Research On Ascertaining The Position Of Fixity Of Tall Buildings With Basements And Some Other Problems

The position of fixity of tall building must be obtained bofore structural analysis, and it is significant to take it correctly. The paper have studied the conditions for taking the top-floor of basement as the fixed of the tall building and some other problems by analysing dynamic calculating models and typical examples in Finite Element Method. The investigations mainly include three aspects as follows:①In order to obtain the rearonable value and calculation methods of stiffness ratio of the basement and the upside adjacent floor, the dynamic performance has been studied primary by analysing simplified models, then it has been checked by analysing the typical examples.②When the tall building has a large basement and some small towers, it has been obtained that the relative rigidity range of large basement and some other applying conditions, which is studied by analysing the typical examples in the assumption of elastic plate.③When the tall building has a basement which is not fixed around, it has been discussed that the top-floor of basement should be considered as the position of fixity of the tall building or notFrom this text, we can acquire conclusion as following:①There are two lateral rigidity calculation methods of basement in Technical specification for concrete structures of tall building and some other teaching material. The two methods contain the seismic force and story drift ratio and the shear stiffness ratio, which are very different from each other. The second one is suggested in the paper.②As to the frame structure and shear structure, when the stiffness ratio of the basement and the upside adjacent floor is more than 2, the error of structure characters is less than 10 percents between two modles, so the top-floor of basement can be taked as the fixed of the tall building.③As to the tall building with a large basement and some small towers, it should be considered that the lateral rigidity of basement under the towers and basement within a given range. The given range can be suggestted as the area between the towers and a point 2 spans from the towers.④When the number of span and the rigidity of structure elements increase in a given area of the basement, the interal force of most vertical elements under the towers will reduce obviously. Which is favourable for the basement resisting the the upside adjacent floor's overstrength and the seismic force of itself.⑤As to the tall building with a non-fixed around basement, it is suggested that the top-floor of basement can be taked as the fixed of the tall building under some conditions.