Variation of Small Amplitude Vibration Dynamic Properties with Displacement in Reinforced Concrete Structures

Correlating damage level and changes in dynamic characteristics of a structure forms the basis for damage detection techniques in structural health monitoring. In reinforced concrete building structures such correlation is not well established. A damage detection technique capable of identifying the structural condition of the system based on its small amplitude vibration response is desirable because such response is easier to obtain. It is a common practice in engineering applications to estimate dynamic parameters from small-amplitude vibrations assuming a linear behavior of the structure. This simplification causes inaccurate estimation of the dynamic properties in reinforced concrete structures due to the presence of nonlinear elastic behavior. In this study no such assumption is made and a linear model is only used for sets of data corresponding to the same displacement amplitude of a nonlinear elastic system. The trends found between small-amplitude vibration dynamic properties and past levels of maximum displacement in various reinforced concrete structures are reported. In addition to analytical and numerical studies, results from a series of laboratory tests are reported to demonstrate the use of the approach. One full-scale three-story reinforced concrete flat-plate building and six small-scale reinforced concrete beams were examined. In this study, small displacements are defined as displacements below an overall drift ratio of 0.03%. The displacement dependence of the dynamic properties is considered explicitly. It was found that while fundamental frequencies of the examined reinforced concrete specimens were found to decrease uniformly as past peak displacement level increased, the equivalent viscous damping ratio was found to increase until the past peak displacement reached the neighborhood of nominal yield displacement and then observed to decrease when the specimens are pushed beyond the nominal yield displacement level, which has not been reported in literature before. Recommendations are provided as to how small amplitude vibration tests should be set up to avoid misleading observations due to nonlinear response at small amplitude response, observations that could lead to erroneous conclusions regarding the damage state of a structure.     

钢筋混凝土结构破坏过程的近场动力学模拟

采用近场动力学(Peridynamics,PD)方法对钢筋混凝土结构破坏过程进行模拟,在"键"型近场动力学模型的基础上,考虑物质点对间的转动以突破泊松比的限制,采用能够描述混凝土材料的拉压异性和断裂特征的损伤模型,引入动态松弛、分级加载、平衡收敛准则和冲击接触等算法,建立了能准确描述钢筋混凝土结构破坏的近场动力学模型。模拟了钢筋混凝土梁在不同荷载作用下破坏的全过程,裂纹扩展路径以及最终破坏形式均与试验结果吻合,为解决工程结构破坏问题提供了一种有效的方法。     

爆炸地震动的反应谱拟合

本文提出采用拟动力试验方法解决爆破工程和防护工程中由于炸药爆炸和核爆炸地震动而引起的安全问题和结构抗震问题,并提出采用该方法时应解决的爆炸地震动的设计反应谱和爆炸地震动人工模拟的反应谱拟合技术。     

Residual stiffness assessment of structurally failed reinforced concrete structure by dynamic testing and finite element model updating

When an under-reinforced concrete beam structure has been loaded to the point where reinforcing steel on the tension side has yielded, it is deemed to have structurally failed and the full load capacity and stiffness can no longer be developed. When unloaded from the point of failure, the residual stiffness of the structure is difficult to estimate. There is a need to establish the serviceability of the structure and ultimately establish the levels of further loading that can be sustained before total collapse. In this paper we present a method for assessing residual stiffness of such a “failed” reinforced concrete structure, through the application of dynamic testing and finite element (FE) model updating. In an experimental study, failed zones in a beam structure were simulated in a FE model. Through a procedure of sensitivity-based updating using the measured modal properties, the stiffness distribution along the failed beam structure was identified.     

高层钢-混凝土混合结构地震作用下的能量反应分析

针对目前高层建筑混合结构中常用的外钢框架-内混凝土核心筒混合结构,从地震能量输入和结构能量耗散的角度对其进行了地震作用下的能量反应分析.通过选取不同的地震动参数对混合结构的总输入能及其在滞回耗能和阻尼耗能之间的分配情况进行了研究,明确了地震动各参数的变化对混合结构能量反应的影响;通过分析不同地震波作用下混合结构的滞回耗能在外钢框架和内混凝土核心筒间的分布以及在结构层间的分部情况知,底层剪力墙是整个混合结构中滞回耗能比较集中的区域,地震破坏最为严重.通过本文研究知,地震反应的能量分析方法是一种能较好反应混合结构在地震作用下的地震反应全过程及其弹塑性性能的方法.     

