10th World Conference on Seismic Isolation, Energy Dissipation and Active Vibrations Control of Structures, Istanbul, Turkey, May 27-30, 2007

DESIGN APPROACH FOR THE SEISMIC STRENGTHENING OF AN EXISTING RC BUILDING WITH BUCKLING RESTRAINED BRACES

Di Sarno, L.1, Manfredi, G.2 and Acanfora, M.31 Department of Engineering, University of Sannio, Benevento, Italy

2 Department of Structural Engineering, University of Naples, Federico II, Italy 3 Consortium T.R.E., Naples, Italy

ABSTRACT

Structural designers may adopt traditional interventions, such as strengthening of members and/or connections or global strategies, or utilize innovative retrofitting schemes. The latter may include base isolation or supplemental damping or, in same cases, a combination of them. Bracing systems were found to be very cost-effective for seismic retrofitting of reinforced concrete (RC) multi-storey frames. In particular, innovative buckling-restrained braces (BRBs) are a viable option to be used because of their stable energy dissipation capacity especially under moderate-to-high magnitude earthquakes. The present paper focuses on the seismic performance of a RC framed building structure retrofitted by means of BRBs. The latter enhance the energy dissipation capacity of the RC structure designed for gravity loads only and increase significantly its damping capacity. As a result, the seismic demand is lowered and the structural capacity is significantly enhanced, which, in turn, minimize the vulnerability of the structural. Detailed non linear static analyses were carried out in order to investigate the inelastic performance of the structure. The results of the performed analyses demonstrate that the use of BRBs is extremely cost-efficient for seismic retrofitting of existing RC framed structures.

1. INTRODUCTION Existing reinforced concrete (RC) buildings were designed mainly for gravity loads only and hence they do not employ adequate earthquake resistance. As a consequence the local and global ductility of such structures is rather poor and seismic retrofitting is deemed necessary to ensure that they will exhibit acceptable - structural and non-structural – performance. The seismic retrofitting of existing structures can be performed by means of traditional and/or innovative strategies (e.g. FEMA 450, 2003). The former strategy consists of the local modification of material properties and/or seismic details (e.g., beam-to-column connections) and the global stiffening and strengthening of the lateral resisting systems.

Figure 1 provides a comparison between the effects of these local and global intervention schemes on the seismic structural performance. The objective of local retrofitting is to increase the deformation capacity of deficient components so that they will not reach their specified limit state as the building responds at the design level (Figure 1). Effective global upgrading strategies should be able to increase the capacity of the structure and/or decrease the demand imposed by the earthquake loads. Structures with enhanced capacities may safely resist the forces and the deformations induced by earthquake response. Generally, global modifications to the structural system are designed so that the design demands (or target displacement) on the existing structural and non-structural components are less than their capacities. Lower demands may reduce the risk of brittle failures in the structure and/or avoid the interruption of its functionality.

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Figure 1. - Characteristics of global (top) and local (bottom) intervention approaches in seismic retrofitting.

On the other hand, innovative seismic retrofitting schemes include generally base isolation or supplemental damping or, in same cases, a combination of them. The following paper focuses on the application of an innovative bracing systems, i.e. buckling-restrained braces (BRBs) to existing RC framed buildings designed for gravity loads only. Bracing is an effective upgrading strategy to enhance the global stiffness and strength of unbraced frames, either in structural steel or in RC (e.g. Sam et al., 1995; Bruneau et al., 1998; Balendra et al., 2001). It can increase the energy absorption of structures and/or decrease the demand imposed by earthquake loads whenever hysteretic dampers, e.g. BRBs, are utilized. Structures with augmented energy dissipation may safely resist forces and deformations caused by strong ground motions. Unbounded braces, as for example those shown in Figure 2, are hysteretic dampers very

popular in Japan and the US (e.g. Watanabe et al., 1988; Housner et al., 1997; Kasai et al., 1998; Iwata et al., 2000; Di Sarno and Elnashai, 2005). These devices are becoming popular also in Italy for seismic retrofitting of RC structures. They consist of a core steel plate encased in a concrete filled steel tube. Yielding of the interior component under reversal axial loads provides stable energy dissipation; the exterior concrete filled steel tube prevents local and member buckling.

