【正文】 most important source among all others,because earthquakes perform the most realistic laboratory tests on the buildings. The author has reevaluated the damages observed in earthquakes during the past 30 years in reevaluation has revealed that more than 90%of the damages can be attributed to one of the following causes or binations of these: made in choosing the building configuration(general configuration or the structural system chosen). detaling and proportioning or errors made in detailing. construction quality caused by inadequate supervision. It is interesting to note that causes of damage grouped into the above three categories seem to apply to earthquake damages observed in other countries three causes will be discussed briefly in the paragraphs to follow. Configuration Seismic resistance should be initiated at the architectural design the general configuration chosen by the architect is wrong,it is very difficult and expensive for the structural engineer to make the building seismic a general principle the floor plan should be as symmetrical as length of wings(T,L,.cross shaped buildings)causing reentrant corners should not be the length of the wings is not short,then these should be separated from the main building by an expansion about the elevational axis is not as significant as the plan ,abrupt changes in building plan along the height of the building are not desirable from the seismic resistance point of are mon vertical irregularities in building cause discontinuities and abrupt changes in strength and seriousness of the setback effect depends on the relative proportions and absolute size of separate parts of the building. In general the designer should try to make changes in strength and stiffness along the building height as small as possible. As far as the structural system is concerned,one can set out some basic rules for better seismic setting out these rules,it would be appropriate to remind the engineers that nonstructural infill walls will influence the frame behaviour significantly unless separated from the frame. Sudden changes in stiffness along the height of the building should be the stiffness of one storey is significantly smaller than the others(soft storey),premature failure can occur due to excessive lateral displacement at this floor shown in Figure 2,changes in the storey stiffness can be caused not only by structural elements,but also by nonstructural elements such as infill walls. Two adjacent buildings should be separated from each other by an adequate distance in order to avoid the damage caused by pounding or reciprocal hammering of the buildings. The vertical load carrying elements in a floor should be so proportioned and arranged that the center of mass and center of resistance should nearly these two centers are away from each other,the resulting eccentricity can cause severe floor torsion,increasing the shear forces at the boundary elements is not only created by structural elements(Figure 3b)but can also be created by infill walls unless separated from the frame,Figure 3a. The maximum shear force which be acting on a column can be found by adding the moment capacities(ultimate moments)at each end of the column and dividing by the column length Figure simply means that,if the length of the column is/5,then the column will carry five times as much this reason,short columns should be avoided whenever it is possible,because of the Figure of shear illustrated in Figure 4,short columns are created by either structural or nonstructural (infill)elements. Structures with flexible floor members(flat plates or joist system with shallow beams)should either have rigid columns or shear walls(or crossbracing)to prevent excessive the vertical load carrying members are not rigid enough,very high second order moments can result as shown in Figure 1967 Adapazari and 1985 Mexico earthquakes numerous failures have been observed in buildings with flexible floors and slender columns. For a more detail discussion on configuration,the reader is directed to Reference 2. and Detailing The dimensions of structural members not only influence the strength,but also the overall stiffnes of the the light of experience gained from the past earthquakes,the author believes that the ratio of the sum of the crosssectional areas of vertical load carrying members to the floor area is an important parameter in seismic ratio will be called theDensity Ratio.The author has studied the variation of this ratio in the monumental historical buildings in Istanbul,which have withstood several severe earthquakes during the past was found out that this ratio varied between and an example,the floor plan of the S252。leymaniye the density ratio was about . Another investigation made on modern reinforced concrete buildings built