Elastic Response Spectra Research Articles

The new seismic hazard model ESHM20 of Europe: Investigating the implications to the seismic design and risk assessment of major industrial facilities across Europe

AbstractThe seismic performance and safety of major European industrial facilities have a global interest for the whole of Europe and its citizens. However, the seismic design of these facilities is based on national, sometimes outdated seismic hazard analyses. The results of the commonly developed, fully harmonized newly released European Seismic Hazard Model ESHM20 provide a pertinent reference for seismic hazard at European scale and have been officially adopted as an “acceptable representation of the seismic hazard in Europe” in the ongoing revision of Eurocode 8. This study presents a large-scale investigation of the impact of the potential adoption of ESHM20 on the design of new industrial facilities as well as on the potential seismic risk of existing facilities at European level with respect to the current seismic codes. The horizontal elastic response spectra using ESHM20 in combination with the revised Eurocode 8 for selected industrial sites are compared with the respective response spectra of the national regulations for return periods of 475, 2500 and 5000 years. In addition, a single containment LNG tank is analysed for an industrial site in Germany using the design approach for liquid filled tanks according to the revised Eurocode 8. Furthermore, a deterministic and probabilistic seismic risk assessment of a vessel installed in a five-storey frame is performed for industrial sites in Greece and Germany. The examples show that a consistent procedure all over Europe would be desirable and a benefit for engineers in terms of comparability and achievement of the same safety targets. Based on the conducted illustrative studies, the consequences of a potential adoption of the revised Eurocode 8 and ESHM20 hazard maps are discussed and summarized with respect to the standard harmonisation process in Europe.

Development of elastic design response spectra with emphasis on far-source earthquakes for low to moderate seismic region

The development of design response spectra is crucial for earthquake design of structures. However, there are disagreements from the engineering community on the suitability of design values proposed by the existing design code which underestimates the long-period responses for flexible soils, typical of far-source earthquakes. This study uses soil response analysis to investigate the effect of near and far sources’ earthquakes on the response spectral acceleration of Malaysia in three seismically different regions, namely Peninsular Malaysia, Sabah and Sarawak. 1923 borehole data have been collected and analysed under 5 near and 4 far sources earthquakes, subjected to the intensity from the probabilistic seismic hazard analysis. The results show that for Peninsular Malaysia, the far-source earthquake will govern the response at a period of more than 1 s, indicating its importance for structures with long periods such as tall buildings. It is also found that the corner period TC is slightly higher than the code recommended and is dependent on the soil property, while TD is significantly higher for far-source earthquakes due to the larger magnitudes. The finding of this research shows that the Eurocode 8 supplemented by the Malaysian National Annex (MS-EN1998-1, 2017) can be used to design structures in Malaysia, with some adjustments to the longer period motion for Peninsular Malaysia. Finally, it is recommended to perform an enhanced analysis for important structures of long periods to ensure their loadings are not underestimated.

Dynamic Modal Analyses of Building Structures Employing Site-Specific Response Spectra Versus Code Response Spectrum Models

This paper is aimed at giving structural designers guidance on how to make use of elastic site-specific response spectra for the dynamic modal analysis of a structure in support of its structural design. The use of response spectra in support of the pushover analysis of an RC building forming part of the non-linear static analysis procedure (that can be used to predict seismic demand without relying on the code-stipulated default R factor) is also presented. Seismic analysis of structures based on the use of site-specific response spectra can help to achieve a more optimised, and cost-effective, structural design compared to the conventional approach employing a response spectrum model stipulated by the code for different site classes. Currently, the methodology is only adopted in major projects in which enough resources are available to engage experts who are skilled in operating the procedure; thus, the use of site-specific response spectra in structural engineering practice is still limited despite the merits of the procedure. Deriving a site-specific response spectrum requires a database of representative ground motion records to be developed. Extra analytical tasks to be undertaken include the processing of bore log data, site response analyses, and selection/scaling of bedrock accelerograms for input into site response analyses. Guidelines for implementing this design methodology are currently lacking. To promote the wide adoption of site-specific seismic design, this article presents the procedure for developing the required site-specific design spectra, as well as guidelines for applying these spectra for seismic design based on analyses of linear, or nonlinear, models of the building. Non-linear analysis can be accomplished by dealing with macroscopic models as illustrated in a case study.

