Erdal Safak - Academia.edu (original) (raw)

Papers by Erdal Safak

Journal of the Engineering Mechanics Division, Apr 1, 1981

A method for analyzing the torsional vibration of along-wind excited structures is presented. The... more A method for analyzing the torsional vibration of along-wind excited structures is presented. The method is based on random vibration concepts and yields the expected maximum translational and torsional responses of a single-mass structure. The approach is similar to the gust factor method. Aerodynamic admittance functions are derived which are used to estimate the spectral density function of the random torque and the cross-spectral density function of the force and torque acting on the structure. These are required for estimating the structure’s translational and torsional mean square responses. Results for several examples indicate that the dynamic torsional response increases as follows: as the width of the structure’s exposed face increases, as the structural or geometric eccentricity increases, and as torsional natural frequency decreases.

AGU Fall Meeting Abstracts, Dec 1, 2015

The paper presents three new parameters to characterize earthquake-induced ground motions. All th... more The paper presents three new parameters to characterize earthquake-induced ground motions. All three parameters evolve from the concept of response spectra, and are termed 3D response spectra, probabilistic response spectra, and P-A response spectra.

Engineering mechanics, 1979

Building an International Community of Structural Engineers, 1996

In this study, we offer a brief overview of the recent developments in the area of structural hea... more In this study, we offer a brief overview of the recent developments in the area of structural health monitoring research, and provide a number of application examples. From these examples, we also infer various present challenges and offer a view for future research. The applications include interpretation of acceleration data collected during strong motion events on two buildings and a bridge, ambient survey and forced vibration testing of a retrofitted reinforced concrete building, and a couple of examples on damage identification using synthetic as well as physical data from a laboratory-scale structure.

Journal of Wind Engineering and Industrial Aerodynamics, 1981

Abstract A method for estimating the torsional dynamic response of a symmetrical structure subjec... more Abstract A method for estimating the torsional dynamic response of a symmetrical structure subjected to a normally-incident wind is presented. The procedure is based on random-vibration concepts and parallels the gust factor approach currently used in engineering design. An analytical expression is derived for the torsional aerodynamic admittance function, and an approximate expression is presented for computing the expected maximum torsional response. Numerical examples are presented which indicate that the torsional response depends on the width of the structure and its torsional natural frequency.

Journal of Wind Engineering and Industrial Aerodynamics, 1987

ABSTRACT A method for analyzing the three-directional coupled dynamic response of wind-excited bu... more ABSTRACT A method for analyzing the three-directional coupled dynamic response of wind-excited buildings is presented. The method is based on a random vibration concept and is parallel to those currently used for analyzing alongwind response. Only the buildings with rectangular cross-section and normally-incident wind are considered. The alongwind pressures and their correlations are represented by the well-known expressions that are available in the literature. The acrosswind forces are assumed to be mainly due to vortex shedding. The torque acting on the building is taken as the sum of the torque due to random alongwind forces plus the torque due to asymmetric acrosswind forces.The study shows the following: (1) amplitude of acrosswind vibrations can be several times greater than that of alongwind vibrations; (2) torsional vibrations are significant if the building has large frontal width, and/or it is asymmetric, and/or its torsional natural frequency is low; (3) even a perfectly symmetric structure with normally incident wind can experience significant torsional vibrations due to the randomness of wind pressures.

Bulletin of the Seismological Society of America, 2002

Approximately 1000 people were killed in the collapse of buildings in Istanbul, Turkey, during th... more Approximately 1000 people were killed in the collapse of buildings in Istanbul, Turkey, during the 17 August 1999 İzmit earthquake, whose epicenter was roughly 90 km east of the city. Most of the fatalities and damage occurred in the suburb of Avcilar that is 20 km further west of the epicenter than the city proper. To investigate this pattern of damage, the U.S. Geological Survey, in cooperation with the Kandilli Observatory & Earthquake Research Institute (KOERI), deployed portable digital seismographs at seven free-field sites in western Istanbul, to record aftershocks during the period from 24 August to 2 September. The primary objective of this deployment was to study the site effects by comparing the aftershock ground motions recorded at sites inside and outside the damaged area, and to correlate site effects with the distribution of the damaged buildings. In addition to using weakmotion data, mainshock and aftershock acceleration records from the KOERI permanent strong-motion array were also used in estimating the site effects. Site effects were estimated using S waves from both types of records. For the weak-motion data set, 22 events were selected according to the criteria of signal-to-noise ratio (S/N ratio) and the number of stations recording the same event. The magnitudes of these events ranged from 3.0 to 5.2. The acceleration data set consisted of 12 events with magnitudes ranging from 4.3 to 5.8 and included two mainshock events. Results show that the amplifying frequency band is, in general, less than 4 Hz, and the physical properties of the geologic materials are capable of amplifying the motions by a factor of 5-10. In this frequency band, there is a good agreement among the spectral ratios obtained from the two mainshocks and their aftershocks. The damage pattern for the 17 August İzmit earthquake is determined by several factors. However, our study suggests that the site effects in Avcilar played an important role in contributing to the damage.

