Seismic Analysis Of Steel Frame Based On The Critical Incidence Angle

According to the present seismic code,structure designs often calculate the seismic action of two main axis directions and combine them with related rule as the standard value.However,the direction of ground vibration in an actual earthquake may occur in any direction and change continuously.Directions in which ground motion produce larger structural responses than structure responses produced by ground motions applied along structure axis may exist.Generally the angle in which produces the maximum structure response is referred to as the critical incidence angle of structure under specific ground motions.The existing studies often consider special structure and irregular structure with seismic action in range of 360°or to use different methods.In this paper,attentions were paid on factors: span number,layer number and randomness of seismic waves.According to present seismic code,a series of models were established with different spans and layers.Base shear force,layer displacements and top displacements were chose as indicators.seismic actions were loaded on all of the models with angle change during the range of 0°~180°,which changed 5°as a step.According to factors,different models were classified for comparison.Some conclusions were drew: 1.The critical incidence angle had little impacts on base shear force,but not did layer displacements and top displacements.2.Change of span numbers had little impacts on the critical incidence angle,but not did layer number.3.Randomness of seismic waves also had huge impacts on the critical incidence angle.The chose three waves loaded on the same structure,the critical incidence angle appeared discreteness.4.the max displacement layer may transfer considering the critical incidence angle.5.In the low-level models,the critical incidence angle occurred near 30 ° ~ 40 °.With the number of the span increasing,the critical incidence angle appear in the vicinity of 0 °;In the high-level models,the critical incidence angle occurred near 90 °.With the number of the layer increasing,the critical incidence angle appear in the vicinity of 90 °,The higher structure is more necessary to consider the most the critical incidence angle.After that the seismic response of the frame is evaluated with the SRSS rule and the 30% rule and the difference between the one-way wave and the two-way wave is compared at the same time.Some conclusions were made: 1.both rules underestimate the structure's response near 20% in an earthquake.2.the one-way wave of SRSS law underestimated Seismic action more when considering the low level frame,the two-way wave of SRSS law underestimated Seismic action more when considering the high level frame.30% rules underestimated Seismic action more than SRSS rules in any case.Finally,this paper analyze the search step of the angle.With date analysis,proposal “the critical incidence angle locating way with length of step change ” : 1.to general structures,check the structure with 45° angle change within the range of 360 °.2.Check the structure with 15° angle change within the 45° angle range given by step1.3.Check the structure last 2 timeswith 5° angle change within the 15° angle range given by step2.Compare with 72 times calculation during 360°,the method had similar accuracy but workload was much less.