Numerical analysis of the primary and secondary structural dynamic interaction effects on elastic floor response spectra

In modern seismic design, the assessment of seismic behavior in secondary structures relies on the evaluation of the primary structure's acceleration at the support of the secondary structure. To enable effective secondary structure design, a thorough understanding of the interaction between the primary and secondary structures is essential. This article conducts an analysis based on parametric data, delving into the dynamic interaction between these structures. In this study, both the elastic primary and secondary structures are represented as single-degree-of-freedom systems. The governing equations of motion for both the coupled and uncoupled systems are derived and solved using the numerical method. Subsequently, the floor response spectrum (FRS) is computed for both coupled and uncoupled configurations. This investigation focuses on the impact of three crucial parameters: the tuning ratio (T_r), the mass ratio (μ), and the damping ratio (ξ_s) on the FRS. The analytical findings reveal that dynamic interaction does not significantly affect the FRS when the mass ratio is very low, at 0.1%. However, for a range of 0.8 ≤ T_r ≤ 1.2, dynamic interaction has a substantial influence on the FRS. Additionally, this study underscores that lower damping ratios in the secondary structure result in a more pronounced coupling effect on the FRS.

Seismic Vulnerability of Code Compliant RC Frame Building with Unreinforced Masonry Infill Walls

Unreinforced masonry (URM) infill walls in Reinforced concrete (RC) buildings are normally designed as non-structural elements. However, in the past, the post-earthquake response of the RC frame building showed that URM infill walls increase the strength and stiffness of the RC frame buildings. Further, it is also observed that URM helped to prevent the disastrous destruction by acting as a structural member in RC frame. In the present study, the performance assessment of a four-storey URM infill RC frame with different infill configurations, namely bare and Open Ground Storey (OGS) is done using fragility analyses. Nonlinear masonry material is modelled as the equitant three strut model in ETABS. Nonlinear static pushover analysis is applied for the analysis of the configured RC frames. The probability of damage states is determined by firstly converting pushover curves into capacity curves and accordingly, performance-point values of Spectral acceleration and Spectral displacement for seismic demand Zone-V (Elastic Response Spectra as per IS 1893:2016) using the capacity-spectrum method (ATC-40) are determined. The fragility function used is an approximation of the continuous to discrete distribution. Fragility curves and mean damage matrix are derived to compare performance with each other. From the vulnerability analyses, it is observed that the OGS framed RC building performs better as compared to the bare frame.

Seismic Response of Monolithic and Precast RC Building for Site-Specific Conditions

This paper investigates the seismic response of a rigid and flexible RC building by performing nonlinear time history analysis (NLTHA). In the present study, the numerical model of a 5-storey monolithic RC (MRC) and 5-storey precast RC (PRC) buildings are developed in SAP2000. The buildings are assumed to be situated in Assam and a real Silchar site from Assam is considered. For the NLTHA, six spectrum compatible time histories (SCTH) are considered. The first set comprises of 3 SCTH matched to the Indian seismic code design spectra (ISCDS) and the second set comprises of 3 SCTH matched to the site-specific elastic response spectra (SSERS). The NLTHA results show that the roof displacement of the MRC building is less as compared to the PRC building for all the considered cases. However, the response for both the structures for SSERS compatible time histories is more significant as compared to the ISCDS compatible time histories. It is interesting to note that when the energy dissipation in the structure is evaluated for ISCDS compatible time histories, the damage in PRC building is negligible as compared to MRC building. Moreover, for SSERS compatible time histories, the damage in both the buildings are comparable. The results show the importance of site-specific study and its response on the structure behavior.

