analysis of rc structures retrofitted with...

ANALYSIS OF RC STRUCTURES RETROFITTED

WITH BUCKLING RESTRAINED BRACES1

Ramazan Özçelik Akdeniz University

Faculty of Engineering Civil Engineering Department

Dumlupınar Bulvarı

07058

Turkey

e-mail: [email protected]

telephone: +90 242 310 63 73

Elif Firuze Erdil Akdeniz University

Faculty of Engineering Civil Engineering Department

Dumlupınar Bulvarı

07058

Turkey

e-mail: [email protected]

telephone: +90 242 310 63 23

Abstract This study examines the strengthening of existing deficient reinforced concrete (RC)

structures with buckling restrained braces (BRBs). In this study, an existing deficient three-

story RC building strengthened with BRBs was analyzed. The analyses were performed by

using Opensees program capable of performing nonlinear time history analysis (NTHA). In

the NTHAs, Duzce ground motion record was used. According to the results of analyses,

inter-story drift ratios (IDR) of the building were 5% and 1.5% before and after

strengthening, respectively. Furthermore, the performance of the structural members

(columns and beams) were also evaluated based on the Turkish Earthquake Code (TEC2007).

These performance values show that the existing RC building has an inadequate level of

performance with its current state and should be retrofitted with a suitable technique. On the

other hand, the performance level of the existing building strengthened with BRBs was at a

sufficient performance level. Consequently, the NTHAs results indicated that instead of

concrete shear wall or jacketing, using BRBs as a strengthening method was found to be

sufficient to improve the seismic performance of deficient RC building.

Keywords: deficient RC structures; seismic retrofit; buckling restrained braces; nonlinear

time history analysis

1The research discussed in this paper was conducted at Akdeniz University-Structural Mechanics Laboratory.

Funding provided by TÜBİTAK (projects no: 112M820) are greatly appreciated.

Scientific Cooperations Journal of Civil Engineering and Architecture, Vol. 3, Issue. 1, December-2017

34

1. Introduction

There are huge number of buildings that are deficient against earthquake effect in the

seismic zone areas all over the world. Lack of knowledge about seismic risk or to malpractice

and insufficient quality control during construction are reasons of deficiencies in the

buildings. The main deficiencies of such buildings are low strength concrete (in the range of 8

to 15 MPa), insufficient spacing of transverse reinforcement in beams, columns and beam-

column joints and insufficient splice length at column critical regions that may result in

excessive bond slip of plain longitudinal reinforcement. It was observed from recent

earthquakes (Northrige 1994, Kobe 1994, Kocaeli 1999, Taiwan 2003, L’aqulia 2009, Van

2011) that such buildings collapsed or had severe damage. Hence, these buildings need to be

upgraded against such seismic effects before a severe earthquake.

The most well-known seismic retrofitting techniques is adding structural shear walls

(Frosch et al., 1996; Altın et al., 1992; Canbay et al., 2003), steel braces (Badoux and Jirsa,

1990; Bush et al., 1991; Pincheira and Jirsa, 1995; Masri and Goel, 1996; Ghobarah and

Abou-Elfath, 2001; Ozcelik and Binici, 2006; Youssef et al., 2007; Ozcelik et al., 2012;

Ozcelik et al., 2013), bonding FRP diagonal braces on the infill walls (Binici et al., 2007;

Erdem et al., 2006). In addition, precast concrete shear walls (Kazunori et al., 1999), external

steel frames (Tsunehisa et al., 1999; Ozcelik et al. 2011) and internal steel frames (Takahiro

and Yasuyoshi, 2005; Ozcelik et al., 2011; Ozcelik et al., 2013) are some other alternative

retrofitting techniques. The most common structural-level strengthening techniques structural

RC shear walls because such walls provide significant lateral stiffness, strength, and energy

dissipation capacity (Jirsa, 1991; Altın et al., 1992; Canbay et al., 2003). On the other hand,

the structural shear wall requires interrupting building use for a substantial period of time and

may conflict with architectural requirements. Moreover, the shear wall retrofitting is restricted

to only perimeter frames due to minimizing the disturbance to occupants (Pincheira, 1993).

Hence, this causes the concentration of the lateral forces in the walls that may require heavy

strengthening works of the foundation (Jirsa, 1991). The choice of the retrofitting technique to

be applied on deficient buildings depends on the economical and architectural concerns,

foundation type of the structure, disturbance limits to owners and occupants, government

permissions, and design codes. Hence, not only one but also more than one technique can be

used for any retrofitting project. The main advantages of strengthening with steel braces are

the addition of minimal mass to the structure, little disturbance on the functioning of the

building, and its ability to accommodate openings for architectural purposes.


