SEISMIC ANALYSIS OF A FULL-SCALE RC BUILDING RETROFITTED WITH CFRP

Reyes García*, Iman Hajirasouliha, Yaser Jemaa, Kypros Pilakoutas, and Maurizio Guadagnini Department of Civil and Structural Engineering,

University of Sheffield, UK *Email: r.garcia@sheffield.ac.uk

ABSTRACT This study compares the predicted response by a finite element program and the real behaviour of a full-scale 3D RC frame tested on a shake table. The structure was designed and built according to typical old pre-seismic construction practice of southern Europe. After the initial tests that caused damage, the building was retrofitted using Carbon FRP (CFRP) before further experiments were carried out. The bare and CFRP-retrofitted building is modelled and analysed using DRAIN-3D software, and the effect of poor joint detailing is explicitly considered. This paper presents the results of the nonlinear time-history analyses. It is found that issues as bar pullout at joints influence significantly the response of existing under-designed RC frames. The CFRP strengthening is shown to reverse some of the damage and enables the structure to fulfil its plastic potential. It is concluded that the retrofitting strategy was successful at improving the building’s performance, and that advanced modelling using DRAIN-3D reasonably predicts the response of the bare and CFRP-retrofitted building. KEYWORDS CFRP, seismic retrofitting, full-scale tests, RC buildings, nonlinear analyses. INTRODUCTION High mortality and extensive damage in recent major earthquakes in developing countries as Pakistan (2005), Peru (2007) and China (2008), have highlighted the seismic vulnerability of many existing RC frames. Many of these buildings have been designed according to old standards and often suffer from inadequate construction practices. Consequently, these structures have deficient lateral load resistance and insufficient energy dissipation that can rapidly lead to collapse during strong earthquakes. As demolishing existing structures could be an uneconomic solution, their seismic vulnerability can be mitigated by upgrading their structural performance. Among the different techniques currently available for seismic strengthening, fibre-reinforced composites (FRP) offer effective and attractive solutions. Most current research is focused on strengthening techniques to enhance the capacity of existing buildings, whilst less attention has been given to the study of analytical tools used in high level seismic assessment of FRP-retrofitted buildings. This study compares the predicted response by a finite element program and the actual behaviour of a full-scale 3D RC frame tested on a shake table. The structure was tested in the CEA-Saclay laboratory in France as part of the EU-funded Ecoleader Project (Chaudat et al, 2006; Papastergiou et al, 2009). The one-bay two-storey RC frame was designed and built according to typical old pre-seismic construction practice of southern Europe and suffered from poor detailing in the beam-column joints; hence it is considered to be representative of typical substandard buildings found in developing countries. After the initial tests, the damage was repaired and the frame was rehabilitated using CFRP for further tests. In this contribution, the bare and rehabilitated buildings are modelled using DRAIN-3D software (Prakash et al, 1994) to perform nonlinear time-history analyses. Constitutive models of steel concrete bond-slip and bond strength degradation under cyclic loading are adopted to simulate deficient joints. Subsequently, the experimental response from the shake table tests is compared with the results from the nonlinear time-history analyses to verify the accuracy of the finite element models. Finally, the effectiveness of the CFRP retrofitting at improving the seismic performance of the building is verified analytically.

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EXPERIMENTAL PROGRAMME

The one-bay two-storey frame was regular in plan and elevation (Figure 1-left), and was designed so that columns and joints experienced damage during the initial shaking. The geometry, element sections and reinforcement are shown in Figure 1-right. The mechanical properties of the materials obtained from tests are, for the reinforcement steel fy=550 MPa and fu=656 MPa, and for concrete fc=20 MPa and Ec=25545 MPa. An additional mass of 9 tonnes was added at each slab by using steel plates.

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Figure 1. View of the bare frame (left), and general geometry of the building (right).

The structure was instrumented with displacement and acceleration transducers at each floor to monitor the response history during the experiments. Uniaxial shake-table tests were carried out on the as-built frame using increasing peak ground accelerations (PGA) ranging from 0.05g to 0.4g. A single artificial record was used, based on the Eurocode 8 soil type C spectrum (CEN, 2004a). The structural periods of the frame were obtained using white noise as input signal before the start of each test. As expected, significant damage was observed at the columns ends and beam-column joints after the initial tests, whilst the beams remained practically undamaged (see Figure 2).

Figure 2. Damage in joints (left), and columns (right) after the test PGA=0.4g

After the initial series of tests, the damaged structure was rehabilitated using CFRP, Figure 3-left. Before the strengthening, the damaged concrete was repaired with mortar and the main cracks filled by injecting epoxy resin. Concrete surfaces were smoothed and prepared using a primer to improve the adherence between the existing concrete and the fibre sheets. Modern seismic design philosophy establishes the need to avoid non-ductile failures; hence, the main purpose of the rehabilitation was to produce a beam-sway mechanism. It is well known that FRP confinement can significantly increase the flexural capacity of columns (fib, 2001; ACI 440, 2008), and therefore, four plies of CFRP were wrapped to increase their flexural capacity. One additional sheet of CFRP was attached vertically at the faces of columns ends to further enhance their flexure strength. Beam-column joints at both floors were also strengthened to avoid premature shear failures. More details on the rehabilitation strategy can be found in Papastergiou et al (2009). After the FRP intervention, further shaking table tests were conducted for increasing PGAs ranging from 0.05g to 0.5g. Damage of the CFRP sheets was first observed at the 2nd floor columns after the PGA=0.2g tests.

