SUMMARYWhen a homogeneous elastic continuum is modelled by a non-uniform finite element grid. the differences in the size of adjacent finite elements can cause a spurious wave reflection which does not exist in the continuum. Extending previous studies in which the spurious wave reflection was investigated for the case of two uniform grids with a sudden element size change from one grid to the other, the present study deals with the case when the two uniform grids of different element sizes are separated by a transition zone through which the element size varies gradually, either by a geometric progression or by an arithmetic progression. The solution is carried out in complex variables and numerical plots of the results are given. When the ratio of wavelength to the largest element size is as low as 4: 1, the spurious wave reflection is very significant, while for the ratios over 10: 1 it is insignificant. Inserting a transition zone with gradually varying element size between the two uniform parts of the grid somewhat mitigates the phenomenon of spurious reflection, but this is significant only when the ratio of element sizes in the uniform grids is small (less than about 1· 5: 1). Varying the element size throughout the transition zone in an arithmetic progression seems slightly better than in a geometric progression. The spurious wave reflection is less pronounced for higher-order elements, in particular for linear strain elements as compared to constant strain elements. The spurious reflection is also less severe for consistent mass than for lumped mass.

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From worldwide observations made after the occurrence of earthquakes, as well as the tremendous amount of experimental, analytical and numerical studies, significant contributions have been made for a better understanding of the characteristics of earthquakes, and effects of earthquakes on existing structural systems. Consequently, seismic design codes are revised in a parallel fashion by integrating new concepts towards more realistic considerations of seismic demand, seismic response and seismic capacity. In this paper, after outlining the performance of existing buildings in Turkey during recent earthquakes (particularly Kocaeli 1999 and Duzce 1999 Earthquakes), and by focusing on the observed common structural deficiencies, a brief summary of the evolution of the Turkish Seismic Design Code in the last decades is presented. It is important to note that the poor seismic performance of existing buildings in Turkey outlined in this study is not directly related to the inefficiency of the relevant seismic design codes, but rather to extremely low quality construction and the absence of a strict inspection system at the time of their construction. It should also be highlighted that the lessons learnt from the catastrophic consequences of recent earthquakes, revisions in the seismic design code and the developments in the material and workmanship characteristics have significantly improved the quality of newer constructions in Turkey in the last decade.

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