NUMERICAL SIMULATION OF CRACK GROWTH OF REINFORCED CONCRETE FLEXURAL MEMBERS BASED ON XFEM1)

基于最大主应力准则,利用扩展有限元法对钢筋混凝土三点弯曲梁的裂纹扩展进行数值模拟。通过与已有模型试验结果的比对,验证了该方法的准确性与有效性。研究发现,带裂纹钢筋混凝土构件的裂纹扩展与力学性能不但受到单裂纹的尺寸、位置、倾角的影响,而且还受多裂纹间位置关系的影响;其中,裂纹横向分布位置和裂纹纵向分布位置是影响带裂纹钢筋混凝土梁裂缝扩展的主要因素。     

基础隔震系统对核电站安全壳抗震的影响

为了确保核电站在遭受破坏性的地震后,安全壳要保持密封性且不被破坏,基于地震波在结构中的传播规律,从隔震技术的原理出发,建立较为准确的三维安全壳有限元模型,运用定性的方法对核电厂安全壳进行数值模拟,对比了极限安全地震动作用下采取隔震技术和不采取隔震技术安全壳的动力响应。采取隔震措施的安全壳的顶点在x、y、z方向的最大加速度分别为2.85、12.84和3.05m/s2,相比于无隔震措施的安全壳,加速度分别降低了79.52%、27.56%和79.47%。结果表明,隔震技术能有效地减小核电站安全壳的地震反应。     

双向拟静力加载规则对钢筋混凝土高墩滞回性能的影响

通过四种不同双向加载规则的钢筋混凝土高墩拟静力试验,初步研究了双向拟静力加载规则对其滞回特性的影响规律。本文主要介绍钢筋混凝土高墩的试验构件设计,加载装置、测试方法和加载规则,着重比较了对角线加载、正方形加载、菱形加载和圆形加载等四种加载规则的钢筋混凝土高墩的滞回性能,包括荷载-位移滞回曲线、荷载-位移骨架曲线、荷载退化、刚度退化以及累积滞回耗能的特点,并分析这些特点与双向加载规则之间的关系。结论认为:对角线加载模式加载效率高且能较好反映高墩双向抗震滞回性能,适合于高墩的双向拟静力试验研究。     

框架梁试验台的研制与应用

介绍了新型框架梁试验台的原理和构造, 该装置能够有效地在梁端产生轴 向和转动约束来模拟框架梁的实际受力状态. 应用该装置对钢筋混凝土框架梁进行了试验研 究, 试验结果证明了理论分析的结果, 并表明该试验台稳定可靠, 测试数据准确.     

冻融和疲劳对预应力受弯构件损伤的影响

通过15根预应力及普通混凝土试验梁在经历不同冻融循环次数后的疲劳试验, 探索试验梁 相对动弹模衰减规律, 以相对动弹模为损伤变量, 通过方差分析方法定量分析冻融和疲劳两 种因素对构件损伤的影响. 研究表明试验梁在冻融和疲劳荷载双因素作用下, 初期以冻融损 伤为主, 之后很快发展为以疲劳损伤为主, 疲劳损伤是构件的主要损伤因素. 施加预应力或 提高预应力度对试验梁疲劳损伤有较明显的抑制作用, 将缩短试验梁全寿命中以疲劳损伤为 主的时间.     

Experimental study and analysis of RC beams strengthened with CFRP laminates under sustaining load

Six reinforced concrete beams strengthened in flexure using carbon fiber reinforced polymer (CFRP) laminates subjected to different sustaining loads were tested. The main goal of the test is to examine the effects of initial load and load history on the ultimate strength of strengthened reinforced concrete beams by externally bonded CFRP laminates. The main experimental parameters include different levels of sustaining load at the time of strengthening and load history. To explain the experimental results in quantitative terms, a theoretical model for flexural behavior of the strengthened reinforced concrete beam is also developed. Test results in the current study show that sustaining load levels at the time of strengthening have important influence on the ultimate strength of strengthened reinforced concrete beams. If the initial load is basically same, the ultimate strength of reinforced concrete beams strengthened with CFRP laminates is almost same regardless of load history at the time of strengthening.     