35

The seismic retrofitting of deficient RC structure by using buckling restrained brace

(BRB) is a new topic for about ten years. The BRBs as a new bracing system have stable

hysteretic behavior under axial compression and tension demands. This means that the BRBs

have almost same compression and tension capacity. They can be yield under tension and

compression without buckling (Ozcelik et al., 2017 (a)). Hence, the BRBs as a lateral load

carrying members as well as energy dissipater become popular in seismic regions all over the

world. In order to investigate the performance of the RC buildings strengthened by using

BRBs, this case study was experienced. The details of the braced building such as connections

and structural members and BRB details and analysis method also given in this paper.

An experimental test frame with and without BRBs has been examined by authors and

then the analytical study has been conducted to calibrate test frame with BRBs by using

Opensees program (another study presented at this conference (Ozcelik and Erdil, 2017)). The

analytical study of the test frame retrofitted with BRBs indicated that the global behavior can

be simulated by using Opensees program. In this study, based on calibrated modelling

strategy of the BRB braced RC frame, an existing three story public building strengthened

with BRBs was evaluated by using procedure suggested in the TEC 2007. The Duzce

earthquake record was utilized in the NTHAs to determine the performance points of the

building before and after retrofitting. This study also explains the installation procedure of the

BRBs to the RC frame.

2. Structural Details of the Building

The performance based design of existing three story RC building located in the city

of Antalya in Turkey is presented. Within this study, a performance evaluation method based

on NTHA was carried out using structural data taken from side observation and material tests.

The strengthening of the building based on the methodology has been described in another

study presented at this conference (Ozcelik and Erdil, 2017). Figure 1a indicates the plan view

of the building. Average uniaxial compressive strength, obtained from core specimens, is 14

MPa (nearly close to test frame). The steel grade of the longitudinal and transverse

reinforcement obtained from tensile test is S220 with the yield strength of 350 MPa. The 8-

14 plain bars and 8 / 20 cm stirrups was determined in the columns by the in-situ method of

column stripping. The transverse bars are bended 90 degrees. The clear cover of the columns

is 4 cm. The top and bottom reinforcement ratios of the beams were determined as 0.006 and

0.003, respectively. The stirrups spacing of the beams are about 20 cm with a clear cover of 4


36

cm. The dimensions of the building in both directions called as x and y are 12.2 m and 28.4 m,

respectively. The dimension of the columns is 40x50 cm and the dimension of the beams in x

and y direction is 30x60 and 40x60 cm, respectively. The orientation and also size of the

beams and columns are shown in Figure 1b. It is important to mention that the stirrup spacing

of columns and beams does not satisfy the current code TEC2007. Furthermore, the in-situ

concrete strength is lower than the code specified minimum.

Figure 1 Three Story Building a) Plan View, b) Column and Beam Dimensions

3. Strengthening Building with BRBs

The RC building modeled with the OPENSEES program was strengthened and

evaluated only in x direction. Figure 2 indicates the strengthened bays of the building. There

were two strengthened cases namely BRB 1 and BRB 2. The area of the BRBs used in the

case of BRB 1 and BRB 2 was 1100 and 2150 mm2, respectively. Due to architectural

reasons, BRBs were added to only 12 bays (Figure 2). BRBs were connected to the structure

in a diagonal configuration. The details of the connection were made in accordance with the

details described in another study presented at this conference (Ozcelik and Erdil, 2017).

Hence, the BRBs were modeled as truss elements in the OPENSEES (Ozcelik et al., 2017 (a);

Ozcelik et al., 2017 (b)). Thus, length of the truss element was taken up to the plastic zone of

the core plate and the other connections (column-beam-BRB-gusset plate) were defined as

rigid.

There is no detailed resource on strengthening reinforced concrete buildings with

BRBs. For this reason, the cross section areas of BRBs which used to strengthen the building

could only be determined by trial and error.


37

Figure 2 a) Strengthened Bays of the Building

The following method has been followed to avoid the loss of time during determining

the area of BRBs. First of all, the total weight of the building was calculated. It was assumed

that the BRBs added to the building carried the entire base shear forces obtained by using R =

1.5 and R = 3 values. Since the BRBs increased the horizontal stiffness of the building, it was

assumed that S(T) reached its maximum value (Based on TEC2007 S(T)=2.5). The building is

in the center of Antalya and therefore it is in the 2nd degree earthquake zone.

