Evaluation of the response of historical structures fitted with seismic-isolation

: The paper highlights the performance of the seismic isolation devices, installed on retrofitted buildings in reducing the seismic response when subjected to earthquakes. Two buildings from the beginning of the XX-th century in Bucharest are chosen from many more, monitored over the city area. We discuss the response of these base-seismic isolated structures, relying on good quality data acquired from the recent strong earthquake (03 November 2022, M W = 5.0). Elastic response spectra computed from recordings at two levels of each structure are used, placed under and right above the isolating layer. At one building the existence of previous recordings and the particularity of the sensors allow a comparison with other two relatively recent medium intensity earthquakes. The assessment is carried out in terms of maximum acceleration a max, measured at certain levels in each structure, spectral acceleration amplitude SA max and spectral-peak corresponding period. We find that the base-isolation methodology is effective in reducing the response of the building right above the isolating layer, an observation valid for both structures, all components of the recordings, and spectral acceleration values. Moreover, the outcomes from the modal evaluation performed prior to rehabilitation and seismic isolation process are presented, by pointing out a higher newly acquired fundamental period of the isolated structures.


Introduction
The instrumented buildings performance over the Bucharest area has already became a continuously undergoing task with valuable results [1][2][3].The level of performance enhancement and the state of damage for a large variety of structures were assessed in terms of years and type of construction, design, usage destination and utility [4][5][6][7].Detailed structural response data provide a potential for adjustments in the design process.The present study is focused on two old, seismically base-isolated buildings and retrofitted, located in Bucharest city area.The selected buildings erected at the beginning of the XX century are hosting administrative and educational activities.
The dynamic behavior during the earthquake of 03 November 2022, MW = 5 [8] is analyzed.This is a continuation of a previous work, where the impact analysis of the methodology used for near-real-time response was carried out [9].The monitoring system has proved its capabilities to ensure the data flow, and make them available in a shorter time after the earthquake, to the authorities, civil protection, decision makers etc. Basically near-real-time (i.e., right after a potentially damaging event and/or during its aftershock sequence) structure response-based status was assessed.The novelty herein consists in an evaluation of the response of a previously seismically isolated building, now endowed with new sensors located above/under the isolating layer.A comparison is made with other old building' response, already having the same type of deployment for the isolated device and sensors.The responses under another two recent earthquakes is brought into attention for the same structures, and a discussion is made from the recorded parameters and spectralrelated characteristics perspective (maximum acceleration amax considered at certain levels in each structure, spectral acceleration amplitude SAmax, spectral-peak corresponding period [3]).