EXPERIMENTAL. MODAL. ANALYSIS OF REINFORCED CONCRETE STRUCTURES

Experimental modal analysis techniques have been shown to be applicable to both laboratory test specimens and in situ test structures made of reinforced concrete. These techniques, in general, apply only to linear structures; however, many concrete structures are designed to remain in the linear, uncracked response region during dynamic excitation (nuclear power plant structures, for example). Data from the experimental analyses agreed well with finite-element modal analysis results, and the numerical models were further refined based on the experimental results. Because of the relatively low excitation levels required, these methods provide engineers with techniques for assessing the as-built condition of a structure without introducing damage into the structure. If a concrete structure is damaged, the experimental modal analysis methods could possibly be used to monitor its deterioration.
Further investigations are needed to evaluate the sensitivity to damage of the experimentally determined modal properties. Also, methods must be found to determine, without prior modal data, if an in-situ structure is in a damaged state. These topics are being pursued by other researchers in the experimental modal analysis field.⁶
In its current form, experimental modal analysis methods can provide both practicing and research engineers with a valuable tool for verifying dynamic properties of reinforced concrete structures.     

Changes in modal parameters of a bridge during fatigue testing

The effects of high-cycle fatigue on the dynamic modal properties of a full-scale bridge test section are examined in this study. A span 30.5 m (100 ft) in length was salvaged from an interstate bridge for experimental testing to establish the fatigue performance of the retrofit procedures which could be used for future field repairs of the fatigue cracks. Horizontal cracks had formed in the webs of the twin bridge girders in the vicinity of the web-flange weld at or near the connections of the girders to the floor beams. An electrohydraulic vibration shaker was used to excite the test span in the torsional and bending modes to promote fatigue crack growth. In addition to the experimental evaluation of the retrofit procedures, the dynamic response characteristics of the test span during the high-cycle fatigue test were monitored. Experimental data have been used to establish mode shapes and modal damping ratios during the four-million-cycle fatigue test. Damping ratios ranged from 1.09 percent to 0.53 percent. Modal stiffnesses were calculated based on the experimental mode shapes. There is a modest correlation between the changes in stiffness and the observed cracking of the steel girders and the subsequent repairs. Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Porland, OR on June 5–10.     

Passive effects of rare-earth permanent magnets on flexible conductive structures

The paper presents a theoretical and experimental study of vibrating structures where paramagnetic or diamagnetic systems interact with rare-earth passive magnets.The theoretical model of the system is focused on the damping properties of permanent magnets and on their interactions with the dynamic behaviour of an Euler–Bernoulli beam. In particular, the magnetic model is based on the analogy of the equivalent currents method in a quasi-static open-circuit-type configuration and it is used to determine the influence of eddy currents on the dynamic behaviour of conducting material structures. The magnetic effects are characterised by a viscous-type damping and by a stiffening dynamic effect of the structure, called “phantom effect”.The authors present the experimental outcomes for uniform cantilever clamped-free beams of different kinds of paramagnetic or diamagnetic conducting materials. It appears that the system frequency response can be modified by the presence of a pair of concordant or discordant permanent magnets of high residual induction settled at the free end.Through the comparison between theoretical and experimental results, the paper demonstrates the validity of the model, that is able to describe both the above mentioned effect of dynamic stiffening of the structure and the considerable localised damping properties in paramagnetic or diamagnetic materials having low electric resistivity.     

