Research On Seismic Performance And Design Method Of New Monolithic Precast UHPC-HSC Beam-column Joint And Its Frame Structure

Since the implementation of the “14th Five-Year” Plan,China has been actively promoting prefabricated buildings to achieve industrialization,automation and highquality development of construction.Initiatives such as the “Belt and Road”construction and the “Revitalization of Northeast China” plan have created a conducive environment for the growth of prefabricated buildings.Previous earthquake damage shows that the connection areas of beam-column joints and the longitudinal bars are the key factors in determining the seismic performance of structures.Despite the current shortcomings in existing prefabricated structures,such as poor integrity,weak seismic performance and complex assembly processes,the introduction of new materials and new structural forms can provide new ideas for improving the overall performance of prefabricated structures and better promote the domestic and international infrastructure construction of prefabricated buildings.This research proposes a new monolithic precast Ultra-High Performance Concrete-High Strength Concrete frame structure(UHPC-HSC frame)with UHPC composite beams and HSC columns.The composite beams are made of prefabricated U-shaped UHPC permanent beam formworks and post-casting concrete beam cores.This combination fully utilizes the characteristics of UHPC material,such as ultra-high strength and high durability.Additionally,the use of permanent beam formwork eliminates the need for traditional formwork and support during construction,reducing formwork turnover,construction waste,and improving construction efficiency.The precast columns made of HSC offer reduced component weight,facilitating transportation and on-site lifting.The post-casting UHPC at the the beam-column joints enhances the connection reliability,and improves seismic performance.For the proposed new monolithic precast UHPC-HSC frame,the technical feasibility requires further verification,the seismic performance and the mechanical properties are still unclear,and the design method has not been established.In addition,there is a lack of experimental and theoretical research at the beam-column joints and frame structure levels.Addressing these scientific issues,this study conducts experimental,theoretical,and numerical simulation research on seismic performance and design method of new monolithic precast UHPC-HSC beam-column joint and its frame structure.The specific aspects of this study are as follows:(1)Material level: The optimization design of the mix ratio for UHPC materials was conducted,resulting in water-binder ratio of 0.16 and steel fiber volume content of1%.The basic mechanical properties of UHPC materials under high-temperature steam curing and natural curing conditions were obtained.The cube compressive strengths of UHPC are 131.4MPa and 105.7MPa under the two curing conditions,with corresponding tensile strengths of 6.2MPa and 5.6MPa,and flexural strengths of15.0MPa and 12.5MPa.Additionally,the basic mechanical properties of the concrete and steel bars used in the specimens associated with the beam-column joints and frame structures were acquired.(2)Joint level: The quasi-static test was conducted on three monolithic precast UHPC-HSC beam-column joints and one fully cast-in-place beam-column joint,the seismic performance was evaluated,and the theoretical analysis model was established.The results show that all four beam-column joints exhibit flexural ductile failure at the beam ends,with the UHPC-HSC joints experiencing smaller damage.The seismic performance of the UHPC-HSC joints is superior to that of the fully cast-in-place joint,demonstrating plump hysteresis curves and better energy dissipation capacity.The loadcarrying capacity of the UHPC-HSC joints is increased by 10.84% to 11.48% compared to the fully cast-in-place joint.The UHPC-HSC joints demonstrate good deformation ability,higher stiffness,and better integrity.In addition,the bending moment-lateral drift model for the monolithic precast UHPC-HSC beam-column joints is established based on the plane-section assumption and considering the tensile strength of UHPC.The predicted results are accurate.(3)Frame level: The quasi-static test was conducted on two frames with 1/2-scale,three-story,and two-bay: one monolithic precast UHPC-HSC frame and one fully castin-place frame.The numerical models of the frames were established.The results indicate that both frames experience the formation of plastic hinges at the beam ends,followed by the appearance of plastic hinges in the beams and columns alternately.Both frames exhibit a mixed sidesway mechanism with beam hinges dominating.The UHPC-HSC frame demonstrates better structural integrity,less damage,and is more suitable for post-earthquake repair.The UHPC-HSC frame shows superior seismic performance.The load-carrying capacity of the UHPC-HSC frame is increased by15.78% compared to the fully cast-in-place frame.The UHPC-HSC frame exhibits better ductility and deformation capacity compared to the fully cast-in-place frame.The whole and interstory ductility coefffcients of UHPC-HSC frame are greater than 3.5.The UHPC-HSC frame exhibit higher stiffness and a more uniform stiffness distribution,as well as a more stable lateral load-carrying capacity.Additionally,the numerical models of the frames established using Open SEES software demonstrate good agreement with the experimental results.The numerical models of the frames are established using Open SEES software,and the finite element analysis results are basically consistent with the experimental results.(4)Design method level: The Direct Displacement-Based Design(DDBD)method was employed for the monolithic precast UHPC-HSC frame.Combined with the four levels of ground motion(seismic fortification level),the six seismic performance levels and corresponding damage states of the UHPC-HSC frame were proposed.Four performance objectives for seismic design of the the UHPC-HSC frame were established,and the inter-story lateral drift limits for the six performance levels were quantified as 0.18%,0.20%,0.55%,1.1%,2.5% and 4%,respectively.The DDBD method for the monolithic precast UHPC-HSC frame structure was developed and applied to a six-story,three-span UHPC-HSC frame,which met the expected performance objectives according to nonlinear Pushover analysis.This verified the accuracy and feasibility of the DDBD method for the monolithic precast UHPC-HSC frame structure.