Rajesh Rupakhety

Can Simple Pulses Adequately Represent Near-Fault Ground Motions?

This article investigates the importance of higher modes in the elastic response of tall buildings subjected to near-fault ground motions. Building structures modeled as generic frames are analyzed using a large set of forward-directivity affected pulse-like ground-motion records. It is found that higher modes contribute significantly to peak interstory drifts at the upper portions of the buildings. The importance of higher modes increases as the height of the frame is increased. Equivalent pulses, often used to characterize structural response to near-fault ground motions, are found to underestimate peak interstory drifts at the roof level by a factor of 1.4.

A saga of the 1896 South Iceland earthquake sequence: magnitudes, macroseismic effects and damage

This paper describes Professor Ambraseys’s work on the quantification of magnitudes of the main events in the 1896 South Iceland earthquake sequence, using original teleseismic data recorded by primitive seismological instruments located in Russia and Italy. This includes an example of his original worksheets showing surface-wave magnitude re-appraisal along with the initial part of the parametric earthquake catalogue of Iceland. A brief description of the macroseismic effects of the earthquake sequence is presented. Earthquake damage to buildings in the most affected area is quantified as fractions of collapsed farmhouses. Spatial distribution of the quantified damage shows good correspondence to the estimated epicentres of the earthquakes of the sequence. The macroseismic effects of the earthquake sequence are described and compared to those of the South Iceland earthquakes in June 2000 and May 2008. It is argued that the magnitude estimate of the largest event of the sequence as obtained by Professor Ambraseys, based entirely on teleseismic measurements, is likely to be more accurate than other estimates based on the extent of the damaged area. This argument is supported by a discussion based on a comparison between the felt area, damaged area, and seismic moment estimates of the June 2000 earthquakes with the documented effects of the 1896 main shock. The observations and discussions presented here highlight the importance of Professor Ambraseys in the study of historic earthquakes.

Rotation-invariant measures of earthquake response spectra

A new procedure for combining the response spectra of two horizontal components of recorded ground motion is presented. The presented formulation accounts for different orientations of accelerometer sensors and derives the maximum and the expected (mean) horizontal response spectra at a site, both of which are invariant to rotation of sensor axes. The maximum response spectrum is derived as the peak resultant response of single degree of freedom oscillators subjected to the as-recorded ground acceleration. The expected spectrum is derived by projecting the displacement response (due to as-recorded motion) along two orthogonal axes to a principal axes in which the displacement responses are uncorrelated. This property is used to formulate an approximation for the expected response spectrum over all possible sensor orientations. A large set of accelerometric data from Europe and the Middle East is used to demonstrate the applicability of the proposed response spectral measures.

Rotation-invariant mean duration of strong ground motion

Strong-motion duration is usually computed separately for three components of recorded ground-motion time series. This results in different values of duration for the three components. Furthermore, the computed duration values are dependent on the sensor orientation. Physically, such dependence is not desirable. In this work, computing duration based on resultant recorded motion instead of individual components is proposed. Such a measure of duration is shown to be rotation-invariant and hence independent of the sensor axes. Furthermore, it is demonstrated that the duration of resultant motion represents the mean duration for all possible arbitrary sensor orientations in three-dimensional space. The results indicate that the apparent difference between duration of horizontal and vertical motion reported in the literature is not universal to all definitions of duration. A set of 462 three-component accelerograms from Europe and the Middle East is used to demonstrate and support the presented findings and arguments.

Spatial variability of strong ground motion: novel system-based technique applying parametric time series modelling

Spatial variability of strong ground motion within the dimensions of a horizontally extended structure is often described in terms of spectral parameters, such as autospectral densities and cross-spectral densities of motion, recorded at an array of closely spaced sensors. Traditionally, windowed and tapered periodogram techniques have been used in processing strong-motion array data, whereby spectral quantities are estimated. This approach involves large variances in the computed estimates, which can be reduced by decreasing the bandwidth of smoothing windows. A major problem in such applications is the selection of an optimal window, for which, as far as we know, no formal mathematical criteria exist. In this paper we propose a novel technique, based on parametric time series modelling, to replace the periodogram technique for estimating spectral quantities relevant to the description of spatial variability of ground motion. By using actual earthquake data recorded by a strong-motion array, we demonstrate that autoregressive (AR) time series modelling can be used in spectral analysis of strong-motion array data. Such models can easily be calibrated using a variant of least squares techniques, and well-defined statistical criteria are used to identify an optimal model to describe the recorded data. The application of AR modelling eliminates the subjective judgement involved in periodogram techniques and provides stabler estimates of lagged coherencies.

