Full-Scale PsD Testing of the Torsionally Unbalanced SPEAR Structure in the 'As-Built' and Retrofitted Configurations

In the framework of the research activity of the ELSA Laboratory of the Joint Research Centre, pseudo-dynamic testing of a real-size plan-wise irregular 3storey frame structure was carried out as the core of the research project SPEAR (Seismic PErformance Assessment and Rehabilitation of existing buildings). The main goal of the SPEAR project was contributing to the improvement of current design, assessment and retrofitting techniques and the development of new simplified approaches for the assessment and rehabilitation of existing building structures, by means of a balanced combination of experimental and numerical activities. The experimental activity carried out at the ELSA Laboratory on the full-size SPEAR specimen consisted of three rounds of PsD tests on the structure in three different configurations: at first, the ‘as-built’ structure was tested; after that, it was retrofitted by means of Fiber Reinforced Polymer (FRP) wraps and then tested again; finally, it was reinforced by means of conventional Reinforced Concrete (RC) jacketing and re-tested to failure. In the paper, the test set-up is presented and the most significant outcomes of the tests are discussed with reference to the open issues of research in the field of torsionally unbalanced buildings. 1 DESCRIPTION OF THE STRUCTURE AND TEST SET-UP A thorough description of the aims and the experimental and numerical activity of the SPEAR project is given elsewhere (Negro et al. 2004), (Molina et al 2004), (Molina et al. 2005). There, the detailed description of the structure can be found, together with that of the test set-up and of the control algorithm; in the present paper, only the most relevant information about these issues is summarized. The SPEAR structure is a simplification of an actual three-storey building representative of old constructions in Southern European Countries, without specific provisions for earthquake resistance. The structure is regular in elevation: it is a three-storey building with a storey height of 3 meters. The plan configuration is non symmetric in two directions (Figure 1), with 2-bay frames spanning from 3 to 6 metres; the presence of a balcony on one side and of a part of the structure 1 metre (in the weak direction) or 0.5 metre (in the strong one) longer than the rest increases the plan irregularity, shifting the centre of stiffness away from the centre of mass. SPEAR Workshop – An event to honour the memory of Jean Donea – Ispra, 4-5 April 2005 140 Figure 1. The SPEAR structure Eight out of the nine columns have a square 250 by 250 mm cross-section; the ninth one, column C6 in Figure 1, has a cross-section of 250 by 750 mm, which makes it much stiffer and stronger than the others along the Y direction, as defined in Figure 1, which is the strong direction for the whole structure. As the structure is regular in elevation and has the same reinforcement in the beams and columns of each storey and its resisting elements in both directions are all of the same kind (frames), the structure belongs to a special class of multi-storey buildings, the so-called regularly asymmetric multi-story structures, meaning that the centre of mass (CM), the centre of stiffness (CR) and the centre of strength (CP) of each storey are located along three vertical lines separated by the distances er and es. The centre of stiffness (CR) (based on column secant-to-yield stiffness) is eccentric with respect to the mass centre (CM) by 1.3 m in the X direction (~13% of plan dimension) and by 1.0 m in the Y direction (~9.5% of plan dimension). The reference system used in the PsD test and the location of the CM of the structure at the first and second floor are shown in Figure 1. The origin of the reference system is in the centreline of column C3. The coordinates of the CM of the first two storeys with respect to this reference system are (-1.58m, -0.85m); at the third storey the coordinates of the CM vary slightly, becoming (-1.65m; -0.94m). Given the doubly non-symmetrical plan configuration of the specimen, the PsD test needed to be bi-directional, with the input applied along two orthogonal directions. As a consequence, three degrees of freedom (DoFs) per storey were taken into account: two translations and one rotation along the vertical axis, as opposed to the single degree of freedom per storey that is usually taken into account in conventional unidirectional PsD testing. The accelerograms used as input were the Montenegro ’79 Herceg-Novi records for the longitudinal and transverse component, artificially fitted to the EC8 spectrum, as shown in Figure 2, scaled to different levels of PGA and applied to the structure acSPEAR Workshop – An event to honour the memory of Jean Donea – Ispra, 4-5 April 2005 141 cording to the combination of direction and orientation that would maximize torsional effects, based on a number of pre-test numerical simulations. Global quantities, such as displacement and rotation at the CM at each floor, base shear, storey shears, storey drifts, absorbed energy at each floor and for each degree of freedom, were measured. Local instrumentation was also set at the top and bottom of a number of columns and beams, in the most meaningful locations MONTENEGRO HERCEG NOVI X 1g PGA -1.00E+00 -8.00E-01 -6.00E-01 -4.00E-01 -2.00E-01 0.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Time [s] G ro un d A cc el er at io n [g ] MONTENEGRO HERCEG NOVI Z 1g PGA -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Time [s] G ro un d A cc el er at io n [g ] HERCEG NOVI RECORDS PGA 1g 5% DAMPING PSEUDOACCELERATION SPECTRA 0.00 5.00 10.00 15.00 20.00 25.00 30.00 0 1 2 3 4 5 Period [s] Ps eu do -A cc el er at io n [m /s /s ] X COMPONENT Y COMPONENT Figure 2. Herceg-Novi records PGA=1g; a) longitudinal component b) transverse component c) acceleration response spectra of the X and Y components 2 EXPERIMENTAL ACTIVITY As mentioned above, the experimental activity carried out during the SPEAR project consisted of three different rounds of PsD tests on the specimen in the original and in two different retrofitted configurations. Each round consisted in three tests at different PGA levels: the 0.02g control test from which the initial modal properties of the structure (first mode shapes, initial frequencies and modal damping values) were estimated, (Molina et al. 1999), the 0.15 and 0.20g PGA tests (for the ‘as-built’ structure) or the 0.20g and 0.30g PGA tests (for the retrofitted specimen), during which the non-linear range of behaviour was entered. In the following, the outcomes of each round of tests will be summarized; for the sake of brevity, only the results from the highest intensity test of each round will be presented. In the end, a comparison between the responses in the different configurations will be carried out, in order to draw some preliminary conclusions on the effectiveness of the retrofitting interventions. SPEAR Workshop – An event to honour the memory of Jean Donea – Ispra, 4-5 April 2005 142 2.1 ‘As-built’ configuration 2.1.1 Displacements time history During the tests, the displacement time-histories in the X, Y and θ directions were measured at the three floors of the specimen. The displacements and the rotation are those of the CM. The measured displacements were significantly different from those predicted by pre-test numerical analyses, in particular the torsional response turned out to be much larger. The maximum displacements were above 100 mm both in the X direction and in the Y direction; the maximum rotation was about 20 mrad at the top storey. 2.1.2 Floor hysteresis loops In Figures 3-5 the storey hysteresis loops for the X, Y and θ directions are reported. The interstorey shears (or torques) are plotted against the interstorey drifts (or rotations). It can be observed that the level mainly involved in the energy dissipation process was level 2 and the direction where the absorbed energy was larger is the Y direction. HERCEG NOVI RECORDS PGA 0.20g HYSTERESIS LOOP X DIRECTION -200 -150 -100 -50 0 50 100 150 200 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 Interstorey Drift [mm] In te rs to re y Sh ea r [ kN ] LEVEL 1 HERCEG NOVI RECORDS PGA 0.20g HYSTERESIS LOOP X DIRECTION -200 -150 -100 -50 0 50 100 150 200 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 Interstorey Drift [mm] In te rs to re y Sh ea r [ kN ]