Seismic Response of a Tall Reinforced Concrete Frame Structure Subjected to both Horizontal and Vertical Strong Motions

The continuous development of urban areas coupled with the ageing of built-stock and the variety of structures built following different design code recommendations may result in different behaviours during seismic events. Hence, reliable information is needed in order to assess the risk of severe damage or collapse during earthquakes. Accurate numerical simulations coupled with analytical models provide essential information on the seismic performance of civil engineering structures. The results obtained from realistic numerical simulations may serve as entries in data-bases designed to train and calibrate early warning systems (EWS). Their continuous development is of paramount importance taking into account their potential to save lives and prevent economic loses in case of a severe seismic event. The paper presents the preliminary results obtained from numerical simulations on the seismic response of a tall reinforced concrete (RC) frame structure subjected to both horizontal and vertical string motions produced by an earthquake. The obtained results, in terms of response spectra in terms of accelerations, may be used for training EWS sensors to identify a seismic event and transmit warning messages at local, regional and even national level.


Introduction
The continuous development of urban areas coupled with the ageing of built-stock and the variety of structures built following different design code recommendations caught the attention of decision makers and researchers around the world.While the primary role of the former is to draft resilience plans in case of natural disasters [1], with particular focus on strong earthquakes, the responsibility of the latter may reside into offering accurate information on the behaviour of different types of structural systems in case of a severe earthquake [2].The first step towards reducing the associated risk, seismic risk in this case, of a natural disaster consists in correctly evaluating the vulnerability of at-risk structures and the possible consequences that may rise after a severe seismic event.The accurate assessment of seismic vulnerability of buildings may result in reducing the associated risk by taking apriori preparatory measures, e.g.retrofitting, strengthening, and by drafting relief and rescue plans and policies in the aftermath of such an event [3].The responsibility of designers and contractors is to follow seismic design code regulations which are continuously updated based on field observations, recorded data and extensive research work [4].Accurate numerical simulations [5][6][7] coupled with analytical models are efficient tools in assessing the seismic behaviour of civil engineering structures [8].Their main advantage is the possibility of varying several parameters to account of all possible scenarios without the need of costly and timeconsuming experimental tests.Another advantage consists in the continuous updating of the seismic scenarios based on the recorded data from recent severe earthquakes from around the world: Japan (Tohoku, 2011), Nepal (2015), Puerto Rico (2020), Croatia (Zagreb, 2020), Haiti (2021), Turkey (Gaziantep, 2023).The results obtained from realistic numerical simulations may serve as entries in data-bases designed to train and calibrate early warning systems (EWS) [9,10].Their continuous development are of paramount importance taking into account their potential to save lives and prevent economic loses in case of a severe seismic event.While such systems are already well developed and IOP Publishing doi:10.1088/1757-899X/1304/1/012004 2 functional in countries such Japan, South Korea and U.S.A., their use in Romania is rather limited to institutions deemed of national interest (e.g.nuclear power plants, oil refineries).Early warning of population is very limited, the main limiting factor being the warning infrastructure itself [11].In view of the recent seismic events and taking into account the ageing of the built stock, the present paper presents the preliminary results obtained from numerical simulations on the seismic response of a tall reinforced concrete (RC) frame structure subjected to both horizontal and vertical strong motions produced by an earthquake.The obtained results, in terms of response spectra of accelerations, may be used for training EWS sensors to identify a seismic event and transmit warning messages at local, regional and even national level.

Numerical Model
The chosen building is based on the RC frame structure type.It consists of a tall ground floor, 4.35 m in height, 7 storeys each measuring 2.75 m in height, and a technical room housing the elevator equipment.It is commonly met in urban areas in Romania, following a modular design pattern adopted during the mid-1980s.

