Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes

Antonello Salvatori; Giovanni Bongiovanni; Paolo Clemente; Chiara Ormando; Fernando Saitta; Federico Scafati

doi:10.3390/infrastructures7020013

Salvatori, Antonello;Bongiovanni, Giovanni;Clemente, Paolo;Ormando, Chiara;Saitta, Fernando;Scafati, Federico

2022-01-21 00:00:00

infrastructures Article Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes 1 2 2 , 3 2 Antonello Salvatori , Giovanni Bongiovanni , Paolo Clemente * , Chiara Ormando , Fernando Saitta and Federico Scafati Department of Civil, Construction-Architectural and Environmental Engineering, University of L’Aquila, 67100 L’Aquila, Italy; antonello.salvatori@univaq.it (A.S.); federico.scafati@graduate.univaq.it (F.S.) Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Casaccia Research Centre, 00123 Rome, Italy; giovanni.bongiovldi8@alice.it (G.B.); fernando.saitta@enea.it (F.S.) Department of Civil Engineering and Computer Science Engineering, University of Rome Tor Vergata, 00133 Rome, Italy; chiara.ormando@uniroma2.it * Correspondence: paolo.clemente@enea.it Abstract: The behavior of a reinforced concrete building, seismically isolated with high damping rubber bearing (HDRB) and sliding devices (SD), observed during the most important events of the seismic sequence that struck central Italy from August 2016 to January 2017, is analyzed. Due to the epicenter distances, all the events had light effects at the site, thus the isolation system was not always put into action. A previous very low energy earthquake and the ambient vibration analysis are used for comparison. The study of the isolation system response is ﬁrst carried out and the variability of the resonance frequencies with the input energy at the site is pointed out. These frequencies are quite close to those of the superstructure considered as ﬁxed base. Small cracks were observed after the sequence in some partition walls of the building. The analysis of the superstructure was performed by means of a ﬁnite element model, assuming a non-linear model for the isolators, based on previous experimental data. The importance of a suitable decoupling between the superstructure and the Citation: Salvatori, A.; Bongiovanni, ground and the contribution of the sliding devices under low energy earthquake is pointed out. G.; Clemente, P.; Ormando, C.; Saitta, F.; Scafati, F. Observed Seismic Behavior of a HDRB and SD Isolation Keywords: seismically isolated buildings; base isolation; experimental seismic behavior; high System under Far Fault Earthquakes. damping rubber bearings; seismic monitoring Infrastructures 2022, 7, 13. https://doi.org/10.3390/ infrastructures7020013 1. Introduction Academic Editor: Carlo Rainieri Seismic isolation was proposed to mitigate the effects of strong earthquakes in struc- Received: 17 December 2021 tures. The idea of separating a building from the ground motion was well-known in ancient Accepted: 17 January 2022 Greece but the ﬁrst engineered isolation techniques appeared only in the second half of the Published: 21 January 2022 nineteenth century [1]. In 1868, Stevenson developed and used in the lighting system in Publisher’s Note: MDPI stays neutral Japan the “aseismatic joint”, which consisted of spherical rollers in niches. In 1870 the same with regard to jurisdictional claims in idea was used by Touaillon, while Cooper proposed natural-rubber bearings to provide a published maps and institutional afﬁl- building with an elastic cushion or a system of springs, and so to mitigate the shocks. The iations. ﬁrst modern steel-rubber isolators, based on a vulcanization process, were produced in England in the 1970s, while the ﬁrst curved surface slider, known as friction pendulum, appeared in the USA in the second half of the 1980s [2]. In the usual application in buildings, seismic isolation consists of the insertion of Copyright: © 2022 by the authors. seismic devices between the foundation and the superstructure. The scope is to increase Licensee MDPI, Basel, Switzerland. the fundamental period of vibration of a building up to a value for which the acceleration This article is an open access article spectral amplitudes are low enough and an elastic behavior of the superstructure is expected distributed under the terms and under the design earthquake. The energy that the ground transmits to the structure is conditions of the Creative Commons substantially reduced and so the seismic effects in it [3,4]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Nowadays, the number of buildings protected by seismic isolation is increasing more 4.0/). and more all over the world, also thanks to the good performances of previous applica- Infrastructures 2022, 7, 13. https://doi.org/10.3390/infrastructures7020013 https://www.mdpi.com/journal/infrastructures Infrastructures 2022, 7, 13 2 of 20 tions [5], both for new and existing structures [6,7]. Actually, the effectiveness of seismic isolation in preserving structures, included non-structural elements and contents, has been pointed out during several earthquakes [8–16]. The large diffusion of seismic isolation points out the problem of its correct use [17]. The main features, to be accounted for in an optimum preliminary design and check, have been pointed out both for high damping rubber bearings [18–20] and for curved surface sliders [21]. The seismic isolators have a supporting function with reference to vertical loads, both in operating and seismic conditions, and a low horizontal stiffness, to allow relative displacements between the superstructure and the foundation during a seismic event. On the other hand, they must also have an adequate stiffness against horizontal actions of small amplitude, in order to avoid continuous vibrations, which could be dangerous for the building, especially for non-structural elements (due to high frequency vibrations), and cause disturbances to the inhabitants (due to wind or trafﬁc-induced vibrations) [22]. These opposite requirements are not easy to satisfy. In practice, a good isolator should be rigid up to a certain value of the horizontal seismic action but exhibit suitable displacements when this value is overpassed. This threshold should be deﬁned on the basis of the seismic capacity of the superstructure, i.e., its strength under seismic action, assuming a very low behavior factor (preferably equal to 1). For curved surface sliders the threshold is ﬁxed by the static friction at the onset of motion. Actually, friction is of uncertain evaluation and depends on the vertical load acting during the quake [23,24]. Furthermore, it varies during time and could be much higher after a period of inactivity. In high damping rubber bearings, the shear modulus G increases when the shear strain diminishes and becomes very high when ! 0, as pointed out by a previous study in which both the effects on the isolation system [25] and the superstructure were analyzed [26,27]. In this paper the behavior of a reinforce concrete building, seismically isolated by means of HDRBs and SDs, observed during the most important events of the seismic sequence that struck Central Italy from August 2016 to January 2017, is analyzed. Due to the epicenter distances, all the events had light effects at the site. However, small cracks were observed in some partition walls after the main event of 30 October 2016. Obviously, this occurrence did not affect the seismic protection of the structure through seismic isolation. A detailed analysis of the recorded behavior during this earthquake is ﬁrst shown. Then, all the considered events are classiﬁed on the basis of the accelerations at the basement; their effects, such as accelerations, relative displacements and resonance frequencies are compared. A previous very low energy earthquake and the ambient vibration analysis, when the isolation system was not put in action, are used to analyze the differences in terms of resonance frequencies. Finally, the analysis of the superstructure was performed by assuming a non-linear model for the isolators, based on previous experimental data. The ﬁnite element model ﬁrst validated using the recordings obtained at the building during the 30 October 2016, Norcia earthquake and used to interpret the experimental behavior. 2. The Forest Ranger Building and the Monitoring System 2.1. The Building and the Isolation System The Forest Ranger building of the Umbria Civil Protection Centre in Foligno is a seismically isolated reinforced concrete building (Figure 1). It has an underground level and two ﬂoors above the ground. The maximum dimensions in plan are about 16 31 m, in x and y direction, respectively. The inter-ﬂoor heights are 3.14, 4.14, and 3.34 m, for the underground, ﬁrst and second level, respectively. Infrastructures 2022, 7, x FOR PEER REVIEW 3 of 19 1. 12 HDRBs of Type 1, located along the perimeter of the main rectangular portion; 2. 4 flat slider devices (SD) with a lubricated steel-PTFE (polytetrafluoroethylene) inter- face, having a nominal friction factor of 1.0%, located at the internal column of the Infrastructures 2022, 7, x FOR PEER REVIEW 3 of 19 main rectangular portion; 3. 4 HDRBs of Type 2, located at the columns external to the main portion. The isolation devices are located at the top of the columns of the underground level. 1. 12 HDRBs of Type 1, located along the perimeter of the main rectangular portion; Infrastructures 2022, 7, 13 3 of 20 The characteristics of the HDRBs are shown in Table 1. Schematic outlines of the isolators 2. 4 flat slider devices (SD) with a lubricated steel-PTFE (polytetrafluoroethylene) inter- are in Figure 3. face, having a nominal friction factor of 1.0%, located at the internal column of the main rectangular portion; 3. 4 HDRBs of Type 2, located at the columns external to the main portion. The isolation devices are located at the top of the columns of the underground level. The characteristics of the HDRBs are shown in Table 1. Schematic outlines of the isolators are in Figure 3. Figure 1. View of the Forest Ranger building (photo P. Clemente). Figure 1. View of the Forest Ranger building (photo P. Clemente). The isolation system is composed by (Figure 2): 1. 12 HDRBs of Type 1, located along the perimeter of the main rectangular portion; 2. 4 ﬂat slider devices (SD) with a lubricated steel-PTFE (polytetraﬂuoroethylene) inter- face, having a nominal friction factor of 1.0%, located at the internal column of the main rectangular portion; Figure 1. View of the Forest Ranger building (photo P. Clemente). 3. 