R

ITSAWVt

))(( 0

)1(

cos12)35.1(

y

torRBRB

f

VA

)2(

Where, Vt: Base shear force, A0: Effective ground acceleration coefficient (2nd degree

earthquake zone (TEC2007)), S(T): Spectrum coefficient (maximum value (TEC2007)), I:

Building importance factor, ABRB(R=1.5): Cross sectional area of the core plate of BRBs for

R=1.5, fy: yield strength, θ: horizontal angle of the BRBs. When R was taken as 1.5, this force

was considered to be carried by the BRBs and so, ABRB(R=1.5) was calculated as 2191 mm2.

Similar calculations were made for R=3 and the ABRB(R=3.0) was calculated as 1095 mm2.

Consequently, the cross section areas of the BRBs determined as ABRB(R=1.5) = 2150 mm2 and

ABRB(R=3.0) = 1100 mm2 is used during the NTHAs of the building.


38

Figure 3 Ground Motion Properties

The calculated cross sectional areas of the BRBs were added to the numerical model

and the NTHAs were performed under the Duzce ground motion record (Figure 3). The strain

values of the concrete and reinforcement at the columns obtained from analyses were

compared with the limit values given in TEC2007 and then the seismic performance of the

RC building was evaluated. In addition, if the axial load demands of the RC columns

exceeded 85 % of the column axial load capacity, these columns were assumed to be

strengthened with local retrofitting techniques such as FRP or jacketing.

4. Results of the Analysis and Performance

Analysis results of the RC building before and after strengthening with BRBs are

shown in terms of top displacement vs. time in Figure 4. Figure 5 indicates the IDRs of the

building before and after strengthening. As seen in the both figures, the BRBs could control

the displacement of the floor displacements as expected. After strengthening with BRBs the

maximum IDR of the building was about 1.5 % (in the case of BRB 1) and 1.0 % (in the case

of BRB 2) while before strengthening it was about 5 %. These results also show that the IDRs

values were decreased in proportion to the cross-sectional area of the BRBs. Figures 6 and 7

show the structural member-based performance evaluations of the building before and after

strengthening with BRBs according to TEC2007. As seen from these figures, the building had

an inadequate level of performance with its current state. Therefore, the existing RC building

should be strengthened by a suitable strengthening method. After the existing RC building

was strengthened with BRBs, the seismic performance was improved to an adequate level

according to the TEC2007. Hence, it was concluded that, the building strengthened with

BRBs has sufficient seismic performance under the Duzce ground motion record.


39

Figure 4 Analysis Results in terms of Top Displacement vs. Time

Figure 5 Analysis Results in terms of IDRs vs. Stories


40

Figure 6 Strain Values of Columns’ Concrete


41

Figure 7 Strain Values of Columns’ Reinforcement


42

3. Conclusion

The NTHA results indicated that the BRBs increased the lateral strength, stiffness and

energy dissipation capacity of the deficient RC frames with respect to cross-sectional area of

the BRBs. Based on the data and model calibrations obtained from the experimental studies, a

case study of existing RC building before and after BRB retrofit was analyzed and evaluated

by utilizing procedures suggested by the TEC2007. It was determined that although

seismically deficient three story RC building did not satisfy the performance levels with

respect to TEC20007, after retrofitting with BRBs its performance level was found to be

adequate. As a result, the BRBs can be considered as a rapid, safe and practical retrofitting

technique.

References

[1] ALTIN, S., ERSOY, U., TANKUT, T. Hysteretic Response of Reinforced-Concrete

Infilled Frames. Journal of Structural Engineering (ASCE). 1992, vol. 118, no, 8, pp.

2133-2150.

[2] BADOUX, M., JIRSA, J.O. Steel Bracing of RC Frames for Seismic Retrofitting.

Journal of Structural Engineering (ASCE). 1990, vol. 116, no. 1, pp. 55-74.

[3] BINICI, B., OZCEBE, G., OZCELIK, R. Analysis and Design of FRP Composites for

Seismic Retrofit of Infill Walls in Reinforced Concrete Frames. Composites Part B:

Engineering. 2007, vol. 38 no. 5, pp. 575-583.

[4] BUSH, T.D., JONES, E.A., JIRSA, J.O. Behavior of the RC Frames Strengthened Using

Structural Steel Bracing. Journal of Structural Engineering (ASCE) 1991, vol. 117, no.

4, pp. 1115-1126.

[5] CANBAY, E., ERSOY, U., OZCEBE, G. Contribution of Reinforced Concrete Infills to

Seismic Behavior of Structural Systems. ACI Structural Journal. 2003, vol. 100, no. 5,

pp. 637-643.

