Fragility functions of blockwork wharves using artificial neural networks

The use of artificial neural networks in the general framework of a performance-based seismic vulnerability evaluation for earth retaining structures is presented. A blockwork wharf-foundation-backfill complex is modeled with advanced nonlinear 2D finite difference software, wherein liquefaction occurrence is explicitly accounted for. A simulation algorithm is adopted to sample geotechnical input parameters according to their statistical distribution, and extensive time histories analyses are then performed for several earthquake intensity levels. In the process, the seismic input is also considered as a random variable. A large dataset of virtual realizations of the behavior of different configurations under recorded ground motions is thus obtained, and an artificial neural network is implemented in order to find the unknown nonlinear relationships between seismic and geotechnical input data versus the expected performance of the facility. After this process, fragility curves are systematically derived by applying Monte Carlo simulation on the obtained correlations. The novel fragility functions herein proposed for blockwork wharves take into account different geometries, liquefaction occurrence and type of failure mechanism. Results confirm that the detrimental effects of liquefaction increase the probability of failure at all damage states. Moreover, it is also demonstrated that increasing the base width/height ratio results in higher failure probabilities for the horizontal sliding than for the tilting towards the sea.     

Seismic assessment of the rigid sliding displacements caused by pulse motions

This paper presents a comprehensive study on the rigid block sliding displacement of slopes subjected to ground motions with large velocity pulses. A comparison of the performance of various existing empirical displacement models is provided through analyses of the displacement residuals of slopes subject to pulse-like motions. Except for the PGA- and PGV-based Saygili and Rathje model (2008, referred to as SR08), positive medians of residuals are observed for selected models, indicating an under-estimation. There is a negative constant shift in the total residuals for the SR08 model, which can be easily fixed by changing the constant term in the predictive equation. The residuals from the SR08 model also have the smallest standard deviation compared to the other models. A modified SR08 model is developed for predicting rigid block sliding displacements for pulse-like motions. The modified predictive model is used in probabilistic seismic displacement analyses of slopes in a hypothetical near-fault region.     

悬臂式抗滑桩加固黏土边坡地震永久位移算法

为了有效地确定悬臂式抗滑桩加固的黏土边坡地震永久位移,基于极限分析上限定理,针对圆弧滑动式土坡破坏模式,通过对设置抗滑桩条件下土坡进行外力功率和内能耗散率的计算,按严格力学定义推导出坡体在地震作用下的安全系数,进而导出与安全系数相对应的边坡地震屈服加速度计算公式,并结合Newmark滑块位移法对边坡产生的转动加速度进行二次积分,推导出与边坡设计安全系数密切相关的坡体地震永久位移的详细计算公式。以5.12汶川地震卧龙测站东向地震波为例,通过对一算例边坡进行分析,给出了边坡永久位移时程曲线以及不同设计安全系数与永久位移的关系,分析了算法与Ambraseys算法的结果,验证了所提计算方法的有效性,并得到不同设计安全系数时边坡土体黏聚力和内摩擦角对坡体永久位移的影响规律。研究结果表明,坡体永久位移随着设计安全系数的增加逐渐呈指数函数式减小变化,在较低设计安全系数下,坡体永久位移受土体抗剪强度参数影响较为敏感,随着设计安全系数的提高,这种敏感性则逐渐降低。     

壁式削壁充填法采场顶板临界跨度计算

针对缓倾斜极薄矿体壁式削壁充填法劳动强度大、生产效率低的问题,提出采取机械化生产。对适应机械化生产的大尺寸采场结构需进行稳定性分析,将崩落废石堆体假设成弹簧支撑件,把采场顶板简化为平面问题的固定端超静定悬臂梁结构,对顶板临界跨度计算。同时,该计算法在矿山得到实际应用,实现了机械化作业。     

Acceptance limits for performance‐based seismic design of RC walls for low‐rise housing

Acceptance limits of the structural response of walls for low‐rise concrete housing were developed. Proposed values are applicable within a performance‐based seismic design framework. Acceptance limits are based on performance indicators of structural response–allowable story drift ratios, width of residual cracks and residual damage index, and expected damage of walls. Cracking limits were defined from parameters obtained at the unloading stage of walls (i.e., residual cracking stage). The residual cracking stage may be used for structural damage evaluation and cost estimation of structural rehabilitation after an earthquake has occurred. The performance indicators proposed herein were derived from test observations and measured response of 39 RC walls' specimens during shaking table and quasistatic testing, as well as from limiting values and results of previous studies. Copyright © 2012 John Wiley & Sons, Ltd.     

