Seismic Behaviour Factor Of Concentrically X-braced Steel Frames

At first this paper generalizes the value of structural influencing coefficient R and development of seismic behavior theory. After comparing with many country's codes for seismic design, Code for Seismic Design of Buildings (GB50011-2001) is unreasonable for taking the same earthquake action and the same value of R coefficient (2.8125) without taking account of different structural systems. The structural influencing coefficient R is different with different style of structure and material. Steel structures have characteristic of good ductility and dissipative ability, so for steel structures the Code (GB50011-2001) sets higher earthquake action and lower value of R coefficient. Concentrically X-braced steel frame is easy to build and have good stiffness to resist lateral force, but it is easy to buckling under rare earthquake action.Elasto-plastic time history analysis is utilized to determine suitable value of R for a concentrically X-braced steel frame. Detailed steps of this method are descripted and the difficulties and importance are mentioned.Based on the Code (GB50011-2001), 10 concentrically X-braced steel frames are designed and after Elasto-plastic time history analysis their structural coefficients are obtained. The result indicates earthquake actions of less than nine stories concentrically X-braced steel frame which is designed according to the Code (GB50011-2001) are higher than 2.8125. Tentative seismic behaviour factor R= 3.0 are proposed for X-braced steel frames under 9 storey. According to statistical processing and analysis of 10 examples, variation regularities of structural coefficient, ductility coefficient, overstrength coefficient and top draft between floors are acquired.