Code Design Spectra Research Articles

Prediction of nonlinear seismic demand of inter-story isolated systems using improved multi-modal pushover analysis procedures

The engineering community requires a simple and engineering-friendly procedure to evaluate the seismic performance of inter-story isolated structures (IIS), considering the widespread application of IIS and the limitations of current static nonlinear analysis procedures. This paper proposes an improved multi-modal pushover analysis procedure (IMMPA) that is compatible with the code design spectrum and can account for the effects of higher mode responses and the strongly nonlinear nature of IIS. Nonlinear response history analysis (NRHA) is unnecessary for a reliable assessment of IIS seismic performance when IMMPA is utilized. The IMMPA involves converting the IIS into a linear non-proportional damping structure to satisfy performance requirements, decoupling it into modal oscillators with the equivalent of the nonlinear structure in the modal coordinates using complex-mode-superposition approaches, adjusting the input response spectrum excitation with its equivalent parameters, and converting the response spectrum excitation into the demand spectrum. The main modal oscillators are identified by the complex modal participation mass ratio; the respective acceleration is used as the weight factor of the lateral pushing force; and the capacity spectrum procedure is employed to determine the performance point. Combining the main modal responses using a complex-mode superposition complete square combination rule (CCQC) produces an effective nonlinear peak response. Finally, the IMMPA procedure is demonstrated using an IIS example and evaluated with NRHA data. The evaluation demonstrated that the IMMPA procedure can overcome conventional pushover limitations and efficiently evaluate IIS seismic requirements.

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.

Evaluating the Use of Multisite Probabilistic Seismic Hazard Analysis: A Case of Sarpol-e Zahab City, Iran.

This paper evaluates the use of multisite (MS) probabilistic seismic hazard analysis (PSHA), which estimates the annual exceedance rate of a given level of ground motion in at least one of several sites as one of several possible results. For this purpose, (1) MS-PSHA is implemented through the Monte Carlo approach, taking into account various area sizes and correlation distances (CDs), and then (2) two proposals are represented as applications of MS-PSHA outcomes, both with reference to Sarpol-e Zahab City, a seismically active region located in the west of Iran. The first proposal attempts to determine the current code design probability of exceedance in at least one site, and the second one defines collapse prevention levels based on different probabilities of exceedance in at least one site. The efficiency of the results is discussed mainly by comparing them to recorded peak ground accelerations (PGAs) of three earthquakes, including the 2017 Sarpol-e Zahab 7.3 Mw event that largely exceeded the code design spectrum. MS-PSHA results demonstrate reasonable performance both in determining design ground motions and evaluating current design code when the exact seismic parameters of the study area are used in the analysis. Moreover, developed code-type design spectra based on MS-PSHA provided safety against collapse compared to a recently occurring low-probability event. MS estimates for various CDs and probabilities of exceedance in at least one site can also provide flexible design strategies regarding the importance of a structure and expected damage on a regional scale.

Study on standard response spectrum parameters of special long-period ground movements

Multiple earthquake damages show that the long-period ground movements have an amplification effect on the dynamic response and seismic damage of long-period structures. To form an improved code design spectrum suitable for long-period ground movements, the standard response spectra consisting of four segments for near-fault pulse-like (NFPL) and far-field harmonic (FFH) ground movements are proposed. Firstly, two types of special long-period ground movements with reliable information are selected for this research. Then, the fundamental period of a single-degree-of-freedom (SDOF) system is extended to 16 s with the damping ratio of 5 %, and the seismic response spectrum and the normalized response spectrum are analyzed. Next, the normalized acceleration spectrum of long-period ground movements is calibrated to the standard response spectrum of regular variation. Finally, the parameters of standard response spectrum about long-period ground movements and the ones of the current code design spectrum are discussed. The results show that the current code design spectrum overestimates the structural seismic response in the short period under NFPL and FFH ground movements, while it underestimates the structural seismic response in the long period. The fundamental period of a SDOF system of china’s current code design spectrum should be extended to consider the influence on long-period structures acted by a long-period earthquake. Further, it accounts for the resonance effect of long-period ground movements which ensures the seismic design safety of long-period structures.

