| Step-terrace frame strucures constructed in mountain areas exist natural vertical and torsional irregularity,causing the loading and deformation features as well as failure patterns different from ordinary moment frames supported on flat ground.The way how to improve irregularity and failure patterns is the key problem need to be solved for the seismic behavior of step-terrace frame structures.The way how to avoid the upper embedding lateral force resisting members always failure prior to other members is the direction what design theories of step-terrace frame structures pursue.In this study,steel bracings was introduced to step-terrace frame structures to form steel bracing-step-terrace frame structures,referring the idea that steel bracings have been widely used to enhance ordinary moment frames.Elastic response spectrum analysis and dynamic time history analysis were conducted to investigate the influence of different layout schemes and lateral stiffness ratios of steel bracings on seismic performance of step-terrace frame strcutures,using SAP2000 software.Steel bracings with rational arrangement were verified to be able to improve the structural irregularity and lead stepterrace frame structures to form a more rational failure pattern.In addition,the practical stiffness based design method for steel bracing-step-terrace frame structures was proposed.The main work and conclusions were summarized as follow:(1)The rational layout scheme of steel bracing was investigated.For the horizontal layout,steel bracings should be adopted to the spans connected to the exterior corner columns near the lower ground.It could balance the structural stiffness distribution and control torsion effect of upper stories.For the vertical layout,steel bracings should be adopted continuously from the lower ground to the weak story in both down and cross slope directions.It can significantly lower torsion displacement ratios and inter story drift ratios near the second upper embedding story without increasing the structural stiffness and loads too much.(2)The rational lateral stiffness ratio between steel bracings and step-terrace frame structure was studied.By analyzing the results such as torsion displacement ratios,inter story drift ratios,shear load distributions and plastic hinge distributions,it was found that when the lateral stiffness ratio kept between 0.5~1.5,the structural torsion displacement ratios and inter story drift ratios decreased significantly.Meanwhile,steel bracings would share more story shear loads without increasing the structural loads obviously.In addition,the proportion of plastic hinges beyond limit states declined significantly when lateral stiffness ratio kept between 0.5~1.5.Hence,the rational lateral stiffness ratio range was suggested to be 0.5~1.5.(3)The limit design value of inter story drift ratios for step-terrace frame structures were determined.Considering the enhancement from steel bracings,the main body stepterrace frames could be weakened in the preliminary design.By investigating the way how steel bracing-step-terrace frame structures' maximum inter story drift ratios change under frequent and rare earthquakes when the maximum elastic inter story drift ratio of main body step-terrace frames change,the limit inter story drift ratios of main body stepterrace frames corresponding to lateral stiffness ratios of 0.5,1.0 and 1.5 were found to be 1/510,1/490,1/480.(4)A stiffness based design method for steel bracing-step-terrace frame structures was proposed.First,find the weak story and maximum inter story drift ratios of the main body step-terrace frame in both down and cross slope directions during the preliminary design.Then choose the rational lateral stiffness ratios according to the maximum inter story drift ratios.Areas of steel bracings can be calculated by the lateral stiffness ratios and story stiffness of the main body step-terrace frame.The design method was verified to be feasible and rational by analysis of the structural example. |
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