Earthquake Engineering & Structural Dynamics, Feb 1, 1988

A simple model for the description of strong ground motions is introduced. The model shows that r... more A simple model for the description of strong ground motions is introduced. The model shows that response spectra can be estimated by using only four parameters of the ground motion, the RMS acceleration, effective duration and two corner frequencies that characterize the effective frequency band of the motion. The model is a windowed band-limited white noise, and is developed by studying the properties of two functions, cumulative squared acceleration in the time domain, and cumulative squared amplitude spectrum in the frequency domain. Applying the methods of random vibration theory, the model leads to a simple analytical expression for the response spectra. The accuracy of the model is checked by using the ground motion recordings from the aftershock sequences of two different earthquakes and simulated accelerograms. The results show that the model gives a satisfactory estimate of the response spectra.

During an earthquake, the presence of free surface generates waves, known as surface waves, which... more During an earthquake, the presence of free surface generates waves, known as surface waves, which propagate along Earth’s surface. In sedimentary basins, the conversion of seismic body waves at basin edges also generates surface waves, which travel across the basin. Surface waves are characterized by long-periods and much slower velocities than shear waves. Consequently, they are more visible in displacements than in accelerations, and show up much later in a record. In triggered seismic networks, the recorders are triggered on and off based on acceleration amplitudes. In most earthquakes, recorders are turned off before the arrival of surface waves, and the post-event memory is typically set too short to record them. This paper investigates the effects of surface waves on structural response, and presents observational and analytical confirmations of their significance. Recent data from realtime monitored structures clearly show the importance of surface waves. Because their energy is concentrated at long periods, surface waves are particularly critical for longperiod structures, such as tall buildings and base-isolated structures. Also, because of their long duration, surface waves can push a damaged structure into complete collapse because of the additional hysteretic deformation cycles that the structure goes through.

Springer eBooks, 1991

Stochastic-adaptive filters provide excellent tools to analyze nonstationary and noisy signals, b... more Stochastic-adaptive filters provide excellent tools to analyze nonstationary and noisy signals, because of their superior time-tracking and noise handling abilities. This paper gives a brief summary of the theory of stochastic-adaptive filtering, and presents three applications in earthquake engineering. The applications are on modeling earthquake ground motions, site amplification, and separation of translational and torsional motions in buildings. An example for each application is presented by using real-life data.

Soil Dynamics and Earthquake Engineering, Apr 1, 1989

ABSTRACT Optimal filtering techniques have been used successfully in various areas in science and... more ABSTRACT Optimal filtering techniques have been used successfully in various areas in science and engineering. They are based on statistical properties of the signal and the noise, and stochastic approximation theory. In addition to filtering, optimal filters can also be used for smoothing, prediction, and system identification. This paper introduces some applications of optimal filtering techniques to earthquake engineering by using the so-called ARMAX models. Three applications are presented: (a) spectral modelling of ground accelerations, (b) site amplification (i.e., the relationship between two records obtained at different sites during an earthquake), and (c) source scaling (i.e., the relationship between two records obtained at a site during two different earthquakes). A numerical example for each application is presented by using recorded ground motions. The results show that the optimal filtering techniques provide elegant solutions to above problems, and can be a useful tool in earthquake engineering.

Structural Engineering in Natural Hazards Mitigation, 1993

Methods alternative to spectral ratios are introduced to characterize site amplification. The met... more Methods alternative to spectral ratios are introduced to characterize site amplification. The methods are developed by using a range of models, from the simple constant amplification model to the time-varying filter model. Examples are given for each model by using a pair of rock- and soil-site recordings from the Loma Prieta earthquake.

Earthquake Spectra, Aug 1, 2019

The paper presents a methodology to analyze plane frame structures using the Spectral Element Met... more The paper presents a methodology to analyze plane frame structures using the Spectral Element Method (SEM) with and without considering Soil-Structure Interaction (SSI). The formulation of spectral element matrices based on higherorder element theories and the assemblage procedure of arbitrarily oriented members are outlined. It is shown that SEM gives more accurate results with much smaller computational cost, especially at high frequencies. Since the formulation is in the frequency domain, the frequency-dependent foundation impedance functions and SSI effects can easily be incorporated in the analysis. As an example, the dynamic response of a plane frame structure is calculated based on the Finite Element Method (FEM) and SEM. FEM and SEM results are compared at different frequency bands, and the effects of SSI on the dynamic response are discussed.