UB-MCEER-EERI Seminar Series at the University at Buffalo
The EERI student chapter of the University at Buffalo (UB-EERI), the MCEER Student Leadership Council, the Networking and Education Programs of MCEER, and the University at Buffalo's Department of Civil, Structural and Environmental Engineering jointly sponsor a series of seminars on a variety of topics related to earthquake hazard mitigation. The purpose of the seminar series is to widen accessibility to timely, technical presentations by students, researchers, visitors and affiliates of MCEER. The seminars are broadcast over the Internet in real-time, and can be viewed anytime at http://civil.eng.buffalo.edu/webcast . UB's Professor Andrei Reinhorn and undergraduate student Jason Hanley, from Computer Science and Engineering, arrange the webcasts.
Frequency Domain Analysis of Long-Span Bridges Subjected to Non-Uniform Seismic Motion
Mohammed Ettouney, Weidlinger Associates, Inc.
Faculty and students from UB's Department of Civil, Structural and Environmental Engineering enjoyed a luncheon with Dr. Ettouney prior to his seminar presentation.
Dr. Mohammed Ettouney, Weidlinger Associates, Inc., gave a seminar entitled "Frequency Domain Analysis of Long-Span Bridges Subjected to Non-Uniform Seismic Motion," at the University at Buffalo on Friday, November 30, 2001. His presentation was included as the sixth in the ongoing seminar series at UB on earthquake engineering topics. The MCEER Information Service prepared handouts with references related to the topic of Dr. Ettouney's presentation.
Ms. Ani Natali Sigaher, Ph.D. candidate in the Department of Civil, Structural and Environmental Engineering and the president of the MCEER Student Leadership Council, opened the seminar by welcoming the audience of about 35 people, and introducing the speaker.
Dr. Ettouney started by giving an introduction to the topic of his presentation. He noted, in particular, three identified reasons for non-uniformity of seismic motion that affect the seismic behavior of long-span bridges. They are: local soil conditions, wave passage and incoherency effects, while other effects, such as extended source and attenuation, are relatively small. He discussed the importance of non-uniform seismic motions, especially for sensitive and important structures and how it has led to the development of several methods of analysis. These methods can be subdivided into two general categories, deterministic and stochastic. Due to the inherent uncertainty of the non-uniform seismic motions, deterministic methods, mainly time integration methods, can be computationally inefficient while stochastic methods are generally very efficient. The stochastic methods have been based mainly on modal analysis methods where the input/output are either described in terms of power spectral density or response spectra.
Dr. Ettouney then elaborated on a direct frequency domain method that is based on formulating the whole soil-structure problem and showed its applications to non-uniform seismic support motions on a simple two-dimensional bridge and a long-span suspension bridge. The results indicate that non-uniform support motions may result in a large shifting of resonant frequencies of the structure. Also, a large redistribution of bridge responses and internal forces was observed when the results for non-uniform and uniform seismic motion, respectively, were compared.
Dr. Ettouney's presentation was received by generous applause, after which Ms. Sigaher moderated the discussions. A number of interesting questions were posed from the audience.
Submitted by Benedikt Halldorsson, UB-EERI secretary
Incorporating Effects of Near Fault Tectonic Deformation into Design Ground Motions
Norman Abrahamson, Pacific Gas and Electric Company
Dr. Norman Abrahamson discussed a new method to develop design ground motions that incorporates fling effects.
The University at Buffalo Earthquake Engineering Research Institute Student Chapter (UB-EERI) invited seismologist Dr. Norman Abrahamson of the Pacific Gas and Electric Company, California, to give a presentation as part of its ongoing seminar series. His presentation, "Incorporating Effects of Near Fault Tectonic Deformation into Design Ground Motions," was held on October 26, 2001. His visit was sponsored by the Friedman Family Visiting Professionals Program of the EERI, whose purpose is, among others, to enhance the understanding of the multidisciplinary nature of earthquake engineering.
The event began with Darren Vian, UB-EERI president for 2001-2002, opening the seminar by welcoming attendants (over 35 people) to the seminar and providing a brief introduction of the purpose of the seminar series and its sponsors. Subsequently, Benedikt Halldorsson, the UB-EERI secretary for 2001-2002 and past president, introduced the speaker with a few background remarks.
Dr. Abrahamson started with an introduction of two near fault effects on long period ground motions: directivity and fling, explaining how both effects result in large velocity pulses in the near fault ground motion while having very different causes. He showed examples of fling effects observed in recent earthquakes e.g., the 1999 Turkey and Taiwan earthquakes.
He emphasized that existing ground motion attenuation relations do not include fling effects a separate ground motion model needs to be developed for this phenomena. The total ground motion would then be computed by combining the ground motion from attenuation relations with the ground motion from the fling.
Subsequently, he introduced a preliminary model based on a single sine-wave cycle to model the fling in acceleration. He explained the parameters needed for the model and presented preliminary estimates of these parameters based on existing data. He then went on to show an example of the application of the method for a magnitude 7.2 strike-slip earthquake at a distance of 5 km.
Finally, Dr. Abrahamson gave a summary of the presentation and posed a question regarding if fling has a significant effect on the response of structures. Incorporating fling into the ground motion adds complexity to the development of the ground motion. It has not yet been determined which classes of structures are affected by fling, thus justifying this additional complexity.
Dr. Abrahamson's presentation was received by generous applause, after which the UB-EERI secretary moderated discussions. A number of interesting questions were posed from the active audience, the vast majority of which were engineers.
As with all of the seminars in this series, Dr. Abrahamson's presentation was webcast in real time, and archived for later viewing. In particular, webcasting Dr. Abrahamson's presentation made it possible for other EERI student chapters to benefit from the Friedman Family Visiting Professional Program.
Submitted by Benedikt Halldorsson, UB-EERI secretary