Equivalent linear model for seismic damage evaluation of single-degree-of-freedom systems representing reinforced concrete structures considering cyclic degradation behavior

In this study,a novel equivalent damping ratio model that is suitable for reinforced concrete(RC)structures considering cyclic degradation behavior is developed,and a new equivalent linearization analysis method for implementing the proposed equivalent damping ratio model for use in seismic damage evaluation is presented.To this end,Ibarra’s peak-oriented model,which incorporates an energy-based degradation rule,is selected for representing hysteretic behavior of RC structure,and the optimized equivalent damping for predicting the maximum displacement response is presented by using the empirical method,in which the effect of cyclic degradation is considered.Moreover,the relationship between the hysteretic energy dissipation of the inelastic system and the elastic strain energy of the equivalent linear system is established so that the proposed equivalent linear system can be directly integrated with the Park-Ang seismic model to implement seismic damage evaluation.Due to the simplicity of the equivalent linearization method,the proposed method provides an efficient and reliable way of obtaining comprehensive insight into the seismic performance of RC structures.The verification demonstrates the validity of the proposed method.

Evaluation of collapse resistance of RC frame structures for Chinese schools in seismic design categories B and C

According to the Code for Seismic Design of Buildings （GB50011-2001）, ten typical reinforced concrete （RC） frame structures, used as school classroom buildings, are designed with different seismic fortification intensities （SFIs） （SFI=6 to 8.5） and different seismic design categories （SDCs） （SDC=B and C）. The collapse resistance of the frames with SDC=B and C in terms of collapse fragility curves are quantitatively evaluated and compared via incremental dynamic analysis （IDA）. The results show that the collapse resistance of structures should be evaluated based on both the absolute seismic resistance and the corresponding design seismic intensity. For the frames with SFI from 6 to 7.5, because they have relatively low absolute seismic resistance, their collapse resistance is insufficient even when their corresponding SDCs are upgraded from B to C. Thus, further measures are needed to enhance these structures, and some suggestions are proposed.

Analytical Modeling for Corrosion-Induced Cover Cracking of Corrosive Reinforced Concrete Structures

An analytical model for predicting the corrosion-induced cracking of concrete cover of reinforced concrete(RC) structures was developed.The effects of influence factors such as practical initial defects,corrosion rate,strength and elastic modulus of concrete on the corrosion-induced cracking of concrete cover were investigated.It was found that the size of practical initial defects was the most effective factor.Therefore,improving the compactness of concrete is an effective way to improve the durability of RC structures.It was also demonstrated that the accelerated corrosion tests may be unfavorable in the study of the relationship between cracking time and crack width.

Nonlinear damage model for seismic damage assessment of reinforced concrete frame members and structures

A nonlinear damage model based on the combination of deformation and hysteretic energy and its validation with experiments are presented.Also,a combination parameter is defined to consider the mutual effect of deformation and hysteretic energy for different types of components in different loading stages.Four reinforced concrete (RC) columns are simulated and analyzed using the nonlinear damage model.The results indicate that the damage evolution evaluated by the model agrees well with the experimental phenomenon.Furthermore,the seismic damage evolution of a six-story RC frame was analyzed,revealing four typical failure modes according to the interstory drift distribution of the structure;the damage values calculated using the nonlinear damage model agree well with the four typical failure modes.

Achievements of Truss Models for Reinforced Concrete Structures

Achievements are presented for truss models of RC structures developed in previous years: 1. Two constitutive models, biaxial and triaxial, are based on regular trusses, with bars obeying nonlinear uniaxial σ-ε laws of material under simulation;both models have been compared with test results and show a dependence of Poisson ratio on curvature of σ-ε law. 2. A truss finite element has been used in the nonlinear static and dynamic analysis of plane RC frames;it has been compared with test results and describes, in a simple way, the formation of plastic hinges. 3. Thanks to the very simple geometry of a truss, the equilibrium equations can be easily written and the stiffness matrix can be easily updated, both with respect to the deformed truss, within each step of a static incremental loading or within each time step of a dynamic analysis, so that to take into account geometric nonlinearities. So the confinement of a RC column is interpreted as a structural stability effect of concrete. And a significant role of the transverse reinforcement is revealed, that of preventing, by its close spacing and sufficient amount, the buckling of inner longitudinal concrete struts, which would lead to a global instability of the RC column. 4. The proposed truss model is statically indeterminate, so it exhibits some features, which are not met by the “strut-and-tie” model.

