Response of frames seismically protected with passive systems in near-field areas

Transcription

1 326 Int. J. Structural Engineering, Vol. 5, No. 4, 2014 Response of frames seismically protected with passive systems in near-field areas Dora Foti Department of Civil Engineering and Architecture, Polytechnic of Bari, Via Orabona 4, Bari, Italy Abstract: The present paper analyses the behaviour of moment resisting frames in reinforced concrete subject to earthquakes recorded in near-field areas. In these areas, the signals show impulsive-type accelerograms, with velocity and displacement peaks higher than in far-field zones especially for the fault-normal component of the ground motion velocity in the direction of propagation of the wave, which shows large-amplitude pulses. In the following, seven near-field signals scaled in agreement with the design spectrum of the Italian code (NTC, 2008) have been adopted to perform a nonlinear analysis on a six-story frame. The response has also been determined for the same frame protected once with hysteretic-type energy dissipaters and once with base isolators. The aim of the present study is to acquire quantitative knowledge on the near-field ground motion effects on frame buildings and on their damage also in the presence of passive seismic protection systems. Keywords: structures in near-field areas; near-field seismic motions; moment resisting frames; passive protection; probabilistic assessment. Reference to this paper should be made as follows: Foti, D. (2014) Response of frames seismically protected with passive systems in near-field areas, Int. J. Structural Engineering, Vol. 5, No. 4, pp Biographical notes: Dora Foti is a Professor at the Department of Civil Engineering and Architecture, Polytechnic of Bari. She received her BS and MS degrees from Polytechnic of Bari, while she received her PhD from Universitat Politecnica de Catalunya, Barcelona, Spain. Her research interests are principally focused on the anti-seismic behaviour of structures, vulnerability and risk assessment of existing structures and new recycled materials to improve concrete behaviour. 1 Introduction In the 90s, the devastating earthquakes of Northridge (1994), Kobe (1995), Kokaeli Turkey (1999) and Chi-Chi Taiwan (1999) aroused great interest among researchers due to the high number of collapsed buildings with significant costs in terms of human lives. Currently, the extensive monitoring network spread all over the globe allows to recording several measurements of ground motions, which differ both for distances from the source and for site conditions. So, it is possible to verify that, in all the earthquakes mentioned before, the source of the earthquake was located beneath an urbanised area, Copyright 2014 Inderscience Enterprises Ltd.

2 Response of frames seismically protected with passive systems 327 while most of the victims of the earthquake were located in areas close to the fault (near-field), i.e., within a few kilometres from the projection of the fault on the surface of the ground. In particular, if the monitoring station is located in the area of fracture propagation of the fault, the near-field motions are significantly different from the usual motions recorded in areas away from faults (far-field). In near-field areas, the component of the motion normal to the fault is impulsive, with a high pulse period of acceleration (Bray and Rodriguez-Marek, 2004). To get better knowledge on near-field ground motions, well-known concepts and results obtained from far-field motions have been extended to near-field ones. Comparisons of the responses of SDF systems subject to the two mentioned motions have been developed, based on elastic and inelastic response spectra (Chopra and Chintanapakdee, 2001a). In addition, a drift spectrum has been developed and the elastic response of structures to near- and far-field ground motions has been determined. In fact, the drift spectrum provides important information to near-field ground motions that cannot be obtained from the response spectrum. Anyway, the drift spectrum seems to be restricted to structures that can be idealised as a shear beam (Chopra and Chintanapakdee, 2001b). The impulsive component of the motion in near-field areas is very pronounced, and does not appear in the recorded ground motions distant from the epicentral zone. Therefore, it is clear the importance of considering the effects of near-field motions in the earthquake resistant structural design (Alavi and Krawingler, 1999, 2000; Tirca, 2000; Xu and Agrawal, 2010; Yi and Zhang, 2007). Unfortunately, the current seismic codes only refer to a design in far-field or intermediate areas (as distance from the epicentre), where the characteristics of the ground motion are definitely known and more documented. In conclusion, the ground model adopted in the current design methodology, based on motions recorded in far-field areas, cannot be used in an appropriate way to describe the action of the earthquake in a near-field zone. The differences between near-field and far-field areas can be summarised in the following points: The direction of propagation of the fault has a major influence in a near-field area, the stratification of soil having minor effects. On the contrary, in the case of far-field zone, the stratification of the soil and the site conditions are of primary importance for the horizontal components of the seismic waves. In near-field areas, the ground motion time-history acceleration plot shows a pulse in the field of low frequencies and a pronounced pulse in the velocity and displacement time-histories. In this case, the motion is of short duration; on the contrary, in far-field areas, the acceleration, velocity and displacement recordings have the characteristic of a cyclic movement, with a long duration action (Mazza and Vulcano, 2010; Mazza and Mazza 2012). In near-field areas, there are very high velocities; in such areas the velocity appears to be the most significant parameter in the design of structures, replacing the acceleration, which represents the most significant parameter in the structural design in far-field areas.

