Evaluation of the Response of Historical Structures Fitted with Seismic-Isolation

1. Introduction

The instrumented buildings performance over the Bucharest area has already became a continuously undergoing task with valuable results (see, for example [1,2,3,4]. In these works 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 [5,6,7,8]. 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 03.11.2022, Mw = 5 [9] 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 [8,10,11,12,13]. 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 spectral–related characteristics perspective (maximum acceleration a_max considered at certain levels in each structure, spectral acceleration amplitude SA_max, spectral-peak corresponding period [3,4].

2. 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 [14,15,16,17,18,19,20,21,22,23]. 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 [24,25,26,27]. 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 [28,29,30,31,32,33].

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. 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 biaxial-hysteretic behavior (Studiu Instrumentarea Cladirii PMB si ASE). 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 [34]. (Studiu Instrumentarea Cladirii PMB si ASE).

For the ES building the preliminary study employed the same computation program (ETABS NON-LINEAR v.8.4.5., (Studiu Instrumentarea Cladirii PMB si ASE), 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 0.08-0.79 s 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 solid-rigid 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. (Studiu Instrumentarea Cladirii PMB si ASE).

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 31.12.2022) [35]. The instrumented and isolated buildings characteristics are given in Table 1.

The sensors are of the same types, the recordings and data processing are performed according to the standard procedure, by the same automated Antelope seismological system, (BRTT, 2018) installed at the National Data Centre (NDC) of the National Institute for Earth’s Physics, in Magurele [35].

The characteristics of the most recent considered earthquake were as follows: date and triggered time 03.11.2022, 06:50:25, local time, lat. 45.4895° N, long. 26.5262° E, focal depth 148.8 km, M_W = 4.9 (ROMPLUS Catalogue, 2023 [36], 122 km epicentre distance for Bucharest and M_W = 5 according to near-real time release of the (Internal Report, 2022 [9]). 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 [37,38]. 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 28.10.2018, 03:38:11 local time, lat. 45.6079°, long. 26.4068°, focal depth 148 km, M_W = 5.5 and 31.01.2020, 04:26:48 local time, lat. 45.6937°, long. 26.6918°, focal depth 118 km, M_W = 4.8 (Table 2) [36]). The intensity on the Mercalli scale was VI and IV respectively, in the epicentre zone [37,38].

3. 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.

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 Figure 1 and Figure 2 and Table 3, display lower values above the isolator. The corresponding oscillation periods are all in a low and narrow range 0.09 – 0.22 s, 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) [2,3,4]. 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 (Figure 1 and Figure 2, Table 4) for the 31.01.2020, M_W = 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%, M_W = 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 [28,39,40]. 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 (see, for example, [6,42,43]). The structures design and dynamic features are also taken into consideration [7,8,22,23,41].

According to 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!) [44,45,46], it can be stated that the technique has accomplished this particular point. According to the current Romanian seismic design code P100-1/2013 [47] three values of the control period T_c=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 T_c=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 (see, for example [48,49]) are in the 0.7-1.9 s domain [50,51]. In particular, values between 0.08 – 0.79 s 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.

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 31.01.2020, for the horizontal components.

Turning back to the BCH structure, one may notice as a 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 [3,4,6]. This situation corresponds to the two strongest earthquakes discussed here (03.11.2022, M_W = 5.0 and 28.10.2018, M_W = 5.5).