Research On The Seismic Performance Of A Presressing Precast Frame Joint Based Grouped Energy Dissipation Bars

Current researches and applications of precast concrete in China are mostly concentrated on the field of structures with wet-type connections,which are under the design criteria of‘emulative’.This wet-type structure style reserves cast-in-place concrete and can obtain integrity and safety to the maximum extent,but there are still some unsolved controversies on the mechanism of the bond between the fresh and hardened concretes.In addition,the assembly of reinforcing steel bars in the cast-in-situ place will involve extra work in structural designing,processing and installing,which offsets the advantages of precast structures in energy saving and constructing efficiency to some content.Therefore it is reasonable to develop a new direction of precast structure to satisfy the diversified demands on structure.In fact,a new structure style with dry-type connection,which does not need cast-in-situ concrete,can offer the engineers a new choice in precast concrete.Therefore,a new type precast frame joint with local unbonded prestressing strings is proposed in this paper,along with an energy dissipation device of a buckling-restrained longitudinally weakened round steel bar.The new frame joint composes of precast beams,columns and local unboned prestressing strings.The beam and the column are connected by the compressing force of the prestressing strings.The line type of the prestressing strings is crooked,which makes them go through the joint center and come out and be anchored at the beams’ side faces.The new energy dissipators composes of an inner longitudinally weakened round bar and an outer restrained tube,which is a typical configuration of buckling restrained braces.The energy dissipation bars are set at the beams’ top and bottom surfaces,and use the open and close of the beam-column connection to dissipate energy.This paper divides the whole research into several parts of researches of mechanic performances of single energy dissipator,researches of installing constructions of energy dissipator groups,and researches of the seismic capacity of the whole structures.The specific research contents are shown as followed:1)The research on the Miniature Bar-typed Structural Fuses(MBSF for short)is divided into three research levels of core bar study,restraint tube study and performance improvement study.In chapter 2,components with different area cutting ratios,different section types and different weakened lengths are conducted to quasi-static tests and analyzed to demonstrate the effects of core bar type differences on the MBSF’s mechanic performance.The generation mechanism of local buckling of the upper free segment is discussed,and it is found that the cutting area ratio should be larger than 0.35 to avoid the upper segment’s plastifying and local buckling.The effects of section type play a rather important role in component’s mechanic capacity,and the components with three-side-cutting core bars have the best performance.The lengths of the weakened region have a limited effect on the whole components,but the lengthening in core bars might increase the adverse effects of friction.Then,an analysis model of the restraint tube is established based on the interaction format between three-side-cutting core bar and the restraint tube.A safety factor calculation method to avoid tube local bulging is proposed by the mechanic analysis based on the tube mechanic model.A recommended valueγ≥0.283 of the tube’s thickness-radius ratio is proposed for the MBSFs’ primary design,as well.Based on the two analysis of the core bar and the restraint tube,a new test of comparing the effects of Teflon spacers is conducted.It is found that the Teflon spacers can effectively improve the mechanism capacity of components with one-side-cutting and two-side-cutting core bars.2)A new installation construction of MBSF groups is proposed,along with three derivative constructions of hierarchical energy dissipation systems(HED): various lengths construction,Belleville spring construction and frictional sliding construction.In chapter 3,the feasibility of the installation construction and the HED systems are examined by an idealized rotation beam test.The effect of hierarchical energy dissipation is analyzed by a series of dynamic analyses on single degree of freedom models.Based on the IDA results,a design method based on structural displacement for HED system is proposed.3)The research on the new prestressing precast frame joint composes of experimental tests,theoretical analysis and finite element model simulations.In chapter 4,quasi-static loading test is conducted on 5 joint components to examine the seismic capacity of the new prestressing proposed frame joint.It is found that damages on the corbels will lower down the joints’ recentering capacity.Even though in the case of ultra-high performance concrete(UHPC)corbel,damages will accumulate under repeated loadings.In chapter 5,the forced mechanism of the new prestressing joint is discussed carefully with an improved design method of the new prestressing frame joint including normal structure and HED structure,and corresponding finite element model simulation of the tested components is conducted by the open source FEM software Open Sees(Open System for Earthquake Engineering Simulation).The differences between the two model establishment methods of trussing model and rotation spring is also compared.In chapter 6,based on the model method of rotation spring,full frame models of the new prestressing frame joint,normal frame joint and the prestressing frame joint with HED design are established.Based on the results of pushover analysis and dynamic analysis,it is indicated that the residual deformation of the new prestressing frame is extremely small compared to the normal frame,while other dynamic responses of base shear force and ductility demand are almost the same.Followed seismic vulnerability indicates that the possibility of moderate and serious failure in dry-type frame is smaller than normal frame.Similar results are obtained in the analysis between the new prestressing frame and HED-designed frame.The failure possibilities of moderate,serious and completed damage in the frame with HED design are modified compared to the original prestressing frame.