Progressive collapse resisting capacity of reinforced concrete load bearing wall structures

Reinforced concrete(RC) load bearing wall is widely used in high-rise and mid-rise buildings. Due to the number of walls in plan and reduction in lateral force portion, this system is not only stronger against earthquakes, but also more economical. The effect of progressive collapse caused by removal of load bearing elements, in various positions in plan and stories of the RC load bearing wall system was evaluated by nonlinear dynamic and static analyses. For this purpose, three-dimensional model of 10-story structure was selected. The analysis results indicated stability, strength and stiffness of the RC load-bearing wall system against progressive collapse. It was observed that the most critical condition for removal of load bearing walls was the instantaneous removal of the surrounding walls located at the corners of the building where the sections of the load bearing elements were changed. In this case, the maximum vertical displacement was limited to 6.3 mm and the structure failed after applying the load of 10 times the axial load bored by removed elements. Comparison between the results of the nonlinear dynamic and static analyses demonstrated that the "load factor" parameter was a reasonable criterion to evaluate the progressive collapse potential of the structure.

New Factor to Characterize Mechanism of “Strong Column-Weak Beam” of RC Frame Structures

Most reinforced concrete(RC)frame structures did not achieve the "strong column-weak beam" failure mode in recent big earthquakes, resulting in a large number of casualties and significant property loss. To deal with this serious problem, a new column-beam relative factor was proposed to characterize the relative yield situation of column ends and beam ends. By limiting the column-beam relative factor, RC frame structures could achieve the "strong column-weak beam" failure mode under the excitation of strong ground motions. The limit values of column-beam relative factor were calculated, analyzed and verified by using structural simulation models for corner columns in the bottom story of structures, which are destroyed most seriously in earthquakes. The results show that the limit values should be analyzed under bi-directional ground motion and with different axial compression ratios of columns. The peak ground acceleration(PGA)of ground motions has no significant effect on the limit values, while the type of strong ground motions has a significant effect on the limit values.

Estimation of Aleatory Randomness by S_(a)(T_(1))-Based Intensity Measures in Fragility Analysis of Reinforced Concrete Frame Structures

Based on the multiple stripes analysis method,an investigation of the estimation of aleatory randomness by S_(a)(T_(1))-based intensity measures(IMs)in the fragility analysis is carried out for two typical low-and mediumrise reinforced concrete(RC)frame structures with 4 and 8 stories,respectively.The sensitivity of the aleatory randomness estimated in fragility curves to various S_(a)(T_(1))-based IMs is analyzed at three damage limit states,i.e.,immediate occupancy,life safety,and collapse prevention.In addition,the effect of characterization methods of bidirectional ground motion intensity on the record-to-record variability is investigated.It is found that the damage limit state of the structure has an important influence on the applicability of the ground motion IM.The S_(a)(T_(1))-based IMs,considering the effect of softened period,can maintain lower record-to-record variability in the three limit states,and the S_(a)(T_(1))-based IMs,considering the effect of higher modes,do not show their advantage over S_(a)(T_(1)).Furthermore,the optimal multiplier C and exponentαin the dual-parameter ground motion IM are proposed to obtain a lower record-to-record variability in the fragility analysis of different damage limit state.Finally,the improved dual-parameter ground motion IM is applied in the risk assessment of the 8-story frame structure.