Quantitative Modeling of Pulse-Like Ground Motions Using Imperialist Competitive Optimization Algorithm

Increasing attention in quantitative and qualitative models has been paid to the dominant velocity pulse of the pulse-like ground motions. Manual trial and error approach for fitting of time history and response spectrum of the actual record and simulated pulse has been employed to obtain pulse parameters of these models. In this study, a mathematical model and the imperialist competitive algorithm (ICA) are used to overcome the limitations of the previous studies. For this purpose, an objective function is defined as the norm deviation of the time history and response spectrum of the actual record and simulated pulse. The proposed approach is capable of identifying and extracting the dominant pulse of any pulse-like record, and consists of 91 pulse-like records used in this study. In all of 91 records, the highest accordance with time history and elastic response spectrum of actual records and extracted pulses is achieved. As a case study, the extracted pulses of the 1999 Chi-Chi earthquake are used to generate artificial pulse-like records. The dynamic responses of the 3-story moment frame in SAC project subjected to generated records are investigated. The results show that the proposed approach can be used in parametric studies of pulse-like ground motions.

Effects of stochastic generation on the elastic and inelastic spectra of fully non-stationary accelerograms

Nonlinear dynamic analyses are a state-of-the-art tool to assess the performance of earthquake-resistant structures. Inevitably, the validity of the predicted seismic response depends on the fidelity of the computational model to the actual structural behavior and the representativeness of the time histories of ground acceleration as realizations of the seismic hazard for the site under consideration. The generation of artificial time histories is generally allowed by international seismic codes and represents a valid alternative to recorded accelerograms, provided that the key features in the expected seismic input are preserved in the generated signals. Different stochastic generation methods of fully non-stationary accelerograms have been proposed in the literature. Two alternative randomization strategies are compared in this paper, based on (i) wavelets analysis and (ii) evolutionary power spectral density (PSD) functions. The analyses are focused on the aleatory variability observed in the generated elastic and inelastic response spectra in relation to different modeling choices, offering qualitative and quantitative information to designers using stochastically generated accelerograms.

Site specific seismic hazard analysis of monumental site Dharahara, Kathmandu, Nepal

This study was carried out to estimate the seismic requirement for analyzing and designing the monumental structure, Dharahara, in Kathmandu, Nepal. A probabilistic seismic hazard analysis of the study region was performed to develop the elastic response spectrum for the bedrock site. The obtained spectrum was compared with the existing seismic codes, NBC 105:2020 and IS 1893:2016, to estimate the target response spectrum. One-dimensional equivalent linear (EQL) and non-linear (NL) ground response analyses were performed using DEEPSOIL v7, incorporating a suite of ground motions. The three borehole logs exhibited very soft soil in the region. Due to the presence of soft soil strata in the region under study, strength-corrected modulus reduction and damping curves were adopted in the current study to overcome the effect of larger shear strain. The average response spectra from EQL analyses predicted higher values than NL analyses. Finally, a comparison of the proposed design response spectrum with the code spectra was performed. Comparisons revealed that the mean EQL and NL outputs fall in between the design spectra of similar soil classes in NBC 105:2020 and IS 1893:2016 seismic codes.

Multi-pulse decomposition for nonlinear seismic analysis of structural systems

Large sympathetic, resonance-like, structural behaviour to earthquake excitations with analogous frequency content often plays a critical role in determining its maximum seismic response. Earthquake excitation typically contains a broad spectrum of non-stationary frequency content, wave packets, which are difficult to observe from the recorded time series. Therefore, identifying the root cause of large responses (which act sympathically with the input but do not achieve full-resonance) of a structure is problematic. Hence, this paper proposes a new multi-pulse decomposition method of ground motions, through which components of a ground motion within a specific period range are determined. In this method, a ground motion is approximated with a Gauss-Fourier wave packet series. The decomposed components, wave packets, contain information about its time-position, frequency, amplitude, pulse width and phase angle. Unlike the Ricker (Morlet) wavelet the Gauss-Fourier wave packet is not limited to symmetrical pulses. One ensemble of 40 near-fault ground motions and one ensemble of 44 far-fault ground motions are used to demonstrate the application and efficiency of the proposed method. The method is shown to be precise in reconstructing the original ground motion using its decomposed components. It is also concluded that the method is accurate in replication of elastic and inelastic response spectra of ground motions within a specific period range. It is demonstrated that for some structure/ground motion combinations, only a few Gauss-Fourier components are required to faithfully describe response behaviour. This highlights that, for these systems, most of the recorded earthquake time series acts like noise on a much simpler wave-packet signal.

