1、Structural Systems to resist lateral loads Commonly Used Structural Systems With loads measured in tens of thousands kips, there is little room in the design of high-rise buildings for excessively complex thoughts. Indeed, the better high-rise buildings carry the universal traits of simplicity of th
2、ought and clarity of expression. It does not follow that there is no room for grand thoughts. Indeed, it is with such grand thoughts that the new family of high-rise buildings has evolved. Perhaps more important, the new concepts of but a few years ago have become commonplace in todays technology. O
3、mitting some concepts that are related strictly to the materials of construction, the most commonly used structural systems used in high-rise buildings can be categorized as follows: 1. Moment-resisting frames. 2. Braced frames, including eccentrically braced frames. 3. Shear walls, including steel
4、plate shear walls. 4. Tube-in-tube structures. 5. Tube-in-tube structures. 6. Core-interactive structures. 7. Cellular or bundled-tube systems. Particularly with the recent trend toward more complex forms, but in response also to the need for increased stiffness to resist the forces from wind and ea
5、rthquake, most high-rise buildings have structural systems built up of combinations of frames, braced bents, shear walls, and related systems. Further, for the taller buildings, the majorities are composed of interactive elements in three-dimensional arrays. The method of combining these elements is
6、 the very essence of the design process for high-rise buildings. These combinations need evolve in response to environmental, functional, and cost considerations so as to provide efficient structures that provoke the architectural development to new heights. This is not to say that imaginative struc
7、tural design can create great architecture. To the contrary, many examples of fine architecture have been created with only moderate support from the structural engineer, while only fine structure, not great architecture, can be developed without the genius and the leadership of a talented architect
8、. In any event, the best of both is needed to formulate a truly extraordinary design of a high-rise building. While comprehensive discussions of these seven systems are generally available in the literature, further discussion is warranted here .The essence of the design process is distributed throu
9、ghout the discussion. Moment-Resisting Frames Perhaps the most commonly used system in low-to medium-rise buildings, the moment-resisting frame, is characterized by linear horizontal and vertical members connected essentially rigidly at their joints. Such frames are used as a stand-alone system or i
10、n combination with other systems so as to provide the needed resistance to horizontal loads. In the taller of high-rise buildings, the system is likely to be found inappropriate for a stand-alone system, this because of the difficulty in mobilizing sufficient stiffness under lateral forces. Analysis
11、 can be accomplished by STRESS, STRUDL, or a host of other appropriate computer programs； analysis by the so-called portal method of the cantilever method has no place in todays technology. Because of the intrinsic flexibility of the column/girder intersection, and because preliminary designs should
12、 aim to highlight weaknesses of systems, it is not unusual to use center-to-center dimensions for the frame in the preliminary analysis. Of course, in the latter phases of design, a realistic appraisal in-joint deformation is essential. Braced Frames The braced frame, intrinsically stiffer than the
13、momentresisting frame,finds also greater application to higher-rise buildings. The system is characterized by linear horizontal, vertical, and diagonal members, connected simply or rigidly at their joints. It is used commonly in conjunction with other systems for taller buildings and as a stand-alon
14、e system in low-to medium-rise buildings. While the use of structural steel in braced frames is common, concrete frames are more likely to be of the larger-scale variety. Of special interest in areas of high seism city is the use of the eccentric braced frame. Again, analysis can be by STRESS, STRUD
15、L, or any one of a series of two or three dimensional analysis computer programs. And again, center-to-center dimensions are used commonly in the preliminary analysis. Shear walls The shear wall is yet another step forward along a progression of ever-stiffer structural systems. The system is charact
16、erized by relatively thin, generally (but not always) concrete elements that provide both structural strength and separation between building functions. In high-rise buildings, shear wall systems tend to have a relatively high aspect ratio, that is, their height tends to be large compared to their w
17、idth. 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,
18、 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 partic
19、ularly 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 seism city. The analysis of shear wall systems is made complex because of t
20、he inevitable presence of large openings through these walls. Preliminary analysis can be by truss-analogy, by the finite element method, or by making use of a proprietary computer program designed to consider the interaction, or coupling, of shear walls. Framed or Braced Tubes The concept of the fr
21、amed or braced or braced tube erupted into the technology with the IBM Building in Pittsburgh, but was followed immediately with the twin 110-story 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 s
22、hear 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 overall moment of inertia is increased and stiffness is very high. The a
23、nalysis of tubular structures is done using three-dimensional concepts, or by two- dimensional analogy, where possible, whichever method is used, it must be capable of accounting for the effects of shear lag. The presence of shear lag, detected first in aircraft structures, is a serious limitation i
24、n the stiffness of framed tubes. The concept has limited recent applications of framed tubes to the shear of 60 stories. Designers have developed various techniques for reducing the effects of shear lag, most noticeably the use of belt trusses. This system finds application in buildings perhaps 40st
25、ories and higher. However, except for possible aesthetic considerations, belt trusses interfere with nearly every building function associated with the outside wall； the trusses are placed often at mechanical floors, mush to the disapproval of the designers of the mechanical systems. Nevertheless, a
26、s a cost-effective structural system, the belt truss works well and will likely find continued approval from designers. Numerous studies have sought to optimize the location of these trusses, with the optimum location very dependent on the number of trusses provided. Experience would indicate, however, that the location of these trusses is provided by the optimization of mechanical systems and by