4 HDRBs of Type 2, located at the columns external to the main portion. Figure 2. First floor with isolation system and vertical section, with the accelerometers deployment. Figure 2. First floor with isolation system and vertical section, with the accelerometers deployment. Figure 2. First ﬂoor with isolation system and vertical section, with the accelerometers deployment. Infrastructures 2022, 7, 13 4 of 20 The isolation devices are located at the top of the columns of the underground level. The characteristics of the HDRBs are shown in Table 1. Schematic outlines of the isolators are in Figure 3. Table 1. Nominal characteristics of the two types of HDRBs. Characteristic Type 1 Type 2 Number of devices 12 4 Diameter (mm) 700 550 Total rubber thickness (mm) 284 300 Thickness of a single rubber layer (mm) 7 5 Shear modulus of rubber at = 1 (N/mm ) 0.4 0.4 Equivalent horizontal stiffness at = 1 (N/mm) 541 317 Infrastructures 2022, 7, x FOR PEER REVIEW 4 of 19 Equivalent damping factor at = 1 (%) 10 10 Maximum displacement (mm) 379 395 Figure 3. Schematic sections of (a) an elastomeric isolator and (b) a ﬂat slider device. Figure 3. Schematic sections of (a) an elastomeric isolator and (b) a flat slider device. The substructure is composed by the foundation beams (120 75 cm) and 16 columns (80 Th 80 cm), e substruct on which ure the iisolators s compo ars eed placed. by th The e fo superst undation ructur beam e is a s frame (120str × uctur 75 cm e ) and 16 columns (columns 40 40 cm, beams 40 64 or 80 34 cm). The ﬂoors are reinforced concrete and (80 × 80 cm), on which the isolators are placed. The superstructure is a frame structure hollow tiles mixed ﬂoors, while the external cornices are in reinforced concrete as well as the (columns 40 × 40 cm, beams 40 × 64 or 80 × 34 cm). The floors are reinforced concrete and stairs between the ﬁrst ﬂoor and the second ﬂoor. The elevator shaft is a light steel structure hollow tiles mixed floors, while the external cornices are in reinforced concrete as well as linked to the ﬁrst, second and covering ﬂoors and suspended to them. Additionally, the stairs between the underground and the ﬁrst ﬂoor are in steel and hanged to the elevator the stairs between the first floor and the second floor. The elevator shaft is a light steel steel structure. structure linked to the first, second and covering floors and suspended to them. Addition- The Forest Ranger building was designed following the prescriptions of the technical ally, the stairs between the underground and the first floor are in steel and hanged to the code in force at the age of the construction [28]. elevator steel structure. A speciﬁc analysis of the seismic hazard, carried out by ENEA, allowed to consider a peak ground acceleration equal to 0.28 g for a non-exceedance probability of 10% in 50 years. Based on the results of downhole tests, the subsoil was classiﬁed as type B (shear Table 1. Nominal characteristics of the two types of HDRBs. wave velocity between 360 and 800 m/s), for which a soil ampliﬁcation factor equal to 1.25 was given by the code. Furthermore, with the building being a strategic structure according Characteristic Type 1 Type 2 to the Italian code, the acceleration spectrum ordinates were ampliﬁed by an importance Number of devices 12 4 factor equal to 1.4. Considering all these ampliﬁcations, a peak ground acceleration at the site equal to 0.49 g was obtained. Diamet In Figur er (m em) 4 the elastic response spectra at the Ultimate 700 550 limit state (ULS) are shown for two values of the damping factor equal to 5% and 10%, Total rubber thickness (mm) 284 300 respectively. The ﬁrst value corresponds to the damping associated with the structure; the Thickness of a single rubber layer (mm) 7 5 latter is the value considered in the design phase for the isolation system. Shear modulus of rubber at γ = 1 (N/mm ) 0.4 0.4 Equivalent horizontal stiffness at γ = 1 (N/mm) 541 317 Equivalent damping factor at γ = 1 (%) 10 10 Maximum displacement (mm) 379 395 The Forest Ranger building was designed following the prescriptions of the technical code in force at the age of the construction [28]. A specific analysis of the seismic hazard, carried out by ENEA, allowed to consider a peak ground acceleration equal to 0.28 g for a non-exceedance probability of 10% in 50 years. Based on the results of downhole tests, the subsoil was classified as type B (shear wave velocity between 360 and 800 m/s), for which a soil amplification factor equal to 1.25 was given by the code. Furthermore, with the building being a strategic structure accord- ing to the Italian code, the acceleration spectrum ordinates were amplified by an im- portance factor equal to 1.4. Considering all these amplifications, a peak ground accelera- tion at the site equal to 0.49 g was obtained. In Figure 4 the elastic response spectra at the Ultimate limit state (ULS) are shown for two values of the damping factor equal to 5% and 10%, respectively. The first value corresponds to the damping associated with the struc- ture; the latter is the value considered in the design phase for the isolation system. The design fundamental period of the isolated building was 2.57 s (fundamental fre- quency equal to 0.39 Hz). According to this value and to a damping factor equal to 10%, the value of the spectral acceleration is 0.19 g, while the value of the spectral displacement is 0.31 m. These values are relative to the initial nominal values of the device characteris- tics, without aging effects. Infrastructures 2022, 7, x FOR PEER REVIEW 5 of 19 Infrastructures 2022, 7, 13 5 of 20 1.5 0.6 Se ξ = 5% Se ξ = 10% SDe ξ=5% 1 SDe ξ=10% 0.4 0.5 0.2 0 0 0 1 2 3 4 T (s) Figure 4. Acceleration spectra (continuous line) and displacement spectra (dotted line) at the ULS. Figure 4. Acceleration spectra (continuous line) and displacement spectra (dotted line) at the ULS. The design fundamental period of the isolated building was 2.57 s (fundamental 2.2. Behavior under Ambient Vibrations frequency equal to 0.39 Hz). According to this value and to a damping factor equal to 10%, the value of the spectral acceleration is 0.19 g, while the value of the spectral displacement The structure was first dynamically characterized using ambient vibrations. For this is 0.31 m. These values are relative to the initial nominal values of the device characteristics, purpose, a temporary network of 12 velocimeter sensors deployed in the same location of without aging effects. the accelerometers of the permanent network, was used (Figure 2). Data were analyzed in 2.2. the Behavior frequency under dom Ambient ainV ev ibrations aluating the power spectral densities (PSD) of all the recording and The the str cross uctur sp e was ectra ﬁrst l dens dynamically ities (CDS) characterize of all th d using e sign ambient ificant vi coup brations. les of For sen this sors. The follow- purpose, a temporary network of 12 velocimeter sensors deployed in the same location of ing first three resonance frequencies were extracted by means of the peak picking tech- the accelerometers of the permanent network, was used (Figure 2). Data were analyzed in nique: 3.13 Hz, 3.71 Hz and 3.88 Hz. These resonance frequencies are related to the super- the frequency domain evaluating the power spectral densities (PSD) of all the recording structure modes because the isolation system was not excited by ambient vibrations. The and the cross spectral densities (CDS) of all the signiﬁcant couples of sensors. The following analysis of the phase factors of CSDs allowed to state that the first frequency is associated ﬁrst three resonance frequencies were extracted by means of the peak picking technique: 3.13 Hz, 3.71 Hz and 3.88 Hz. These resonance frequencies are related to the superstructure with a torsional mode, while the second and the third ones are associated with transla- modes because the isolation system was not excited by ambient vibrations. The analysis tional modes. They will be compared with the frequencies recorded during the seismic of the phase factors of CSDs allowed to state that the ﬁrst frequency is associated with events, in order to check the decoupling of the motion between the superstructure and the a torsional mode, while the second and the third ones are associated with translational ground. modes. They will be compared with the frequencies recorded during the seismic events, in order to check the decoupling of the motion between the superstructure and the ground. 2.3. The Permanent Accelerometer Network 2.3. The Permanent Accelerometer Network The permanent monitoring system is composed by a data acquisition system Kine- The permanent monitoring system is composed by a data acquisition system Kine- metrics K2 and 12 accelerometric sensors Kinemetrics FBA11. The sensors are deployed metrics K2 and 12 accelerometric sensors Kinemetrics FBA11. The sensors are deployed as follows (Figure 2): as follows (Figure 2): 1. Three accelerometers, A01, A02 and A03, are at the basement (level 0, L0) in x, vertical 1. Three accelerometers, A01, A02 and A03, are at the basement (level 0, L0) in x, vertical and y direction, respectively; and y direction, respectively; 2. Five accelerometers are on the slab above the isolation interface (level 1, L1), as follows: A04 and A08 in x direction, A05 in y direction, and A06, A07 and A09 in the 2. Five accelerometers are on the slab above the isolation interface (level 1, L1), as fol- vertical direction; lows: A04 and A08 in x direction, A05 in y direction, and A06, A07 and A09 in the 3. Three accelerometers are at the top of the building (level 2, L2), as follows: A10 and vertical direction; A12 in x direction and A11 in y direction. 3. Three accelerometers are at the top of the building (level 2, L2), as follows: A10 and A short term average/long term average (STA/LTA) logic is used to recognize seismic events. The mean value of a signal in a short-time interval of 6 s is compared with the A12 in x direction and A11 in y direction. mean value of the same signal in a long-time interval of 60 s. If the ﬁrst one is greater A short term average/long term average (STA/LTA) logic is used to recognize seismic than four times the second one, then a trigger command is activated by the sensor. If the events. The mean value of a signal in a short-time interval of 6 s is compared with the mean value of the same signal in a long-time interval of 60 s. If the first one is greater than four times the second one, then a trigger command is activated by the sensor. If the trigger command is activated simultaneously by at least two sensors at the base and two sensors at the top of the building, the signals are recorded starting from 30 s before the trigger commands activation. The recording stops 30 s after the sensors, which activated the trig- ger command, measure a signal amplitude lesser than the 40% of the trigger value. Se (g) SDe (m) Infrastructures 2022, 7, 13 6 of 20 trigger command is activated simultaneously by at least two sensors at the base and two sensors at the top of the building, the signals are recorded starting from 30 s before the trigger commands activation. The recording stops 30 s after the sensors, which activated the trigger command, measure a signal amplitude lesser than the 40% of the trigger value. 