According to the requirements of seismic fortification target, the seismic design of the structure should meet the requirements of three levels of seismic design. The current seismic design code adopts two-stage design method to achieve the above objectives:
The first stage is designed to check the bearing capacity of the structural members by the basic combination of the small earthquakes and other load effects, and to verify the elastic deformation of the structures to the action of small earthquakes. In particular, under the premise that the scheme is in accordance to the principle of seismic design, the seismic action value under the mass intensity (small earthquake) is used as the fortification index, assuming that the structure and the member are in the elastic working state, calculating the seismic action effect (internal force and Deformation), checking the seismic bearing capacity of structural members, and taking the necessary seismic measures. This is not only in the first level to meet the necessary bearing capacity (small earthquake is not bad), while meeting the second level of fortification requirements (damage repair). In addition, for the frame structure and frame - shear wall structure and so gentle structure, but also check the value to the value of the elastic layer under the displacement, to control the lateral deformation under the action of a small earthquake will not be too large. For most of the structure, only the first stage to the design, and through the concept of design and seismic construction measures to meet the third level of design requirements.
The second stage during the design is (mid-earth repairable) elastic-plastic deformation check, the special requirements of the building and the earthquake when the collapse of the structure, in addition to the first stage of design, but also according to the role of the earthquake when the weak parts of the bomb Plastic layer deformation check and take the appropriate construction measures to achieve the third level (earthquakes) of the fortification requirements. The first is to find the weak layer or weak part of the structure according to the actual design cross section (the larger displacement of the interlayer or the first yielding site), and then calculate and control the displacement of the elasto-plastic interlayer under the action of the earthquake and take structural deformation capacity of the structural measures to achieve the purpose of earthquakes.
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