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Figure 2. – Typical unbounded brace (courtesy of FIP Industriale, Italy). The following work deals with the application of BRBs for an existing RC buildings which was designed to resist gravity lads only. In so doing, detailed non linear static analyses were carried out in order to investigate the inelastic performance of the framed system. It is proved that the use of BRBs is extremely cost-efficient for seismic retrofitting of existing RC framed structures. 2. RETROFITTING OF EXISTING RC FRAMES: A CASE STUDY 2.1. Building description The sample RC building is located in Avellino (ag = 0.25 g and soil type B), in South of Italy; it was built about 30 years ago using only gravity loads. The plan layout of the building comprised two T-shaped blocks and a connecting rectangular block. The structural system consists of three stories used for school rooms and an additional roof floor utilized for insulation purposes. The ground floor is 3.08m high and hosts mainly laboratories; the first and second floors are 3.65m high (Figure 3). The top floor has an inclined tiled roof as shown in Figure 4; its height varies between 0.2m and 1.90m. The lateral resisting system of the sample school building consists of a multi-storey RC frame with deep beams. The column cross-sections are summarised in Table 1; the frame employs 30x65 deep beams.

Table 1 – Geometry of the columns of the sample frame (dimensions in cm).

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Floor

35x30 35x30 30x30 30x30 35x40 35x35 30x35 -- 40x40 30x40 30x40 30x40 40x55 40x45 40x40 --

Figure 3 - Longitudinal cross-section of the sample building.

Figure 4 - Aerial view of the sample building.

2.2. Material properties An extensive experimental test program was carried out to characterized the mechanical properties of the existing RC building. In particular, the performed tests are as follows:

• Compression tests on 19 concrete coupons (cylinders with diameter φ100 mm) from beams and columns;

• Compression tests on 19 concrete coupons (cylinders with diameter φ60 mm) from beams and columns;

• Ultrasonic tests on concrete (72 tests on beams, columns and foundations); • Tensile tests on steel reinforcement bars (2 tests on bars from beams and columns).

The concrete strength was estimated by using the following relationships: )cm/daN( 35RR 2

med,c1,ck −= ……………………… (1)

)cm/daN( 35RR 2min,c2,ck += …………………… … (2)

The computed characteristic concrete strength is for the cylinders with diameter equal to 100 mm and which are very close. The mean value derived from the ultrasonic tests is equal to . The concrete strength is conservatively assumed equal to .

2ck cm/daN185R ≅

2ck cm/daN195R ≅

2ck cm/daN216R ≅

2ck cm/daN185R ≅

The tensile tests carried out on the reinforcement bars give to a value of fyk = 2800 daN/cm2. 2.3. Seismic response of the existing structure It is instructive to note that the following preliminary interventions were used to minimize the irregularities of the lateral resisting system of the sample RC building. Such interventions include:

• Structural joints between the low-rise structure (with rectangular layout) and high-rise blocks (with T-shape) thus obtaining three independent regular buildings;

• Strengthening of the RC walls at the basement with gunite and additional light steel mesh.

