Seismic Performance Research Of Freeze-thaw Damaged Rc Column And RC Frame Structures

Reinforced concrete(RC)structures are damaged due to insufficient durability,causing significant losses to people's lives and property in all countries around the world.Among them,the degradation of concrete mechanical properties caused by freeze-thaw damage has attracted widespread attention from scholars at home and abroad,and certain research progress has been made.However,based on the research results of freeze-thaw damage at the concrete material level,it is still difficult to objectively predict the degradation of mechanical and seismic performance of RC members and even structures after subjected to freeze-thaw cycles.There is few research concentrated on the cross-fields of concrete freeze-thaw durability and seismic performance of components at home and abroad.Therefore,it is necessary and urgent to conduct research on the seismic performance of reinforced concrete structures under freeze-thaw environment.The research on the seismic performance of RC columns damaged by freezing and thawing is taken as the starting point,and then extended to the seismic performance of RC frame structures with RC columns as lateral resistance members.The main contents this research work are listed as follows:(1)The artificial climatic environment accelerated freeze-thaw experiment technology was used to perform freeze-thaw cycle tests on 13 RC column specimens.The development process of freeze-thaw damage at the material level and the component level were observed during the procedure of freeze-thaw test.Then the specimens were subjected to pseudo-static testing.The test results show that freeze-thaw cycles would significantly influence the seismic performance of RC columns,in terms of failure patterns,hysteresis curve,bearing capacity,deformation capacity and energy consumption capacity.Also,the seismic behavior of RC columns under different design parameters varies with the freeze-thaw damage.(2)Based on the test results of mechanical properties of freeze-thaw concrete materials presented in this research and in existing literatures,a calculation method of equivalent freeze-thaw cycles under different freeze-thaw test environments was established.Also,a model was proposed for deteriorating mechanical properties of concrete considering the progression and extent of freeze-thaw damage.Combined with the fiber section analysis method,a freeze-thaw damaged fiber beam-column model was developed.Overall,the proposed model were verified to capture the envelope of test hysteresis curve relatively well.(3)Based on the calculation method of bond slip in the anchorage area of RC members,a bond slip model considering the the progression and extent of freeze-thaw damage was established through theoretical derivation.The model was validated by comparing the data from frozen-thawed bar pullout experiments.Then the proposed model was implemented to the zero-length section for formulating the modeling method for RC columns considering the uneven distribution of freeze-thaw damage and reinforcement slip effect.The simulation results were verified to agree well with the experimental hysteresis curve.(4)The calculation formulas of the characteristic points of the RC column shear skeleton curves at home and abroad were collected.And based on the model of the mechanical properties of concrete considering the progression of freeze-thaw damage established in this research,the calculation method of the shear skeleton curve parameters of the freeze-thaw RC column was built.The shear limit curve was modified to account the degradation effect of freeze-thaw RC column shear capacity with increasing horizontal loading displacement.A numerical simulation method for freeze-thaw RC columns suitable for flexure-shear or shear-type failure was proposed by introducing the shear spring element into the freeze-thaw damaged fiber beam-column model considering reinforcement slip effect.The simulation results were verified to agree well with the experimental hysteresis curve of specimens failed in flexure-shear mode.(5)The division method of RC column failure mode and the selection method of seismic performance index of RC column under different failure modes were proposed.Seismic performance level of RC columns were divided according to the degree of material damage.Furthermore,based on the numerical simulation method of RC column considering the progression of freeze-thaw damage,the failure mode discrimination method as well as the calculation formula for the seismic performance index of freeze-thaw RC columns under different failure modes were established through pushover analysis with the coupling effect of the degree of freeze-thaw damage and design parameters.Finally,considering the uncertainty of the mechanical properties of the materials,the Monte Carlo sampling method was used to establish the fragility curve of the freeze-thaw damaged RC column components.(6)The RC frame structure designed in accordance with Chinese current standards is taken as the research object.Considering the main parameters affecting the seismic performance of the structure,six typical RC frame structures with different fortification intensity and number of layers were designed.Based on the beam-column fiber model that considers the effects of freeze-thaw damage penetration,a numerical model of the typical structure is established,and the probabilistic seismic demand analysis of each typical structure is realized by inputting the amplitude ground motion record set.According to the vulnerability of frozen-thawed RC column,the thresholds of different damage states of corresponding RC frame structures are determined,and finally the vulnerability curves of RC frame structures with different freeze-thaw damage levels,different layers and different fortification intensity are established.