Synthesis of accelerograms compatible with the Chinese GB 50011-2001 design spectrum via harmonic wavelets: artifi cial and historic records

A versatile approach is employed to generate artificial accelerograms which satisfy the compatibility criteria prescribed by the Chinese aseismic code provisions GB 50011-2001. In particular, a frequency dependent peak factor derived by means of appropriate Monte Carlo analyses is introduced to relate the GB 50011-2001 design spectrum to a parametrically defined evolutionary power spectrum （EPS）. Special attention is given to the definition of the frequency content of the EPS in order to accommodate the mathematical form of the aforementioned design spectrum. Further, a one-to-one relationship is established between the parameter controlling the time-varying intensity of the EPS and the effective strong ground motion duration. Subsequently, an efficient auto-regressive moving-average （ARMA） filtering technique is utilized to generate ensembles of non-stationary artificial accelerograms whose average response spectrum is in a close agreement with the considered design spectrum. Furthermore, a harmonic wavelet based iterative scheme is adopted to modify these artificial signals so that a close matching of the signals＇ response spectra with the GB 50011-2001 design spectrum is achieved on an individual basis. This is also done for field recorded accelerograms pertaining to the May, 2008 Wenchuan seismic event. In the process, zero-phase high-pass filtering is performed to accomplish proper baseline correction of the acquired spectrum compatible artificial and field accelerograms. Numerical results are given in a tabulated format to expedite their use in practice.

Topographic seismic effects and avalanche hazard:A case study of Mount Siella(L’Aquila,Central Italy)

In mountainous areas,snow avalanches could be triggered by the shaking produced by earthquakes.The forces induced by the earthquake can cause an irregular increase of shear strength load down the slope,for the presence of complex surface and buried morphologies.Topographic irregularities generate maximum effects of waves amplification linked to wavelengths comparable to the horizontal dimension of the topographic feature.For this reason,the selected time-histories represent an appropriate input for the two-dimensional numerical response analyses when a dynamic phenomenon produce the resonant motion of a whole mountain.This represents an important earthquake-induced hazard in snow-covered mountain areas with high probability of seismic events.Some valleys are located in regions with scare ground motion data and investments on infrastructures are not always accompanied by adequate protection against earthquake-induced avalanches.The paper points out a simple deterministic approach for selecting a set of real accelerograms applied to a real case of Siella Mountain(Central Italy)where a large avalanche destroying a tourist facility of Rigopiano resort on 18 January 2017.The selected time histories were used as input for the two-dimensional numerical model of the subsoil to evaluate the topographic seismic amplification in ridge and compare it with the results of other authors.These methods suggest that morphology-related inertial effects should be considered as an overload action on snow layers when controlling multi-hazard studies and spatial planning.

PGA and structural dynamics input motion at a given site

The computation of the representative ground motions,to be used as input for the dynamic analyses of a struc- ture at a particular site,can be approached by several methods.The choice of the approach depends on two factors:the da- ta available and the type of problem to be solved.This paper reports the experience of the authors in approaching a specific case study:the Southern Memnon Colossus,located in Luxor,Egypt.The results are of interest when the hazard analysis estimation in developing countries and the safeguard of cultural heritage are concerned.Monuments have to be treated as important structures,due to their historical and economical value.Hence,standard procedures of probabilistic seismic haz- ard analysis for the seismic classification of common buildings have to be disregarded.On the other hand,the consequences of the collapse of a monument are not comparable to those related to structures such as nuclear power plants and large dams, for which the deterministic seismic hazard analysis provides a straightforward framework for evaluation of the worst case ground motions.An'intermediate'approach,which requites a lower amount of input data with respect to the deterministic one,is adopted.Its stochastic component can eapture significant eharacteristics of earthquakes,primarily the frequency contents which depend on the magnitude(often referred to as the earthquake scaling law).