3 328 D. Foti In contrast to what happens in the far-field zones, in near-field areas vertical components may be higher than the horizontal ones (Tirca and Gioncu, 1998; Mazza et al., 2012). Studies have also been conducted on the behaviour of protected buildings in near-fault areas (Filiatrault et al., 2002; He and Agrawal, 2008). This field is not much explored and most research is centred on buildings in far-field areas (Foti et al., 1998; Ordoñez et al., 2003), protected with different types of devices [examples are those described in Foti et al., (2010, 2013a, 2013b)]. Just a few studies consider the performance in near-field areas of base isolated buildings (Greco et al., 1999; Mazza and Vulcano, 2009, 2012) and buildings protected with energy dissipaters (Tirca et al., 2003; Xu et al., 2007; Dicleli and Mehta, 2007). Evaluations have also been studied for protected bridges in near-field areas (Dicleli and Buddaram, 2007; Tan et al., 2005). Anyway research is very limited especially if addressed to highlight the differences in the behaviour of base isolated and protected frames sited close to a seismic fault. In the present paper, the response of a frame that has been protected ones with hysteretic-type energy dissipaters and then with reinforced elastomeric base isolators in a near-field area has been considered. The unprotected building (bare) is a six-story reinforced concrete frame subject to seven different earthquake signals, acquired at stations located in near-field areas and arranged in the direction of propagation of the fault. The signals have been scaled according to the design spectrum of the Italian code (NTC, 2008). The design of the structure has been conducted in accordance with the requirements of Eurocode 2 (2004), and a nonlinear analysis has been applied. Comparisons have been developed on the behaviour of the bare frame and the seismically protected ones. From the results, it is noted that the passive protection systems are able to significantly reduce the response of the frame structures subject to near-field motions. 2 Description of the frames A reinforced concrete building frame with a fixed base has been considered. In the following, it will be referred to as bare frame [frame (a) in Figure 1]. The building consists of six floors and two equal bays. The total height of the frame is 19 m, while the length of each bay is 6 m. The beams have all the same size, base b = 30 cm and height h = 40 cm, while the columns have a cross-section variable with the height. The types of section of the columns are shown in Figure 1(a), while the respective dimensions are shown in Table 1. Table 1 Dimensions of the columns of the frames Base [cm] Height [cm] COL COL COL COL