3. Observed Seismic Behavior The permanent monitoring system recorded all the seismic events that struck Central Italy between August 2016 and January 2017. Among these only the most representative earthquakes were chosen to analyze the behavior of the building. The features of the selected earthquakes are summarized in Table 2, where I represents the Arias intensity at the base of the structure, obtained by the formula (a , a and a are the accelerations in 0x 0y 0z the three directions at the basement of the building recorded between the initial time t and the ﬁnal time t of the event) [29]: h i 2p 2 2 2 I = a (t) + a (t) + a (t) dt (1) 0x 0y 0z Table 2. Data of the selected seismic events. Epicentral Magnitude Event Date Duration D (s) I (cm/s) I /D (cm/s ) A A Distance (km) (Mw or Ml) 4 5 SH008 2015.05.21 50 3.4 9.0 2.73 10 3.03 10 TX040 2016.08.24 53 6.0 17.3 5.19 3.00 10 1 2 TX053 2016.08.24 41 5.4 15.6 5.00 10 3.21 10 3 4 TX064 2016.08.24 59 4.1 16.3 2.02 10 1.24 10 2 3 TX066 2016.08.24 41 4.4 11.0 1.89 10 1.72 10 UP036 2016.10.26 36 5.4 10.3 2.59 2.51 10 UP041 2016.10.26 35 5.9 20.3 3.46 1.70 10 UP166 2016.10.30 36 6.5 15.8 17.5 1.11 1 3 UR115 2017.01.18 68 5.5 23.0 2.25 10 9.78 10 In the last column of Table 2, the ratio between I and the event duration D is reported, which was evaluated as the time interval between the two instants corresponding to 5% and 95% of I , respectively. As one can see, in some cases a greater I does not correspond A A to a greater I /D. In the following, the behavior recorded during the 30 October 2016, Norcia earthquake, which induced the maximum effects at the site, is carefully analyzed. Then, the results obtain from the selected seismic events are brieﬂy compared to each other. 3.1. The 30 October 2016 Norcia Earthquake The 30 October 2016 Norcia earthquake was the event with the maximum magnitude in the sequence, but also the event that induced the maximum effects at the site of the building. In Figure 5 the acceleration time histories recorded in x and y direction, respectively, at the basement L0 (A01 and A03), at the ﬁrst ﬂoor above the isolation interface L1 (A04 and A05) and at the top of the building L2 (A10 and A11) are plotted. The peaks of the acceleration, for the same levels, are PBA = 0.098 g, PIA = 0.054 g and PTA = 0.059 g, respectively. The absence of ampliﬁcation from the basement to the top, which characterizes isolated structures, is clearly recognizable. On the contrary, there is a reduction in acceleration between the basement (L0) and the ﬁrst ﬂoor above the isolation system (L1). Infrastructures 2022, 7, x FOR PEER REVIEW 6 of 19 3. Observed Seismic Behavior The permanent monitoring system recorded all the seismic events that struck Central Italy between August 2016 and January 2017. Among these only the most representative earthquakes were chosen to analyze the behavior of the building. The features of the se- lected earthquakes are summarized in Table 2, where IA represents the Arias intensity at the base of the structure, obtained by the formula ( a , and a are the accelerations 0 x 0 y 0 z in the three directions at the basement of the building recorded between the initial time ti and the final time tf of the event) [29]: 2 2 2 2  I = a t + a t + a t dt ( ) ( ) ( ) (1) A 0 x 0 y 0 z  g i In the last column of Table 2, the ratio between IA and the event duration D is re- ported, which was evaluated as the time interval between the two instants corresponding to 5% and 95% of IA, respectively. As one can see, in some cases a greater IA does not correspond to a greater IA/D. In the following, the behavior recorded during the 30 October 2016, Norcia earth- quake, which induced the maximum effects at the site, is carefully analyzed. Then, the results obtain from the selected seismic events are briefly compared to each other. 3.1. The 30 October 2016 Norcia Earthquake The 30 October 2016 Norcia earthquake was the event with the maximum magnitude in the sequence, but also the event that induced the maximum effects at the site of the building. In Figure 5 the acceleration time histories recorded in x and y direction, respec- tively, at the basement L0 (A01 and A03), at the first floor above the isolation interface L1 (A04 and A05) and at the top of the building L2 (A10 and A11) are plotted. The peaks of the acceleration, for the same levels, are PBA = 0.098 g, PIA = 0.054 g and PTA = 0.059 g, respectively. The absence of amplification from the basement to the top, which character- Infrastructures 2022, 7, 13 7 of 20 izes isolated structures, is clearly recognizable. On the contrary, there is a reduction in acceleration between the basement (L0) and the first floor above the isolation system (L1). Foligno FOR UP166 0.6 A10 A04 A01 Infrastructures 2022, 7, x FOR PEER REVIEW 7 of 19 0.0 30 40 50 60 t (s) and (b) y direction (the sign of record at A03 has been changed; therefore, it is named –A03 in the (a) legend). Foligno FOR UP166 0.6 Table 2. Data of the selected seismic events. A11 Epicentral Dis- Magnitude Duration IA IA/D Event Date tance (km) (Mw or Ml) D (s) (cm/s) (cm/s ) A05 −4 −5 SH008 2015.05.21 50 3.4 9.0 2.73 × 10 3.03 × 10 −1 TX040 2016.08.24 53 6.0 17.3 5.19 3.00 × 10 –A03 -A03 −1 −2 0.0 TX053 2016.08.24 41 5.4 15.6 5.00 × 10 3.21 × 10 −3 −4 30 40 50 60 TX064 2016.08.24 59 4.1 16.3 2.02 × 10 1.24 × 10 t (s) −2 −3 TX066 2016.08.24 41 4.4 11.0 1.89 × 10 1.72 × 10 −1 (b) UP036 2016.10.26 36 5.4 10.3 2.59 2.51 × 10 −1 UP041 2016.10.26 35 5.9 20.3 3.46 1.70 × 10 Figure 5. Time histories at the basement (green), at the ﬁrst ﬂoor above the isolation system (blue) and Figure 5. Time histories at the basement (green), at the first floor above the isolation system (blue) UP166 2016.10.30 36 6.5 15.8 17.5 1.11 at the top of the building (red) during the 30 October 2016, Norcia earthquake in (a) x direction and and at the top of the building (red) during the 30 October 2016, Norcia earthquake in (a) x direction −1 −3 UR115 2017.01.18 68 5.5 23.0 2.25 × 10 9.78 × 10 (b) y direction (the sign of record at A03 has been changed; therefore, it is named –A03 in the legend). Th The e Four Fourier ier s spectra pectra for forth the e horizo horizontal ntal se sensors nsors ar ar e e pl plotted otted in in Fig Figur ure 6. e 6 Th . The e sensor sensors s in th ine the same same diredir ction, ection, deploy deployed ed in thein supe therstructure superstructur , show e,a show peak o af p amplit eak of ude amplitude for the same for the same frequency, equal to 0.95 Hz in x direction and 0.81 Hz in y direction. These frequency, equal to 0.95 Hz in x direction and 0.81 Hz in y direction. These resonance frequenc resonance iesfr are equencies much low areer much than lower the resonance than the fre resonance quencies fr reco equencies rded unr der ecor am ded bient under vi- ambient vibrations and related to the superstructure. The spectrum rotates of the recordings brations and related to the superstructure. The spectrum rotates of the recordings ob- obtained at the couples A04–A05 and A10–A11 conﬁrmed the presence of two different tained at the couples A04–A05 and A10–A11 confirmed the presence of two different fre- frequencies along the two main directions (Figure 7). quencies along the two main directions (Figure 7). 4 4 1.4E+04 1.4E+04 1.4 10 1.4 10 Foligno For UP166 Foligno For UP166 A03 A01 A05 A04 A11 A10 3 3 7.0E+03 7.0E+03 0.7 10 0.7 10 0.0E+00 0.0E+000 0 0 5 10 15 0 5 10 15 f (Hz) f (Hz) (a) (b) Figure 6. Fourier spectrum amplitude at different levels obtained during the 30 October 2016 Norcia Figure 6. Fourier spectrum amplitude at different levels obtained during the 30 October 2016 Norcia Earthquake in (a) x direction and (b) y direction. Earthquake in (a) x direction and (b) y direction. Figure 7. Spectrum rotates of the couples of recordings A04-A05 and A10-A11, obtained during the 30 October 2016 Norcia earthquake. FFT a (g) a (g) FFT Infrastructures 2022, 7, x FOR PEER REVIEW 7 of 19 and (b) y direction (the sign of record at A03 has been changed; therefore, it is named –A03 in the legend). Table 2. Data of the selected seismic events. Epicentral Dis- Magnitude Duration IA IA/D Event Date tance (km) (Mw or Ml) D (s) (cm/s) (cm/s ) −4 −5 SH008 2015.05.21 50 3.4 9.0 2.73 × 10 3.03 × 10 −1 TX040 2016.08.24 53 6.0 17.3 5.19 3.00 × 10 −1 −2 TX053 2016.08.24 41 5.4 15.6 5.00 × 10 3.21 × 10 −3 −4 TX064 2016.08.24 59 4.1 16.3 2.02 × 10 1.24 × 10 −2 −3 TX066 2016.08.24 41 4.4 11.0 1.89 × 10 1.72 × 10 −1 UP036 2016.10.26 36 5.4 10.3 2.59 2.51 × 10 −1 UP041 2016.10.26 35 5.9 20.3 3.46 1.70 × 10 UP166 2016.10.30 36 6.5 15.8 17.5 1.11 −1 −3 UR115 2017.01.18 68 5.5 23.0 2.25 × 10 9.78 × 10 The Fourier spectra for the horizontal sensors are plotted in Figure 6. The sensors in the same direction, deployed in the superstructure, show a peak of amplitude for the same frequency, equal to 0.95 Hz in x direction and 0.81 Hz in y direction. These resonance frequencies are much lower than the resonance frequencies recorded under ambient vi- brations and related to the superstructure. The spectrum rotates of the recordings ob- tained at the couples A04–A05 and A10–A11 confirmed the presence of two different fre- quencies along the two main directions (Figure 7). 