M.，and Kono S., Seismic Strengthening Using Precast Prestressed

Concrete Braces，The 13th World Conference on Earthquake Engineering, Vancouver, August，Paper 3406, 2004.

2. Architecture Institute of Japan, 1997. Design guidelines for earthquake resistant reinforced concrete buildings based on inelastic displacement concept. (In Japanese)

3. Architecture Institute of Japan, 2004. Guidelines for performance evaluation of earthquake resistant reinforced concrete buildings. (In Japanese)

Fig. 13. Experiment on the specimens with the proposed brace system.

seismic performance of rc structures strengthened with ... · elastic modulus （gpa） d6 360 515...

Documents