4 Response of frames seismically protected with passive systems 329 Figure 1 (a) Bare frame (b) Frame protected with dissipaters (c) Frame protected with base isolators (a) (b) (c) Each beam is subject to a uniformly distributed load of 40 kn/m, which includes the dead load the live load (2 kn/m) equal to the part of the floor attributable to each beam. The design of the bare frame has been checked in accordance with the requirements of Eurocode 2. In addition, the same frame has been considered seismically protected with two different passive protection systems: hysteretic energy dissipaters (frame b in Figure 1), and base isolators made with reinforced elastomeric bearings (frame c). The three frames have been analysed in the nonlinear field utilising seven near-field signals (see par. 3). In Table 2, the values of the first three periods of vibration of each frame are reported. Table 2 First three periods of vibration of the bare frame and the protected frames Bare With dissipaters Base isolated T 1 [s] T 2 [s] T 3 [s] Protected buildings Energy dissipaters Shear-link (SL) energy dissipaters (Chais et al., 2001) were installed at each floor of the frame in order to seismically protect it. In particular, they were positioned between the intersection of two steel diagonals and the beam of the upper floor [Figure 1(b)]. SL devices dissipate energy through the plasticisation of mild steel. The device presents an H-shaped cross section (Figure 2) realised with a steel strip provided with transverse and longitudinal stiffeners at a close spacing each other. The total height of the dissipating connection is about 30.4 cm including the thickness of the base plates. It is important noting that in the system the sizes can be easily varied to obtain different yield forces, i.e., the force that activates the energy dissipation.

5 330 D. Foti Figure 2 SL dissipater, (a) vertical view (b) plan view (a) (b) The energy is dissipated, in fact, by the plastic deformation due mainly to the shear stress in the central part of the cross section. Stable hysteresis curves under cycles of loading and unloading are generated. The dissipater is very rigid in its plane, but flexible in the orthogonal direction. Consequently, it has the advantage of starting to dissipate energy in correspondence of very small deformations, with interstory drifts lower than those commonly considered for other types of dissipaters. The geometry of SL device allows an evenly dissipation throughout the central area, avoiding the occurrence of local stresses that reduce the dissipation capacity. The presence of stiffeners avoids also the occurrence of local buckling phenomena of the central part. Preliminary cyclic tests aimed at the characterisation of the hysteretic curve of SL dissipater were performed at the laboratory of the Department of Architecture and Construction Engineering, University of Girona (Spain). As previously mentioned, the tests showed that the shape of the hysteretic curve remains unchanged till the failure of the dissipater, showing a stable dissipative behaviour and a constant stiffness. Next phase of testing involved the dynamic characterisation performed at the laboratory ISMES in Seriate, Bergamo (Italy). In this phase, a series of shaking-table tests were carried on a one-floor one-bay steel frame in real scale, protected with SL devices. Each SL device shows the hysteresis loop of Figure 3, with the characteristic shear behaviour. It should be noted that, after the failure of the thinnest parts of the device, the dissipater exhibits further strength capacity, as the behaviour changes in a flexural-type one. The behaviour of the dissipaters used in this study was modelled in accordance with the above-mentioned trend and a nonlinear dynamic analysis was performed. For each dissipater, the following characteristics have been assumed: Yield strength Fy = KN; Stiffness in the elastic range K = KN/cm. The number of SL dissipaters to install at each floor has been obtained by applying a distribution of horizontal static equivalent seismic forces (according to NTC, 2008) to the

6 Response of frames seismically protected with passive systems 331 bare structure reinforced by mean of steel diagonals with a mm tubular square section. The horizontal component of the maximum force transmitted by the diagonals to the beam at the connection node has been determined. The dissipaters were designed considering a yielding strength (shear) F y equal to 75% of this maximum horizontal force. So it has been necessary to install at each floor the number (N) of dissipaters listed in Table 3. K TOT is the total stiffness of the dissipaters at each floor, TOT while F y is the total yielding force per level. Figure 3 Hysteresis loop of an SL dissipater Table 3 Number of dissipaters per floor and their total characteristics Level K TOT TOT [KN/cm] F y [KN] N 1 2, , , , , Base isolators The base isolation system of the analysed frame was made with high damping bearings in neoprene reinforced with metal plates. In fact, each support is made by rubber layers alternating with metal plates. The plates have a thickness of about 2 mm and are inserted to increase the vertical stiffness of the isolator and support the weight of the superstructure with small deformations. The metal plates are vulcanised in the rubber layers, so as to obtain a single monolithic element with the function of reinforced rubber bearing. The bearing devices meet all the requirements contained in the Instructions CNR-UNI (Istruzioni CNR 10018, 1999) and other requirements relating to: the vertical load,