A near-fault probabilistic seismic hazard assessment for Yasouj, located in the Kazerun fault system, southwest Iran

The Kazerun fault system (KFS) is located in the central part of the Zagros, the most seismically active orogenic belt in the Iranian plateau. The city of Yasouj is located in the midst of the KFS and thus exposed to high earthquake hazard. In this study, we conduct a probabilistic seismic hazard analysis (PSHA) which includes near-fault effects. For this purpose, different ground motion prediction equations (GMPEs) are ranked using the deviance information criterion (DIC) data-driven method against a dataset of the regional recorded strong motions. Then, the most appropriate GMPEs are selected for use in a logic tree framework to consider the epistemic uncertainty. In addition, the shear wave velocity is estimated using experimental data and the local site conditions are evaluated for different parts of the city. In addition, the seismicity parameters are obtained using the maximum likelihood methodology considering the magnitude uncertainty. The results show large variations in the spectral accelerations among different locations at short oscillator periods, while no significant difference is observed at longer periods. Moreover, due to the local site conditions, the highest ground motions are expected in the northeastern part of the Yasouj city. Comparing the results of this study with the Iranian seismic code design spectra shows that the design spectra of the Iranian seismic code are underestimating the ground motions at short spectral periods. The results of this study can be useful for both engineers and decision-makers to reduce earthquake risk for the region under study.

Seismic hazard analysis and local site effect of the 2017 Mw 7.3 Sarpol-e Zahab, Iran, earthquake

A strong earthquake occurred on November 12, 2017, in Sarpol-e Zahab city, western Iran, with the moment magnitude ( $$M_{{\text{w}}}$$ ) of 7.3 and a focal depth of 18 km. The maximum horizontal peak ground acceleration of 0.69 g was recorded at the Sarpol-e Zahab station. Significant damages were observed in frame and masonry buildings, while the damage distribution was non-uniform throughout this small city. The preliminary site reconnaissance revealed that numerous engineering structures collapsed or considerably damaged in some regions, contrary with those non-structural masonry buildings in other regions which remained intact during earthquake. This paper represents a preliminary reconnaissance report prepared through the site visit done by the authors, a few days following the earthquake occurrence. Then, the data recorded by the strong ground motion stations in the affected city and the surrounding regions together with the geotechnical data gathered from the available boreholes in Sarpol-e Zahab are incorporated for probabilistic seismic hazard and local site effect analyses. The observed response spectra at two stations and distance-dependency of ground acceleration are compared with those predicted by some attenuation models. The results of probabilistic seismic hazard analysis in the return periods of 475 and 2475 years are compared with the observed ground response and the design spectra recommended by the Iranian seismic code (for site classes Types I and II). Several geotechnical boreholes from the previous works in the affected area were analyzed through the equivalent-linear site response approach in order to obtain the seismic response at the soil surface. The results are then compared with the code design spectra for the site classes of Types III and IV. It is demonstrated that the calculated response spectra are generally larger than those recommended by the Iranian seismic code, especially for the 4–7 stories buildings.

Effects of the use of the surface spectrum of a specific region on seismic performances of R/C structures

The present study pertains to the estimation of the significance of the use of the local acceleration spectrum of a region rather than the design spectra based on the linear time-history analyses of real structures in the region. The study was realized in three stages: (1) development of the target spectrum of the bedrock level accounting for the possible devastative earthquake, the distance of the region to the main fault and the earthquake magnitude; (2) establishment of the surface (local) spectrum of the region based on the bedrock spectrum and soil profile; and (3) the linear time-history analyses of the existing R/C building structures from the studied region using earthquake records that were produced by spectral matching according to the site-depended local spectrum and the design spectra of the seismic code. For this objective, the city of Kirikkale, located in mid-Anatolia, was adopted as the study region owing to the high seismic risk of the region and the presence of abundant poorly designed and poorly or non-engineered structures. The analyses indicated that the use of the local spectrum instead of the code design spectra is rather crucial in R/C structures with irregular story plans. In addition, structures with high eccentricity were obtained to be rather affected from earthquake motions compared to those with regular story plan. The use of the code design spectra provides closer and more conservative base shear, base overturning moment and lateral displacement estimates in R/C structures with regular story plans.

Application of predictive models to assess failure of museum artifacts under seismic loads

Abstract In recent years, there has been a growing interest in the protection of cultural heritage structures and artifacts from seismic excitations. Nevertheless, although the vulnerability of museum exhibits under seismic excitations has been repeatedly verified, it has not been given proper attention. In this work, emphasis is placed on efforts for mitigating seismic risk of museum artifacts elucidating the necessity to identify artifact failure not only based on code design spectra that mainly account for far-fault conditions but also considering near-source phenomena. A general methodology is proposed and demonstrated with representative examples. The methodology considers the detailed geometry of the artifacts, its support conditions, relative distance from the soil surface, the fundamental frequency of the housing structure as well as relevant seismological data, such as vicinity with active faults and soil type, and provides the critical distance from an active fault within which the artifact could fail. The proposed methodology can serve as an easy-to-apply analytical means to assess the seismic risk of museum exhibits for preserving cultural heritage.