Sliding response of gravity dams including vertical seismic accelerations

Seismic safety assessment of gravity dams has become a major concern in many regions of the world while the effects of vertical seismic accelerations on the response of structures remain poorly understood.This paper first investigates the effect of including vertical accelerations in the sliding response analysis of gravity dams subjected to a range of historical ground motion records separated in two groups according to their source-to-site distance.Analyses showed that the incidence of vertical accelerations on the sliding response of gravity dams is significantly higher for near-source records than for far- source records.The pseudo-static 30% load combination rule,commonly used in practice to account for the non-simultaneous occurrence of the peak horizontal and vertical accelerations,yielded good approximations of the minimum safety factors against sliding computed from time-history analyses.A method for empirically estimating the vertical response spectra based on horizontal spectra,accounting for the difference in frequency content and amplitudes between the two components is investigated.Results from analyses using spectrum compatible horizontal and vertical synthetic records also approximated well the sliding response of a gravity dam subjected to series of simultaneous horizontal and vertical historical earthquake records.

Earthquake accelerogram simulation with statistical law of evolutionary power spectrum

By using the technique for evolutionary power spectrum proposed by Nakayama and with reference to the Kameda formula, an evolutionary spectrum prediction model for given earthquake magnitude and distance is established based on the 80 near-source acceleration records at rock surface with large magnitude from the ground motion database of western U.S.. Then a new iteration method is developed for generation of random accelerograms non-stationary both in amplitude and frequency which are compatible with target evolutionary spectrum. The phase spectra of those simulated accelerograms are also non-stationary in time and frequency domains since the interaction between amplitude and phase angle has been considered during the generation. Furthermore, the sign of the phase spectrum increment is identified to accelerate the iteration. With the proposed statistical model for predicting evolutionary power spectra and the new method for generating compatible time history, the artificial random earthquake accelerograms non-stationary both in amplitude and frequency for certain magnitude and distance can be provided.

Synthesis of accelerations of the 1976 Tangshan earthquake(M_S 7.8)in near-and far-field by using semi-empirical method

In this paper a heterogeneous fault model of the Tangshan earthquake is suggested, which consists of two southern sub faults striking N30°E and two northern sub faults striking N50°E. Total length of the main shock fault is 114 km and seismic moment is about 1.4×10 20 N·m. The epicentre of the main shock is located at the southern part, near the intersection of the two bands. Accelerations of two aftershocks ( M L 5.5, M S 6.9 ) were used as empirical Green's functions to synthesize the accelerations of the main shock in near and far field. A method that small events and main shock are considered not satisfying the similarity relationship in the improved empirical Green's function is also applied in this paper. Peak values, duration and response spectra of synthesized accelerations in far field are in agreement with the observed records. The synthesized results in near field are also in agreement with the epicentral intensity distribution of the main shock. The results show that the peak acceleration of Tangshan earthquake in epicentral region exceeds 1.1 g. It is consistent with the peak accelerations recorded in some large earthquakes occurred in recent years.

Influence of Soil-Foundation Interaction Properties on Oscillations of the System “Building-Building” and “Building-Stack-Like Structure”

Seismic oscillations of the “building-building” system which is interconnected buildings built close to each other, and “building-stack-like structure” system which is adjacent and connected in different ways to existing building are considered in the paper. Different types of connections, such as dampers, including the ones suggested by the authors, are studied. Seismic impact is given as a harmonic function and various existing accelerograms, including synthesized ones. Distinctive feature of this paper from previously published ones [1] [2] is the fact that the emphasis falls on the influence of soil-foundation interaction properties, which are described using various models of load-displacement connections. Calculation results are compared in the case of representation of the building as concentrated masses and spatial systems. Ways to reduce seismic response of buildings during the earthquakes are pointed out. Results of experimental studies are given in the paper and are compared with calculations.

Use of probabilistic and deterministic measures to identify unfavorable earthquake records

This study introduces measures to identify resonant (concentration of energy in a single or a few frequencies) or unfavorable earthquake ground motions. Probabilistic measures based on the entropy rate and the geometric properties of the power spectral density function (PSDF) of the ground acceleration are developed first. Subsequently, deterministic measures for the frequency content of the ground acceleration are also developed. These measures are then used for identifying resonance and criticality in stochastic earthquake models and 110 acceleration records measured at rock, stiff, medium and soft soil sites. The unfavorable earthquake record for a given structure is defined as the record having a narrow frequency content and dominant frequency close to the structure fundamental natural frequency. Accordingly, the measures developed in this study may provide a basis for selecting records that are capable of producing the highest structural response. Numerical verifications are provided on damage caused to structures by identified resonant records.