7 332 D. Foti the maximum displacement and stability, the equivalent viscous damping, the effects of cyclic loads and, consequently, of degradation. Figure 4 shows a high damping neoprene isolator of ALGA; these isolators are widely used both in Italy and abroad. Figure 4 High damping rubber bearing (ALGA) The rubber isolators are installed under each column. The connection of the isolators with the structure must be a reversible mechanical-type one having to realise a perfect fix constraint. In particular, the replacement of the devices does not require the lifting of the building, but only the transfer to the sub-structure of the load imposed on the isolator to be replaced from the super-structure. In the present study, the bare frame has been protected with Type 600 isolators, installed at the base of the lateral columns; Type 1200 isolators were installed at the base of the central columns. Such isolators show a nonlinear behaviour in the horizontal plane, i.e., for displacements and deformations in the two directions of the plane, while they present a linear behaviour for deformations and displacements along the vertical direction. Figure 5 shows the hysteresis loop of a Type 600 rubber isolator installed in the bare frame, subject to Kokaeli 60 earthquake simulated signal. Figure 5 Hysteresis cycle of a Type 600 isolator

8 Response of frames seismically protected with passive systems 333 The mechanical properties of Type 600 and Type 1200 rubber bearings are summarised in Table 4. Table 4 Characteristics of the base isolators Type 600 Type 1200 Shear stiffness K KN/m 1,200 KN/m Axial stiffness 600,000 KN/m 1,200,000 KN/m Max vertical load (lift) 1,000 KN 2,000 KN Damping 40 KN s/m 80 KN s/m Nonlinear properties F y 60 KN 120 KN K/K Note: F y is the yielding force and K is the stiffness after yielding. 3 Near-field earthquakes The study of the seismic recordings in near-field areas shows that, when the fracture generated by a seismic shock propagates with a velocity close to that of the shear waves, most of the energy released during the earthquake produces a high intensity pulse. This happens at the beginning of the phenomenon and for a short duration. The strong impulse is mainly found in the component of the motion along the direction perpendicular to the fault. Consequently, the motion that develops in the direction of propagation of the fault is characterised by pulses of high amplitude. This characteristic of the motion is evidenced in the velocity and displacement time-history diagrams. On the contrary, the motion recorded in the opposite direction, does not show impulsive characteristics. Moreover, the peak value of the velocity component of the motion in the direction of propagation of the fault is even three times larger than that relating to the component of motion in the opposite direction. In addition, the peak values of the acceleration and displacement are higher. For the dynamic analysis of the structures considered in this study, the following seven near-field accelerograms have been considered: Duzce-Turkey, November 1999, two accelerograms recorded respectively in Duzce and Bolu Kobe-Japan, 1995, two accelerograms recorded respectively in Kobe-JMA and Takatori Northridge-California, 1994, two accelerograms recorded at Rinaldi and Sylmar, respectively Kokaeli-Turkey, August 1999, recorded in Yarimka. In the analyses, the components at 90 or impulsive type of the near-field signals have been considered. Figure 6 shows the fast Fourier transforms (FFT) of the seven accelerograms used for the dynamic analysis, evaluated for 5% damping. From the FFT of these signals, it is