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Additional damage in the columns or joints was only evident after the 0.5g tests; however, as shown in Figure 3-right, significant damage occurred at the beam ends. Consequently, the adopted retrofitting strategy seemed to change effectively the plastic hinge mechanism from column-sway to beam-sway, which is in line with the intended rehabilitation target.

Figure 3. View of CFRP-retrofitted frame (left), and damage at beam ends after tests on the retrofitted frame

(right); Chaudat et al, (2006). ANALYTICAL MODELLING FOR TIME-HISTORY ANALYSES

Numerical models of the bare and CFRP-retrofitted frame were developed in DRAIN-3D for nonlinear time-history analyses. Due to the structural symmetry and for computational efficiency, only half of the building was modelled in 2D (Figure 4-left). Beams and columns were modelled using a fibre element of distributed plasticity (Powell and Campbell, 1994) using centre-to-centre lengths of members. The section comprises discrete steel and concrete fibres, which increase the accuracy of the flexural analysis (Figure 4-right). The behaviour of reinforcement and concrete was characterised by the constitutive models given in Eurocode 2 (CEN, 2004b). Vertical nodal loads were assigned along the beams to simulate the distributed dead load from slabs and beams. To simulate the actual geometry of columns and joints, additional nodes were added at the top and bottom of the columns. The masses at each floor were lumped at the two corresponding exterior nodes and computed assuming a concrete density of 24 kN/m3. Only the first two translational modes of vibration were considered in the analysis. Elastic damping was adopted through a mass damping coefficient and an element stiffness coefficient using a Rayleigh damping model (Chopra, 2001). Trial values of 3 to 5% were assigned to the 1st mode of vibration, and 2 to 4% for the 2nd mode. Second order (P-Δ) effects were also included in the analysis. To model the rehabilitated frame, additional nodes were added to define the CFRP-retrofitted zones of columns (Figure 4-left). The effect of the FRP confinement was introduced by using the constitutive model for confined concrete proposed by Model Code 90 (CEB-FIP, 1993). The MC90 model was adopted because it depends mainly on the volume of the confinement material and, therefore, can be easily used for FRP and other innovative materials. The ultimate compression strength and ultimate strain used for the analysis were f*cc= 30 MPa and ε*cc=0.010, respectively. Vertical CFRP retrofitting at columns ends was modelled using a fibre element and the material characteristics provided by the manufacturer (Figure 4-right). The bond degradation parameters were reduced to reflect the effect of the additional confinement provided by the CFRP. Previous research showed the need of considering the additional deformations generated by stiffness degradation and slippage of the reinforcing bars (Kyriakides, 2008; Jemaa et al, 2008; Garcia et al, 2009). As such, the stress-strain concrete relationship considered stiffness degradation to simulate the effect of damage accumulation during the analyses (Figure 5-left). To consider bar slippage, deformations occurring at the joints were specified using zero-length connection hinges at column ends. The fibre properties used for the elements were chosen to model bond stress-bar slip within the beam-column joint (Figure 5-centre), and included stiffness and strength degradation factors (Powell and Campbell, 1994). Gap properties were assigned at the connection face to simulate crack opening (Figure 5-right).

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Figure 4. Frame models of bare structure and CFRP-retrofitted frame (left), and fibre elements used in the

models developed in DRAIN-3D (right).

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the analyses (adapted from Powell and Campbell, 1994). COMPARISON OF RESULTS FROM ANALYSES AND EXPERIMENTS

The results from the shaking table tests are compared with the results from nonlinear time-history analyses of the analytical models developed in DRAIN-3D. Maximum drift is selected as the response parameter for comparison purposes because it is considered a reliable indicator of possible damage. The structural period obtained from white noise tests are compared with modal analysis results in Table 1. It is shown that the dynamic properties of the bare and retrofitted frame are well captured by the analytical models for the 1st and 2nd modes of vibration.

Table 1. Structural period of the frame obtained from experiments and analyses (in sec)

Mode No. Bare frame CFRP-retrofitted Experiments Analysis Experiments Analysis

1 0.53 0.51 0.73 0.70 2 0.18 0.19 0.23 0.25

The experimental and analytical displacement histories of the 1st and 2nd floors for the bare and CFRP-retrofitted building are compared in Figures 6 and 7, respectively. Due to space limitations, only the results for PGA of 0.05g, 0.2g and 0.4g are presented in this work, which are representative of the elastic and inelastic range of behaviour. In spite of some differences, the illustrated results indicate that the predicted and measured displacements compare reasonably well along the entire time duration of the excitation. The test results show that the 2nd floor displacements of the bare frame are larger than those of the 1st floor. Conversely, for the CFRP-retrofitted frame, 2nd storey displacements are smaller than the 1st storey displacements. Notice that in Figure 6, the experimental response of the 2nd storey becomes horizontal at PGA=0.4g due to the failure of the displacement transducer.

