Nonlinear seismic analysis of reinforced concrete frames using the force analogy method

A method of nonlinear seismic analysis for RC framed structures considering full‐range factors, including stiffness and strength degradation, geometric nonlinearity, and structural member failure, is established based on the fundamental concept of the force analogy method. The strong material nonlinearity, large geometric deformation, and internal forces redistribution due to the member failure can be depicted by the proposed local plastic mechanisms, the rotation hinges at the member ends and the slide hinges assigned to the columns, of which the measurement relationships are moment versus plastic rotation and shear force versus shear plastic deformation, respectively. They are capable of evaluating the exact response of RC structures. Because only unchanging initial stiffness matrices are used through the whole computation process, the state‐space formulation was used for solving the equations of motion. The advantages of the force analogy method, such as high efficiency and stability, are still retained. The exactness of the proposed local plastic mechanisms is verified against a group of tests data, and the application of the proposed procedure is performed to an RC framed structure to simulate the full‐range nonlinear response by increasing the excitation step by step until failure of partial structural members appear. Copyright © 2014 John Wiley & Sons, Ltd.     

Centrifuge modeling of rocking‐isolated inelastic RC bridge piers

Experimental proof is provided of an unconventional seismic design concept, which is based on deliberately underdesigning shallow foundations to promote intense rocking oscillations and thereby to dramatically improve the seismic resilience of structures. Termed rocking isolation, this new seismic design philosophy is investigated through a series of dynamic centrifuge experiments on properly scaled models of a modern reinforced concrete (RC) bridge pier. The experimental method reproduces the nonlinear and inelastic response of both the soil‐footing interface and the structure. To this end, a novel scale model RC (1:50 scale) that simulates reasonably well the elastic response and the failure of prototype RC elements is utilized, along with realistic representation of the soil behavior in a geotechnical centrifuge. A variety of seismic ground motions are considered as excitations. They result in consistent demonstrably beneficial performance of the rocking‐isolated pier in comparison with the one designed conventionally. Seismic demand is reduced in terms of both inertial load and deck drift. Furthermore, foundation uplifting has a self‐centering potential, whereas soil yielding is shown to provide a particularly effective energy dissipation mechanism, exhibiting significant resistance to cumulative damage. Thanks to such mechanisms, the rocking pier survived, with no signs of structural distress, a deleterious sequence of seismic motions that caused collapse of the conventionally designed pier. © 2014 The Authors Earthquake Engineering & Structural Dynamics Published by John Wiley & Sons Ltd.     

LFM信号检测混凝土缺陷研究

针对超声混凝土缺陷检测信噪比低、穿透深度浅的问题,提出利用线性调频（LFM）信号检测混凝土缺陷。通过在发射端发射时宽和带宽的积大于1的LFM信号,在接收端将回波脉冲压缩,得到主瓣较窄,信噪比较高的压缩信号。理论分析了换能器带宽对压缩效果的影响。数值计算结果表明,在一定幅度的噪声存在条件下,回波信号经过脉冲压缩后仍能够得到好的压缩效果。实际检测结果表明,压缩后的LFM信号较压缩前检测信号明显提高了信噪比。     

新型框架结构弹塑性地震反应的等效线性化方法研究

等效线性化方法是计算结构弹塑性地震反应的实用方法.通过对几种等效线性化方法的比较分析,提出了基于FEMA440 （2005）计算等效刚度和等效阻尼比的等效线性化方法;分别采用静力弹塑性分析方法和等效线性化方法,分析了局部使用高性能纤维增强混凝土新型框架结构的弹塑性地震反应.结果表明,两种方法所得结构的地震反应均比较接近.因此,本文建议的等效线性化方法能够较好地预测新型框架结构的弹塑性地震反应.     

A practice‐oriented estimation of the failure probability of building structures

In the presented practice‐oriented probabilistic approach for the seismic performance assessment of building structures, the SAC‐FEMA method, which is a part of the broader PEER probabilistic framework and permits probability assessment in closed form, is combined with the pushover‐based N2 method. The most demanding part of the PEER probabilistic framework, that is incremental dynamic analysis, is replaced by the much simpler N2 method, which requires considerably less input data and much less computational time, but which can, nevertheless, often provide: acceptable estimates for the mean values of the structural response. Using some additional simplifying assumptions that are consistent with seismic code procedures, an explicit equation for a quick estimation of the annual probability of “failure” (i.e. the probability of exceeding the near collapse limit state) of a structure can be derived, which is appropriate for practical applications, provided that predetermined default values for the dispersion measures are available. In the paper, this simplified approach is summarized and applied to the estimation of the “failure” probability of reinforced concrete frame buildings representing both old structures, not designed for earthquake resistance, and new structures designed according to Eurocode 8. The results of the analyses indicate a high probability of the “failure” of buildings, which have not been designed for seismic loads. For a building designed according to a modern code, the conservatively determined probability of “failure” is about 30 times less but still significant (about 1% over the lifetime of the structure). Copyright © 2011 John Wiley & Sons, Ltd.     

