Study On The Simplified Methods To Analyze Framed-Tube Structures Of High-rise Buildings

The framed tube system is widely accepted as an economic solution for tall building structures over large range of building heights. In its basic form, the system consists of closely spaced perimeter columns tied at each floor level by deep spandrel beams to form a tubular structure. Unlike a real tube with the normal "plane sections remain plane" assumption, the basic beam bending action of the framed tube is complicated by the occurrence of shear lag, as a result of the flexural and shear flexibilities of the frame members. Occurrence of negative shear lag in framed tube building under lateral loading is reported in the literatures, but comprehensive studies of it is relatively lacking. The primary concern in this investigation is to develop an analysis method, in which the positive and negative shear lag effects are both taken into account, for framed tubes that is simple to use and yet reasonably accurate. At first, developed orthotropic membrane analogy of transforming the framework panels into equivalent orthotropic membrane each with elastic properties so chosen to present the axial and shear behavior of the actual framework. Then analyzed the equivalent membrane tubes by assuming the axial deformation distributions to be cubic and parabolic in the web and flange panels respectively and using energy formulation to derive the governing differential equations. An analogy between shear lag behavior of uniform box girder and uniform framed tube is established. The present paper separates axial-stress distribution in a equivalent membrane tube into two modes: one contributing to the positive shear lag , which produces the shear lag-aftereffect, and the other to negative shear lag. The net shear lag, positive or negative, is the resultant of the axial-force distributions in these modes. Moreover, expressions are derived for the axial stress. An equivalent column analogy method with the negative shear lag effects taken into account is proposed on the basic of the research of the equivalent membrane tube. The expressions for obtaining the axial forces of the columns in flange panels are also proposed. The equivalent column coefficient is dependent on loading case,ratio of sections of corner column to those of mid column and relative shear stiffness parameter of the panels. Further, an analogy between shear-leg behavior of a uniform cantilevered box girder and uniform framed-tube building is established by interpreting structural parameters occurring in the closed-form solution of a box girder in terms of those of framed-tube building. Then expressions are derived for the equivalent column coefficient and for distance of the section of shear lag reversal from the free end under uniformly distributed forces. And then an analysis method for estimating the effects of axial deformation of columns in wall-frame. This method is suitable for quick evaluation during the preliminary design stage and giving an initial assessment of the structural behavior and in the final design stage for manual checking of the computer analysis results. Numerical examples are given to demonstrate the ease of application and accuracy of the proposed method.