The abovementioned three independent structures possess lower seismic vulnerability and can be assessed separately. The high-rise blocks may safely carry the gravity loads, however, they are inadequate to sustain the seismic displacement demand imposed on the building. The seismic response of the existing structure was computed by means of inelastic static (pushover) analysis. The construction site (Saint Thomas suburb in outskirt of the municipality of Avellino) is characterized by the bedrock acceleration equal to 0.25g. The soil type corresponds to the type B in the EC8 (2006) and hence S=1.25; the importance factor is γI = 1.20. Therefore, at the damageability limit state the peak ground acceleration is equal to 0.375g (=0.25x1.25x1.20). A lumped plasticity model was utilized to discretize the RC multi-storey frame. The plastic hinges are located at the edges of beams and columns, where inelasticity is expected under the combined actions of vertical (gravity) and horizontal (seismic) loads. Moment-rotation curves were defined for all beams and columns to define the flexural behaviour of the structural members. Values of bending moments and flexural rotations were evaluated at different limit states, e.g. cracking, yielding and collapse. The finite element (FE) model of spatial framed of the existing structure is shown in Figure 4. The FE software used for the simulations is SAP2000 (CSI, 2005).

Figure 4 – Finite element model of the existing structure. Modal and response spectrum analyses of the sample buildings were carried out on the FE model in Figure 4. It is found that the fundamental period of the structure is 0.47sec and the modes of vibration are coupled. The response curves of the existing sample building are provided in Figure 5. The displacement demands at the limit states of limit and severe damage are also included.

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Figure 5 – Capacity curves of the sample building: loading along the X-direction (top) and Y-direction (bottom).

The sample structural system does not possess adequate lateral resistance to withstand earthquakes and hence a seismic retrofitting strategy is needed as discussed hereafter.

2.4. Seismic retrofitting strategy The sample school building in Avellino was retrofitted by means of special dampers, i.e. buckling-restrained braces. The selection of dampers, their location and number, their geometric and mechanic properties was based on the following:

• Dampers located along the perimeter of the structure to minimize the interruption of the building functionality and occupancy. The axial forces in the dampers are transferred to the RC walls along the perimeter of the lateral resisting system, at ground floor;

• The cross-sections of the dampers minimize the axial loads transferred to the foundation and are adequate for the installation within the brick infill;

• Dampers are installed within the existing infills to minimize the impact of the structural retrofitting elements on the facades of the building;

• The layout of the dampers is compliant with the large openings of the building facades;

• The layout of the dampers in elevation is aimed at regularizing the dynamic response of the retrofitted earthquake-resistant structural system;

• The design of the dampers is aimed at minimizing the level of tensile actions in the RC columns of the existing frame.

The layout of the BRBs is displayed in Figure 6 for the sample T-shaped block of the building school.

Figure 6 – Layout of the buckling-restrained braces along the perimeter of the sample building frame. The design of the BRBs is based on the allowable interstorey drifts that are compliant with the maximum axial deformations of the dampers. As a result, BRBs with 40 mm (= ± 20 mm) axial deformations were selected. The maximum storey lateral displacement is assumed

equal to 10mm; this value is however very small for the ultimate limit state. The RC frame equipped with BRBs exhibits high lateral stiffness at damageability limit state as discussed in the next section.

• Ground floor: Steel hysteretic dampers with circular hollow section (De=140mm, Di=120mm); damper label BRAD 48/40;

• First floor: Steel hysteretic dampers with circular hollow section (De=120mm, Di=104mm); damper label BRAD 41/40.

The cross-sections utilized for the hysteretic dampers possess a slenderness ratio (d/t) lower than 15, which is much smaller that the threshold values provided in the seismic codes , i.e. d/t<36 (EC3, 2006). The hysteretic damper BRAD 48/40 has the following properties:

Fy=350 kN; Fmax= 420 kN; dmax=±20 mm; kel= 305 kN/mm, and the damper BRAD 41/40 is as below:

Fy=300 kN; Fmax= 360 kN; dmax=±20 mm; kel= 250 kN/mm. The steel grade utilized for the tubular profiles is S275 and hence the values of the Euler critic load (Ncrit.=A⋅π2⋅E/λ2) are given by:

• Tube with De=140 mm: λ=lo/ρmin=76 - A=40,84 cm2 - σcrit= 3470 daN/cm2 and Ncrit.=1417 kN.

• Tube with De=120 mm: con λ=lo/ρmin=88 - A=28,15 cm2 - σcrit = 2575 daN/cm2 and Ncrit.=725 kN.