9 334 D. Foti possible to detect the frequency ranges where the acceleration is higher (frequency content). Figure 6 Acceleration response spectrum of the seven near-field signals (see online version for colours) In this article, the seven near-field signals to be used in the analysis have been scaled as follows. Known the design spectrum according to the new Italian code (NTC, 2008) for a Type B soil in seismic zone 1, it was determined the value of the spectral acceleration at the fundamental period of the r.c. bare building. Then, all signals have been scaled for such acceleration, in order to have a maximum reference value of the peak acceleration. 4 Results A nonlinear analysis has been performed by mean of a FE software, where the behaviour of the frame is shear-type and kept linear, while nonlinearities are concentrated only in the dissipaters. Since for a severe earthquake the structure must remain in the linear field, the forces that activate the protection devices are equal or less than those induced by strong inputs and the main frame is designed to withstand such loads without yielding. In the following, the results of the analysis are reported for different mechanical characteristics such as damage index, acceleration amplification, interstory drifts, shear force at each floor, ductility capacity. Then the probabilistic values related to the characteristics examined are also reported. 4.1 Damage index In this study, the three frames are similar in the design and are kept in the linear elastic field; therefore it is interesting to evaluate the possibility of damage by mean of an index,

10 Response of frames seismically protected with passive systems 335 defined as damage index, obtained as the ratio of the maximum displacement at the top of the frame (Dmax) and the total height of the frame (H). From the results, a strong reduction of the damage index is detected for frames with dissipaters, especially at Kobe 90 and Sylmar 228 signals (Figure 7). Figure 7 Damage index for the bare and the protected frames (see online version for colours) As a result of Kobe-JMA signal, however, the frames with dissipaters show a higher damage index if compared to the bare and isolated frames. Also as a result of Duzce 90 signal, a different behaviour is noticed: the higher reduction of the damage index is shown in the isolated frame. 4.2 Acceleration amplification The maximum acceleration amplification of the base acceleration is obtained in the frames protected with dissipaters, as the presence of the diagonals makes the structure stiffer (Figure 8). Only with Sylmar 52 and Kobe 90 signals, there is a reduction of the amplification with respect to the bare frame. This is probably due to the type of frequency content of the spectra of these signals (Figure 6). In fact, in correspondence of the first period of vibration of the bare frame, the spectral acceleration values for these signals are higher than those at the frame with dissipaters. Therefore, the acceleration response is higher in the bare frame. A reduced amplification is noticed for the isolated frame and for all the near-field signals.

11 336 D. Foti Figure 8 Maximum acceleration amplification of the base acceleration (see online version for colours) 4.3 Interstory drifts The plots representing the interstory drifts have a similar shape for the bare frames and the frames with dissipaters; this is not the case for the isolated frames (Figure 9). In fact, the displacement of the base produces a large increase in the interstory drifts at the first level. In the other two cases, however, the interstory drifts are higher at more central levels, approaching to zero at the first and last levels. In addition, for the frames with dissipaters, the value of the interstory drift at the second, third, fourth and fifth levels is practically constant for all the near-field signals used in the analysis. Finally, for this type of frames, the interstory drifts show the minimum values. In the case of base isolated frames, the interstory drifts show a strong reduction at the top floors. So, higher values at the first level are obtained. As already pointed out before, it depends on the strong displacement produced by the isolators; instead, from the second level onwards, the displacements go on reducing up to minimum values at the sixth level. The lowest values are obtained for Duzce 90 register, while the highest values are obtained for Kokaeli 60 signal, showing very high values at the first level. 4.4 Shear force at each floor In general, it is noticed that the shear force (V) is lower in the protected frames respect to the bare frames (Figure 10).

12 Response of frames seismically protected with passive systems 337 Figure 9 Maximum interstory drifts, (a) bare frame (b) frame with dissipators (c) base isolated frame (a) (b) Notes: d = displacement at the floor level; h = floor height (c)

13 338 D. Foti Figure 10 Maximum shear force at each floor, (a) bare frame (b) frame with dissipators (c) base isolated frame (see online version for colours) (a) (b) (c) Note: W = total weight of the frame.