RC frame structure
The 3D view of the numerical model is shown in Figure 1.Columns have different cross-section depending on their position along the height of the structure, changing every two storeys.The numerical model considered also the presence of the elevator shaft, acting as a central core, and the staircase.The floor plan is shown in Figure 2.Although the rim of the floor may be symmetric with respect to Y axes, the presence of the staircase and the elevator shaft resulted in the structure being non-symmetric with respect to both in-plane axes.Therefore, a coupled displacement and rotation fundamental mode of vibration is expected, with the dominant component being the lateral displacement along Y axis.A C18/22.5 concrete strength class was used in all structural and non-structural elements.The design compressive strength considered in the initial project was 14 MPa and the modulus of elasticity was 28.5 GPa.

Loading scenarios
The structure was subjected to both horizontal and vertical ground motions recorded for Vrancea 1977 [12] and Turkey 2023 earthquakes [13].The former is one of the reference earthquakes taken into Staircase Elevator shaft X Y account in terms of seismic design in Romania while the latter became significant in view of the destructions and high number of casualties in its wake.The time histories of both Vrancea and Turkey earthquakes, with their components along East-West (EW), North-South (NS) and vertical (Z) directions, are presented in Figure 3 and Figure 4, respectively.The characteristics of the two seismic events are summarized in Table 1.The structure was assumed to be located near the epicentre of both earthquakes such that the vertical ground motion was also significant.Consequently, the following loading cases were considered: both EW and NS components acting simultaneously, all three components acting simultaneously, EW and NS components having a 0.5 seconds delay compared to the Z component.The last scenario was intended to simulate the fact that different seismic waves travel at different velocities and therefore, they reach the site at different time stamps.
The EW component of both earthquakes was applied along X axis of the model (Figure 2) while the NS component was applied along Y axis (Figure 2).The considered loading cases are summarized in Table 2.However, the numerical model will be improved during the subsequent stages of the research to account for the real location of the building with respect to the traveling direction of seismic waves.Consequently, different angles will be applied to the seismic motions with respect to the X an Y axes of the model.

Event Loading case designation
Component along the axes of the model

Results
The obtained results are presented and discussed in terms response spectra determined for the spectral acceleration (SA) of the investigated structure, corresponding to each storey of the building.The level of SA on the response spectrum corresponding to the fundamental period of vibration can offer insightful information on the expected damage of the structure due to the seismic motion.At the same time, amplification phenomena can be rendered evident.
Response spectra are very useful tools for the analysis of performance of structures and/or equipment during seismic events.Therefore, based on the fundamental period of vibration or fundamental frequency of vibration, researchers can estimate the response of a structure by reading the value of the SA from the response spectra of the ground motion.This is a quick manner of assessing whether or not the resonance phenomenon is to be expected.The fundamental period of vibration of the considered structure was determined as 0.7863 seconds.

Ground motion spectra
The response spectra of Vrancea 1977 earthquake expressed in terms of spectral acceleration (SA) are shown in Figure 5.The SA spectra are plot with respect to the period of vibration.The fundamental period of vibration of the structure is shown as vertical line.From the intersection of the vertical line with each of the response spectra, the expected acceleration values of the ground can be determined.The obtained values are summarized in Table 3.The vertical line corresponds to the fundamental period of vibration of the selected structure and it indicates the intensity of the ground acceleration an oscillator having the same dynamic characteristics of the considered structure would be subjected to.
For small values of damping, such as the ones usually considered in civil engineering, the values of spectral acceleration (SA) are almost identical with the values of the pseudo-spectral acceleration (PSA).
The response spectra of Turkey 2023 earthquake expressed in terms of spectral acceleration (SA) are shown in Figure 6.
Comparing the results presented in the two figures it can be concluded that the NS component of Vrancea 1977 earthquake would affect longer period structures with a fundamental period of vibration of 1.15 seconds, the Tukey 2023 seismic event would significantly affect structures with a fundamental period of vibration of 0.3 seconds for both its EW and NS components.Moreover, the seismic event of 2023 being a shallow depth earthquake, it would result in higher values of the spectral acceleration due to significantly lower damping of seismic waves amplitudes.