水工结构高强钢筋混凝土梁受剪性能试验研究

通过对12根配置高强箍筋的混凝土梁在集中荷载作用下的受剪试验,研究梁的挠度、箍筋应变、斜裂缝扩展规律及破坏形态。依据现行《水工混凝土结构设计规范》,对比分析试验梁斜截面承载力及考虑荷载长期作用的正常使用阶段斜裂缝宽度。研究结果表明,配置高强箍筋的混凝土梁斜向开裂规律和受力特征与普通钢筋混凝土梁基本相同,受剪承载力可按现行《水工混凝土结构设计规范》公式计算,且有较高安全储备。当考虑荷载长期作用,HRBF500钢筋在水工结构中抗拉强度设计值取360MPa时,斜裂缝宽度满足正常使用极限状态要求。     

基于无单元法的平面框架地震反应分析

无单元法在动力计算领域中的应用目前多以梁、板等简单构件的自振特性分析为主,对建筑结构的动力计算,尤其是地震反应分析尚未涉及.文中推导了无单元法的离散动力方程,以轴力杆、欧拉梁模型为基础,将简单构件的无单元动力计算推进到实际结构的地震反应分析,并给出了数值实现的基本流程.最后以一个十一层钢筋混凝土高层结构为算例,实现了多层多跨平面框架的无单元地震反应分析,达到了预期的效果,验证了无单元法应用于平面框架地震反应分析的可行性和有效性.     

FATIGUE LIFE PREDICTION OF RC BEAMS STRENGTHENED WITH PRESTRESSED CFRP UNDER CYCLIC BENDING LOADS

The application of prestressed carbon reinforced polymer(prestressed CFRP)in reinforced concrete(RC)members can improve the mechanical properties of strengthened structures and strengthening efficiency.This paper proposed a semi-empirical prediction formula of fatigue lives of the RC beams strengthened with prestressed CFRP under bending loads.The formula is established based on the fatigue life prediction method of RC beams and fatigue experimental data of non-prestressed CFRP reinforced beams done before.Fatigue effect coefficient of the formula was confirmed by the fatigue experiments of the RC beams strengthened with prestressed carbon fiber laminate(prestressed CFL)under cyclic bending loads.Fatigue lives of the strengthened beams predicted using the formula agreed well with the experimental data.     

防护结构设计中应变速率效应问题

在防护结构设计中，通常是采用材料的动力强度和动力弹性模量以考虑应变速率效应问题。应用所提出的应变速率相关材料模型，对核武器及常规武器爆炸作用下的钢结构和钢筋混凝土结构进行速率相关分析，并与考虑材料强度提高的速率无关分析对比。两种分析结果表明，对于钢结构，两种计算结果相差不大；而对于钢筋混凝土结构．两种计算结果相差较大，而且目前防护结构设计方法偏于保守。     

计及剪切影响的RC梁的非线性分析

为提高钢筋混凝土RC梁的计算效率和精度,提出了一种基于梁截面弯矩-曲率关系的宏观有限元方法,可用于各种跨高比RC梁的材料非线性分析。首先假定了混凝土和钢筋的非线性应力-应变关系,然后引入经过修正的Rodriguez截面模型,根据边界顶点把截面划分成若干梯形单元,利用quasi-Newton法求解由两个变量耦合而成的截面非线性平衡方程,由此建立RC截面的弯矩-曲率关系。在此基础上利用Timoshenko梁弯曲理论建立考虑横向剪切变形影响的RC梁的有限元分析模型。通过对试验梁的分析对比验证了所提出的分析方法的适用性。     

Experiment as an aid to structural seismic design

An overview of the available methods for simulating or estimating seismic effects on buildings and similar structural models as well as onin situ structures is given. The motivation is provided by noting severe damage even to modern buildings in the wake of strong earthquakes. The complex inelastic behavior of members and connections must be determined experimentally. Such information is essential for developing mathematical design models. Four available test methods, with examples, are described. One of these is the pseudo-static method of testing where a specimen is subjected to apriori selected cyclic forces or displacements. The second, the pseudo-dynamic method of testing, combines the above procedure with an on-line computer. In employing this approach it is possible to apply a random sequence of displacements corresponding to a previously recorded earthquake while taking into account the continuously changing structural stiffness. The third procedure employs a shaking table programmed to apply scaled earth-quake input based on available accelerograms. The fourth, a nondestructive method of testing of actual structures, consists of recording and interpreting ambient or forced vibrations in the elastic regime.