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1、High Rise Buildings Introduction It is difficult to define a high rise building One may say that a low rise building ranges from 1 to 2 stories A medium rise building probably ranges between 3 or 4 stories up to 10 or 20 stories or more Although the basic principles of vertical and horizontal subsys
2、tem design remain the same for low medium or high rise buildings when a building gets high the vertical subsystems become a controlling problem for two reasons Higher vertical loads will require larger columns walls and shafts But more significantly the overturning moment and the shear deflections p
3、roduced by lateral forces are much larger and must be carefully provided for The vertical subsystems in a high rise building transmit accumulated gravity load from story to story thus requiring larger column or wall sections to support such loading In addition these same vertical subsystems must tra
4、nsmit lateral loads such as wind or seismic loads to the foundations However in contrast to vertical load lateral load effects on buildings are not linear and increase rapidly with increase in height For example under wind load the overturning moment at the base of buildings varies approximately as
5、the square of a buildings may vary as the fourth power of buildings height other things being equal Earthquake produces an even more pronounced effect When the structure for a low or medium rise building is designed for dead and live load it is almost an inherent property that the columns walls and
6、stair or elevator shafts can carry most of the horizontal forces The problem is primarily one of shear resistance Moderate addition bracing for rigid frames in short buildings can easily be provided by filling certain panels or even all panels without increasing the sizes of the columns and girders
7、otherwise required for vertical loads Unfortunately this is not is for high rise buildings because the problem is primarily resistance to moment and deflection rather than shear alone Special structural arrangements will often have to be made and additional structural material is always required for
8、 the columns girders walls and slabs in order to made a high rise buildings sufficiently resistant to much higher lateral deformations As previously mentioned the quantity of structural material required per square foot of floor of a high rise buildings is in excess of that required for low rise bui
9、ldings The vertical components carrying the gravity load such as walls columns and shafts will need to be strengthened over the full height of the buildings But quantity of material required for resisting lateral forces is even more significant With reinforced concrete the quantity of material also
10、increases as the number of stories increases But here it should be noted that the increase in the weight of material added for gravity load is much more sizable than steel whereas for wind load the increase for lateral force resistance is not that much more since the weight of a concrete buildings h
11、elps to resist overturn On the other hand the problem of design for earthquake forces Additional mass in the upper floors will give rise to a greater overall lateral force under the of seismic effects In the case of either concrete or steel design there are certain basic principles for providing add
12、itional resistance to lateral to lateral forces and deflections in high rise buildings without too much sacrifire in economy 1 Increase the effective width of the moment resisting subsystems This is very useful because increasing the width will cut down the overturn force directly and will reduce de
13、flection by the third power of the width increase other things remaining cinstant However this does require that vertical components of the widened subsystem be suitably connected to actually gain this benefit 2 Design subsystems such that the components are made to interact in the most efficient ma
14、nner For example use truss systems with chords and diagonals efficiently stressed place reinforcing for walls at critical locations and optimize stiffness ratios for rigid frames 3 Increase the material in the most effective resisting components For example materials added in the lower floors to the
15、 flanges of columns and connecting girders will directly decrease the overall deflection and increase the moment resistance without contributing mass in the upper floors where the earthquake problem is aggravated 4 Arrange to have the greater part of vertical loads be carried directly on the primary
16、 moment resisting components This will help stabilize the buildings against tensile overturning forces by precompressing the major overturn resisting components 5 The local shear in each story can be best resisted by strategic placement if solid walls or the use of diagonal members in a vertical subsystem Resisting these shears solely by vertical members in bending is usually less economical since achieving sufficient bending resistance in the columns and connecting girders will require more mat