4 1.4E+04 1.4E+04 1.4 10 1.4 10 Foligno For UP166 Foligno For UP166 A03 A01 A05 A04 A10 A11 7.0E+03 7.0E+03 0.7 10 0.7 10 0.0E+00 0.0E+00 0 0 0 5 10 15 0 5 10 15 f (Hz) f (Hz) (a) (b) Infrastructures 2022, 7, 13 8 of 20 Figure 6. Fourier spectrum amplitude at different levels obtained during the 30 October 2016 Norcia Earthquake in (a) x direction and (b) y direction. Infrastructures 2022, 7, x FOR PEER REVIEW 8 of 19 The presence of two different resonance frequencies in the two directions could be related to a non-perfect symmetry of the isolation system. The wavelet transforms [30], plotted in Figure 8 for sensors A10 and A11, show that the dominant frequencies vary during the seismic event and the frequencies related to the isolation system are particu- larly evident only during a small portion of the time histories. One can deduce that the resonance frequencies changed during the earthquake and that the isolation system did not work for the entire recording. These occurrences justify the presence of more peaks in the spectra around the resonance frequency. Furthermore, between 46 s and 50 s, vibra- tions prev Figure 7. Spec ail i tn x direc rum rotates tion. of the couples of recordings A04-A05 and A10-A11, obtained during the Figure 7. Spectrum rotates of the couples of recordings A04-A05 and A10-A11, obtained during the 30 Oc Ot tober her 201 peak 6 No s are rcial ik earth ely quak due e. to the change of the dominant frequency of the earthquake 30 October 2016 Norcia earthquake. during the event. In Figure 9, the cross spectral densities (CSD), plotted in terms of amplitude and The presence of two different resonance frequencies in the two directions could be phase factor, and the corresponding coherence functions between sensors in x direction at related to a non-perfect symmetry of the isolation system. The wavelet transforms [30], the different levels (L0 and L1, L1 and L2) and between parallel sensors at the same level plotted in Figure 8 for sensors A10 and A11, show that the dominant frequencies vary are shown. The CSDs of sensors in y direction are plotted in Figure 10. The analysis of the during the seismic event and the frequencies related to the isolation system are particularly CSDs show that in correspondence of the already pointed out frequencies, the coherence evident only during a small portion of the time histories. One can deduce that the resonance function is always close to one. Furthermore, the values of the phase factor are equal to frequencies changed during the earthquake and that the isolation system did not work for zero both for couples of sensors placed at different levels and couples of sensors placed at the entire recording. These occurrences justify the presence of more peaks in the spectra the same level (for x direction). So, it appears that the superstructure moves as a rigid around the resonance frequency. Furthermore, between 46 s and 50 s, vibrations prevail in bo x dir dy a ection. nd the first fundamental modes are translational modes. (a) (b) Figure 8. Time-frequency analysis for (a) A10 and (b) A11. Figure 8. Time-frequency analysis for (a) A10 and (b) A11. By means of a double integration in the frequency domain, the time histories of the Other peaks are likely due to the change of the dominant frequency of the earthquake displacements (Figure 11) were obtained from the acceleration time histories. The maxi- during the event. mum values of the horizontal displacements of the gravity centers at L0, L1 and L2 were In Figure 9, the cross spectral densities (CSD), plotted in terms of amplitude and phase 27.3, 31.8 and 32.0 mm, respectively. factor, and the corresponding coherence functions between sensors in x direction at the The relative horizontal displacements between L1 and L0, and between L2 and L1 different levels (L0 and L1, L1 and L2) and between parallel sensors at the same level are were obtained for the 4 corner points, which correspond to the position of the isolation shown. The CSDs of sensors in y direction are plotted in Figure 10. The analysis of the devices CSDs show Is01, that Is04, in Iscorr 13 and espondence Is16. Theof rethe lative already particl pointed e motions outof frequencies, these points the are coher plot ence ted in function Figure is 12, always while th close e max to imum one. Furthermor values are e, summ the values arized of in the Tabl phase e 3. As factor one can are equal see, thto e zero both for couples of sensors placed at different levels and couples of sensors placed at maximum displacement of the building does not exceed 32 mm. the same The superstr level (for ucture x direction). has a maximum So, it appears relative that d the ispl superstr acemen uctur t of abou e moves t 2.2 as mm a rigid betwe body en and the ﬁrst fundamental modes are translational modes. L2 and L1. This value is much lower than the limit value allowed by the Italian technical code at the serviceable limit state for strategic seismic isolated buildings, which is equal to h/450, h being the inter-story height. The displacement is concentrated at the level of the isolation system, where the maximum relative displacement in the isolation devices is about 9.3 mm, corresponding to a shear strain of 0.033. In Figure 12b it is clearly recog- nizable the rotation of the top of the building, with respect to L1, around a point external with respect to the building plan. FFT FFT Infrastructures 2022, 7, x FOR PEER REVIEW 9 of 19 Infrastructures 2022, 7, 13 9 of 20 Infrastructures 2022, 7, x FOR PEER REVIEW 9 of 19 A01-A08 A04-A08 Cross Phase Coher Cross Phase Coher A01-A08 A04-A08 3 Cross Phase Coher 4 Cross Phase Coher 5.0 10 1.E 1 +10 04 180 5.0E+03 180 3 4 5.0 10 1.E 1 +10 04 180 5.0E+03 180 90 90 3 3 2.5E+03 0 5.E+03 0 2.5 10 5 10 3 3 2.5E+03 0 5.E+03 0 2.5 10 5 10 –90 -90 –90 -90 –-90 90 –90 -90 0.0E+00 -180 –180 –180 0.E+00 -180 0.0E+00 –-180 180 0.E+00 – -180 180 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 f (Hz) f (Hz) f (Hz) f (Hz) A08-A12 A10-A12 Cross Phase Coher Cross Phase Coher A08-A12 A10-A12 Cross Phase Coher 4 Cross Phase Coher 1.E+04 180 1.E 1 +10 04 180 1 10 1.E+04 180 1.E 1 +10 04 180 1 10 90 90 5.E+03 0 5.E+03 0 5 10 5 10 3 5.E+03 0 5.E+03 0 5 10 5 10 –90 -90 – -9 90 0 –90 –90 -90 -90 0.E+00 – -1 180 80 –180 0.E+000 -180 –180 –180 0.E+00 -180 0.E+00 -180 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 f (Hz) f (Hz) f (Hz) f (Hz) (a) (b) (a) (b) Figure 9. CSD of sensors in x direction (a) at different levels and (b) at the same level (the coherence Figure 9. CSD of sensors in x direction (a) at different levels and (b) at the same level (the coherence Figure 9. CSD of sensors in x direction (a) at different levels and (b) at the same level (the coherence varies in [0,1]). varie varies s in [ in [0,1]). 0,1]). –A03-A05 A05-A11 Cross Phase Coher Cross Phase Coher –A03-A05 A05-A11 Cross Phase Coher 4 Cross Phase Coher 5.0 10 5.0E+03 180 1.1 E +10 04 180 5.0 10 5.0E+03 180 1.1 E +10 04 180 2.5E+03 0 3 2.5 10 5.E+03 0 5 10 2.5E+03 0 3 2.5 10 5.E+03 0 5 10 –-9 90 0 – -9 90 0 –90 -90 –90 -90 0.0E+00 –-1 180 80 0 0.E+00 – -1 180 80 0.0E+00 –-1 180 80 0 0.E+00 – -1 180 80 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 f (Hz) f (Hz) f (Hz) f (Hz) Figure 10. CSD of sensors in y direction at different levels (the coherence varies in [0,1]). Figure 10. CSD of sensors in y direction at different levels (the coherence varies in [0,1]). Figure 10. CSD of sensors in y direction at different levels (the coherence varies in [0,1]). By means of a double integration in the frequency domain, the time histories of the displacements (Figure 11) were obtained from the acceleration time histories. The maximum values of the horizontal displacements of the gravity centers at L0, L1 and L2 were 27.3, 31.8 and 32.0 mm, respectively. 2 2 2 2 2 2 2 2 2 2 2 2 CSD ((cm/s ) /Hz) CS CS D ( D ( (c(m cm /s/s) )/H /H z)z) CS CS D ( D ( (c(m cm /s/s) )/H /H z)z) CSD ((cm/s ) /Hz) Ph Pa hs ae s e ( () ) Phase ( ) Phase ( ) Ph Pa hs ae s e ( () ) 2 2 2 2 2 2 2 2 2 2 2 2 CSD ((cm/s ) /Hz) CC SS DD (( ( c(m cm /s/s) )/H /H z)z) CS CS D ( D ( (c(m cm /s/s) )/H /H z)z) CSD ((cm/s ) /Hz) Ph Pa hs ae s e ( () ) Ph Pa hs ae s e ( () ) Phase ( ) Phase ( ) Infrastructures 2022, 7, x FOR PEER REVIEW 10 of 19 Infrastructures 2022, 7, 13 10 of 20 Infrastructures 2022, 7, x FOR PEER REVIEW 10 of 19 30 Foligno FOR UP166 Foligno FOR UP166 30 30 Foligno FOR UP166 Foligno FOR UP166 A10 A11 A10 A11 A04 A05 A04 A05 A01 –A03 -A03 A01 -–A A0 03 3 –-3 30 0 – -3 30 0 – 30 -30 – -3 30 0 30 40 50 60 70 80 90 30 40 50 60 70 80 90 30 40 50 60 70 80 90 30 40 50 60 70 80 90 t (s) t (s) t (s) t (s) (a) (b) (a) (b) Figure 11. Time histories of the absolute displacements at different level in (a) x direction (b) y di- Figure 11. Time histories of the absolute displacements at different level in (a) x direction (b) y di- Figure 11. Time histories of the absolute displacements at different level in (a) x direction (b) y rection (the sign of record at A03 has been changed; therefore, it is named –A03 in the legend). rection (the sign of record at A03 has been changed; therefore, it is named –A03 in the legend). direction (the sign of record at A03 has been changed; therefore, it is named –A03 in the legend). Table 3. Maximum relative displacement d1-0 between L1 and L0, and d2-1 between L2 and L1, at the Table 3. Maximum relative displacement d1-0 between L1 and L0, and d2-1 between L2 and L1, at the The relative horizontal displacements between L1 and L0, and between L2 and L1 points corresponding to the position of the corner isolation devices. points corresponding to the position of the corner isolation devices. were obtained for the 4 corner points, which correspond to the position of the isolation Isolator Is01 Is04 Is13 Is16 devices Is01, Is04, Is13 and Is16. The relative particle motions of these points are plotted Isolator Is01 Is04 Is13 Is16 d1-0 (mm) 9.28 8.80 8.81 8.38 in Figure 12, while the maximum values are summarized in Table 3. As one can see, the d1-0 (mm) 9.28 8.80 8.81 8.38 maximum d2-1 (mm displacement ) of the 1.49 building does 1.35 not exceed 32 mm. 2.19 2.12 d2-1 (mm) 1.49 1.35 2.19 2.12 Figure 12. Relative particle motions at the points corresponding to the position of the corner isola- Figure 12. Relative particle motions at the points corresponding to the position of the corner isolation Figure 12. Relative particle motions at the points corresponding to the position of the corner isola- tion devices between (a) L1 and L0, and (b) L2 and L1. devices between (a) L1 and L0, and (b) L2 and L1. tion devices between (a) L1 and L0, and (b) L2 and L1. d (mm) d (mm) d (mm) d (mm) Infrastructures 2022, 7, 13 11 of 20 Infrastructures 2022, 7, x FOR PEER REVIEW 11 of 19 Table 3. Maximum relative displacement d between L1 and L0, and d between L2 and L1, at the 1-0 2-1 points corresponding to the position of the corner isolation devices. Isolator Is01 Is04 Is13 Is16 3.2. Comparison of the Structure Behavior under Different Seismic Events d (mm) 9.28 8.80 8.81 8.38 1-0 d In (mm) Figure 13 the 1.49 magnitudes of th 1.35 e different even 2.19 ts are plotted 2.12 versus the epicenter 2-1 distance, for the selected events listed in Table 2. The sizes of the circles are proportional to the Arias intensity but are not representative for events with M < 5. The influence of the The superstructure has a maximum relative displacement of about 2.2 mm between L2 and L1. This value is much lower than the limit value allowed by the Italian technical code magnitude and the epicenter distance on the Arias intensity is apparent. at the serviceable limit state for strategic seismic isolated buildings, which is equal to h/450, In Figure 14, the maximum accelerations in x and y direction occurred at L0 (A01 and h being the inter-story height. The displacement is concentrated at the level of the isolation A03), L1 (A04 and A05) and L2 (A10 and A11), respectively, and are plotted versus IA for system, where the maximum relative displacement in the isolation devices is about 9.3 mm, all the recorded seismic events. Both the horizontal components of the accelerations at the corresponding to a shear strain of 0.033. In Figure 12b it is clearly recognizable the rotation three levels increase with IA. The acceleration at the basement (PBA) is not always greater of the top of the building, with respect to L1, around a point external with respect to the building than the plan. acceleration at L1 and L2, as shown also in Figure 