Seismic Performance Tests And Structural Design Method On Prefabricated Concrete-Filled Double-skin Steel Tube Frames With Infill Walls

The concrete-filled double-skin steel tube(CFDST)arrives from the concrete-filled steel tube(CFST),and the main difference is CFDST has double layer steel tubes.Due to the concrete is restrained by the steel tubes,thus the strength of the concrete can be strengthened.Though the CFDST has been studied in literature,very few investigations have been conducted on the CFDST frame wall system,and such investigations lack supports from experiments.Besides,the traditional weld-bolt connections are still used for CFDST structures in engineering practice currently.Therefore,this paper proposes the prefabricated CFDST frame beam-only-connected wall system(PCDSTF),and investigates the constructional technology,seismic test,design method and seismic risk of the proposed system.The contents and results include:1.To realize the fully-prefabricated construction of the CFDST structures,this paper proposes a splicing type connection for column-to-column joint,a block-strengthened type connection for beam-to-column joint and prefabricated frame-to-wall connections accoridng to the mechanical characteristics of the CFDST structures subjected to lateral loads.The constructional details of the proposed connections are also proposed taking the basis of the demand of the joints,and a PCDSTF system with independent intellectual property rights is developed.According to the performed experimental investigations,the detailed construction processes from the manufacture of the prefabricated components in factory,the transporting process of the components and the construction of the prefabricated components in buildings at in-site are presented.Such presentations could provide efficient scheme and construction experiences for the future engineering practice of the PCDSTF structural system.2.Based on cyclic tests on three CFDST frames with different construction details and six CFDST frame with beam-only-connected shear walls are investigated.The results shown that: the proposed block-strengthened beam-to-column connection could enhance the capacity and ductility of the CFDST frames effectively;the splicing type column-to-column connection had very few influence on capacity and initial stiffness on the CFDST frame,itslightly reduced the ductility of the CFDST frame,however,it is still a desired connection for CFDST frames;the partial tensile fields could be formed to apply lateral stiffness and lateral resistance for CFDST frames;the failure characteristic of the BRW was like the“shear-compression” failure mode of the RC beams;the BCW exhibited the advantages of the BSW and the BRW,and the capacity of the BCW specimens increased slowly due to the combination of the BSW and the BRW.The prefabricated connection of the BSW,BRW and BCW with the CFDST frames is reliable,and it could effectively enhance the performance of the CFDST frames.3.Based on the unified strength theory,the complex stress conditions of the CFDST columns and the mechanical characteristics of the CFDST frames and the CFDST frame-walls subjected to lateral loads are considered,and then the simplified mechanical model of them are developed.The results show that: three typical mechanical stages can be concluded for the CFDST frame due to lateral loads,and the proposed beam-to-column connections could ensure the plastic development on the steel beams;the mechanical model of the BSW can be developed based on the “partial tensile field” model;the BRW behaviors like a deep beam,and its mechanical model is arrived from the mechanical model of deep beams;the mechanical model of the BCW is developed according to the composite shear wall model.By comparing with the theoretical results with the test results on ultimate loading capacity,the maximum error is relative small,indicating that the proposed methods have relative high accuracy.4.Taking the basis of the seismic risk assessment methodology,the seismic risks of the CFDST frames as well as the CFDST frame-walls are quantified.Three typical limit states of the performance levels of the CFDST structures are determined in accordance with the cyclic tests and the FEMA 356.Then,finite element(FE)models of the CFDST frames are built by considering with material degradation,nonlinear behavior of beam-column connections and panel zones,and a shear wall is simplified as a two crossed nonlinear springs with Pinching04 hysteretic model.Lastly,such numerical models are verified by tests,numerical models on mid-rise CFDST frames and CFDST frame-walls are established.Typical ground motions areselected from the FEMA/SAC,and the seismic responses of these models are investigated using nonlinear time-history analyses,and the fragility curves are developed for these models.Lastly,the annual probabilities of each limits and the collapse probabilities in 50 years for these models are calculated and compared.Such results provide risk information for performance-based seismic design of the CFDST structures in this paper.Taking the basis of the experimental and the theoretical results,it can be found that the proposed fabricated joints are reliable.The joints could not only realize the prefabricated construction of the CFDST frame with shear wall systems,but also meet the performance demands of the connections between the parts of the structures.Furthermore,the BSW,the BRW and the BCW could effectively enhance the seismic performance of the CFDST frames,and thus they could be treated as the effective methods to strengthen the CFDST frames.