Methodology, procedure and buildings characteristics
Isolation devices are capable of sustaining dynamic strength at strong displacements induced by the ground motions.The soil particularity consisting in soft, weak-consolidated, without cohesiveness mechanical characteristics, usually included in sedimentary layering or basins, can be an issue in building design.Nonlinear phenomena in these cases are an added weight to the general dynamic behavior.The difficulties that have to be surpassed consist in different amplification at the ground level, high oscillating site period or strong variability of the involved parameters over the interest area.Therefore, a thorough seismic hazard, correct site response and geotechnical information knowledge are compulsory for these zones.As regard the concept of isolation procedure, it must rise the flexibility and damping, and withstand to service loads.
The undesired response of the structures may be avoided by taking necessary measures in terms of design, as emerged from a thorough understanding of the seismicity and ground motion characteristics.The seismic isolating devices can be used in order to reduce the structure vulnerability.In this regard, among the techniques largely used in some countries the base isolating method has been proved successful (USA, Japan, Italy, and New Zealand), in relation to the seismicity specificity, geophysical and geological characteristics of the interest areas [10][11][12][13][14][15][16][17][18][19].In general practice of seismic isolation, the main aim is to reduce the seismic demand on the structure.The buildings considered in this study were endowed with the above mentioned base-isolating systems.The isolation system does not absorb the earthquake energy, which is deflected through the dynamics of the system.The technique involves a certain level of damping that is helpful to avoid possible resonance.By decoupling a structure from the direct action of the horizontal components of a ground motion, it acquires a fundamental frequency that is much lower than its fixed-base frequency and the usual predominant frequencies of the ground motion.Based on the modal evaluation of the fundamental vibration period, an insight into the benefits of using base isolators in structures could be gained by considering the special case of a two degrees of freedom structure, which is separated from the ground by some type of isolating device [20][21][22].The study relies either on a single degree of freedom model, with possible nonlinearities included, or, more exactly, on a system of coupled elastic oscillators [23][24][25][26][27][28].
Some buildings host costly equipment and contents that must be protected against earthquakes and be operational after a severe ground shaking; such buildings are those designated for research, health care, telecommunication, nuclear power plants, etc. Buildings constructed by following old seismic codes, with conventional resistant design approaches, cannot protect anymore the people or the valuable equipment that are contained.The constructions evaluated in this study were not randomly chosen, but taking into account their age, importance and design.One of the chosen buildings is an administrative one, hosting the Bucharest City Hall (BCH), the other being "Victor Slavescu" building, belonging to Bucharest University of Economic Studies (ES).For both, the isolating devices were implemented at certain elevation point in the basement, according to design and retrofitting specification (Table 1).By this procedure, the supra-structure was decoupled to some extent from its foundation, which continues to move rigidly with the soil during an earthquake.
For the modeling of the isolating system of the BCH building some types of constitutive laws were used, such as those characterizing a linear elastic or biaxialhysteretic behavior [29].The preliminary analysis of the isolated building involved dynamic linear and non-linear computation of time-history type.A direct integration of the differential equations of motion based on recorded accelerograms for the 1977, 1986, 1990 strong earthquakes was employed, imposing the input condition of 0.24 g for the scaled maximum acceleration at the ground level.The data were taken at a location which is the only one where recordings for the 1977 earthquake exist; also, it has the advantage of similar local soil features.The results show a reduction of 11-12 times for the relative displacement values on one horizontal direction, and 7-8 times on another, of the seismic action at a vibration period of the isolated building of 3.3 s.As regards the stress reduction, the values are 4 and 3 times smaller for the two horizontal seismic action directions.At the same time, the relative displacements distribution at the level of each floor on the vertical, shows a general solid-rigid trend of displacement, with a general displacement at isolated interface level of approximately 20-22 cm and 28-30 cm for horizontal directions of the seismic action, under the assumption of peak ground acceleration of 0.24 g, according to (P100-1/2006), which was the code in force at that time [29,30].
For the ES building the preliminary study [31] employed the same computation program (ETABS NON-LINEAR v.8.4.5.) relying on finite element method.Modal analysis for the resistance structure has considered first 6 (six) vibration modes, as follows: three of one horizontal translational direction, two of the other horizontal direction (perpendicular on the former) and one of general torsion.All these vibration periods are in the 0.08-0.79s range.Following the reduction of stress and displacements values, stress values diminished of 2.4 and 2.5 times for two horizontal seismic action directions and relative displacements reduced by 3.5 at story levels, 2.5 at ground level respectively, for both horizontal seismic action directions.The vibration period of the isolated structure was 2.8 s.At the same time, the relative displacements distribution at each level on vertical shows a general solidrigid trend of displacement, with a general displacement at isolated interface level of approximately 15 cm and 20 cm for horizontal directions of the seismic action [31].
Table 1.Seismic isolated buildings (general characteristics) [5].The comparison was made to the performance of the both buildings using the same analyses carried out by employing specific design parameters without taking into account the isolation devices characteristics.

No. Name of building/monitored period
The BCH building was seismically monitored with 4 sensors located on the isolated structure from the year 2017; starting with year 2022 one sensor was deployed above/under the isolator layer.The ES building is permanently monitored by the National Institute for Earth's Physics (NIFP) from the year 2011, with seismic accelerometers at two different levels (status at 31 December 2022) [32].
The characteristics of the most recent considered earthquake were as follows: date and triggered time 03 November 2022, 06:50:25, local time, lat.45.4895° N, long.26.5262° E, focal depth 148.8 km, MW = 4.9, ROMPLUS Catalogue, 2023 [33], 122 km epicentre distance for Bucharest and MW = 5 according to near-real time release of the Internal Report [8].Herein, a magnitude 5 is considered, as for the other two earthquakes the values in ROMPLUS are identical to those in the Internal Reports released in the very short aftermath of the recordings [33][34][35].Moreover, the data processing and computation of the parameters are based on the mentioned References, according to the purpose of the work that involved a near-real time evaluation of the response.
The earthquake mentioned above belongs to the intermediate-depth Vrancea seismic region, and was felt with intensities about V on MSK scale in the epicentre area and III-IV in Bucharest.For the other two seismic events the characteristics are: date and triggered time 28