15, where the maximum accel- erations, obtained as vector sum of the components along x and y, are shown. However, 3.2. Comparison of the Structure Behavior under Different Seismic Events the structural amplification in these cases is quite low. It is worth pointing out also that In Figure 13 the magnitudes of the different events are plotted versus the epicenter PBA does not always increase with IA. In some cases, this occurrence can be justified by distance, for the selected events listed in Table 2. The sizes of the circles are proportional to means of IA/D ratio (UP036, see Table 2), in other cases by the presence of a very high peak the Arias intensity but are not representative for events with M < 5. The inﬂuence of the in the acceleration time history. magnitude and the epicenter distance on the Arias intensity is apparent. Foligno FOR UP166 TX040 UP041 UR115 TX053 5 UP036 TX066 TX064 SH008 30 40 50 60 70 Epicentral distance (km) Figure 13. Magnitude versus epicenter distance for the recorded events. The areas of the circles are Figure 13. Magnitude versus epicenter distance for the recorded events. The areas of the circles are proportional to I , except for events with M < 5. proportional to IA, except for events with M < 5. In Figure 14, the maximum accelerations in x and y direction occurred at L0 (A01 and 0.10 0.10 A03), L1 (A04 and A05) and L2 (A10 and A11), respectively, and are plotted versus I for A01 A03 all the recorded seismic events. Both the horizontal components of the accelerations at 0.08 0.08 A04 A05 the three levels increase with I . The acceleration at the basement (PBA) is not always A10 A11 greater than the acceleration at L1 and L2, as shown also in Figure 15, where the maximum 0.06 0.06 accelerations, obtained as vector sum of the components along x and y, are shown. However, the structural ampliﬁcation in these cases is quite low. It is worth pointing out also that 0.04 0.04 PBA does not always increase with I . In some cases, this occurrence can be justiﬁed by means of I /D ratio (UP036, see Table 2), in other cases by the presence of a very high peak 0.02 0.02 in the acceleration time history. 0.00 0.00 -4 -2 0 2 -4 -2 0 2 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1.E-04 1.E-02 1.E+00 1.E+02 1.E-04 1.E-02 1.E+00 1.E+02 IA (cm/s) IA (cm/s) Figure 14. Maximum absolute acceleration at the basement PBA (A01 and A03 in x and y direction, respectively), above the isolation system PIA (A04 and A05), and at the top of the building PTA (A10 and A11), for all the recorded seismic events. Max Acc (g) Magnitude Max Acc (g) Infrastructures 2022, 7, x FOR PEER REVIEW 11 of 19 3.2. Comparison of the Structure Behavior under Different Seismic Events In Figure 13 the magnitudes of the different events are plotted versus the epicenter distance, for the selected events listed in Table 2. The sizes of the circles are proportional to the Arias intensity but are not representative for events with M < 5. The influence of the magnitude and the epicenter distance on the Arias intensity is apparent. In Figure 14, the maximum accelerations in x and y direction occurred at L0 (A01 and A03), L1 (A04 and A05) and L2 (A10 and A11), respectively, and are plotted versus IA for all the recorded seismic events. Both the horizontal components of the accelerations at the three levels increase with IA. The acceleration at the basement (PBA) is not always greater than the acceleration at L1 and L2, as shown also in Figure 15, where the maximum accel- erations, obtained as vector sum of the components along x and y, are shown. However, the structural amplification in these cases is quite low. It is worth pointing out also that PBA does not always increase with IA. In some cases, this occurrence can be justified by means of IA/D ratio (UP036, see Table 2), in other cases by the presence of a very high peak in the acceleration time history. Foligno FOR UP166 TX040 UP041 UR115 TX053 5 UP036 TX066 TX064 SH008 30 40 50 60 70 Epicentral distance (km) Infrastructures 2022, 7, 13 12 of 20 Figure 13. Magnitude versus epicenter distance for the recorded events. The areas of the circles are proportional to IA, except for events with M < 5. 0.10 0.10 A01 A03 0.08 A04 0.08 A05 A10 A11 0.06 0.06 0.04 0.04 0.02 0.02 0.00 0.00 -4 -2 0 2 -4 -2 0 2 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1.E-04 1.E-02 1.E+00 1.E+02 1.E-04 1.E-02 1.E+00 1.E+02 IA (cm/s) IA (cm/s) Figure 14. Maximum absolute acceleration at the basement PBA (A01 and A03 in x and y direction, Figure 14. Maximum absolute acceleration at the basement PBA (A01 and A03 in x and y direction, Infrastructures 2022, 7, x FOR PEER REVI respec EW ti vely), above the isolation system PIA (A04 and A05), and at the top of the building 12 of PT 19 A respectively), above the isolation system PIA (A04 and A05), and at the top of the building PTA (A10 (A10 and A11), for all the recorded seismic events. and A11), for all the recorded seismic events. 1.E-01 -1 1 10 PBA PIA PTA -2 1.E-02 1 10 1.E-03 -3 1 × 10 1.E-04 -4 1 × 10 SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 Figure 15. Maximum absolute acceleration at the basement PBA, above the isolation system PIA and Figure 15. Maximum absolute acceleration at the basement PBA, above the isolation system PIA and at the top of the building PTA for the selected seismic events. at the top of the building PTA for the selected seismic events. In Figures 16 and 17, the maximum absolute displacements at the three levels and In Figures 16 and 17, the maximum absolute displacements at the three levels and the rth elative e relat displacements ive displacemen d ts ,d between 1-0, between the gravity the gravity centers cent of ers L1 of and L1 an L0, d and L0, d and ,d between 2-1, between the 1-0 2-1 the gravity centers of L2 and L1, are shown in increasing order of IA. The displacement at gravity centers of L2 and L1, are shown in increasing order of I . The displacement at the the basement does not increase always with IA and the maximum displacement does not basement does not increase always with I and the maximum displacement does not occur always occur al at wthe ays top at th of e the top building. of the buAs ilding. already As alre pointed ady po out, inted thisout, occurr this ence occcan urren bece justiﬁed can be with justifi aed dif w fer ith ent a dD ifferent or the D pr or esence the pre of sen a ce very of a high very peak high in pethe ak in time the time history histor . Instead, y. Instethe ad, Infrastructures 2022, 7, x FOR PEER REVIEW 13 of 19 relative displacement d is always higher than d , also for the lowest energy earthquakes. the relative displacement d1-0 is always higher than d2-1, also for the lowest energy earth- 1-0 2-1 Acceleration and displacement values are synthesized in Table 4. quakes. Acceleration and displacement values are synthesized in Table 4. For all the recorded events a frequency domain analysis was performed. In Figure 18 1.E+02 1 10 the first two resonance frequencies are plotted versus IA. As one can see, in a lower energy L0 range, the resonance frequency is independent of the seismic energy and varies in a small L1 1.E+01 1 10 range around the value related to the superstructure. In these cases, the isolation system L2 was not activated probably because of the friction forces of the SDs. For higher values of IA, 1inste .E+00ad, the seismic isolation system was put in action and the first resonance frequen- 1 10 cies, related to the isolation system, decrease almost linearly with Log(IA). As one can see, -1 for low energy earthquakes the resonance frequencies approach those of the superstruc- 1 1. E10 -01 ture and there was no decoupling of motion. -2 1 10 1.E-02 Table 4. Maximum accelerations, displacements, relative displacements of the gravity centers and maximum relative displacements at the corner isolators for the selected seismic events. -3 1 1 .E10 -03 SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 Event SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 Figure 16. Maximum absolute displacement between the gravity centers at L0, L1 and L2 for the Figure 16. Maximum absolute displacement between the gravity centers at L0, L1 and L2 for the PBA (g) 0.0005 0.0012 0.0054 0.0094 0.0175 0.0706 0.0291 0.0575 0.0975 considered seismic events. considered seismic events. PIA (g) 0.0008 0.0014 0.0043 0.0111 0.0134 0.0379 0.0397 0.0300 0.0539 PTA (g) 0.0011 0.0017 0.0053 0.0136 0.0136 0.0411 0.0373 0.0327 0.0589 1.E+01 1 10 L1 - L0 1-0 PBD (mm) 0.0066 0.0271 0.2439 2.2917 1.1813 3.3642 9.7588 15.415 27.302 L2 - L1 PID (mm) 0.0138 0.0378 0.2467 2.3663 1.3072 5.5737 14.443 16.213 31.809 2-1 1.E+000 1 10 PTD (mm) 0.0171 0.0417 0.2524 2.3821 1.3349 5.8568 14.367 15.765 31.956 d1-0 (mm) 0.0116 0.0230 0.0728 0.2907 0.6282 5.0766 6.7810 4.9572 8.7882 d2-1 (mm -1 ) 0.0046 0.0107 0.0261 0.1503 0.1453 0.3685 0.6432 1.1301 1.6426 1.E-01 1 10 d1-0 Is01 (mm) 0.0116 0.0224 0.0744 0.2869 0.6686 5.0513 6.6656 5.2281 9.2833 d1-0 Is04 (mm) 0.0142 0.0236 0.0727 0.2847 0.6206 5.0211 6.7040 4.7085 8.7999 1.E-02 -2 1 10 d1-0 Is13 (mm) 0.0114 0.0234 0.0730 0.2968 0.6384 5.1327 6.8636 5.2482 8.8119 d1-0 Is16 (mm) 0.0149 0.0251 0.0738 0.2947 0.5880 5.1030 6.9015 4.7466 8.3750 d2-1 Is01 (mm) 0.0045 0.0107 0.0279 0.1439 0.1424 0.4136 0.5993 0.9671 1.4852 1.E-03 -3 1 10 SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 d2-1 Is04 (mm) 0.0046 0.0111 0.0292 0.1385 0.1310 0.4408 0.5747 0.9542 1.3457 Figure 17. Maximum relative displacement between the gravity centers at L1 and L0, and at L2 and d2-1 Is13 (mm) 0.0047 0.0113 0.0272 0.1735 0.1680 0.3127 0.7596 1.5602 2.1928 L1, for the considered seismic events. d2-1 Is16 (mm) 0.0049 0.0109 0.0264 0.1631 0.1531 0.3454 0.7973 1.5626 2.1249 4.5 f (A10) f (A11) 3.0 1.5 0.0 -4 -2 0 2 1 1.E ×-0 10 4 1 1.E × - 10 02 1 1.E ×+0 100 1 1.E × 10 +02 IA (cm/s) Figure 18. First resonance frequencies versus IA for all the recorded seismic events. 4. Non-Linear Modelling of the Isolation System The design frequency of the structure is 0.39 Hz, while during the seismic events in which the isolation system was activated, the resonance frequency varies between 0.8 Hz and 2.5 Hz. This effect is to be related to the change of the stiffness of the isolation devices, which depends on the angular strain. A suitable model to represents this behavior was set up with reference to another building, seismically isolated with HDRBs produced by the same manufacturer, which is also part of the Civil Protection Centre at Foligno [27]. The devices used for the type tests at time of construction, and properly conserved, were subjected to the displacement time history obtained in the devices on site during the 30 October 2016 Norcia earthquake. In Max Acc (g) Magnitude a (g) d (mm) d (mm) f (Hz) Max Acc (g) Infrastructures 2022, 7, x FOR PEER REVIEW 13 of 19 1.E+02 1 10 L0 L1 1.E+01 1 10 L2 1 1 .E +10 00 -1 1 1. E10 -01 -2 1 1. E10 -02 -3 1 1 .E10 -03 SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 Infrastructures 2022, 7, 13 13 of 20 Figure 16. Maximum absolute displacement between the gravity centers at L0, L1 and L2 for the considered seismic events. 