Cyclic performance of large‐scale corroded reinforced concrete beams

Nine large‐scale beam specimens were constructed. Of which, one was used as the control, whereas the other eight ones were divided into four sets. Each set had two specimens and was subjected to accelerated corrosion using an imposed current for the same time interval. Following the corrosion, a specimen in each set was tested using cyclic loading to examine the seismic performance, whereas the other one was demolished to examine the extent of corrosion. Cyclic loading results indicated that with an increasing corrosion level, the ultimate drift, ductility, plastic rotation capacity, and energy dissipation of the beams initially increased and later decreased. The failure mode switched from flexural failure, largely owing to buckling of the longitudinal reinforcement to flexural‐shear failure, which is mainly caused by fracturing of the transverse reinforcement. Corrosion increased shear deformation and the spread of plasticity of the plastic hinge region. The residual flexural strength, as estimated by an empirical equation based on the maximum pit depth in the longitudinal reinforcement, closely corresponds to experimental values. Furthermore, the residual shear strength estimated based on the minimum reduced cross‐sectional area of transverse reinforcement correlates better with the experimental observations than that based on the weight loss. Copyright © 2011 John Wiley & Sons, Ltd.     

An efficient performance‐based seismic design method for reinforced concrete frames

In this paper, a practical method is developed for performance‐based design of RC structures subjected to seismic excitations. More efficient design is obtained by redistributing material from strong to weak parts of a structure until a state of uniform deformation or damage prevails. By applying the design algorithm on 5, 10 and 15‐storey RC frames, the efficiency of the proposed method is initially demonstrated for specific synthetic and real seismic excitations. The results indicate that, for similar structural weight, designed structures experience up to 30% less global damage compared with code‐based design frames. The method is then developed to consider multiple performance objectives and deal with seismic design of RC structures for a design spectrum. The results show that the proposed method is very efficient at controlling performance parameters and improving structural behaviour of RC frames. Copyright © 2011 John Wiley & Sons, Ltd.     

火灾下空心混凝土楼板内温度场分析模型及算法

计算火灾下空心混凝土楼板内温度场的关键是计算空腔内的热量传递。假设火灾过程中空腔内气体压力为常数,空气流动引起的热量损失忽略不计,空腔内的热量传递通过壁面辐射和空气传导实现。基于奥本海姆网络法,本文提出了计算空心混凝土楼板空腔内辐射热流的算法,推导了矩阵形式的空腔内有效辐射方程,考虑到系数矩阵的对称性,采用LDLT分解法求解有效辐射方程。利用空腔边界上的有效辐射热流,计算空腔边界上单元的温度节点荷载。这种方法已经嵌入作者开发的温度场分析软件TFIELD之中,用TFIELD对底面按ISO834曲线进行加热的空心混凝土楼板进行计算,结果表明,本文算法十分有效,对研究空心混凝土楼板的抗火性能具有积极意义。     

火灾作用下预应力型钢混凝土简支梁承载性能试验研究

进行了两根足尺预应力型钢混凝土简支梁在火灾高温和竖向恒定加载耦合作用下的耐火性能试验,分析了试验梁的温度场分布规律、竖向变形特点和承载性能劣化过程。通过在试验梁端部预设压力传感器,实测了升温过程中梁端部预压力与升温时间的关系曲线。试验结果表明,预压力对火灾高温作用较为敏感,当标准升温100 min时预压力已降至初始预压力的一半,试验中一根梁出现了侧向弯曲变形。因此,对于预应力混凝土结构,火灾高温作用下应考虑构件可能的侧向变形影响。     

表面设置防火涂料高强混凝土柱的耐火极限

前期试验表明,表面设置厚度20 mm的某非膨胀型防火涂料可较好地抑制高强混凝土的高温爆裂。本文通过试验反算给出了该涂料对应不同温度区间的平均导温系数;随后,利用结构抗火分析软件SAFIR对表面设置该涂料的高强混凝土柱的耐火极限进行了计算分析,考察了截面尺寸、轴压比、荷栽偏心率及配筋率等因素的影响,在此基础上提出了相应的高强混凝土柱耐火极限实用计算公式。研究结果表明：①该涂料的平均导温系数随温度升高有所降低;②表面设置厚度20 mm的该涂料可明显提高高强混凝土柱的耐火极限,提高幅度达40%～350%。