14 Response of frames seismically protected with passive systems 339 In addition, the maximum shear force must be generally decreasing from the base to the upper floors. It should be noted that, for frames with dissipaters, the shear force was determined by excluding the contribution of the diagonals, so as to obtain values more comparable with those obtained for the bare frames and the isolated frames. Comparing the frames with dissipaters and those with isolators, it is possible to notice that the shear force, in general, is higher in the first ones, except for Kokaeli 60, Kobe 90, Sylmar 52 signals. For Kokaeli 60, moreover, the values of the shear force for base isolated frames are close to those obtained for the bare frames, which, as already noted, are the highest values. Under Kobe 90 signal, in frames with dissipaters the shear force shows the lowest values in all the floors. The same result is found for Sylmar 52 register from the second level onwards. It should be noted, finally, that in frames with dissipaters there is always a decrease in the shear force at the second level, followed by an increase at third level, and then it goes on decreasing up to the last level. This can be probably explained considering that on the third level the dissipaters are under dimensioned, while maintaining the same design criterion utilised for all the floors. It is not possible, in fact, to use fractions of dissipaters. 4.5 Ductility capacity In Figure 11, the values of the maximum rotations at the beam ends are plotted. It is plotted the maximum value at each level. This value is an index of the ductility demand of the structure. The ductility capacity of a structure represents the possibility of a structure or a structural element to stand plastic deformations without failure. Comparing the three frames, it is possible to note that the highest values of the ductility capacity are obtained for the bare frame, except for Kokaeli 60 at the first and second levels, Duzce 270 and Sylmar 52 at the first level, where the rotations are higher for the isolated frame. Instead, under Kobe-JMA signal at the first, second and sixth levels higher rotations are obtained in the frame with dissipaters. Comparing the protected frames, instead, it is noted that the highest values of ductility demand are found in the isolated frame at the first, second and third levels, while from the fourth to the sixth level the rotations at the ends of the beams are higher in the frames with dissipaters. 4.6 Probabilistic values The results of the average values added to the respective standard deviation (SD), confirm the observations previously exposed in this chapter. The damage index is maximum for the bare frame, while it is minimum for the frame with dissipaters (Figure 12). This is an expected result, given that the latter frame, due to the presence of braces, shows a reduced displacement at the points where the dissipaters are installed. In Figure 12, Dmax is the maximum top displacement relative to the base and H is the total height of the frame.

15 340 D. Foti Figure 11 Ductility capacity, (a) bare frame (b) frame with dissipaters (c) base isolated frame (a) (b) (c)

16 Response of frames seismically protected with passive systems 341 Figure 12 Average + SD for the damage index and the amplification Notes: Dmax = top displacement; H = total height Figure 13 Average + SD for interstory drifts For the same reason, the acceleration amplification is larger in the frame with dissipaters, minimum in the bare frame.

17 342 D. Foti Figure 14 Average + SD for the shear force at the floors Figure 15 Average + SD for beam rotations

18 Response of frames seismically protected with passive systems 343 Regarding the interstory drifts, as previously shown, the maximum deformed shape presents a reduction at the first and last levels both for the bare frame and for the frame with dissipaters (Figure 13). The base isolated frame, instead, shows a sharp increase in the interstory drifts between the first level and the base. From the second level onwards, there is an increasing reduction up to almost coincide with the value of the interstory drift between the fifth and the sixth level. The same result is also obtained for the frame with dissipaters. Also, the shear force at the floors is decreasing from the base to the top level (Figure 14). In general, the shear force is higher for the bare frame, and minimum for the protected frames. On the first and sixth levels, the maximum shear value is obtained for the frame with dissipaters, while at the second and fourth levels it is obtained for the frame with base isolators. In reference to the maximum rotation at the ends of the beams, the highest values are obtained for the bare frame, except at level 1, where the maximum rotation occurs in the beam ends of the isolated frame (Figure 15). The maximum absolute rotation is found in the bare frame at the second level. Comparing the protected frames, it is noticed that the isolated frame shows the highest values except at the last two levels. 5 Conclusions The present paper addresses the analysis of r.c. frames with and without passive seismic protection to earthquakes in near-field areas, i.e., next to the epicentral area of the fault rupture. The r.c. frames have six levels and two equal bays. A frame without protection (bare), a frame with hysteretic-type dissipaters, and a frame with rubber base isolators reinforced with steel plates have been analysed. A nonlinear analysis has been performed using the signals of seven earthquakes recorded in near-field areas, scaled according to the response spectrum of the Italian seismic code (NTC 2008). From the results, it is noted that passive protection devices are able to effectively reduce the seismic response of structures subject to near-field ground motions compared to the bare frames. In general, there is some more advantage for frames with dissipaters compared to the isolated ones. Finally, a topic of considerable interest for future research is the evaluation of the increase in yield strength due to the high velocity of the seismic forces in near-field areas. Under these conditions, there is a significant decrease in the available local ductility. Because of the increased ductility demand, as a result of the impulsive nature of the seismic force and the response reduction due to the high velocity components, the balance request-response may be low. Therefore, the need to determine the ductility, in function of the velocity of the seismic action, is an important topic to be developed in future research in this field.

