Study On The Theory And Preliminary Application Of Direct Displacement-based Seismic Design Approach Of Partially Prestressed Concrete Frame Structures

As huge economic losses caused by earthquake, how to ensure the safety under rare earthquake and how to reduce the economic losses should be one of the goals for seismic design. As multiple performance levels supplied for owners in the displacement-based seismic design, in which the structure displacement is taken as the design parameter, it can control the structure deformation under the earthquake preferably. So it can ensure the seismic goals to be realized. In this paper, the theory and preliminary application of direct displacement-based seismic design approach for partially prestressed concrete frames has been illuminated through theoretical analysis and pushover analog computation by OpenSees. Base on the work of the research group, several aspects have been researched further, such as the damping ratio of yielding moment, equivalent reduction factors for the elastic flexural stiffness of beam and column under rare earthquake, the DDBD approach how to satisfy multiple performances levels, how to control the target displacement effectively. The insufficiency of the approach has been pointed out, and some recommendations have been given for improvement. The main research work is illustrated as follow:â‘ Yielding stiffness. In the paper, many hysteretic curves of prestressed concrete frames with single storey and single span under horizontal alternate load have been collected and analyzed. The damping ratio at the yielding moment is proposed to be 0.1; Also refer to the GB 50011-2010, to propose the seismic effect coefficientαmaxat 6 degree,7 degree(0.1g),7 degree(0.15g),8 degree(0.2g),8 degree(0.3g),9 degree respectively equal to 0.11,0.21,0.32,0.42,0.63,0.84. According to the damping ratio and seismic effect coefficient at yielding moment, the yielding stiffness can be calculated through the displacement response spectrum.②Equivalent reduction factors for the elastic flexural stiffness of beams and columns under rare earthquake. The stiffness of beams and columns has been degenerated under the rare earthquake, so stiffness degeneration should be considered when to calculate the internal force under the earthquake. In the paper an approach to adopt the reduction factors for the stiffness of beams and columns has been proposed to reflect the nonlinear features of the structures under the rare earthquake, so the equivalent elastic approach can be used to calculate the internal force under the rare earthquake. Through the pushover analysis and equivalent elastic analysis of four prestressed concrete frames under rare earthquake, the equivalent reduction factors of the elastic flexural stiffness for internal force calculation under the rare earthquake can be suggested to take as: for beam from 0.3 to 0.45, for column from 0.7 to 0.8.③DDBD approach of multiple performances levels. Based on the demand of multiple performances for owners, a DDBD approach of multiple performance levels has been proposed in this paper. The design method of"basic intact"performance level is same as the force-based approach of the current code. The standard combination and standard value of material is adopted under the"collapse prevention"performance level. The live load combination coefficient is taken as 0.5. In the"collapse prevention"performance level, the internal force adjusting coefficient and bearing capacity adjusting coefficient is also considered which is different from the force-based approach of the current code. To aim at the multiple performances levels, the seismic design under multiple performances levels and non-seismic design can be carried out respectively; then based on the results of the several reinforcement, the maximum beam-column reinforcement will be the last result. The validity of the approach has been preliminary proved through the analysis of two prestressed concrete frames in the region of 8 degree (0.2g).④Effective control of target displacement. Base on the state that performance point displacement is less than the design target displacement, the principal factors have been summarized through the analysis of two prestressed concrete frames in the region of 8 degree (0.2g). The principal factors can be described as: the obvious overstrength of the structures and the great differences between the displacement pattern at performance point and the design displacement pattern. In the numerical examples the formula for reduction factorΩin which the strong column coefficient, the difference between average value and standard value of materials, the bearing capacity adjusting coefficient and detailing measure have been taken account, have been brought to reduce the base shear. Also the displacement pattern of the performance point has been adopted to modify the displacement pattern, so the control of design displacement has been improved obviously.