1.E+01 1 10 L1 - L0 1-0 L2 - L1 2-1 1.E+00 1 10 -1 1.E-01 1 10 1.E-02 -2 1 10 -3 1.E-03 1 10 SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 Figure 17. Maximum relative displacement between the gravity centers at L1 and L0, and at L2 and Figure 17. Maximum relative displacement between the gravity centers at L1 and L0, and at L2 and L1, for the considered seismic events. L1, for the considered seismic events. 4.5 Table 4. Maximum accelerations, displacements, f (r A elative 10) displacements of the gravity centers and maximum relative displacements at the corner isolators for the selected seismic events. f (A11) 3.0 Event SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 PBA (g) 0.0005 0.0012 0.0054 0.0094 0.0175 0.0706 0.0291 0.0575 0.0975 PIA (g) 0.0008 0.0014 0.0043 0.0111 0.0134 0.0379 0.0397 0.0300 0.0539 1.5 PTA (g) 0.0011 0.0017 0.0053 0.0136 0.0136 0.0411 0.0373 0.0327 0.0589 PBD (mm) 0.0066 0.0271 0.2439 2.2917 1.1813 3.3642 9.7588 15.415 27.302 PID (mm) 0.0138 0.0378 0.2467 2.3663 1.3072 5.5737 14.443 16.213 31.809 PTD (mm) 0.0171 0.0417 0.2524 2.3821 1.3349 5.8568 14.367 15.765 31.956 0.0 d (mm) 0.0116 0.0230 0.0728 0.2907 0.6282 5.0766 6.7810 4.9572 8.7882 -4 -2 0 2 1-0 1 1.E ×-0 10 4 1 1.E × - 10 02 1 1.E ×+0 100 1 1.E × 10 +02 d (mm) 0.0046 0.0107 0.0261 0.1503 IA (cm/s) 0.1453 0.3685 0.6432 1.1301 1.6426 2-1 d Is01 1-0 0.0116 0.0224 0.0744 0.2869 0.6686 5.0513 6.6656 5.2281 9.2833 Figure 18. First resonance frequencies versus IA for all the recorded seismic events. (mm) d Is04 1-0 0.0142 0.0236 0.0727 0.2847 0.6206 5.0211 6.7040 4.7085 8.7999 4. Non-Linear Modelling of the Isolation System (mm) d Is13 1-0 The design frequency of the structure is 0.39 Hz, while during the seismic events in 0.0114 0.0234 0.0730 0.2968 0.6384 5.1327 6.8636 5.2482 8.8119 (mm) which the isolation system was activated, the resonance frequency varies between 0.8 Hz d Is16 1-0 0.0149 0.0251 0.0738 0.2947 0.5880 5.1030 6.9015 4.7466 8.3750 and 2.5 Hz. This effect is to be related to the change of the stiffness of the isolation devices, (mm) d Is01 which depends on the angular strain. 2-1 0.0045 0.0107 0.0279 0.1439 0.1424 0.4136 0.5993 0.9671 1.4852 (mm) A suitable model to represents this behavior was set up with reference to another d Is04 2-1 building, seismically isolated with HDRBs produced by the same manufacturer, which is 0.0046 0.0111 0.0292 0.1385 0.1310 0.4408 0.5747 0.9542 1.3457 (mm) also part of the Civil Protection Centre at Foligno [27]. The devices used for the type tests d Is13 2-1 0.0047 0.0113 0.0272 0.1735 0.1680 0.3127 0.7596 1.5602 2.1928 at time of construction, and properly conserved, were subjected to the displacement time (mm) history obtained in the devices on site during the 30 October 2016 Norcia earthquake. In d Is16 2-1 0.0049 0.0109 0.0264 0.1631 0.1531 0.3454 0.7973 1.5626 2.1249 (mm) For all the recorded events a frequency domain analysis was performed. In Figure 18 the ﬁrst two resonance frequencies are plotted versus I . As one can see, in a lower energy range, the resonance frequency is independent of the seismic energy and varies in a small range around the value related to the superstructure. In these cases, the isolation system was not activated probably because of the friction forces of the SDs. For higher values of I , instead, the seismic isolation system was put in action and the ﬁrst resonance frequencies, related to the isolation system, decrease almost linearly with Log(I ). As one can see, for low energy earthquakes the resonance frequencies approach those of the superstructure and there was no decoupling of motion. d (mm) d (mm) f (Hz) Infrastructures 2022, 7, x FOR PEER REVIEW 13 of 19 1.E+02 1 10 L0 L1 1.E+01 1 10 L2 1 1 .E +10 00 -1 1 1. E10 -01 -2 1 10 1.E-02 -3 1 10 1.E-03 SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 Figure 16. Maximum absolute displacement between the gravity centers at L0, L1 and L2 for the considered seismic events. 1.E+01 1 10 L1 - L0 1-0 L2 - L1 2-1 1.E+00 1 10 -1 1.E-01 1 10 1.E-02 -2 1 10 1.E-03 -3 1 10 SH008 TX064 TX066 UR115 TX053 UP036 UP041 TX040 UP166 Infrastructures 2022, 7, 13 Figure 17. Maximum relative displacement between the gravity centers at14 L1 an of 20 d L0, and at L2 and L1, for the considered seismic events. 4.5 f (A10) f (A11) 3.0 1.5 0.0 -4 -2 0 2 1 1.E ×-0 10 4 1 1.E × - 10 02 1 1.E ×+0 100 1 1.E × 10 +02 IA (cm/s) Figure 18. First resonance frequencies versus I for all the recorded seismic events. Figure 18. First resonance frequencies A versus IA for all the recorded seismic events. 4. Non-Linear Modelling of the Isolation System 4. Non-Linear Modelling of the Isolation System The design frequency of the structure is 0.39 Hz, while during the seismic events in which the isolation system was activated, the resonance frequency varies between 0.8 Hz The design frequency of the structure is 0.39 Hz, while during the seismic events in and 2.5 Hz. This effect is to be related to the change of the stiffness of the isolation devices, which the isolation system was activated, the resonance frequency varies between 0.8 Hz which depends on the angular strain. and 2.5 Hz. This effect is to be related to the change of the stiffness of the isolation devices, A suitable model to represents this behavior was set up with reference to another building, seismically isolated with HDRBs produced by the same manufacturer, which is which depends on the angular strain. also part of the Civil Protection Centre at Foligno [27]. The devices used for the type tests A suitable model to represents this behavior was set up with reference to another at time of construction, and properly conserved, were subjected to the displacement time building, seismically isolated with HDRBs produced by the same manufacturer, which is history obtained in the devices on site during the 30 October 2016 Norcia earthquake. In the also part of the Civil Protection Centre at Foligno [27]. The devices used for the type tests force-displacement diagram, several cycles were selected. Each of them was approximated by an elliptical curve with center in the origin and, with reference to this ellipse, the at time of construction, and properly conserved, were subjected to the displacement time equivalent stiffness and viscous damping were calculated. history obtained in the devices on site during the 30 October 2016 Norcia earthquake. In As usual, in the type tests the devices were subject to six sets of ﬁve sinusoidal cycles, each set having the same frequency of 0.5 Hz but different amplitudes (corresponding to = 0.05, 0.30, 0.50, 0.70, 1.00 and 2.00), keeping an axial pressure of 6.0 N/mm . These tests allowed to obtain the shear modulus and the equivalent viscous damping of the isolation devices for = 0.05 2.00. The additional test, with the seismic recorded time histories, gave the values of G for < 0.05. The procedure used and the results of all these tests are extensively described in previous papers, in which a very simple formulation was also proposed to relate the shear modulus and the damping factor to the shear strain [27]: 0.43 G = G g (2) 0.6 = g (3) where is the shear modulus and G = 0.4 N/mm and = 10% are the shear modulus 1 1 and the damping factor corresponding to = 1, respectively. From Equation (2), the relation between the horizontal force F and the shear modulus can be obtained [27]: 0.57 F = G Ag (4) where A is the area of the cross section of the isolation device. In Figure 19, the horizontal force and the shear modulus are plotted versus the shear strain. d (mm) d (mm) f (Hz) Infrastructures 2022, 7, x FOR PEER REVIEW 14 of 19 the force-displacement diagram, several cycles were selected. Each of them was approxi- mated by an elliptical curve with center in the origin and, with reference to this ellipse, the equivalent stiffness and viscous damping were calculated. As usual, in the type tests the devices were subject to six sets of five sinusoidal cycles, each set having the same frequency of 0.5 Hz but different amplitudes (corresponding to γ = 0.05, 0.30, 0.50, 0.70, 1.00 and 2.00), keeping an axial pressure of 6.0 N/mm . These tests allowed to obtain the shear modulus and the equivalent viscous damping of the isolation devices for γ = 0.05  2.00. The additional test, with the seismic recorded time histories, gave the values of G for γ < 0.05. The procedure used and the results of all these tests are extensively described in pre- vious papers, in which a very simple formulation was also proposed to relate the shear modulus and the damping factor to the shear strain [27]: −0.43 GG= (2) −0.6 =  (3) where γ is the shear modulus and G1 = 0.4 N/mm and ξ1 = 10% are the shear modulus and the damping factor corresponding to γ = 1, respectively. From Equation (2), the relation between the horizontal force F and the shear modulus can be obtained [27]: 0.57 F= G A (4) where A is the area of the cross section of the isolation device. In Figure 19, the horizontal force and the shear modulus are plotted versus the shear strain. 5. Comparison between the Observed Behavior and the Numerical Analysis Using the software Midas Gen, a finite-element model of the superstructure was set up to compare the experimental results with the numerical ones. The numerical model was set up on the basis on the design documents. Only frame elements were used to model Infrastructures 2022, 7, 13 the structural elements, while the floors and the slabs were considered 15 ofon 20 ly as a perma- nent weight. 2 10 0.57 F=G1Aϒ0.57 F = G A F=G1Aϒ F = G A 1.5 -0.43 G=G1ϒ-0.43 G = G  0.5 0 0 0 0.5 1 1.5 2 Figure 19. Force and shear modulus versus the shear strain. Figure 19. Force and shear modulus versus the shear strain. 