Integrated geophysical-geological model for seismic local site response: the Caldes alpine slope case (Southern Alps, NE Italy)

We present the seismic local response analysis of an alpine slope based on an integrated geophysical and geological model. The studied site is located in Caldes, in the lower Val di Sole valley in the Trentino region, Southern Italian Alps. Caldes represents the typical alpine valley formed by glacial and fluvio-glacial processes characterized by complex geomorphological, lithological and tectonic settings. A detailed reconstruction of the valley slope geology (subsoil geometry and stratigraphy) was obtained using electrical tomography resistivity soundings, controlled source multi-channel analysis of surface waves, microtremor single station measurements, borehole investigations, and geotechnical analyses on undisturbed samples. The geological model was shaken with real accelerometric series to obtain 1D (linear equivalent approach) and 2D (finite element approach) local seismic responses. The results, considering the uncertainties of the geophysical measurements, show different seismic responses between 1D and 2D approaches due to relevant geometrical amplifications, with clearer variations in the 2D approach. The comparison of the results with the building code design spectra shows the limits of the simplified approaches, suggesting that detailed characterizations are necessary, especially for such populous alpine environments.

Comparing of the effects of scaled and real earthquake records on structural response

Time history analyses have been preferred commonly in earthquake engineering area to determine earthquake performances of structures in recent years. Advances in computer technology and structural analysis have led to common usage of time history analyses. Eurocode 8 allows the use of real earthquake records as an input for linear and nonlinear time history analyses of structures. However, real earthquake records with the desired characteristics sometimes may not be found, for example depending on soil classes, in this case artificial and synthetic earthquake records can be used for seismic analyses rather than real records. Selected earthquake records should be scaled to a code design spectrum to reduce record to record variability in structural responses of considered structures. So, scaling of earthquake records is one of the most important procedures of time history analyses. In this paper, four real earthquake records are scaled to Eurocode 8 design spectrums by using SESCAP (Selection and Scaling Program) based on time domain scaling method and developed by using MATLAB, GUI software, and then scaled and real earthquake records are used for linear time history analyses of a six-storied building. This building is modeled as spatial by SAP2000 software. The objectives of this study are to put basic procedures and criteria of selecting and scaling earthquake records in a nutshell, and to compare the effects of scaled earthquake records on structural response with the effects of real earthquake records on structural response in terms of record to record variability of structural response. Seismic analysis results of building show that record to record variability of structural response caused by scaled earthquake records are fewer than ones caused by real earthquake records.

Effects of eccentric steel bracing systems on seismic fragility curves of mid-rise R/C buildings: A case study

In this study, the seismic reliability of a mid-rise reinforced concrete (R/C) building retrofitted using eccentric steel braces is investigated through fragility analysis. As a case study, a six storey mid-rise R/C building was selected. The design of selected sample building was made with reference to 1975 version of the Turkish Seismic Code. The effectiveness of using different types of eccentric steel braces in retrofitting the building was examined. The effect of distributing the steel bracing over the height of the R/C frame on the seismic performance of the retrofitted building was studied. For the strengthening of the original structure, D, K, and V type eccentric bracing systems were utilized and each of these bracing systems was applied with four different spatial distributions in the structure. For fragility analysis, the study employed a set of 200 generated earthquake acceleration records compatible with the elastic code design spectrum. Nonlinear time history analysis was used to analyze the structures subjected to this set of earthquake accelerations generated in terms of peak ground accelerations (PGA), whilst monitoring four performance limit states. The fragility curves were developed in terms of PGA for these limit states; namely: slight, moderate, major, and collapse with lognormal distribution assumption. The improvement of seismic reliability achieved through the use of D, K, and V type eccentric braces was evaluated by comparing the median values of the fragility curves of the existing building before and after retrofits. As a result of this study, the improvement in seismic performance of this type of mid-rise R/C building resulting from retrofits by different types of eccentric steel braces was obtained by formulation of the fragility reduction.