Results. response analysis
The instrumental data from the two selected structures, subjected to a medium intensity earthquake, were processed in terms of peak recorded accelerations and spectral accelerations, at two levels, below and right above the isolating devices.Recordings from three-component accelerometers, installed on these levels, consisting in acceleration time-histories are pre-processed: baseline corrected and filtered using a 4th order Butterworth band pass (0.2-25 Hz) filter.The limits were set for obtaining a good signal to noise ratio, and also a taper function was applied on the data to allow the spectral-related calculation.The sensors are of the same types, the recordings and data processing are performed according to the standard procedure, by the automated Antelope seismological system, developed by Boulder Real Time Technologies [36]) installed at the National Data Centre (NDC) of the National Institute for Earth's Physics [32].It includes program applications and module units that are ran in order to ensure data acquisition, automatic seismic events detection, location, magnitudes and other parameters computation or evaluation.The ground shaking and buildings seismic response parameters processing (in terms of peak ground acceleration, maximum buildings recorded acceleration, spectral acceleration and related fundamental or oscillation period) are the tasks that are accomplished through this system.
The processing of the data releases elastic response spectra in terms of spectral pseudo-acceleration, with 5% damping (Figures 1 and 2).The information are depicted as engineering parameters, that are maximum acceleration amax recorded on three directions (two horizontal, NS, EW, and one vertical Z), maximum spectral acceleration (SAmax) from elastic response spectra, and corresponding oscillation periods TSAmax (Tables 3 and 4).In Table 3 a clear decrease of the peak acceleration recorded by the seismic sensors above the isolating device can be seen.This is valid for all components of the recordings and at both buildings.Also, the maximum spectral accelerations, as shown in Figures 1 and 2 and Table 3, display lower values above the isolator.The corresponding oscillation periods are all in a low and narrow range 0.09-0.22s, excluding the danger of possible resonance effects.Another observation is higher values for recordings at the ES building in comparison with the other building (BCH), at both instrumented levels.We note that this phenomenon is encountered at the ES building for other earthquakes of comparable magnitude, in comparison to any other instrumented structure in Bucharest city area (Table 4) [2,3].The higher recorded values at the ES building for low and medium magnitudes can be a matter of local soil conditions, or due to the building dynamic characteristics themselves.
Moreover, at this building more data of good quality are available, both under and above isolating layer; therefore one can infer the dependence to the earthquake magnitude.From Table 4 it may be seen the increase of the recorded values with earthquake' source level of strength, for all measured and processed parameters.
We can also note the higher values of the oscillation period corresponding to spectral amplitude (Figures 1 and 2, Table 4) for the 31 January 2020, MW = 4.8 earthquake, which is the lower magnitude considered in this study.These higher values are encountered for horizontal components and above the isolated layer.The other values (i.e., under isolation, and both for the vertical Z component) follow the general tendency of laying in a rather low and confined range, as it is expected for these type of earthquakes.It is worth the focal depth range of ~122-149 km, for these three earthquakes, besides sharing same focal area (Vrancea-intermediate depth) and moderate magnitude (Table 2).
In Table 5 the reduction of the seismic amplitudes is shown in terms of maximum recorded accelerations, in order to have a quantitative representation for the outcome of the employed technique.For this purpose the ES building is considered, for which more recordings exist with this type of isolating devices.It is the first building that benefited of this technique and a proper sensors deployment, allowing for this type of analysis.All three earthquakes are of medium magnitude, originating in the same focal area.The reduction coefficient represents the ratio for under/above values and the reduction percentage stands for its corresponding percent in the weight reduction.As it may be seen, the average for three relatively recent seismic events is over 50% on the two horizontal components, and below this value for vertical ones.The maximum average reduction is attained for the N-S component with 59% the higher value, corresponding to the strongest earthquake (74%, MW = 5.5).The consequence of the base-isolating procedure consists in raising the fundamental period for the supra-structure, or the shift of the base-supra-structure ensemble and splitting in two values if they are considered as two coupled oscillators [23,37,38].In some cases this increase is considered as a goal itself.However, the benefits of this outcome and its limits of applicability are discussed in correlation to various types of parameters, from the source characteristics to local effects and specific soil response [5,37].The structures design and dynamic features are also taken into consideration [6,7,18,19,39,40].
According to Iordachescu and Iordachescu [29], Marmureanu et al. [31] previous studies a shift towards 2.5-3 s for fundamental periods can be obtained.Given the oscillation periods of the strong and damaging earthquakes that hit Bucharest, of over ~1 s, but not exceeding 1.6 s (until now!) [41][42][43][44], it can be stated that the technique has accomplished this particular point.According to the current Romanian seismic design code P100-1/2013 [45] three values of the control period Tc = 0.7; 1.0; 1.6 s are considered in the design response spectra, provided by this regulation regarding the whole Romanian territory.In this respect for the Bucharest city a control period of Tc = 1.6 s and 0.3 g the value for the design ground acceleration are recommended.
At the same time the values for the soil predominant periods accepted for the city area [46,47] are in the 0.7-1.9s domain [48].In particular, values between 0.08-0.79s for the first 6 (six) normal modes of vibration are suggested.It follows that the value of ~2.8 s for the ES building fulfills the objective of extracting the structure from the dangerous range of maximum amplitude of the response spectral at that site [29,31].
From Table 4 one may see the lower range for this parameter (oscillation period for the maximum spectral amplitude) at both buildings, for all components (0.09-0.42 s).Among them, the higher values correspond to the ES building, and are encountered for the less strong earthquake of 31 January 2020, for the horizontal components.
Turning back to the BCH structure, one may notice as specificity the higher values on vertical component Z in terms of maximum acceleration and spectral acceleration, in comparison with the horizontal ones, as long as, usually for this component (Z), the values are the lowest, at both buildings.This situation corresponds to the two strongest earthquakes discussed here (03 November 2022, MW = 5.0 and 28 October 2018, MW = 5.5).

