Research On Seismic Performance Of Steel Reinforced UHTCC Columns Under Complex Seismic Earthquakes

UHTCC has the characteristics of tensile strain hardening and multi crack development,which also has advantages in crack control and structural seismic resistance.The application of UHTCC instead of ordinary concrete in structural short columns can improve the deformation performance and brittle failure mode to a certain extent,but the previous research is limited to the standard test method,that is,3 loading cycles are used to simulate the typical type II earthquake with short duration and less cycle times,while for the type I earthquake with long duration and many cycle times,as well as other complex earthquake situations such as encountering aftershocks again after the main earthquake,it is almost not involved.For this reason,in order to provide basic data support for improving the seismic design theory of RUHTCC columns,the evolution process of seismic damage and the changes of performance indexes of reinforced UHTCC columns under the action of simulated type I and type II earthquakes and aftershocks are studied.The specific research work and main conclusions are as follows:Referring to the simulation method of type I and type II earthquakes in Japanese performance-based seismic design: current situation and subject,the low cycle reciprocating seismic tests of six UHTCC columns with different shear span ratios and different cycles are carried out,and the tests of two UHTCC columns under monotonic static loading are completed at the same time,and the influence of loading modes and number of loading cycles on the seismic performance of UHTCC columns are analyzed.The results show that for the specimens with higher shear span ratio,the stiffness degradation is slow,the displacement ductility is poor,the bearing capacity at each characteristic point is low,and the deformation is large;the relative ratio of cyclic loading and monotonic loading showed that the skeleton curves under cyclic loading for three times(type II)at each stage of displacement have little difference with the load deformation curves under monotonic loading,but the ultimate deformation of the specimen under cyclic loading is earlier than that under monotonic loading.When the number of cyclic loading for each stage of displacement increases to 5 or 10 times(type I),the skeleton curves decrease faster than that under monotonic loading,and the ductility is obviously reduced;with the increase of the number of loading cycles,the more obvious pinching effect is shown in the hysteretic curve of the specimen,and the ductility decreases gradually.Compared with the specimen with 3 loading cycles(type II),the ductility of the specimen with 10 loading cycles(type I)decreases by 33% and 22% respectively.It can be seen that the deformation performance of specimens under the action of type I earthquake is significantly worse;the number of loading cycles has little effect on the pre peak performance of the specimens,but it can advance the post peak ultimate deformation.In conclusion,it can be seen that the 3 times(type II)cyclic loading in the code standard test can not reflect the seismic damage process of the specimen under the high cycle times(type I),which is more unsafe.In order to study the seismic behavior of UHTCC columns which suffered aftershocks again after the main earthquake,three UHTCC columns with different degrees of damage and one ordinary concrete(RC)contrast piece are completed in this paper.The results show that: with the increase of damage before loading,crack control,the bearing capacity,hysteretic performance,stiffness,cumulative energy consumption and other indexes of the second loading under the same displacement amplitude are significantly reduced compared with that of the first loading;although the performance indexes of UHTCC columns are reduced after damage,the performance of UHTCC columns is still the same as that of unloading,even after 3 times yield displacement damage,the performance of UHTCC columns is better than that of RC columns;the difference of bearing capacity and displacement ductility between UHTCC columns with different damage degree and those without damage is not significant,which shows that UHTCC columns can still maintain good bearing capacity and deformation performance after damage.Based on the test data of UHTCC columns,different damage states are defined and compared with the actual failure states of specimens.The applicability of Kratzig seismic damage model based on energy consumption contribution and Park-Ang damage model considering deformation and cumulative energy consumption to UHTCC columns is discussed.The modified Kratzig damage model is given by introducing ? value,and the different damage states of UHTCC columns are determined Damage index range of state: basically intact: 0-0.15,slight damage: 0.15-0.35,moderate damage: 0.35-0.55,relatively serious damage: 0.55-0.90,damage: 0.90-1.The modified damage model has a clear damage range,which is in good agreement with the test.