The seismic response of cylindrical steel tank at Al Hoceïma city in Morocco

The aim of this article is determination of seismic response of cylindrical steel tank situated at Al Hoceïma city in Morocco. When a storage facility containing liquid starts moving due to earthquake loading, the fluid filling exerts additional hydrodynamic pressure on storage facility solid domain. The seismic effect of a liquid–filled storage facility converts the contained liquid into and equivalent spring-mass system representing impulsive mode together with convective mode of vibration of the storage facility - liquid system. The seismic response of cylindrical steel tank subjected to seismic loading was calculated by using the elastic response spectrum for locality Al Hoceïma city in Morocco. The rules of Eurocode 8-4 were respected for documented seismic analysis of storage facility containing liquid.

Recalibration of low seismic excitations in Brazil through probabilistic and deterministic analyses: Application for shear buildings structures

AbstractBrazil has a low level of seismicity compared to several other regions in the world; however, some significant earthquakes occurred. In this article, four techniques have been used to define new seismic inputs: probabilistic seismic hazard, elastic spectra, synthetic spectra, and artificial accelerograms. To carry out the probabilistic analysis, new seismogenic zones for southeast Brazil established in 2018 have been implemented. Elastic response spectra follow the Brazilian code, whereas the synthetic response spectra have been defined by using attenuation equations. Finally, the artificial accelerograms have been developed using analytical models. Modified Brazilian elastic response spectra by considering the results of the probabilistic seismic hazard analyses are proposed. Artificial accelerations, consistent to the non‐modified and modified elastic response spectra, show that for non‐modified spectra the structural response increases of ~2.50 times. Data for seismogenic zones and specific analytical equations to obtain the probability of non‐exceedance of the spectral accelerations have been adapted for this study. An application for shear buildings was analyzed.

Simulation of near-field pulse-like ground motions using a correlated bimodal fractional stochastic model

This paper introduces a two-part stochastic method to simulate completely nonstationary pulse-like records and capture several peaks in the power spectrum. Based on the evolutionary power spectral density compatibility of the acceleration ground motion in different stages, the model is fitted to the target records. The proposed model is flexible enough to consider the decay rate variability of the high-and low-frequency content of the ground motions. Also, using the circular shift and IFFT techniques, the discretization of the model is performed such that the simulation cost is significantly reduced. The horizontal components of 178 near-filed pulse-like ground motions with a rupture distance of less than 10 km, with strike-slip and dip-slip mechanisms, are simulated using the proposed method. The model's efficiency is investigated using different criteria such as the elastic response spectra, Arias intensity, zero-crossing, Fourier spectrum, and the 2DOF system response. Moreover, the simulated records' pulse-like characteristics are investigated, and the model's capability to generate an ensemble of desirable ground motions is studied. Comparing the results with the previous models reveals that the model can generate more compatible records with the seed ones according to various aspects.

Comparative Study of Seismic analysis of Bridge Substructure in different Seismic Zones as per IRC Guidelines

