Ragnar Sigbjörnsson

Probabilistic assessment of wind related accidents of road vehicles: A reliability approach

A general probabilistic model for assessment of road vehicle accidents in windy environments is presented. The limit states of safe performance are outlined and the accident point is defined in the space of basic variables. The probability of accident is evaluated using a so-called safety index approach. The theory outlined is used to analyse a particular bus accident by setting up scenarios based on available information. The methodology presented has several potential applications, such as in accident analysis, and in improving the design of roads and highways by pointing out potential accident spots as well as in devising preventive measures to improve traffic safety in windy environments.

A Fast and Efficient Simulation of the Far-Fault and Near-Fault Earthquake Ground Motions Associated with the June 17 and 21, 2000, Earthquakes in South Iceland

The two Mw 6.5 earthquakes on June 17 and 21, 2000, respectively, in the populated South Iceland Seismic Zone (SISZ) significantly augmented the Icelandic database of strong ground motions, and several strong velocity pulses were recorded at near-fault sites. The strong motions are interpreted via the Specific Barrier Model (SBM) and a mathematical model of near-fault velocity pulses. The data indicates self-similar source scaling and significantly greater attenuation of seismic waves than in other interplate regions. Through inversion of the data a new attenuation function for the SISZ has been adopted, which results in unbiased simulations. For the first time, the characteristics of the recorded near-fault pulses have been identified and compared to the worldwide database of such records. The SBM and the near-fault pulse model combine naturally in a fast and efficient synthesis of realistic, broad-band strong ground motions in the far-fault and near-fault region. Such simulations are showcased for the June 2000 earthquakes and indicate that the modeling approach adopted in this study is an effective tool for the estimation of realistic earthquake ground motions in the SISZ.

A THEORETICAL ATTENUATION MODEL FOR EARTHQUAKE-INDUCED GROUND MOTION

A theoretical attenuation model of earthquake-induced ground motion is presented and discussed. This model is related directly to physical quantities such as source and wave motion parameters. An attenuation formula for rms acceleration of ground motion is derived and verified using acceleration data from moderate-sized earthquakes recorded in Iceland from 1986 to 1997. The source parameters and the crustal attenuation are computed uniformly for the applied earthquake data. Furthermore, attenuation formulas for peak ground acceleration are put forward.

Concerning Baseline Errors in the Form of Acceleration Transients When Recovering Displacements from Strong Motion Records Using the Undecimated Wavelet Transform

Abstract This paper discusses the progression of a novel algorithm that uses a wavelet‐transform approach. The transform is a generalization of the decimated, discrete wavelet transform (DWT) that is the undecimated DWT or stationary wavelet transform (SWT) also known as the undecimated a trous algorithm. It forms the basis for recovering displacements from acceleration time histories. The approach recovers a low‐frequency fling that is usually an almost sinusoidal or cosinusoidal pulse responsible for the big ground motions in strong motion events. The algorithm implements a well known and non‐linear, denoising scheme and is applied to the low‐frequency sub‐band and, in particular, succeeds in recovering the acceleration‐fling pulse. The progression is that in order to obtain estimates of displacements, the algorithmic baseline‐correction scheme can now locate an acceleration transient (i.e., a spike), which creates the DC shift in velocity and the linear trend in displacement, and is therefore the baseline error. Once this acceleration transient is corrected for or eliminated, double‐time reintegration recovers the velocity‐fling pulse and residual displacement. The paper infers that these acceleration transients may be due to ground rotation, embedded in the translational data. The scheme provides for easier integration once the low‐ and higher‐frequency accelerations are extracted. Online Material: Additional results for the Chi‐Chi TCU068 (1999) station, the New Zealand Darfield Station (2010), and the Olfus Earthquake (2008) in Iceland.

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.

HAZARD ASSESSMENT OF DUBAI, UNITED ARAB EMIRATES, FOR CLOSE AND DISTANT EARTHQUAKES

The paper presents an earthquake hazard study for Dubai, United Arab Emirates (UAE). The seismicity of the area is outlined and discussed in terms of available earthquake data and published earthquake catalogues. Strong motion estimation models are also discussed and a specific model is selected and extended to meet the defined requirements for the study. The hazard assessment is carried out to evaluate peak ground acceleration and uniform hazard response spectra corresponding to 5% and 2% probability of being exceeded in 50 years. Simulated earthquake ground acceleration records are derived for the purposes of dynamic response history analysis. The implications of the hazard from near and far earthquakes is explored with regard to the dynamic response of tall buildings and other long-period structures common in the study area.

Daylight availability: a poor predictor of depression in Iceland

Objectives. To test the hypothesis that the unexpectedly low prevalence of winter depression in Iceland is explained by Icelanders enjoying more daylight, during the winter months, than allocated to them by latitude. Methods. A conventional photometer was applied to measure illuminance on a horizontal surface at 64°8.8’ N and 21°55.8’ W every minute throughout the year. The illuminance thus measured was compared with computed illuminance, based on theoretical upper bounds. Results. Daylight availability proved to be, on average, 60% of the theoretical upper bounds derived using clear sky conditions. Snow cover did not, on average, cause a significant increase in daylight availability. Great variability was observed in illuminance from day to day, as well as within days. Conclusions. Average daylight availability does not explain the lower than expected prevalence of winter depression in Iceland. The great variability in illuminance might, however, affect the expression of winter depression, as could daylight quality and genetic factors. (Int J Circumpolar Health 2004; 63(3):267–276)

Spectral analysis of nonlinear wave load effects on offshore platforms

Abstract This paper deals with stochastic theory of nonlinear wave forces emphasizing their effects on the structural response of offshore towers. The analysis of forces and response is limited to second-order statistics expressed in terms of power spectral densities. Numerical examples are included to demonstrate the potential of the theory and to quantify the effects of nonlinear forces on the structural response.

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.