Journal of Seismology and Earthquake Engineering
Year:2005 | Volume:7 | Issue:2|Papers Count:7

The seismic performance of unrestrained objects is critically dependent on the displacement demand behaviour of the building floor. The risk of an object overturning can be estimated from the dual independent criteria of object width and height, as opposed to the usual single criterion of the object aspect ratio (or slenderness ratio) based on static analysis. An object is at risk from overturning if the displacement demand of the floor exceeds one-third of the width of the object. According to floor amplification clauses in earthquake codes of practice, the filtering effects of a building amplify ground motions up its height. However, the building may also behave as an isolation medium, which attenuates the transmitted motions. These two perceptions seem contradictory. This paper aims to resolve this significant dilemma and hence contribute to improving the fundamental understanding of the dynamical processes of damage to building contents. Floor spectra of buildings, as presented in the paper, demonstrate both amplification and isolation actions.

Natural phenomena act out of our control and most of the time the environment and people do not act as though they have been instructed. The existing know-how, research, guidelines, codes, and recommendations are not being completely used, implemented or put to work. A quick overview of the past events can substantiate this claim. After every major earthquake, it has been generally concluded that: “It was a surprising earthquake in the sense of ...; the ground motion characteristics being different than expected; the soil behavior and soil-structure interaction being different than expected; the failures were due to poor design and construction and due to ignorance of the ABC’s of engineering principles, further code modifications, and more code refinements are required; and finally more research and funding are needed.” These types of statements will be repeated, almost, after each earthquake.To overcome this difficulty, I believe that “Earthquake Engineering Scientists” should modify their views of the present methodologies and approaches in favor of simplified, doable, affordable and socially acceptable solutions that answer and meet people’s needs in a more direct and to the point approach. Analyzing the success of medical doctors, their influence on people by creating an acceptable level of trust between themselves and the patients, shows that most of people’s demands are being met by the use of simple and almost deterministic solutions based on pharmaceutical research which goes back to Avicenna’s time. Prescription of medicine to a human body, with more complexity than earth’s interior, is what engineers should envisage doing for the safety of the built environment using new technologies such as nanotechnology, smart materials, etc. If simple answers and understandable solutions are available to people, they will respond differently. Definitely the majority of people cannot effectively use the existing manuals and guidelines which they do not understand. They have never understood the causes of many diseases; However, they have used the prescribed medicines due to the confidence and trust that has been built (in most cases) between the doctors and people throughout the history.

The concept of response spectrum was proposed by Biot in 1932 for analysis and design of earthquake-resistant structures. It remained in the academic sphere of research for almost 40 years, finally gaining wide engineering acceptance during the early 1970’s. Success in recording large number of excellent accelerograms during the 1971 San Fernando earthquake, and publication of Brune [18] paper marked the beginning of the modern era for the Response Spectrum Method (RSM). This paper outlines the ideas which helped develop the modern RSM and describes the role Brune’s Spectrum played in this process.

During recent seismic events, non-ductile failure modes of many existing structures occurred. Retrofit of these structures before the earthquake provides a feasible cost-effective approach to reduce the hazard to occupants' safety and owners' investment. The response of two reinforced concrete frames was examined under seismic excitation. The 9-storey and 18-storey frames are part of the lateral load resisting system in two office buildings that were designed according to the 1960s code provisions. The frames were analyzed assuming flexible joint response by considering the joint shear deformation or assuming traditional rigid joints. Two rehabilitation techniques were proposed to improve the dynamic response of these frames. Fibre reinforced polymer (FRP) jackets were used as a local rehabilitation technique to enhance the joint shear strength and ductility. As another option, X-steel braces were installed in the middle bay of the frame along its height as an alternate lateral load resisting system. For each frame, failure sequence and interstorey drift were examined. It was found that FRP wrapping eliminated the brittle failure modes without significant change in the structural response. However, steel bracing significantly contributed to the structural stiffness and reduced the maximum interstorey drift of the frames.

Attenuation relations are developed based on information in the Iranian acceleration data bank (IADB) containing 279 entries from about 30 seismogenic areas across the country. The peak ground horizontal (PGH) and peak vertical accelerations (PGV), varies from a few cm/s2 to over 1000 cm /s2. Moment magnitudes (Mw) vary from about 3.0 to 7.4; and earthquake depths vary from near surface to over 100 km; however, except a majority of depth that are kept at 33 km in the locating process, most depths are about 10 km. Epicentral distances (EPD) vary from 2 km to nearly 250 km. The data bank also includes four site conditions, S. The least squared multi-stage regression solutions for acceleration attenuation are calculated for three cases. The predicted PGH and PGV accelerations are compared with uncorrected high accelerations components near the source of Bam earthquake of 2003; the high PGH acceleration is reproduced, but the estimated PGV acceleration is lower by a factor of 2 to 3. In addition, estimations based in this work are compared with several other studies and discrepancies are discussed.