Analysis On Seismic Behaviour Of Steel Portal Frames With Tapered Members

With lots of advantages, lightweight steel portal frames structures with tapered members has been broad used and become one of major structures used in these years. The span and the height of the structure are become more and more large, while the seismic analyze of the structure is seldom. The prescriptions of seismic design in "Code for design of steel structures" (GB 500017-2003), "Code for seismic design of buildings" (GB50011-2001) and "Technical specification for steel structure of light-weight buildings with gables frames" (CECS 102:2003) have some deficiency. So it is very necessary to do research on the behaviors of these structures under seismic load, the system can be designed to work safely when earthquake happens.Based on the theory of FEM, used the program ANSYS approach combined with material nonlinear, this paper theory calculated and analyzed the structures with tapered members whose spans were 12m～72m, applied to modal analysis and obtained the natural frequency and vibration mode. To testify the accuracy of this paper's calculation model, it is used to compare with the outcome of quadrilateral steel frame of documents; the result shows that the both conform to each other. 11 difference spans and 5 difference slenderness ratios portal frame lightweight steel structure were statically analyzed separately in the condition of frequently occurred and seldom occurred earthquakes with the combinations of horizontal and vertical seismic force. The structure's seismic response is appreciable distinction under two types of load operations.We can conclude that the lager spans the less natural frequencies of the structures were. As to a same span, the small span the change is bigger in natural frequencies. The whole ductility behavior of lightweight steel portal frames structures with tapered members are good in all. It is the snow load which makes the deformation of the large span/slenderness ratios frames exceeded the limitation. So in these cases, the limitation in the relative rules should be modified.The mid-span deflection of lightweight steel portal frames structures with tapered members cannot obtain from related domestic specifications or standards, and in the earthquake the mid-span deflection always exceeded the limitation. Therefore, in the independent seism design, the method in the paper is in effect.