Study on the Uncertainty of Earthquake Damage Prediction Based on Damage Indices for RC Structures

Seismic damage indices of structure are widely used to quantificationally analyze structural damage levels under earthquake action. In this paper, a five-storey building model and a seventeen-storey building model are established. According to seven typical indices and different earthquake-inputs, a structural damage prediction is performed, with the results showing serious uncertainty of structural damage prediction due to different indices. Understanding of this phenomenon aids the comprehension and application of the results of earthquake damage prediction.

考虑填充墙的RC框架结构性能指标限值确定方法

填充墙钢筋混凝土(RC)框架结构是我国应用最为广泛的结构形式之一,而我国规范中给出的此类结构性能指标限值并未充分考虑填充墙对整体结构破坏状态的影响。遵循我国规范中对结构破坏状态的定义原则,提出基于构件破坏率的填充墙RC框架结构性能指标限值确定方法,为此类结构破坏状态与工程需求参数对应关系的研究提供新的思路。以典型结构为例建立数值分析模型,采用该文方法计算考虑填充墙的RC框架结构性能指标限值,并与不考虑填充墙的“纯框架”限值进行了对比分析。相较于“纯框架指标”,该文指标完好、轻微破坏、中等破坏和严重破坏四个破坏状态的层间位移角上限值均更小,其中,完好状态的上限值为1/832,远小于规范中规定的弹性层间位移角限值1/550,说明判定填充墙RC框架结构的破坏状态时,直接应用规范限值会低估结构的破坏程度。

柔性连接填充墙RC框架结构非线性有限元分析

为了研究橡胶作为新型柔性连接材料对填充墙钢筋混凝土(RC)框架结构抗震性能的影响,在7榀1/2缩尺填充墙RC框架结构低周反复加载试验的基础上,采用DIANA有限元软件分析了填充墙与框架之间的连接方式、砌块材料、柔性连接材料以及砂浆强度对填充墙RC框架结构抗震性能的影响。结果表明:与刚性连接相比,以橡胶作为新型柔性连接材料,能减缓结构的强度和刚度退化,极大降低结构的破坏程度,显著提高结构的抗震性能;高弹性橡胶和高阻尼橡胶作为柔性连接材料均表现出优异的抗震性能,而聚苯板(EPS)作为柔性连接材料的抗震性能要比橡胶差;刚性连接方式下不同砌块材料对结构抗震性能影响很大,当使用强度较高的砌块材料时建议采用柔性连接方式以减弱墙-框相互作用;合理降低砂浆的强度等级,可以改变填充墙的破坏模式,对抗震有利。

基于卷积神经网络的RC框架通信机楼震后损伤评定方法

为解决震后大量钢筋混凝土框架通信机楼损伤评定需求,基于卷积神经网络研究从构件层次至整体结构的损伤评定方法。首先对汶川地震、鲁甸地震、芦山地震等多次地震后大量钢筋混凝土框架结构损伤调查图片筛选处理,建立了钢筋混凝土框架梁、柱损伤评定数据集。然后通过对3个关键问题的研究建立了钢筋混凝土框架基于卷积神经网络的震损评定方法:训练和建立YOLOv5网络模型完成从结构震害照片中检测识别出梁、柱构件的任务,并改进优化了YOLOv5网络模型的检测性能;优选比较3种网络模型(ResNet50、MobileNetV2和AlexNet模型)对梁、柱构件损伤水平评定的精确性,最终建立了基于ResNet50的梁、柱构件损伤评定模型;给出了从构件层次到整体结构的损伤水平确定方法,并通过对一栋实际震损框架进行损伤评定验证了文中方法的可用性。结果表明,文中方法与专家的损伤评定结论一致性高,优化后的卷积神经网络模型精确度和稳定性好,对震后钢筋混凝土框架结构损伤评定具有良好的适用性。

