Research On Cumulative Seismic Damage Of CFT Columns

The main purpose of seismic design is to prevent structures from collapse during earthquake to reduce loss of lives and property. Under seismic actions, the induced unrecoverable deformation in critical regions of structures could be attributed to plasticity caused by repeated cyclic loading in the post-yield strain range. There are usually two damage criteria for collapsed structures after earthquake, one is the first passage failure, and the other is cumulative damage failure. Structure collapse is under control of the first criterion when earthquake intensity is well beyond the seismic design level. But for most of collapsed structures, failure is a process from local damage to progressive collapse after a number of cycles. The cumulative damage process can be described as: When a structural is subjected to cyclic loading, it can be assumed that every cycle, whose amplitude exceeds certain threshold amplitude, will cause microstructural changes that bring the structure closer to a state of failure. Although these microstructural changes may not alter visibly the overall response, they constitute damage that accumulates from cycle to cycle. Once the accumulated damage exceeds a specific limit value, failure will take place. So it's necessary to investigate the performance of post-earthquake structures. Usually it is related with some ranges of damage, such as elastic limit, minor damage, repair limit, collapse prevention, et al. This kind of research is quite important for assuring life and property safety.As a type of composite structure, concrete filled steel tubes (CFT) are becoming more widely used in structural applications due to the fact that higher compressive strength and favorable ductile performance is achieved through composite action that is mobilized between the steel tube and concrete core. To investigate the seismic behavior of CFT members, numerous cyclic tests have been carried out in the past decade. Nearly all of them are based on standard loading protocols to obtain the cyclic capacity without considering the effects of amplitude and number of cycles on damage accumulation. In this dissertation, the primary focus is on low-cycle fatigue behaviors and the cumulative damage of CFT columns under simulated seismic loading is studied in detail.Experiments show that CFT columns exhibit stable hysteresis behavior and energy-dissipation capacity. There is no seriously degradation in stiffness, strength, and ductility for CFT columns designed with code even under large plastic deformation. But for CFT columns under cyclic loading with constant amplitude, degradation in hysteresis curves under larger lateral deformation become more quickly, and energy-dissipation capacity decreases too. It proves that larger deformation at CFT columns exerts more damage on it. Test data from strain gauge shows that strain grows quickly at plastic hinge area of CFT columns when drift ratio increases. The unrecoverable plastic strain of steel tube at the maximum deformation increased gradually as the number of cycles adds when under symmetrical constant cyclic loading. Test data reveals that the maximum strain of steel tube reaching fracture state is much lower than that of same kind of steel under monotonic test. The reason is that cyclic tension and comparison stress on steel tube have exerted fatigue damage on it. As damage is cumulated to certain state, the steel tube of CFT columns is fractured by tension.Based on the test, fatigue lives of CFT columns with thicker steel tube and thinner ones are quite different at small deformation. Experimental results showed that CFT specimens with thicker steel tube perform much better than the counterpart specimens with thinner steel tube corresponding to small deformation, whereas the opposite trend was true for cyclic tests under larger deformation. The cyclic drift amplitude has a significant effect on the energy dissipation capacity of CFT columns. Increasing the plastic drift amplitude reduces the energy dissipation capacity of the specimens. The imposed loading path has an effect on the total energy absorption capacity of CFT columns, but it has marginal effect on the cumulative damage of the column. The damage per cycle on CFT columns varies even if the component is subjected to constant drift amplitude. Test results indicate that a linear relationship exists for the energy-based damage parameter and cycle number in a log-log coordinate space. A fatigue life equation was obtained for CFT columns based on the Manson-Coffin relationship. The curves about deformation and fatigue life are gained with the new model. Comparative study showed that the proposed equation provides a reasonable estimation to the fatigue lives of testing results from literature with acceptable conservatism in most of the cases. The cumulative damage behaviors are studied with the famous damage model Miner's rule. Finally a modified fatigue life equation and an energy based cumulative damage model are obtained for CFT columns. Compared with other damage models, the proposed model has the advantage of being simple to apply and permits seismic performance assessment for any arbitrary loading history. Based on the concepts of continuum damage mechanics and standard inelastic analysis of CFT columns, a lumped damage model is provided for modeling the hysteretic behavior of CFT columns. The CFT columns are modeled as the assemblage of an elastic beam-column and two inelastic hinges. Three sets of damage variables, which measure the state of damage of the member, are introduced. An expression for the flexibility matrices and the complementary strain energy of a member are proposed as a function of these internal variables. The damage evolution laws for damaged CFT member under monotonic loading and cyclic loading are formed. The model is verified by simulating experimental data, and satisfying results are gained. The model is welldefined in physics and mechanic, and also it is very simple in nature which makes it convenient for damage analysis of CFT columns.