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
2、 subsystem 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
3、and the shear deflections produced 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
4、same vertical subsystems must transmit 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
5、the base of buildings varies approximately as 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 alm
6、ost an inherent property that the columns , walls , and 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 p
7、anels ) without increasing the sizes of the columns and girders 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
8、 to be made and additional structural material is always required for 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 o
9、f floor of a high-rise buildings is in excess of that required for low-rise buildings . 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 late
10、ral forces is even more significant . With reinforced concrete , the quantity of material also 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 inc
11、rease for lateral force resistance is not that much more since the weight of a concrete buildings helps 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 ef
12、fects . In the case of either concrete or steel design , there are certain basic principles for providing additional 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 subsyste
13、ms . This is very useful because increasing the width will cut down the overturn force directly and will reduce deflection 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
14、to actually gain this benefit. 2. Design subsystems such that the components are made to interact in the most efficient manner . For example , use truss systems with chords and diagonals efficiently stressed , place reinforcing for walls at critical locations , and optimize stiffness ratios for rigi
15、d frames . 3. Increase the material in the most effective resisting components . For example , materials added in the lower floors to the flanges of columns and connecting girders will directly decrease the overall deflection and increase the moment resistance without contributing mass in the upper
16、floors where the earthquake problem is aggravated . 4. Arrange to have the greater part of vertical loads be carried directly on the primary moment-resisting components . This will help stabilize the buildings against tensile overturning forces by precompressing the major overturn-resisting componen
17、ts . 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 col
18、umns and connecting girders will require more material and construction energy than using walls or diagonal members . 6. Sufficient horizontal diaphragm action should be provided floor . This will help to bring the various resisting elements to work together instead of separately . 7. Create mega-fr
19、ames by joining large vertical and horizontal components such as two or more elevator shafts at multistory intervals with a heavy floor subsystems , or by use of very deep girder trusses . Remember that all high-rise buildings are essentially vertical cantilevers which are supported at the ground .
20、When the above principles are judiciously applied , structurally desirable schemes can be obtained by walls , cores , rigid frames, tubular construction , and other vertical subsystems to achieve horizontal strength and rigidity . Some of these applications will now be described in subsequent sectio
21、ns in the following . 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 transmit lateral loads , such as wind or seismic loads
22、 , 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 the square of a buildings may vary as
23、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 stair or elevator shafts can
24、 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 otherwise required f
25、or vertical loads. With reinforced concrete , the quantity of material also 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 f
26、orce resistance is not that much more since the weight of a concrete buildings helps 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
27、 of either concrete or steel design , there are certain basic principles for providing additional resistance to lateral to lateral forces and deflections in high-rise buildings without too much sacrifire in economy . Increase the effective width of the moment-resisting subsystems . This is very usef
28、ul because increasing the width will cut down the overturn force directly and will reduce deflection 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
29、benefit.Design subsystems such that the components are made to interact in the most efficient manner . Remember that all high-rise buildings are essentially vertical cantilevers which are supported at the ground . When the above principles are judiciously applied , structurally desirable schemes can be obtained by walls , cores , rigid frames, tubular construction , and other vertical subsystems to achieve horizontal strength and rigidity . Some of these applications will now be described in subsequent sections in the following .