[6] ERDEM, I., AKYUZ, U., ERSOY, U., OZCEBE, G. An Experimental Study on Two

Different Strengthening Techniques for RC Frames. Engineering Structures. 2006, vol.

28, no. 13, pp. 1843-1851.

[7] FROSCH, R.J., LI, W., JIRSA, J.O., KREGER, M.E. Retrofit of Non-Ductile Moment-

Resisting Frames Using Precast Infill Wall Panels. Earthquake Spectra. 1996, vol. 12,

no. 4, pp. 741-760.


43

[8] GHOBARAH, A., ELFATH, H.A. Rehabilitation of A Reinforced Concrete Frame Using

Eccentric Steel Bracing. Engineering Structures. 2001, vol. 23 no. 7, pp. 745-755.

[9] JIRSA, J.O. Divergent issues in rehabilitation of existing buildings. Earthquake Spectra.

1991, vol. 10, no. 1, pp. 95-112.

[10] KAZUNORI, I., NORIMITSU, H., KIYOSHI, H., KEI, H. Development of Seismic

Strengthening Technology Using the Fitting Shear Wall Construction Method and the

External Frame Method. Kumagai Technical Research Report. 1999, vol. 58, no. 1, pp.

61-67. (in Japanese).

[11] MASRI, A.C., GOEL, S.C. Seismic Design and Testing of an RC Slab-Column Frame

Strengthened by Steel Bracing. Earthquake Spectra. 1996, vol. 12, no. 4, pp. 645-666.

[12] OZCELIK, R. Seismic Retrofit of Deficient RC Structures with Internal Steel Frames.

Advances in Structural Engineering. 2011, vol.14, pp. 1205-1222.

[13] OZCELIK, R., BINICI, B., AKPINAR, U. Seismic Retrofit of Non-Ductile Reinforced

Concrete Frames with Chevron Braces. Proceedings of the Institution of Civil Engineers

- Structures and Buildings. 2013, vol. 166, pp. 326-341.

[14] OZCELIK, R., BINICI, B. Application of Steel Retrofit Schemes for Deficient Buildings

in Turkey. First European Conference on Earthquake Engineering and Seismology,

Geneva, 2006.

[15] OZCELIK, R., BINICI, B., KURC, O. Pseudo Dynamic Testing of an RC Frame

Retrofitted with Chevron Braces. Journal of Earthquake Engineering. 2012, vol. 16, no.

4, pp. 515-539.

[16] OZCELIK, R., BINICI, B., KURC, O. Seismic Retrofitting of Deficient RC Structures

with Internal Steel Frames by Using Pseudo Dynamic Testing Procedure. 15. World

Conference of Earthquake Engineering, Lisbon, Portugal, 24-28 October 2012, pp. 1-10.

[17] OZCELIK, R., BINICI, B., KURC, O. Pseudo Dynamic Test of a Deficient Reinforced

Concrete Frame Upgraded with Internal Steel Frames. Earthquake Engng Struct. Dyn.

2013, vol. 42 no. 5, pp. 763-778.

[18] OZCELIK, R., DIKICIASIK, Y., ERDIL, E.F. (a) The Development of the Buckling

Restrained Braces with New End Restrains. Journal of Constructional Steel Research.

2017, vol. 138, pp. 208-220.

[19] OZCELIK, R., DIKICIASIK, Y., ERDIL, E.F. (b) 2017. Buckling Restrained Braces

with Composite Design Technique. 3rd International Workshop on Civil Engineering and

Architecture.


44

[20] OZCELIK, R., ERDIL, E.F. Nonlinear Time History Analysis of RC Frame Retrofitted

with Buckling Restrained Braces. 3rd International Workshop on Civil Engineering and

Architecture. Istanbul, Turkey. December 16-17, 2017.

[21] PINCHEIRA, J.A., JIRSA, J.O. Seismic Response of RC Frames Retrofitted with Steel

Braces or Walls. Journal of Structural Engineering (ASCE). 1995, vol. 121, no. 8, pp.

1225-1235.

[22] Turkish Earthquake Code. Specifications for structures to be built in seismic areas.

Ministry of Public Works and Settlement, Ankara, Turkey, 2007.

[23] TSUNEHISA, M., KAZUYUKI, S., TSUNEAKI, I. A Study on Seismic Strength Method

Using Brace of Outer Frame. Technical Research Report of Hazama Corporation. 1999,

vol. 31, pp. 17-25. (in Japanese).


45

analysis of rc structures retrofitted with...

Documents