附设黏滞阻尼器的RC框架结构抗震韧性评估

为研究黏滞阻尼器对钢筋混凝土(reinforced concrete,RC)框架结构抗震韧性影响,基于增量动力时程分析方法,对附设黏滞阻尼器的RC框架结构开展抗震韧性能力评估。结合FEMA P-58以及我国抗震韧性评价标准,将主要受损构件的修复费用、修复时间曲线以及易损性信息按实际情况进行相应修正,并对无控结构和有控结构的修复费用、修复时间和人员伤亡等主要抗震韧性指标进行对比分析,明确了黏滞阻尼器对RC框架结构抗震韧性的影响。研究表明,在RC框架结构中适当设置黏滞阻尼器不仅能够减小结构地震响应、降低结构破坏概率,还能有效提升结构抗震韧性。

功能可恢复RCS混合框架结构研究进展

钢筋混凝土柱-钢梁(RCS)混合框架结构以其结合了混凝土优异的抗压性能及钢材优异的抗弯性能而受到广泛关注,而可恢复功能结构以其震后快速恢复使用功能的能力,也成为地震工程界研究的新热点。功能可恢复RCS混合框架结构可以在弯矩较大的梁端和柱脚部位设置可更换构件,实现结构的功能可恢复能力。文中简述了近年来功能可恢复RCS混合框架结构研究进展,重点关注各类型功能可恢复钢梁、功能可恢复摇摆柱脚的构造研究,介绍了功能可恢复RCS混合框架结构性能分析研究进展。最后,对功能可恢复RCS混合框架结构仍需深入研究的问题进行了展望。

多台地掉层RC框架结构扭转效应及控制研究

山地掉层结构考虑坡地开挖工程量、边坡稳定性等因素,常需设置多个接地端,导致结构平面和立面不规则,扭转效应显著。为研究3个不等高接地端的多台地掉层钢筋混凝土(reinforced concrete,RC)框架结构的扭转效应及控制措施,通过拟静力试验和振动台试验结果验证了有限元模型的可靠性,对建立的16个不同结构参数的有限元算例模型进行了弹性和弹塑性动力有限元分析,研究了此类框架结构扭转效应受接地端跨数、总掉层数、中接地端掉层数等因素的影响程度;通过算例分析对比了不同扭转控制措施的效果。研究结果表明:弹性状态下,3台地掉层RC框架结构在中接地层和上接地层处扭转效应显著,总体扭转效应小于对应的2台地掉层RC框架结构;而当结构进入弹塑性状态后,通过降低结构掉层部分长度在顺坡向总长度上的占比,可有效缓解结构的扭转效应;在3台地掉层RC框架结构中设置水平接地构件、钢支撑和黏滞阻尼器均能改善扭转效应,对于岩质边坡,设置水平接地构件对各扭转效应指标的改善最好。

The Lightweight Design of Low RCS Pylon Based on Structural Bionics

A concept of Specific Structure Efficiency (SSE) was proposed that can be used in the lightweight effect evaluation ofstructures.The main procedures of bionic structure design were introduced systematically.The parameter relationship betweenhollow stem of plant and the minimum weight was deduced in detail.In order to improve SSE of pylons, the structural characteristicsof hollow stem were investigated and extracted.Bionic pylon was designed based on analogous biological structuralcharacteristics.Using finite element method based simulation, the displacements and stresses in the bionic pylon were comparedwith those of the conventional pylon.Results show that the SSE of bionic pylon is improved obviously.Static, dynamic andelectromagnetism tests were carried out on conventional and bionic pylons.The weight, stress, displacement and Radar CrossSection (RCS) of both pylons were measured.Experimental results illustrate that the SSE of bionic pylon is markedly improvedthat specific strength efficiency and specific stiffness efficiency of bionic pylon are increased by 52.9% and 43.6% respectively.The RCS of bionic pylon is reduced significantly.

