Journal of Seismology and Earthquake Engineering
Volume:6 Issue: 2, Summer 2004

The recorded strong motion data in Iran since 1994 till the end of 2002 are investigated in order to investigate the well recorded (high signal to noise ratio) accelerometric data, specially in the long period ranges. The records having low signal to noise ratio in the frequency range of less than 0.3 Hz are excluded, and the rest of 100 records are processed and analyzed in this study. The site classification for these records is performed based on the receiver function method (estimating the H/V ratio for each of the recorded motions). According to this procedure, the number of the selected records were 46, 10, 16 and 28 for the site classes 1, 2, 3 and 4 respectively. The frequency contents of most of the records show dominant amplitudes between frequencies 0.2 and 10Hz. The Fmax values are systematically greater for the vertical components comparing to the horizontal ones. This selected catalog of 100 accelerograms obtained from 15 earthquakes provides a basis for the good quality/recent strong motion records in Iran, having good quality low frequencies contents. The response spectra of selected records having a PGA>0.05g are presented in the article and the normalizedmean spectra for the 4 site classes are introduced. The predominant periods of site classes 1, 2, 3 and 4 are found to be 0.09, 0.1, 0.13 and 0.2 sec for horizontal components and 0.08, 0.06, 0.11 and 0.15 sec for vertical components respectively

Frequently long-span bridges provide deep valley crossings, which require special consideration due to the possibility of local amplification of the ground motion as a consequence of topographical irregularities and local soil conditions. This does in fact cause locally enhanced seismic input with the possibility for the bridge piers to respond asynchronously. This introduces special design requirements so that possible out-of-phase ground displacements and the associated large relative displacements of adjacent piers can be accommodated without excessive damage. Assessment of the local variability of the ground motion due to local lateral heterogeneities and to attenuation properties is thus crucial toward the realistic definition of the asynchronous motion at the base of the bridge piers. We illustrate the work done in the framework of a large international cooperation to assess the importance of non-synchronous seismic excitation of long structures. To accomplish this task a complete synthetic accelerogram dataset was computed by using as input a set of parameters that describes, to the best of our knowledge, the geological structure and seismotectonic setting of the investigated area. The results show that lateral heterogeneities can produce strong spatial variations in the ground motion even at small incremental distances. In absolute terms, the differential motion amplitude is comparable with the input motion amplitude when displacement, velocity and acceleration domains are considered. Thus, on the base of the existing empirical regression relations between Intensity and peak values of ground motion a general result of our modeling is that the effect of the differential motion can cause an increment greater than one unit in the seismic intensity experienced by the bridge, with respect to the average intensity affecting the area where the bridge is built

A formulation for the response of the secondary systems subjected to multicomponent earthquake acceleration has been developed, using the random vibration theory. The method accounts for interaction between the primary and the secondary systems as well as the nonproportionality of the combined primaryseondary system damping. The required formulations for the calculation of the autocorrelation function, the power spectral density function, the response spectrum and the critical angle have been obtained. The formulation has been arranged in such a way that the floor response spectrum can be calculated directly from the earthquake response spectra of multicomponent input. The floor response spectra of torsional frames subjected to average response spectrum of 20 earthquake records of Iran have been calculated. Variations of the spectra to various structure parameters such as eccentricity, mass ratio, and nonproportional damping have been studied. Results show that for large eccentricities the effect of multicomponentness of earthquake becomes important and can not be neglected.

Probabilistic Risk Analysis (PRA) of fan type cable stayed bridges is presented to determine their probabilities of failure under random ground motion. Seismic input to the bridge support is considered to be a risk consistent response spectrum. The bridge deck is modeled as a beam supported on springs at different points. The coupled stiffness matrix of the springs is determined by a separate 2D static analysis of cable-tower-deck system in which flexibility of the tower base due to soil-structure interaction is included. Damping due to soil is incorporated by the equivalent modal energy method. The response of the bridge deck is obtained by the response spectrum method of analysis for multi-degree of freedom system. The PRA includes uncertainties of responses due to the variation in ground motion, material property, modeling and method of analysis, and uncertainties of the capacity due to the variation of ductility factor and damage concentration effect. Failure mode of the bridge is assumed to be bending failure of the bridge deck at the point of maximum bending moment. Probability of failure of the bridge deck is determined by First Order Second Moment theory of reliability analysis. A three span double plane symmetrical fan type cable stayed bridge is used as an illustrative example. The fragility curves for the bridge deck failure are obtained under a number of parametric variations. The parameters include, base flexibility, degree of correlation of ground motion, angle of incidence of earthquake, ratio of the components of ground motion, and seismic input. The study shows that flexible base condition provides significantly less value of probability of failure as compared to the fixed base. Further, angles of incidence, degree of correlation, ratio of components of ground motion and input response spectrums have considerable effects on the probability of failure.

Two existing highway bridges in a low seismic area in Eastern U.S. are studied for seismic assessment. Transverse reinforcement spacing and detailing do not satisfy the 1996 AASHTO seismic design specifications. Transverse response of selected two column bents under the action of Nahanni and El Centro earthquakes scaled to 0.15g show the level of lateral force demand to be significantly below yield level. Preliminary seismic assessment based on pier analysis is presented.