【正文】 as to provide the needed resistance to horizontal loads. In the taller of highrise buildings, the system is likely to be found inappropriate for a standalone system, this because of the difficulty in mobilizing sufficient stiffness under lateral forces. Analysis can be acplished by STRESS, STRUDL, or a host of other appropriate puter programs。 analysis by the socalled portal method of the cantilever method has no place in today?s technology. Because of the intrinsic flexibility of the column/girder intersection, and because preliminary designs should aim to highlight weaknesses of systems, it is not unusual to use centertocenter dimensions for the frame in the preliminary analysis. Of course, in the latter phases of design, a realistic appraisal injoint deformation is essential. Braced Frames The braced frame, intrinsically stiffer than the moment –resisting frame, finds also greater application to higherrise buildings. The system is characterized by linear horizontal, vertical, and diagonal members, connected simply or rigidly at their joints. It is used monly in conjunction with other systems for taller buildings and as a standalone system in lowto mediumrise buildings. While the use of structural steel in braced frames is mon, concrete frames are more likely to be of the largerscale variety. Of special interest in areas of high seismicity is the use of the eccentric braced frame. Again, analysis can be by STRESS, STRUDL, or any one of a series of two –or three dimensional analysis puter programs. And again, centertocenter dimensions are used monly in the preliminary analysis. Shear walls The shear wall is yet another step forward along a progression of everstiffer structural systems. The system is characterized by relatively thin, generally (but not always) concrete elements that provide both structural strength and separation between building functions. In highrise buildings, shear wall systems tend to have a relatively high aspect ratio, that is, their height tends to be large pared to their width. Lacking tension in the foundation system, any structural element is limited in its ability to resist overturning moment by the width of the system and by the gravity load supported by the element. Limited to a narrow overturning, One obvious use of the system, which does have the needed width, is in the exterior walls of building, where the requirement for windows is kept small. Structural steel shear walls, generally stiffened against buckling by a concrete overlay, have found application where shear loads are high. The system, intrinsically more economical than steel bracing, is particularly effective in carrying shear loads down through the taller floors in the areas immediately above grade. The sys tem has the further advantage of having high ductility a feature of particular importance in areas of high seismicity. The analysis of shear wall systems is made plex because of the inevitable presence of large openings through these walls. Preliminary analysis can be by trussanalogy, by the finite element method, or by making use of a proprietary puter program designed to consider the interaction, or coupling, of shear walls. Framed or Braced Tubes The concept of the framed or braced or braced tube erupted into the technology with the IBM Building in Pittsburgh, but was followed immediately with the twin 110story towers of the World Trade Center, New York and a number of other buildings .The system is characterized by three –dimensional frames, braced frames, or shear walls, forming a closed surface more or less cylindrical in nature, but of nearly any plan configuration. Because those columns that resist lateral forces are placed as far as possible from the cancroids of the system, the ove