5. Comparison between the Observed Behavior and the Numerical Analysis The structure was subjected to the self-weight of the structural elements, the addi- Using the software Midas Gen, a ﬁnite-element model of the superstructure was set tional permanent loads and a percentage of the variable loads, which were likely present up to compare the experimental results with the numerical ones. The numerical model was set up on the basis on the design documents. Only frame elements were used to during the seismic events. The assumed values are listed in Table 5. model the structural elements, while the ﬂoors and the slabs were considered only as a permanent weight. The structure was subjected to the self-weight of the structural elements, the additional permanent loads and a percentage of the variable loads, which were likely present during the seismic events. The assumed values are listed in Table 5. Table 5. Self-weight, permanent loads and variable load in seismic conditions. Self-Weight Permanent Load Partition Walls Variable Load Floor 2 2 2 2 (kN/m ) (kN/m ) (kN/m ) (kN/m ) First 4.0 2.4 0.8 0.60 Second 4.0 4.9 0.8 0.60 Third 4.0 2.7 0.0 0.00 The isolation devices were modelled by means of elastic link elements, assigning a linear behavior. The effects of aging were accounted for increasing the shear modulus of the rubber by 15% (G = 1.15 G = 0.46 N/mm ) [31,32]. Actually, about 12 years have 1a 1 passed from the construction of the building to the seismic events. In Table 6, the results of the modal analysis are shown. The ﬁrst period is equal to 2.55 s (frequency of 0.39 Hz), which is very close to that assumed in the design phase. The ﬁrst two modal shapes are translational, while the third one is torsional. The higher frequencies are related to the superstructures and are a little lower than those obtained from the ambient vibration analysis. This occurrence can be related to the contribution of the non-structural elements during the ambient vibration tests. Table 6. Results of the modal analysis. Mode Frequency (Hz) Period (s) 1 0.391 2.554 2 0.392 2.552 3 0.493 2.026 4 2.952 0.339 5 3.394 0.295 6 3.447 0.290 F/(G A) G/G 1 Infrastructures 2022, 7, x FOR PEER REVIEW 15 of 19 Table 5. Self-weight, permanent loads and variable load in seismic conditions. Self-Weight Permanent Load Partition Walls Variable Load Floor 2 2 2 2 (kN/m ) (kN/m ) (kN/m ) (kN/m ) First 4.0 2.4 0.8 0.60 Second 4.0 4.9 0.8 0.60 Third 4.0 2.7 0.0 0.00 The isolation devices were modelled by means of elastic link elements, assigning a linear behavior. The effects of aging were accounted for increasing the shear modulus of the rubber by 15% (G1a = 1.15 × G1 = 0.46 N/mm ) [31,32]. Actually, about 12 years have passed from the construction of the building to the seismic events. In Table 6, the results of the modal analysis are shown. The first period is equal to 2.55 s (frequency of 0.39 Hz), which is very close to that assumed in the design phase. The first two modal shapes are translational, while the third one is torsional. The higher fre- quencies are related to the superstructures and are a little lower than those obtained from the ambient vibration analysis. This occurrence can be related to the contribution of the non-structural elements during the ambient vibration tests. Table 6. Results of the modal analysis. Mode Frequency (Hz) Period (s) 1 0.391 2.554 2 0.392 2.552 3 0.493 2.026 4 2.952 0.339 5 3.394 0.295 6 3.447 0.290 Finally, a non-linear time history analysis was carried out applying, at the base of the structure, the horizontal acceleration time histories recorded by the sensors A01 and A03 during the 30 October 2016 Norcia earthquake. For this purpose, a non-linear constitutive law was assumed for the hysteretic isola- tors that simulate the elastomeric devices. It is represented by Equation (4) and was ap- proximated by a trilinear curve (Figure 20). The approximation was made to match Equa- Infrastructures 2022, 7, 13 16 of 20 tion (4) in the range of small shear strain (0–0.03), which was of interest for the observed results during Norcia earthquake. Sliding devices were modelled using sliding bearings having a very high initial stiff- Finally, a non-linear time history analysis was carried out applying, at the base of the structure, the horizontal acceleration time histories recorded by the sensors A01 and A03 ness, able to simulate the presence of a static friction, and a dynamic friction factor of 2%, during the 30 October 2016 Norcia earthquake. slightly amplified with reference to the initial one. The chosen increased stiffness of For this purpose, a non-linear constitutive law was assumed for the hysteretic isolators HDRBs and friction factor of SDs allowed to optimize the correspondence between exper- that simulate the elastomeric devices. It is represented by Equation (4) and was approxi- mated by a trilinear curve (Figure 20). The approximation was made to match Equation (4) imental and numerical results. Both these assumptions can be justified as effects of the in the range of small shear strain (0–0.03), which was of interest for the observed results aging [31,32]. during Norcia earthquake. 0.15 0.1 0.05 0 0.01 0.02 0.03 Figure 20. The non-linear relationship between force and shear strain for the elastomeric isolators, which accounts for the aging effects, approximated by a trilinear curve. Sliding devices were modelled using sliding bearings having a very high initial stiff- ness, able to simulate the presence of a static friction, and a dynamic friction factor of 2%, slightly ampliﬁed with reference to the initial one. The chosen increased stiffness of HDRBs and friction factor of SDs allowed to optimize the correspondence between experimental and numerical results. Both these assumptions can be justiﬁed as effects of the aging [31,32]. The results of the dynamic numerical analysis were compared to the seismic behavior observed during the Norcia earthquake. The good agreement between the acceleration time histories is apparent (Figure 21). Little discrepancies in terms of amplitudes can be attributed to a little lower stiffness of the numerical model with respect to the real structure. For the same reason the numerical displacement peaks are a little higher than those recorded during the earthquake (Figure 22). However, the frequency content of the recordings obtained during the event is well reproduced by the numerical model, as both the acceleration and displacement time histories show. As already said, some crack patterns were observed on the partition walls of the ﬁrst ﬂoor after the main event of 30 October 2016. In order to investigate this aspect, the relative displacements between the lower and upper beams were analyzed. It can be seen that an excursion greater than 2.0 mm around the value under vertical loads with a high frequency content, which occurred during the quake, can justify the cracks (Figure 23). F/(G A) 1 Infrastructures 2022, 7, x FOR PEER REVIEW 16 of 19 Infrastructures 2022, 7, x FOR PEER REVIEW 16 of 19 Figure 20. The non-linear relationship between force and shear strain for the elastomeric isolators, Figure 20. The non-linear relationship between force and shear strain for the elastomeric isolators, which accounts for the aging effects, approximated by a trilinear curve. which accounts for the aging effects, approximated by a trilinear curve. The results of the dynamic numerical analysis were compared to the seismic behavior The results of the dynamic numerical analysis were compared to the seismic behavior observed during the Norcia earthquake. The good agreement between the acceleration observed during the Norcia earthquake. The good agreement between the acceleration time histories is apparent (Figure 21). Little discrepancies in terms of amplitudes can be time histories is apparent (Figure 21). Little discrepancies in terms of amplitudes can be attributed to a little lower stiffness of the numerical model with respect to the real struc- attributed to a little lower stiffness of the numerical model with respect to the real struc- ture. For the same reason the numerical displacement peaks are a little higher than those ture. For the same reason the numerical displacement peaks are a little higher than those recorded during the earthquake (Figure 22). However, the frequency content of the re- recorded during the earthquake (Figure 22). However, the frequency content of the re- cordings obtained during the event is well reproduced by the numerical model, as both cordings obtained during the event is well reproduced by the numerical model, as both the acceleration and displacement time histories show. the acceleration and displacement time histories show. As already said, some crack patterns were observed on the partition walls of the first As already said, some crack patterns were observed on the partition walls of the first floor after the main event of 30 October 2016. In order to investigate this aspect, the rela- floor after the main event of 30 October 2016. In order to investigate this aspect, the rela- tive displacements between the lower and upper beams were analyzed. It can be seen that Infrastructures 2022, 7, 13 tive displacements between the lower and upper beams were analyzed. It can be seen 17 th of at 20 an excursion greater than 2.0 mm around the value under vertical loads with a high fre- an excursion greater than 2.0 mm around the value under vertical loads with a high fre- quency content, which occurred during the quake, can justify the cracks (Figure 23). quency content, which occurred during the quake, can justify the cracks (Figure 23). 0.05 0.05 Foligno FOR A05 Foligno FOR A04 0.05 0.05 Foligno FOR A04 Foligno FOR A05 0.00 0.00 0.00 0.00 0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 Earthquake Earthquake Earthquake Earthquake t (s) t (s) –0.05 Numerical –0.05 - 0.05 - 0.05 Numerical t (s) t (s) –0.05 Numerical –0.05 - 0.05 - 0.05 Numerical 0.05 0.05 Foligno FOR A10 Foligno FOR A11 0.05 0.05 Foligno FOR A10 Foligno FOR A11 0.00 0.00 0.00 0.00 0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 Earthquake Earthquake Earthquake Earthquake t (s) t (s) –0.05 –0.05 Numerical - 0.05 Numerical - 0.05 t (s) t (s) –0.05 –0.05 Numerical - 0.05 Numerical - 0.05 (a) (b) (a) (b) Figure 21. Comparison between the earthquake and numerical accelerations (a) in x direction and Figure 21. Comparison between the earthquake and numerical accelerations (a) in x direction and (b) Figure 21. Comparison between the earthquake and numerical accelerations (a) in x direction and (b) in y direction. in y direction. (b) in y direction. 12 12 Foligno FOR A04–A01 Foligno FOR A05–(–A03) 12 12 Foligno FOR A04–A01 Foligno FOR A05–(–A03) 0 0 0 0 0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 Earthquake Earthquake Earthquake Earthquake t (s) t (s) - –1 12 2 Numerical - –1 12 2 Numerical Infrastructures 2022, 7, x FOR PEER REVItEW (s) t (s) 17 of 19 - –1 12 2 Numerical - –1 12 2 Numerical (a) (b) (a) (b) Figure Figure 22. 22. Comparison Comparisonbetween between the the earthquake earthquake and and numerical numericr al elative relativ displacements e displacements between betwee L1n L1 and L0, (a) in x direction and (b) in y direction. and L0, (a) in x direction and (b) in y direction. 1.5 Foligno FOR Beam 5-6 1.0 0.5 0.0 15 20 25 - 0.5 –0.5 -– 1 1.0 .0 -– 1 1.5 .5 t (s) Figure Figure 23. 