Response spectra for nuclear structures on rock sites considering the near-fault directivity effect

Near-fault ground motions, potentially with large amplitude and typical velocity pulses, may significantly impact the performance of a wide range of structures. The current study is aimed at evaluating the safety implications of the near-fault effect on nuclear power plant facilities designed according to the Chinese code. To this end, a set of nearfault ground motions at rock sites with typical forward-directivity effect is examined with special emphasis on several key parameters and response spectra. Spectral comparison of the selected records with the Chinese and other code design spectra was conducted. The bi-normalized response spectra in terms of different corner periods are utilized to derive nuclear design spectra. It is concluded that nuclear design spectra on rock sites derived from typical rupture directivity records are significantly influenced both by the earthquake magnitude and the rupture distance. The nuclear design spectra specified in the code needs to be adjusted to reflect the near-fault directivity effect of large earthquakes.

COMPLEX MODE SUPERPOSITION ALGORITHM FOR SEISMIC RESPONSES OF NON-CLASSICALLY DAMPED LINEAR MDOF SYSTEM

This paper deals with a complex mode superposition method for the seismic responses of general multiple degrees of freedom (MDOF) discrete system with complex eigenvectors and eigenvalues. A delicate general solution, completely in real value form, for calculat-ing seismic time history response of the MDOF system which cannot be uncoupled by normal modes, is deduced based on the algorithms of the complex superposition method. This solution comprises of two parts which are in relation to the Duhamel integration to sine and cosine function respectively. The related term of the Duhamel integration to sine function is actually the displacement response of the oscillator with corresponding modal frequency and the damping ratio. The other can be transferred into a combina-tion of the displacement and velocity responses of the same oscillator. In order to meet the practical needs of seismic design based on code design spectra for various kinds of structures equipped by viscous dampers, the complex complete quadratic combination (CCQC) method is deduced following similar procedures such as the well-known CQC method, in which a new modal velocity correlation coefficient, together with a new modal displacement-velocity correlation coefficient are involved besides the modal displacement correlation coefficient in normal CQC formula. The new algorithm of CCQC is not only as concise as that of the normal CQC but also has explicit physical meaning. The results obtained from complex mode superposition approaches are discussed and verified in some examples through step by step integration computation under a prescribed earth-quake motion input. From these examplary analyses, it may be pointed that the CCQC algorithm normally yields conservative outcome and that the forced mode uncoupling approach has good approximation even the discussed examplary structures are strongly non-proportional.

Simplified R-Factor Relationships for Strong Ground Motions

Recent studies have demonstrated the need to consider the ground motion frequency content in the development and use of R−μ−T relationships. Results from two different approaches to determining these relationships are unified in the present paper. Two bilinear R−μ−T/Tg relationships are recommended for most strong ground motions and structural systems. One is more accurate, while the other, more conservative relationship is used in FEMA 273, ATC-32, and the simple version of the N2 method. Both relationships are indexed by the characteristic period of the ground motion, Tg. Simple methods to determine Tg from smoothed design spectra and recorded ground motions are provided. Neither recommended relationships are applicable to the nearly harmonic ground motions that may be generated at sites containing soft lakebed deposits. An example illustrates the application of these relationships to a code design spectrum in both the acceleration-displacement and yield point spectra formats.

BUILDING DUCTILITY DEMAND: INTERPLATE VERSUS INTRAPLATE EARTHQUAKES

The inelastic seismic response behaviour for a range of simplified single-degree-of-freedom models has been analysed using 180 random phase angle synthetic accelerograms with different frequency contents and different durations and 105 real accelerograms collected from different regions worldwide. Results from the analyses have identified that the frequency content of the excitation can greatly influence the ductility demand ratio due to inelastic amplification effects. Consequently, results derived from intraplate earthquake records (typically of higher frequency content) were generally different to those from interplate records. However, the commonly used El Centro accelerogram has significantly lower ductility demand in the low period range than the average of records with similar elastic response spectral shape. Apart from this, there was little evidence to suggest any inherent differences in the inelastic response behaviour of buildings from intraplate and interplate earthquakes which possessed similar frequency content. Thus, the average ductility demand ratios from future earthquakes in an area can be predicted by interpolation of the results presented in this paper assuming the elastic response spectrum has been defined. Ductility demand ratios derived from the synthetic accelerograms and the real accelerograms with similar frequency content have been shown to be consistent. However, results from synthetic records derived for the idealised code design spectra (such as the Uniform Building Code and the Australian Standard AS1170.4) indicate a significantly higher ductility demand in the long period range.