Discussion
During the last decade certain types of structures were selected, according to their specificity (old buildings, retrofitted, destination and functionality, etc.), being continuously seismic monitored.Several instrumented buildings from Bucharest city, and one from epicentre area (city of Focsani, Vrancea region) have provided important data about the seismic performance of earthquake protection systems and checked out the performance goals and design issues in major earthquakes [3,6].Out of them, two buildings, Bucharest City Hall (BCH) and "Victor Slavescu" (ES) were constructed at the beginning of the XXth century under inappropriate seismic design regulations.These structures, together with another one, a historical monument (Arch of Triumph) have been retrofitted in order to endure the future strong earthquakes.Thereafter the decision was taken for these buildings to be equipped with base-isolating systems (seismic isolators and viscous dampers) for reducing the lateral forces induced by strong seismic movement.One of them (BCH) has been recently endowed with a pair of seismic sensors at the basement level, above/under the isolating system.The response of the two buildings BCH and ES, subjected to medium intensity earthquakes, was analyzed in terms of peak accelerations and spectral accelerations.Improvement in seismic response of these isolated buildings in Bucharest was evaluated.

Conclusions
According to our analysis the main objective of the base isolating system is considered accomplished, as the efficiency of this type of isolation device during Vrancea-intermediate depth-originating earthquakes was proved.
The methodology has proved its aim to reducing the response of the building right above the isolating layer, in comparison with values under the device.The observation is valid for both structures, all components of the recordings, and for the spectral acceleration values.
These results can be useful for quantifying the benefits and implications of seismic isolation, subjected to moderate seismic events that occur in Vrancea seismic region, in terms of response spectra analysis.
The seismic monitoring of buildings has proved its capability to give a rapid damage assessment after a strong seismic event, based on the level of accelerations the buildings experienced, therefore mitigating the seismic risk for densely populated areas in Romania.Detailed structural response data provide the potential for adjustments of the design process.For the earthquake protection systems, as is seismic base isolation case, the improvements in terms of structural response can be quantified and the performance of the isolator devices can be assessed, as based on measured data.
For the specific situation of Bucharest city, including geology and corresponding site effects, the dynamic parameters must be assessed accordingly, in order to estimate the proper seismic response.
The goal pursued was to take advantage from this performance while proving the potential benefits of this certain type of anti-seismic isolating technique applied to old/historical structures located in the Bucharest city downtown.The practical implications of the findings are pertaining the seismic risk management for this highly populated zone.
Evaluating the performance of base-isolating technique, through comparing the response under and right above isolating devices may be useful for implementing this procedure on other structures sharing the same similarities in terms of age or design.
Data from this paper could help in the future practitioners and policymakers to decide if using a seismic isolation system for a certain building could mitigate its vulnerability to earthquakes.
Local amplification effects and site effects are important as they determine seismic ground motion specificity and dynamic buildings behavior ultimately.Therefore these retrofitting techniques should be considered and recommended for certain urban areas in tight connection to regional seismicity and seismic source specificity, considering typical seismic structural response and local effects-related parameters.

Figure 1 .Figure 2 .
Elastic response spectra for the BCH building above (a) and under (b) isolating layer from recordings of the 03 November 2022 earthquake MW= 5 [9].Legend: red: N-S, blue: E-W, green: Z (vertical) components of recording.Elastic response spectra for the ES building above (a) and under (b) isolating layer from recordings of the 03 November 2022 earthquake MW= 5 [9].Legend: red: N-S, blue: E-W, green: Z (vertical) components of recording.

Table 5 .
Base-isolated device performance in terms of Reduction coefficient and corresponding reduction percentage for the ES building for three seismic events.