Abstract: The capacity design philosophy has currently become design norm for the seismic design of structural systems. it is necessary to assess the overstrength capacity of piers before proceeding with the design of the foundation and superstructure. This paper is devoted to developing deterministic procedures for the seismic analysis of substructure and foundation. Therefore, a moment-curvature approach is analysed. A parametric study is then conducted to investigate the factors that causes the seismic forces in the system. A simplified analysis methodology is put forward based on IRC SP 114; 2018. It is applicable for seismic design of bridges with a design service life of 100 years, considering Design Basis Earthquake (DBE). It has covered the seismic map and spectral acceleration graphs as specified in IS: 1893-Part-I- 2016. It also adopts the method prescribed for evaluation of liquefaction possibility, as specified in IS: 1893-Part-I- 2016. For the evaluation of seismic forces, Elastic Seismic Acceleration method, Elastic Response Spectrum method and Linear Time History method are specified. The IRC Guidelines describe the various types of special investigations to be carried out for bridges to be constructed in near field zones, skew, and curved bridges and so on. For loads and load combinations, IRC 6-2017 provides the guidelines and specifications. Objective of this code is to provide common procedure for design of bridges. It deals with the various loads such as vehicular loads, braking forces, wind load, water current forces and their combinations. Keywords: Seismic design of Bridge Substructure, IRC guidelines, Seismic design, Seismic analysis, seismic zones.

An Overview of the Seismic Elastic Response Spectra and Their Application According to Mexican, U.S., and International Building Codes

The design of earthquake-resistant structures by applying traditional or performance-based seismic analysis methods involves two large variables: the seismic risk of the area and the physical characteristics of the structure. Before analyzing any building, it is required to determine the seismicity in the location to evaluate the accelerations that the structure must withstand by deterministic or probabilistic methods. The typical results of a seismic study are the uniform hazard spectra in the rock layer related to a specific return period and structural damping. The building codes use different methods to obtain the elastic response spectra, seeking to offer simple procedures by using parametric factors to consider the soil type; however, the procedure tends to return conservative estimates. On the other hand, site-specific spectra offer accurate and less conservative acceleration values, with the disadvantage of an extensive and costly mathematical process, justifying their application mostly to important structures. This review article gives the state of the art of seismic elastic response spectra using probabilistic seismic hazard maps as inputs, taking into account the importance of the structure and the soil type, according to Mexican, U.S., and international building codes.

Ground motion of the Ms7.0 Jiuzhaigou earthquake

Abstract Eighty-eight free-field strong ground motion observation datasets obtained by the National Strong Motion Observation Network System of China for the Jiuzhaigou Ms7.0 earthquake were used to study the characteristics of the strong ground motion and the site response. In this study, we calculated the VS30 of the near field station and compared the observed values of the horizontal peak acceleration and peak velocity with the CB2014 ground motion prediction models developed by Campbell et al. (2014). The results indicate that the observed values of the horizontal peak acceleration and peak velocity of the Jiuzhaigou earthquake are both smaller than the predicted values obtained from the ground motion prediction equations. By regressing the spatial variation curves of the D SR (5–95%) and D SR (5–75%) ground motion durations, and comparing them with Bommer’s (2009) ground motion duration prediction curve, it was found that the duration of the Jiuzhaigou earthquake was greater than that obtained from the global empirical prediction equation. The scope of the source duration corresponding to the D SR (5–75%) duration is 2.76–4.28 s, and the scope of the source duration corresponding to the D SR (5–95%) duration is 8.88–10.36 s, which are close to the peak time and completion time of the seismic moment release during the source rupture process. The linear elastic acceleration response spectra of 11 stations within 100 km of the fault were calculated for comparison with the design spectrum. It was concluded that the range of the predominant period of the response spectrum was 0.05–0.26 s, which is less than the natural vibration period of the local multistory building. However, the response spectrum value recorded by station 51JZZ is greater than the design spectrum of the 8° rare earthquake and the observation values of the nearby strong ground motion stations. Through analysis of the H/V spectrum ratio and the case of station 51JZZ, which is closest to the epicenter, this phenomenon was concluded to be related to the magnification effect of the near-surface soil layer and the nonlinear response of the site under the action of strong earthquakes.