The dimensionless slenderness (λ=λ/λY con λY=π(E/fy)0.5) is very small and fulfills the code limits for seismic applications (OPCM 3431, 2005; EC8, 2006), In fact, λ=λ/λY=76/98=0,8 for tube with De=140 mm and λ=λ/λY=88/98=0,9 for tubes with De=120 mm. The ratio of the critic load Ncrit and the yield resistance is higher than 1.3 for each brace (Ncrit./Fmax= 3,37 for tubes with De=140 mm and Ncrit./Fmax= 2,01 for tubes with De=120 mm) and hence the damper will dissipate energy as hysteretic mechanisms. 2.5. Seismic response of the retrofitted structure The seismic response of the framed system retrofitted with BRBs is assessed by using the computer program SAP2000 (CSI, 2005). Elastic static and dynamic (response spectrum) analyses were carried out with respect to the national and European standards for earthquake resistant structures (OPCM, 3431, 2005; EC8, 2006). Capacity curves and nonlinear response history analyses were also carried out to assess the actual strength and ductility of the sample structures. A suite of seven natural spectrum-compatible records were selected to perform the nonlinear time-history analyses and to compute the mean values of the response parameters, either force-based or deformation-based. The FE model utilized to perform the aforementioned analyses is shown in Figure 7.

Figure 7 – Finite element model of the retrofitted structure with buckling-restrained braces.

The modelling of the inelastic response of the BRBs is simplified by assuming a bi-linear discretization of the hysteretic response as displayed in Figure 8, where the constitutive relationships for both BRAD 48/40 and BRAD 41/40 are provided.

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Figure 8. – Modelling of inelastic response of the buckling-restrained braces.

The response curve is shown in Figure 5. It is evident that the retrofitted building presents higher lateral stiffness due to the presence of the axial stiffness of the braces. Moreover, the structure possesses enough strength, global over-strength (1.28 and 1.32 in the X- and Y-direction, respectively) and ductility (3.13 and 4.30 in the X- and Y-direction, respectively) to satisfy all the threshold values for the different limit states considered in the assessment.

2.6. Seismic detailing A number of seismic details were designed to provide an efficient connection of the braces to the existing RC members of the sample frame. In particular, due to the low shear reinforcement in the columns, , e.g. stirrups φ6/20, few members were reinforced with

additional structural steel angle profiles located at the cross-section corners. Additionally, Fiber reinforced polymers, e.g. with carbon, were utilized to wrap the zones of the beams connected to the BRBs. A single layer of 300g uniaxial carbon fiber was utilized. Beton plaque was used to strengthen the top of the beams and to facilitate the construction of the brace-to-beam joint. Figure 9 provides, for example, the seismic details for the bays of the frame incorporating buckling-restrained braces. Columns with and without additional corner angles are also shown. In Figure 10 shows a typical layout utilized for the brace-to-beam joints. The design of all seismic details was based on capacity-design rules and was dimensioned in such a way to minimize the cost of the strengthening of the existing multi-storey frame.

Figure 9. – Seismic details for the bays of the frame incorporating buckling-restrained braces.

Figure 10. – Typical layout of the detail of the brace-to-beam joint.

3. CONCLUSIONS The present papers deals with the application of buckling-restrained braces (BRBs) to an existing reinforced concrete (RC) framed building designed for gravity loads only. It is demonstrated that the use of BRBs is very efficient to enhance the energy dissipation capacity of the RC frame and increase significantly its damping capacity under moderate-to-high magnitude earthquakes. However, further studies, both analytical and experimental, are deemed necessary to provided a cost-effective design procedure for such type of devices, especially when elastic (static and/or response spectrum) are utilized. ACKNOWLEDGEMENTS

This work was financially supported by the Italian Consortium of Laboratories RELUIS, funded by the Italian Federal Emergency Agency. Any opinions, findings and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect those of the Consortium RELUIS. REFERENCES

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