近海大气环境下RC框架结构时变地震易损性分析

由于近海大气环境中存在的氯离子会侵蚀RC框架结构,造成材料性能不断劣化,最终降低了RC框架结构的抗震性能。为研究近海大气环境下RC框架结构地震损伤风险变化规律,基于钢筋均匀锈蚀模型及混凝土开裂前后钢筋锈蚀速率的变化规律,提出了一种改进的钢筋锈蚀率概率模型,并构建材料性能劣化模型。在此基础上,根据解析易损性分析理论,建立考虑氯离子侵蚀的近海大气环境RC框架结构时变易损性分析方法,并构建典型结构,分析不同损伤状态下的时变易损性曲线,评估其时变易损性与损伤状态。研究结果表明:当地震动强度指标取值相同时,各损伤状态结构在服役龄期内的地震易损性呈现出非线性增大趋势,抗震性能不断退化;以易损性指数作为结构损伤指标时,按照我国规范设计的RC框架结构,在30 a服役龄期内能够满足我国三性能抗震设防水准。

近场地震动对我国RC框架结构地震响应影响分析

近场地震常含有丰富的脉冲成分,其短时间内产生的冲击效应会对近场地区建筑结构造成严重破坏。钢筋混凝土(reinforced concrete,RC)框架结构在我国城镇地区应用较多,为此应开展近场地震动对我国框架结构地震响应影响研究。基于近期记录较为完整的土耳其M 7.8级地震,筛选出近场脉冲型和非脉冲型两类地震动各2条作为输入,并对比分析了4条地震动加速度反应谱的形状特征;按8度设防设计并建立了4、7、10层三个不同周期的RC框架结构有限元分析模型,利用现有试验结果验证了该模型的准确性;采用非线性动力时程分析方法,通过比较地震加速度反应谱、结构层间位移角、结构耗能和梁、柱构件利用率等指标,从结构整体到构件层面综合分析了这两类地震动对框架结构地震响应的影响。研究结果表明:所选的脉冲型地震动加速度反应谱平台段较长且双峰和多峰特性明显,非脉冲型地震动加速度反应谱的单峰特性明显;在这两类地震动作用下,短周期结构整体响应以及梁、柱构件的破坏程度均与地震加速度反应谱值呈正相关性;而对于中和长周期结构,相比较非脉冲型地震动,脉冲型地震动作用下会造成加速度反应谱值小,而结构整体响应以及梁、柱构件的破坏程度较大的现象。因此,对该类结构进行抗震设计时应充分考虑近场地震动脉冲效应的影响。

High-temperature Resistance Performance of An Inorganic Adhesive for Concrete Structures Strengthened with CFRP Sheets

Organic epoxy matrices have been widely used in the FRP reinforcing technique, but they have serious disadvantages of poor high-temperature resistance. An inorganic adhesive is invented to replace the organic adhesive. For the inorganic adhesive at normal temperature and different high temperatures, the microstructure and phase composition are investigated by means of X-ray diffraction （XRD） and SEM respectively. Results show that inorganic adhesive can resist at least 600 ℃ high temperature. Fire-resistance performance of inorganic adhesive can meet the requirements of fiber reinforced polymer （FRP） strengthened RC structures.

Seismic damage to structures in the Ms6.5 Ludian earthquake

On 3 August 2014, the Ludian earthquake struck northwest Yunnan Province with a surface wave magnitude of 6.5. This moderate earthquake unexpectedly caused high fatalities and great economic loss. Four strong motion stations were located in the areas with intensity V, VI, VII and IX, near the epicentre. The characteristics of the ground motion are discussed herein, including 1） ground motion was strong at a period of less than 1.4 s, which covered the natural vibration period of a large number of structures; and 2） the release energy was concentrated geographically. Based on materials collected during emergency building inspections, the damage patterns of adobe, masonry, timber frame and reinforced concrete （RC） frame structures in areas with different intensities are summarised. Earthquake damage matrices of local buildings are also given for fragility evaluation and earthquake damage prediction. It is found that the collapse ratios of RC frame and confined masonry structures based on the new design code are significantly lower than non-seismic buildings. However, the RC frame structures still failed to achieve the ＇strong column, weak beam＇ design target. Traditional timber frame structures with a light infill wall showed good aseismic performance.