Modified Partial Capacity Design (M-PCD): achieving partial sidesway mechanism by using two steps design approach

Partial Capacity Design (PCD) has been developed by using magnification factor to keep some columns undamaged during major earthquake. By doing so, the structures will experience the partial side sway mechanism which is also stable, instead of the beam sidesway mechanism. However, in some cases, structures designed by PCD method failed to show the partial side sway mechanism since unexpected damages were still occurred at some columns. In this research, modification of PCD method is proposed by using two structural models in the design process. The first model is used to design beams and columns which are allowed to experience plastic damages, while the second model is used to design columns which are intended to remain elastic when the structure is subjected to a target earthquake. Two nominal earthquakes corresponding to Elastic Design Response Spectrum (EDRS) level with seismic modification factors (R) of 8.0 and 1.6 are used in the first and second structural models, respectively. It should be noted that the second model is identical to the first model except that the stiffnesses are reduced for elements to simulate potential plastic damages. This proposed method is applied to symmetrical 6 and 10 storey buildings with seismic load according SNI 1726:2012 and with soil classification of SE in Surabaya city. A Non-linear Static Procedure (NSP) or pushover analysis and Non-linear Dynamic Procedure (NDP) or time history analysis are employed to evaluate the performance of the structure. The evaluation is conducted at three earthquake levels which are nominal earthquake that is used in second model, earthquake corresponding to EDRS level, and maximum considered earthquake (MCER) specified by the code (50% higher than EDRS level). The building performances satisfy the drift criteria in accordance with FEMA 273. However, the partial side sway mechanism was not achieved at NDP analysis at maximum seismic load, MCER.

Alternative approach in Partial Capacity Design (PCD) by using predicted post-elastic story shear distribution

The Capacity Design Method is an approach widely used to design earthquake resistant structures. It allows the structures to dissipate earthquake energy by forming plastic hinges through beam side sway mechanism. In the design process, the columns need to be designed stronger than the beams connected to them. Several previous studies have been conducted to propose alternative method allowing partial side sway mechanism namely the Partial Capacity Design (PCD) Method. In this method, selected columns are designed to remain elastic and the plastic hinges are allowed to occur only at the columns base. These columns are designed to resist increased forces. Despite of some successful attempts, PCD method still needs to be developed because sometimes the intended mechanism was not observed. This study proposes a new approach to improve the Partial Capacity Design (PCD) method. Symmetrical 6 and 10 story buildings with 7 bays are analyzed using seismic load for city of Surabaya. Structure behavior under non-linear static analysis is well predicted by this approach. However, under non-linear dynamic analysis, a few unexpected plastic hinges of elastic columns were observed at upper stories. But it should be noted that the earthquake used for performance analysis (maximum considered earthquake) is 50% larger than the one used for design (earthquake level corresponding to elastic design response spectrum).

Development of Site Classification System and Seismic Site Coefficients for Korea

ABSTRACT The contemporary two-parameter-based site classification system of Korea, which is depth to bedrock and averaged shear wave velocity of soil deposits overlying the bedrock, has been reported to be ineffective in representing the unique amplification characteristics of shallow bedrock profiles. A new site classification scheme conditioned on site natural period and corresponding intensity-dependent seismic coefficients for Korean site conditions is proposed based on extensive one-dimensional non-linear site response analysis outputs. It is highlighted that the proposed site classification system and site coefficients coupled with a new elastic response spectrum considerably reduce the residuals with the computed site response analysis outputs.

A wavelet-based approach for truncating pulse-like records

The paper discusses the fast seismic performance assessment of structures using truncated pulse-like ground motion records. It is shown that it is possible to truncate a pulse-like signal using a novel wavelet-based definition that identifies the duration of the predominant velocity pulse. The truncated time-history is still able to efficiently reproduce the increased seismic demand that near-field records typically produce. Substituting the original ground motion with the truncated signal, can significantly accelerate structural analysis and design. The truncated signal is the part of the original accelerogram that coincides with the duration of the predominant pulse, which is identified using a wavelet-based procedure, previously proposed by the authors. Elastic and inelastic response spectra and nonlinear time-history analyses for SDOF (single-degree-of-freedom) systems are first studied. Subsequently a nine-storey steel frame is examined in order to demonstrate the performance of the proposed approach on a multiple-degree-of-freedom system. The proposed approach is found very efficient for pulse-like ground motions, while it is also sufficient for many records that are not characterized as such.