Exact analytic solutions for the rotation of an axially symmetric rigid body subjected to a constant torque

New exact analytic solutions are introduced for the rotational motion of a rigid body having two equal principal moments of inertia and subjected to an external torque which is constant in magnitude. In particular, the solutions are obtained for the following cases: (1) Torque parallel to the symmetry axis and arbitrary initial angular velocity; (2) Torque perpendicular to the symmetry axis and such that the torque is rotating at a constant rate about the symmetry axis, and arbitrary initial angular velocity; (3) Torque and initial angular velocity perpendicular to the symmetry axis, with the torque being fixed with the body. In addition to the solutions for these three forced cases, an original solution is introduced for the case of torque-free motion, which is simpler than the classical solution as regards its derivation and uses the rotation matrix in order to describe the body orientation. This paper builds upon the recently discovered exact solution for the motion of a rigid body with a spherical ellipsoid of inertia. In particular, by following Hestenes’ theory, the rotational motion of an axially symmetric rigid body is seen at any instant in time as the combination of the motion of a “virtual” spherical body with respect to the inertial frame and the motion of the axially symmetric body with respect to this “virtual” body. The kinematic solutions are presented in terms of the rotation matrix. The newly found exact analytic solutions are valid for any motion time length and rotation amplitude. The present paper adds further elements to the small set of special cases for which an exact solution of the rotational motion of a rigid body exists.     

Seismic response of multi-tiered reinforced soil retaining walls

In this study, a validated Finite Element procedure was used to investigate the similarities and differences of seismic performances between single- and multi-tiered reinforced soil walls. Three-tiered walls at a total height of 9 m were analyzed together with vertical walls at the same height. It was found from the Finite Element analyses that the resonant frequency of reinforced soil walls might increase with an increase in the tier-offset. The multi-tiered configuration could considerably reduce the residual lateral facing displacement and the average reinforcement load, and the reinforcement load distribution with height was different from that in vertical walls. With the same reinforcement length and spacing, the multi-tiered walls resulted in smaller reinforcement connection loads with the facing blocks. The study filled the gap of seismic behavior of multi-tiered reinforced soil retaining walls and revealed a few unique dynamic properties of this type of earth structures.     

Analytical solution of seismic pseudo-static active thrust acting on fascia retaining walls

This paper presents a limit equilibrium method, based on the approach of Mononobe and Okabe, for calculating the active thrust on fascia retaining walls, where common methods cannot be used owing to the narrowness of the backfill which does not permit the development of the thrust wedge in the shape and sizes predicted by these methods. The proposed method examines three distinct failure mechanisms, called Mechanism 1, Mechanism 2 and Mechanism 3, where the thrust wedge is formed by one, two or three blocks, respectively; separated by plane slip surfaces. The seismic forces have been simulated with the pseudo-static method. For all three mechanisms, the active thrust is obtained in closed form: in particular, with a cubic equation for Mechanism 2, and with a system of two equations, one cubic and the other quartic, for Mechanism 3. Mechanisms with more than three blocks cannot have analytical solutions. The study is completed by an examination of some significant cases from which the higher attenuation of the seismic thrust, with respect to the static, emerges as the backfill width reduces.     

Dynamic analysis of a rigid circular foundation on a transversely isotropic half-space under a buried inclined time-harmonic load

The dynamic analysis of a surface rigid foundation in smooth contact with a transversely isotropic half-space under a buried inclined time-harmonic load is addressed. By virtue of the superposition technique, appropriate Green׳s functions, and employing further mathematical techniques, solution of the mixed-boundary-value problem is expressed in terms of two well-known Fredholm integral equations. Two limiting cases of the problem corresponding to the static loading and isotropic medium are considered and the available results in the literature are fully recovered. For the static case, the results pertinent to both frictionless and bonded contacts are obtained and compared. With the aid of the residue theorem and asymptotic decomposition method, an effective and robust approach is proposed for the numerical evaluation of the obtained semi-infinite integrals. For a wide range of the excitation frequency, both normal and rotational compliances are depicted in dimensionless plots for different transversely isotropic materials. Based on the obtained results, the effects of anisotropy are highlighted and discussed.     

土钉墙地基承载力问题探讨

土钉墙墙底地基土的承载力验算是土钉墙支护设计的一项重要内容。国内的工程实践中,通常将土钉墙地基承载力与坑底土抗隆起验算合并考虑。针对具体案例,通过Plaxis3D有限元数值模拟,分析研究了土钉墙底部土体发生地基承载力失稳的破坏模式、破坏荷载以及土钉墙墙底应力分布特点等,探讨了依据我国相关规程进行土钉墙坑底隆起或地基承载力计算可能存在的问题。借鉴国外加筋土挡墙地基承载力计算的一般方法,将土钉墙作为荷载倾斜、偏心的刚性基础对待,利用荷载倾斜、偏心条件下传统刚性浅基础的地基承载力的Meyerhof解和Vesic解,对土钉墙地基承载力进行了计算和对比,通过对比发现,Meyerhof解更接近实际,据此,提出了土钉墙地基承载力计算的合理模式。