19 344 D. Foti References Alavi, B. and Krawingler, H. (1999) Structural Design Implications of Near-Field Ground Motion, Kajima-CURE Research Report, Kajima Corporation, Tokyo. Alavi, B. and Krawingler, H. (2000) Consideration of near-fault ground motion effects in seismic design, in Proceedings of the 12th WCEE, New Zealand. Bray, J.D. and Rodriguez-Marek, A. (2004) Characterization of forward-directivity ground motions in the near-fault region, Soil Dynamics and Earthquake Engineering, Vol. 24, No. 11, pp Chais, X., Bozzo, L.M., Torres, L. and Foti, D. (2001) Experimental Tests on Hybrid,Semi-active and Passive Devices for Seismic Risk Mitigation, Report No. 7, ISMES, Ed. Giorgio Franchioni Rew. R.T. Severn, C. Taylor, Part 2. Chopra, A.K. and Chintanapakdee, C. (2001a) Comparing response of SDF systems to near-fault and far-fault earthquake motions in the context of spectral regions, Earthquake Engineering and Structural Dynamics, Vol. 30, No. 12, pp Chopra, A.K. and Chintanapakdee, C. (2001b) Drift spectrum vs. modal analysis of structural response to near-fault ground motions, Earthquake Spectra, Vol. 17, No. 2, pp Dicleli, M. and Buddaram, S. (2007) Equivalent linear analysis of seismic-isolated bridges subjected to near-fault ground motions with forward rupture directivity effect, Engineering Structures, Vol. 29, No. 1, pp Dicleli, M. and Mehta, A. (2007) Effect of near-fault ground motion and damper characteristics on the seismic performance of chevron braced steel frames, Earthquake Engineering and Structural Dynamics, Vol. 36, No. 7, pp Eurocode 2 (2004) Design of Concrete Structures. Part 1-1: General Rules and Rules for Buildings, C.E.N., European Committee for Standardization. Filiatrault, A., Tremblay, R. and Wanitkorkul, A. (2002) Performance evaluation of passive damping systems for the seismic retrofit of steel moment-resisting frames subjected to nearfield ground motions, Earthquake Spectra, Vol. 17, No. 3, pp Foti, D., Bozzo, L.M. and Lopez-Almansa, F. (1998) Numerical efficiency assessment of energy dissipaters for seismic protection of buildings, Earthquake Engineering and Structural Dynamics, Vol. 27, pp , Wiley & Sons, Ltd., Chichester, UK. Foti, D., Catalan Goni, A. and Vacca, S. (2013a) On the dynamic response of rolling base isolation systems, Structural Control and Health Monitoring, Vol. 20, No. 4, pp Foti, D., Diaferio, M. and Nobile, R. (2013b) Dynamic behaviour of new aluminum-steel energy dissipating devices, Structural Control and Health Monitoring, Vol. 20, No. 7, pp Foti, D., Diaferio, M. and Nobile, R. (2010) Optimal design of a new seismic passive protection device made in aluminium and steel, An International Journal of Structural Engineering and Mechanics, Vol. 35, No. 1, pp Greco, R., Marano, G. and Foti, D. (1999) Strong motion duration effects on base isolated systems, Physica A Statistical Mechanics and its Applications, Vol. 274, No. 1, p.2, Amsterdam, North Holland. He, W-L. and Agrawal, A.K. (2008) Analytical model of ground motion pulses for the design and assessment of seismic protective systems, Journal of Structural Engineering, Vol. 134, No. 7, pp Istruzioni CNR (1999) Apparecchi di appoggio per le costruzioni, Bollettino Ufficiale, Norme tecniche, anno XXIII, N.190, Roma, Italia. Mazza, F. and Mazza, M. (2012) Nonlinear modeling and analysis of r.c. framed buildings located in a near-fault area, The Open Construction & Building Technology Journal, Vol. 6, pp