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Figure 6. Results from nonlinear time history analysis for 1st (left), and 2nd (right) floors, bare frame.

Figure 7. Results from nonlinear time history analysis for 1st (left), and 2nd (right) floors, CFRP-retrofitted frame. Table 2 compares the maximum inter-storey drifts obtained from the experimental tests and analytical models for different PGAs. In general terms, the analytical models for both bare and retrofitted structures tends to slightly underestimate the storey drift for the 1st storey, while the drift response for the 2nd storey is overestimated. Based on the results, it can be concluded that the analytical models of both bare and retrofitted frame provide a reasonable estimate of the maximum drifts for earthquake excitations with different PGA level. The results also indicate that the application of CFRP significantly decreased the 2nd storey drift, while it increased the 1st storey

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drifts to some extent. This may be attributed to the fact that the rehabilitation strategy changed the dynamic behaviour of the bare frame by preventing the joint failure at the first storey. Finally, it should be noticed that the inter-storey drifts of the rehabilitated frame are less than 2.5% for all cases. As many codes suggest a drift limit of 2.5% to control damage of structural and non-structural elements in buildings, less damage can be effectively expected in the CFRP-retrofitted frame.

Table 2. Inter-storey drift results from experiments and analysis (in %)

PGA Floor No. Bare frame CFRP-retrofitted Experiments Analysis Experiments Analysis

0.05g 2 0.2 0.2 0.1 0.1 1 0.2 0.3 0.4 0.3

0.20g 2 1.2 1.5 0.6 0.8 1 1.4 1.3 1.4 1.3

0.40g 2 3.9 3.9 1.3 2.2 1 1.9 1.6 2.5 2.2

CONCLUSIONS This paper presented an experimental and analytical investigation on the seismic performance of a full scale deficient RC frame and the efficiency of CFRP sheets in upgrading its behaviour.The CFRP rehabilitation strategy has shown to improve the seismic behaviour of the structure by fulfilling its plastic potential aligned with the rehabilitation target. From the results of the analysis, it can be concluded that the advanced modelling using DRAIN-3D reasonably predicts the response of the bare and CFRP-retrofitted frame, and that structural deficiencies such as bond slip in joints can influence significantly the response of existing deficient RC buildings. ACKNOWLEDGMENTS The first author gratefully acknowledges the financial support provided by CONACyT (Mexico) for his PhD research. The second author wishes to acknowledge the financial support provided by the EU through the Marie Curie International Incoming Fellowship. REFERENCES Chaudat, T., Pilakoutas, K., Papastergiou, P., and Ciupala, M.A. (2006) Shaking table tests on RC retrofitted

frame with FRP, 1st European Conference on Earthquake Engineering and Seismology, Geneva, Switzerland.

Papastergiou, P., Pilakoutas, K., Ciupala, M.A. and Chaudat, T. (2009) Enhancement of RC column and joint resistance by using CFRP strengthening, Advanced Composites in Construction Conference, Edinburgh, Scotland.

Prakash, V., Powell,G.H., and Campbell, S. (1994) Drain-3DX: Base program description and user guide, SEEM Report 94/07, University of California-Berkeley, USA.

CEN (2004a) Eurocode 8: Design of Structures for Earthquake Resistance, BS, London, UK. fib (2001) Bulletin 14, Externally Bonded FRP Reinforcement for RC structures, CEB-FIP, Laussane,

Switzerland. ACI 440 (2008) ACI 440.2R-08: Guide for the Design and Construction of Externally Bonded FRP Systems for

Strengthening Concrete Structures, ACI, Farmington Hills, USA. Powell,G.H., and Campbell, S. (1994) Drain-3DX: Element Description and User Guide for Element Type01,

Type04, Type05, Type08, Type09, Type15, and Type17, SEEM Report 94/08, University of California-Berkeley, USA.

CEN (2004b) Eurocode 2: Design of Concrete Structures Part 1-1: General Rules and Rules for Buildings, BS, London, UK.

Chopra, A. K. (2001) Dynamics of structures: theory and applications to earthquake engineering, Prentice-Hall, UK.

Kyriakides, N. (2008) Vulnerability of RC buildings and risk assessment for Cyprus, PhD Thesis, Dept. of Civil and Structural Engineering, University of Sheffield, UK.

Jemaa, Y., Helal, Y., and Garcia, R. (2008) Seismic Performance of a two-storey RC frame building, Concrete Communication Conference, Liverpool, UK.

Garcia, R., Jemaa, Y., Helal, Y., and Pilakoutas, K. (2009) Seismic assessment of under-designed frame buildings, 16th Concrete Conference, Paphos, Cyprus (in Greek).

CEB-FIP (1993) Design Code, Model Code 90, London, UK.

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