23. Relative Relativedisplacem displacem ent ent between betwee the n the beams beams 5-6 of 5-the 6 of ﬁrst the ﬂoor first and floor ofaL1 nd in of vertic L1 in al vert direction. ical direc- tion. 6. Conclusions The behavior of a reinforced concrete building, seismically isolated with high damp- ing rubber bearing (HDRB) and sliding devices (SD), observed during the most important events of the seismic sequence that struck central Italy from August 2016 to January 2017, was analyzed. All the events had quite low effects at the site due to the large epicenter distances; therefore, the isolation system was not always put into action during the events or showed very low displacements. However, small cracks were observed after the main event in some partition walls of the building. This occurrence did not affect the seismic protection of the structure through seismic isolation. A detailed analysis of the recorded behavior during the main event of 30 October 2016 was first shown. Then, the effects of a number of selected events were analyzed and compared. Finally, a suitable finite element model was set up, in which a non-linear model for the elastomeric isolators, based on previous experimental data, and a friction model for the sliders were assumed. The model, first validated comparing the numerical and experimental responses at the sensor locations in terms of accelerations and displacements obtained during the main event, was used to interpret the experimental behavior. The main results can be summarized as follows: 1. The resonance frequencies varied significantly with the energy at the site of the build- ing and approached to the resonance frequencies of the superstructure for the lowest energy events. 2. As a result, there was no suitable decoupling of motion in some cases. This occur- rence must be accounted for in the design of the isolation system and to evaluate the seismic actions in the superstructure. 3. The contribution of sliding devices was very important for the onset of motion under low energy earthquake. Actually, the isolation system was not put in action under very low energy events but only when the maximum friction forces in the sliding devices were not sufficient to face the seismic actions. 4. The contribution of the sliding devices significantly influenced the amplitude of vi- brations and damping. 5. The analysis of the relative vertical displacements between the beams around the damaged partition walls pointed out vibrations at high frequency and amplitudes greater than 2.0 mm. These acted in conjunction with the horizontal vibrations and can justify the observed small cracks. In order to verify all these aspects, non-linear analyses should be recommended to evaluate the effects of earthquakes with different energy at the site. The analyses should account for the non-linear behavior of the elastomeric isolators, with particular reference to the behavior at very small shear strain, and of the sliding devices, with particular ref- a ( ag ()g) a ( ag ()g) d (mm) d (mm) d (mm) a ( ag ()g) a ( ag ()g) d (mm) d (mm) Infrastructures 2022, 7, 13 18 of 20 6. Conclusions The behavior of a reinforced concrete building, seismically isolated with high damping rubber bearing (HDRB) and sliding devices (SD), observed during the most important events of the seismic sequence that struck central Italy from August 2016 to January 2017, was analyzed. All the events had quite low effects at the site due to the large epicenter distances; therefore, the isolation system was not always put into action during the events or showed very low displacements. However, small cracks were observed after the main event in some partition walls of the building. This occurrence did not affect the seismic protection of the structure through seismic isolation. A detailed analysis of the recorded behavior during the main event of 30 October 2016 was ﬁrst shown. Then, the effects of a number of selected events were analyzed and compared. Finally, a suitable ﬁnite element model was set up, in which a non-linear model for the elastomeric isolators, based on previous experimental data, and a friction model for the sliders were assumed. The model, ﬁrst validated comparing the numerical and experimental responses at the sensor locations in terms of accelerations and displacements obtained during the main event, was used to interpret the experimental behavior. The main results can be summarized as follows: 1. The resonance frequencies varied signiﬁcantly with the energy at the site of the building and approached to the resonance frequencies of the superstructure for the lowest energy events. 2. As a result, there was no suitable decoupling of motion in some cases. This occurrence must be accounted for in the design of the isolation system and to evaluate the seismic actions in the superstructure. 3. The contribution of sliding devices was very important for the onset of motion under low energy earthquake. Actually, the isolation system was not put in action under very low energy events but only when the maximum friction forces in the sliding devices were not sufﬁcient to face the seismic actions. 4. The contribution of the sliding devices signiﬁcantly inﬂuenced the amplitude of vibrations and damping. 5. The analysis of the relative vertical displacements between the beams around the damaged partition walls pointed out vibrations at high frequency and amplitudes greater than 2.0 mm. These acted in conjunction with the horizontal vibrations and can justify the observed small cracks. In order to verify all these aspects, non-linear analyses should be recommended to evaluate the effects of earthquakes with different energy at the site. The analyses should account for the non-linear behavior of the elastomeric isolators, with particular reference to the behavior at very small shear strain, and of the sliding devices, with particular reference to the static and dynamic frictions. These analyses would be of fundamental importance to verify a suitable decoupling of motion and a correct working of an isolation system also under low energy earthquakes. Author Contributions: Conceptualization, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); methodology, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); software, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); validation, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); formal analysis, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); investigation, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); resources, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); data curation, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); writing— original draft preparation, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); writing—review and editing, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); visualization, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); supervision, A.S., G.B., P.C., C.O., F.S. (Fernando Saitta) and F.S. (Federico Scafati); project administration, G.B. and P.C.; funding acquisition, G.B. and P.C. All authors have read and agreed to the published version of the manuscript. Infrastructures 2022, 7, 13 19 of 20 Funding: The research work leading to this paper has been developed in the framework of a research project organized and funded by ENEA and Umbria Region. Conﬂicts of Interest: The authors declare no conﬂict of interest regarding the publication of this paper. References 1. Clemente, P. Seismic isolation: Past, present and the importance of SHM for the future. J. Civ. Struct. Heal. Monit. 2017, 7, 217–231. [CrossRef] 2. Calvi, P.M.; Calvi, G.M. Historical development of friction-based seismic isolation systems. Soil Dyn. Earthq. Eng. 2018, 106, 14–30. [CrossRef] 3. Naeim, F.; Kelly, J.M. Design of Seismic Isolated Structures: From Theory to Practice; John Wiley & Sons Inc.: Hoboken, NJ, USA, 1999. [CrossRef] 4. Skinner, R.I.; Robinson, W.H.; McVerry, G.H. An introduction to Seismic Isolation. DSIR Physical Sciences; John Wiley & Sons Inc.: Wellington, New Zealand, 1993. 5. Clemente, P.; Martelli, A. Seismically isolated buildings in Italy: State-of-the-art review and applications. Soil Dyn. Earthq. Eng. 2019, 119, 471–487. [CrossRef] 6. Bongiovanni, G.; Buffarini, G.; Clemente., P.; Saitta, F.; Tripepi, C. Retroﬁt of existing buildings with seismic isolation: Design issues and applications. In Proceedings of the 16th World Conference on Seismic Isolation, Energy Dissipation and Active Vibration Control of Structures, 16WCSI, St. Petersburg, FL, USA, 1–6 July 2019; pp. 111–122. [CrossRef] 7. 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Responses of base-isolated buildings in Tokyo during the 2011 Great East Japan Earthquake. In Proceedings of the 15th World Conf. on Earthquake Engineering (15WCEE), Lisbon, Portugal, 24–28 September 2012. 13. Siringoringo, D.M.; Fujino, Y. Seismic response analyses of an asymmetric base-isolated building during the 2011 Great East Japan (Tohoku) Earthquake. Struct. Control. Health Monit. 2014, 22, 71–90. [CrossRef] 14. Zhou, F.L.; Tan, P.; Heisha, W.; Huan, X.Y. Progress and application of seismic isolation, energy dissipation and control in civil and industrial structures and design codes in China. In Proceedings of the 14th World Conf. on Seismic Isolation, Energy Dissipation and Active Vibration Control of Structures (14WCSI), San Diego, CA, USA, 9–11 September 2015. 15. Zhou, C.; Chase, J.G.; Rodgers, G.W.; Kuang, A.; Gutschmidt, S.; Xu, C. Performance evaluation of CWH base isolated building during two major earthquakes in Christchurch. Bull. N. Z. Soc. Earthq. Eng. 2015, 48, 264–273. [CrossRef] 16. Takayama, M.; Morita, K. Observed performance of seismically isolated buildings during 2016 Kumamoto Earthquake in Japan. In Proceedings of the 17th World Conference on Earthquake Engineering (17WCEE), Sendai, Japan, 28 September–2 October 2021. Paper N C000234. 17. Martelli, A.; Clemente, P.; De Stefano, A.; Forni, M.; Salvatori, A. Recent Development and Application of Seismic Isolation and Energy Dissipation and Conditions for Their Correct Use. In Geotechnical, Geological and Earthquake Engineering; Springer: Berlin/Heidelberg, Germany, 2014; Volume 34, pp. 449–488. [CrossRef] 18. Clemente, P.; Buffarini, G. Base isolation: Design and optimization criteria. SIAPS 2010, 1, 17–40. [CrossRef] 19. Clemente, P.; Bontempi, F.; Boccamazzo, A. Seismic Isolation in Masonry Buildings: Technological and economic issues. 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Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes

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Abstract

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Infrastructures – Multidisciplinary Digital Publishing Institute

Published: Jan 21, 2022

Keywords: seismically isolated buildings; base isolation; experimental seismic behavior; high damping rubber bearings; seismic monitoring

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Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes

Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes

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Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes

Observed Seismic Behavior of a HDRB and SD Isolation System under Far Fault Earthquakes

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