20 Response of frames seismically protected with passive systems 345 Mazza, F. and Vulcano, A. (2009) Nonlinear response of rc framed buildings with isolation and supplemental damping at the base subjected to near-fault earthquakes, Journal of Earthquake Engineering, Vol. 13, No. 5, pp Mazza, F. and Vulcano, A. (2010) Nonlinear dynamic response of r.c. framed structures subjected to near-fault ground motions, Bulletin of Earthquake Engineering, Vol. 8, No. 6, pp Mazza, F. and Vulcano, A. (2012) Effects of near-fault ground motions on the nonlinear dynamic response of base isolated r.c. framed buildings, Earthquake Engineering & Structural Dynamics, Vol. 41, No. 2, pp Mazza, F., Vulcano, A. and Mazza, M. (2012) Nonlinear dynamic response of RC buildings with different base-isolation systems subjected to horizontal and vertical components of near-fault ground motions, The Open Construction & Building Technology Journal, Vol. 6, pp Norme Tecniche per le Costruzioni (NTC) (2008) Italian Ministry of Infrastructures, Nuove norme tecniche per le costruzioni e relative istruzioni, D.M e Circolare , n. 617/C.S.LL.PP. Ordoñez, D., Foti, D. and Bozzo, L.M. (2003) Comparative study of the inelastic structural response of base isolated buildings, Earthquake Engineering and Structural Dynamics, Vol. 32, No. 1, pp Tan, P., Agrawal, A.K. and Pan, Y. (2005) Near-field effects on seismically excited highway bridge equipped with nonlinear viscous dampers, Bridge Structures, Vol. 1, No. 3, pp Tirca, L. (2000) Behaviour of MRFs subjected to near-field earthquakes, in Mazzolani, F.M. and Tremblay, R. (Eds.): Proceedings of 3rd International Conference STESSA 2000, Balkema, Montreal, pp Tirca, L. and Gioncu, V. (1998) The effects of horizontal and vertical ground motion near and far field regions, in Proceedings of 2nd Int. Ph.D. Symposium in Civil Engineering, Budapest, pp Tirca, L., Foti, D. and Diaferio, M. (2003) Response of middle-rise steel frames with and without passive dampers to near-field ground motions, Engineering Structures, Vol. 25, No. 2, pp Xu, Z. and Agrawal, A. (2010) Decomposition and effects of pulse components in near-field ground motions, Journal of Structural Engineering, Vol. 136, No. 6, pp Xu, Z., Agrawal, A.K., He, W-L. and Tan, P. (2007) Performance of passive energy dissipation systems during near-field ground motion type pulses, Engineering Structures, Vol. 29, No. 2, pp Yi, W-J. and Zhang, B. (2007) Damage mechanism of frame structures under action of near-field earthquake, Journal of Natural Disasters, Vol. 16, No. 2, pp