ANALYSIS OF RC STRUCTURES RETROFITTED WITH BUCKLING RESTRAINED BRACES
1Ramazan 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.
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.
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
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.
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
I T S A
Vt W( 0 ( ) )
) 1 (
cos
) 12
3 5 . 1
(
y t or
R
BRB f
A V
) 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.
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.
Figure 4 Analysis Results in terms of Top Displacement vs. Time
Figure 5 Analysis Results in terms of IDRs vs. Stories
Figure 6 Strain Values of Columns’ Concrete
Figure 7 Strain Values of Columns’ Reinforcement
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.
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