From Seismic Input to Damage Scenario: An Example for the Pilot Area of Mt. Etna Volcano (Italy) in the KnowRISK Project

In this paper we present a multidisciplinary approach aimed at assessing seismic risk regarding non-structural damage. The study has been carried out in the framework of the European KnowRISK Project and focuses on the pilot area of Mt. Etna volcano (Italy). Both instrumental data and as well as macroseismic observations provide unique opportunities for testing innovative and classical approaches for assessing seismic risk. Starting from the seismic hazard analysis, we first identify a test site (Zafferana) affected by non-structural damage. We produce seismic scenarios based on macroseismic and ground-motion data and finally obtain the relevant risk map using the Italian census data to classify buildings into vulnerability classes and a model to predict damage distribution.

MODELING THE MECHANICAL RESPONSE OF A LEAD-CORE BEARING DEVICE: DAMAGE MECHANICS APPROACH

Abstract. This paper presents the results of a numerical study aimed at developing a rational approach to assess the degradation of mechanical properties of lead-core bearing devices used for passive seismic isolation. The lead-core rubber bearing is modeled as a multiple-component system. Appropriate constitutive laws and failure criteria are defined for each component of the system. Two alternative constitutive relations are defined for the lead core: bilinear material model by assuming isotropic hardening after yielding, and a model formulated in the framework of continuum damage mechanics. The numerical procedure used to simulate the mechanical response of the device allows the monitoring of accumulated mechanical damage throughout the loading history. Preliminary simulation results obtained by finite element analysis are reported. Specifically, these are shear force-displacement hysteresis loops under various loading conditions in a typical characterization test.

Earthquake Engineering and Structural Dynamics in Memory of Ragnar Sigbjörnsson

Structural engineering for earthquake resistance is undergoing a major revision in its approach toward the fulfillment of seismic safety and utilitarian serviceability in design. Rather than sticking to the established precepts of prescriptive design rules, design has turned toward the achievement of specific results through procedures that are tailored for different buildings and uses. These procedures represent notable research contributions, but they are complicated conceptually for implementation in structural engineering practice, and nonlinear building response estimates, frequently assumed to be performance, can vary within broad limits even for simple applications. In this text we relate the history of code developments. We focus on the two main requirements of earthquake-resistant design of building structures: (1) Life Safety and (2) Protection of the Investment and relate the two demands to current concepts of Performance-Based Building Design. While we provide a personalized vision for the way in which the PBSD framework developed and matured during the last half century, a thorough historiography is not within the scope of the text. We nominate drift to serve as the prime metric for performance judgment.

Experimental Study of a New Bar Damper Device for Vibration Control of Structures Subjected to Earthquake Loads

ABSTRACT A new yielding damper, called Bar Damper (BD), made of a number of solid bars sandwiched between two plates is introduced in this paper. Based on a simplified mechanical modeling, design parameters were formulated. The device is shown to have stable hysteretic behavior under cyclic loads. The device is capable of undergoing large displacements without significant strength and stiffness degradation. The slenderness ratio of the bars is a crucial design parameter controlling the effective stiffness and energy dissipation capacity of the dampers. Strength of the devices estimated from theoretical models developed herein is found to be consistent with experimental results.

Development of a Common (European) Tool to Assess Earthquake Risk Communication

This paper reports work on an on-going EC project called KnowRISK aimed at reducing the seismic risk from non-structural elements in buildings. Specifically it reports work on the development of a European tool to assess the effectiveness of risk communication interventions and awareness raising training with middle and high school children in case study areas in Portugal, Italy and Iceland. It describes the difficulties research teams faced in agreeing a theoretical framework and in devising the survey tool. Although they all agreed it was essential to have a common survey if the findings from the research were to be compared across the three countries, one year into the two-year project two of the teams were moving in different directions. This was significant since some of the pre-intervention surveys had already been conducted. Both theoretical frameworks had merit and each of the questionnaires were capable of assessing the efficacy of the training. However, they were in no way comparable. Finally the paper details how these difficulties were resolved and a common questionnaire was devised that embodied virtues from both surveys. This was then applied in all three countries to provide comparable data, the findings from which will be reported elsewhere.