Response spectra of the structure
The response spectra, in terms of accelerations, at the level of each storey was obtained for each of the loading scenario presented in Table 2.Moreover, the obtained response spectra were obtained with respect to each directional component of the seismic motions EW, NS and Z components, assuming a 5% damping.
From the analysis of the response spectra, the values of the spectral acceleration corresponding to the fundamental period of vibration of the structure were identified.The location of the selected nodes for the assessment of the response spectra of the structure is shown in Figure 7.By looking at the results obtained for the X direction (EW component) it can be observed that they correspond to the peak values of the SA for the fundamental period of vibration of the structure.The values are almost equal to the ones along Y direction of the model (NS component) even though the theoretical applied magnitude of the ground acceleration, considering the response spectra shown in Figure 6, for the EW component is lower than the NS component.This could be attributed to the fact that the structure is stiffer in X direction and therefore less damping of the input acceleration was achieved.

Directional spectral accelerations
The obtained values of spectral acceleration corresponding to the fundamental period of vibration of the structure are summarized in Table 4.The table contains the data obtained for all considered nodes, Figure 7, along the three axes, for each earthquake scenario (Table 2) in case of Vrancea 1977 earthquake.
Table 4. Response spectral accelerations for all considered nodes (unit: m/s 2 )  5 contains the data obtained for all considered nodes, Figure 7, along the three axes, for each earthquake scenario (Table 2) in case of Vrancea 1977 earthquake.
Table 5. Response spectral accelerations for all considered nodes (unit: m/s 2 ) By comparing the data presented in Table 4 with the data corresponding to Vrancea 1977 earthquake from Table 3, it can be noticed that the values of the response SA did not exceed the values of the SA of the seismic motion.This could be explained by the fact that the structure was built after the 1977 seismic event when the design codes were updated based on the information collected in the aftermath of the earthquake.Therefore, the structure would be able to withstand a similar earthquake with minor to moderate damage and, possibly, no casualties.On the other hand, the data presented in Table 5 suggests that the accelerations at level 5 (node 14) and above would exceed the values of the SA of the input seismic motion, suggesting an amplification of the input signal.This could result in moderate to severe damage and higher probability of casualties.Moreover, introducing the vertical component in the numerical simulation, it could be observed that there is a significant amplification of the SA in the vertical direction from the point of view of the response of the structure.
Taking into account the presented information and the fact that the early warning systems (EWS) are getting more and more accurate and fast in alerting the decision makers and general public in case of a severe seismic event, it is now clearer than ever the need for further developing such EWS.The continuous expansion of sensor arrays to monitor seismic motions and record the data could be

Conclusions
The paper presents some preliminary results of a larger research project aimed at developing and testing new early warning system sensors to detect seismic motion.The data presented in this paper highlights the need of such developments by considering a common structural typology present in the urban areas of Romania.The structure was designed and built after the Vrancea 1977 earthquake.The numerical model considers the real dimensions of the structural elements and the material properties specified in the project.The structure is subjected to a combination of seismic waves acting in the longitudinal and transversal direction of the structure coupled with the vertical component of the seismic motions.The results are presented and analysed from the point of view of spectral accelerations of the input motions and the response of the structure.
Based on the obtained results it can be concluded that the structure behaves well in case of a similar seismic event to Vrancea 1977.Response spectral acceleration values are well below the corresponding accelerations of the input motion.However, should a seismic even similar to Turkey 2023 occur, significant amplifications of the input motion is observed in terms of spectral acceleration values.This is due, in part, to the fact that the 2023 earthquake occurred at a shallow depth whereas the 1977 seismic event took place at 96 km below the surface of the earth.Last but not least, the spectra of the two seismic events are different from one another, hence influencing in a different manner the response of the structure.

Figure 1 .
Figure 1.3D view of the numerical model.

Figure 7 .
Figure 7. Selected nodes for assessing the response of the structure.The response spectra of the considered structure obtained at the 8 selected nodes along X, Y and Z directions, for the Turkey 2023 earthquake, are presented in Figure8.

c.
Response spectral acceleration at the selected nodes in Z direction (Z component).

Figure 8 .
Figure 8. Response spectral acceleration at the selected nodes for Turkey 2023 seismic action.

Table 1 .
Characteristics of Vrancea 1977 and Turkey 2023 seismic events.