| Freezing-thawing action is one of the main reasons influencing the durability of concrete structures.The degradation of concrete mechanical properties caused by freezing-thawing damage is also one of the main reasons deteriorating the seismic behavior of reinforced concrete(RC)structures.Since early design specifications for concrete structures did not take frost action into consideration,some existing structures are venerable to frost attack,resulting in the degradation of concrete structures’ mechanical properties and seismic performance.For low-shear-span structures,they are especially easy to fail in bending-shear mode due to freezing-thawing action and seismic attack.China has a large number of severe cold areas,where concrete structures are frequently suffering from freezing-thawing action.In addition,China is prone to earthquakes,quantities of architectural structures are under the threats of both frost damage and seismic attack.Therefore,there is a growing demand for conducting elastoplastic earthquake response analysis on low-shear-span RC structures situated in freezethaw and earthquake-prone zone,in order to study the influence of freezing-thawing action on the seismic behavior of RC structures.The main contents this research work are listed as follows:(1)In order to study the seismic behavior of frost-damaged RC frame beam specimens,accelerated freezing-thawing cycle tests were conducted on 6 low-shear-span RC frame beam specimens in artificial climate laboratory.Then quasi static tests were performed on all the specimens.Freezing-thawing cycle and concrete strength grade were taken as experimental variables.Experimental results indicated that freezing-thawing action has a significant impact on specimens’ failure mode,bearing ability,deformation capacity and hysteretic capacity,etc.In addition,the increase of concrete strength grade can weaken the influence of frost damage on the bearing ability and energy-dissipation capacity of RC frame beam specimens.(2)Based on experimental research results,a tri-line skeleton curve of frost-damaged RC frame beam was established,and corresponding computing formulas for characteristic points of skeleton curve was proposed.On the basis of hysteretic energy dissipation,cyclic degradation parameter was introduced to consider the deterioration of basic strength,post-capping strength,unloading stiffness and accelerated reloading stiffness.Therefore,a hysteretic model applicable to frost-damaged RC frame beams was developed.The proposed hysteretic model was used to simulate the hysteretic behavior of the test specimens.A good agreement was achieved by comparing with the experimental results.(3)On the basis of fiber model,a modified constitutive relation of frost-damage concrete was established to replace the concrete stress-strain relationship of fiber model.In the meantime,a shear model with the consideration of frost action was established.Then,through using Limit State Material,Shear Limit Curve and Zero Length Element,shear model was implemented in Open SEES to simulate the shear deformation.A bending-shear fiber numerical model with the consideration of shear deformation is established.For more accurately modeling the seismic response of frost damaged RC frame beams,a bond-slip model incorporating the frost damage was established to consider bond-slip effect.Based on the established fiber numerical model,through using Pinching4 Material and Zero Length Section Element,bond-slip model was implemented in Open SEES to simulate the bond-slip deformation.Finally,a fiber numerical model was proposed with the consideration of both shear deformation and bond-slip.The seismic response of frost-damaged RC frame beams can be accurately modeled,which lays a basis for the advance of the numerical modeling technology for RC structures based on Open SEES software. |
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