【作者】 张国伟；

【导师】 肖岩； 黄政宇；

【作者基本信息】 湖南大学 ， 结构工程， 2009， 博士

【摘要】 抗震设防目标之一就是确保结构在地震作用下不会倒塌,尽量减少人员的伤亡和财产的损失。地震时,结构受到低周往复荷载的作用,使得结构构件的塑性铰区域产生不可恢复的塑性变形。当地震烈度远远超过了结构的抗震设防烈度时,结构会发生首次超越破坏;大部分结构从产生微小的局部破裂到全部倒塌都需要一个过程,在弹塑性变形下结构往复循环几次乃至十几次才会倒塌,也就是发生累积损伤破坏。因此,需要考察地震后结构及其构件的累积损伤程度,判断是否能够继续使用或者是修复后可以继续使用这些结构,还是需要推倒重建。这项工作对于保障人民的生命和财产安全,减少地震损失具有重要的意义。钢管混凝土作为组合结构的一种组合设计形式,具有良好的力学性能和抗震性能,在地震区建筑中得到了广泛应用。对钢管混凝土抗震性能的研究一直是国内外的研究热点,许多研究者通过抗震试验对钢管混凝土的抗震性能进行了探讨。但是大部分研究者的侧重点都是放在钢管混凝土的承载力、滞回性能以及建立滞回模型的研究上,很少考虑低周往复荷载下钢管混凝土的疲劳损伤,以及由于疲劳损伤的累积而导致的钢管混凝土的累积损伤破坏。本文针对钢管混凝土压弯构件在低周往复荷载下的力学性能和疲劳累积损伤问题进行了较为系统的试验研究和理论分析。试验结果表明钢管混凝土具有良好的抗震滞回性能和耗能能力,即使钢管混凝土产生了较大的变形也不会发生严重的承载力退化现象。钢管混凝土柱滞回曲线的荷载和刚度退化随着侧移率的增大而加快,说明在较大的变形条件下,往复变形一次对钢管混凝土造成的损伤也大;随着侧移率的减小,钢管混凝土的滞回耗能能力明显增强。应变分析也表明,钢管混凝土柱塑性铰区的钢管在屈服后随着侧移率的增大应变增长迅速;而在等幅循环荷载的作用下,钢管的应变增长缓慢,但永久塑性变形随着循环次数的增加在不断增长。试验研究表明,承受低周往复荷载作用的钢管混凝土柱在破坏时钢管断裂处的应变并没有达到单调拉伸时的极限应变,钢管是在拉应力和压应力的反复作用下产生了疲劳损伤,当钢管的损伤累积到一定程度时,最终发生疲劳断裂。根据试验结果,本文建立了反映钢管混凝土柱低周疲劳寿命的改进Manson-Coffin关系模型,得出了钢管混凝土柱变形和疲劳寿命的关系曲线;与试验数据相比,理论曲线能够比较合理地反映这几类钢管混凝土柱在不同变形条件下的疲劳寿命。在试验数据的基础上,建立了基于循环耗能的低周疲劳模型;与试验得出的耗能能力系数相比,耗能能力曲线能够较准确的反映钢管混凝土在低周往复荷载作用下,由于疲劳损伤的累积而导致的力学性能的退化。综合Miner的线性累积损伤理论和钢管混凝土柱的循环耗能规律,建立了钢管混凝土柱的基于耗能的改进线性累积损伤模型;与Miner模型的评估结果相比,基于每次循环耗能所得出的钢管混凝土柱的累积损伤指数具有更高的精度,特别是对于承受变幅位移的钢管混凝土柱来说,由模型可以得到等合理地损伤指数,说明这种累积损伤模型能够用于地震作用后的钢管混凝土的累积损伤评估。基于连续介质损伤力学的基本理论和钢管混凝土的弹塑性分析,建立了钢管混凝土构件在单调荷载和低周往复荷载下的简化损伤梁单元模型。钢管混凝土构件被简化成由中间的弹性梁和两端的集中耗能铰构成的模型,引入损伤参数,分别描述钢管混凝土两端耗能铰的损伤和轴向损伤过程,推导了考虑损伤参数的钢管混凝土构件的柔度矩阵和应变余能方程,给出了钢管混凝土损伤构件在单调荷载和低周往复荷载下的损伤演化过程。该损伤模型形式简单,具有明确的物理和力学意义,比较适合用于地震作用下的钢管混凝土柱的损伤分析。更多还原

【Abstract】 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.更多还原