A simplified model for lateral response of large diameter caisson foundations—Linear elastic formulation

The transient response of large embedded foundation elements of length-to-diameter aspect ratio D/B=2–6 is characterized by a complex stress distribution at the pier–soil interface that cannot be adequately represented by means of existing models for shallow foundations or flexible piles. On the other hand, while three-dimensional (3D) numerical solutions are feasible, they are infrequently employed in practice due to their associated cost and effort. Prompted by the scarcity of simplified models for design in current practice, we here develop an analytical model that accounts for the multitude of soil resistance mechanisms mobilized at their base and circumference, while retaining the advantages of simplified methodologies for the design of non-critical facilities. The characteristics of soil resistance mechanisms and corresponding complex spring functions are developed on the basis of finite element simulations, by equating the stiffness matrix terms and/or overall numerically computed response to the analytical expressions derived by means of the proposed Winkler model. Sensitivity analyses are performed for the optimization of the truncated numerical domain size, the optimal finite element size and the far-field dynamic boundary conditions to avoid spurious wave reflections. Numerical simulations of the transient system response to vertically propagating shear waves are next successfully compared to the analytically predicted response. Finally, the applicability of the method is assessed for soil profiles with depth-varying properties. The formulation of frequency-dependent complex spring functions including material damping is also described, while extension of the methodology to account for nonlinear soil behavior and soil–foundation interface separation is described in the conclusion and is being currently investigated.     

Nonlinear seismic soil–pile–structure interactions: Shaking table tests and FEM analyses

In this paper, a soil–pile–structure model is tested on a shaking table subject to both a sinusoidal wave and the acceleration time history of the scaled 1940 El Centro earthquake. A medium-size river sand is compacted into a 1.7-m-high laminar rectangular tank to form a loose fill with a relative density of 15%. A single-storey steel structure of 2.54 ton is placed on a concrete pile cap, which is connected to the four end-bearing piles. A very distinct pounding phenomenon between soil and pile is observed; and, the acceleration response of the pile cap can be three times larger than that of the structural response. The pounding is due to the development of a gap separation between soil and pile, and the extraordinary large inertia force suffered at the top of the pile also induces cracking in the pile. To explain this observed phenomenon, nonlinear finite element method (FEM) analyses with a nonlinear gap element have been carried out. The spikes in the acceleration response of the pile cap caused by pounding can be modeled adequately by the FEM analyses. The present results suggest that one of the probable causes of pile damages is due to seismic pounding between the laterally compressed soil and the pile near the pile cap level.     

Seismic response of an urban bridge-support system in soft clay

The numerical simulation of an instrumented urban bridge support and its foundation system was conducted. The bridge works as a deck in a surface subway station and was built 12 yr ago in the so-called Lake Zone in Mexico City, where very soft clays, exhibiting low shear strength and high compressibility prevail. Since the beginning of its construction, pile loads, soil–raft contact pressures and the overall response of the foundation system have been monitored. Within this period, several earthquakes have occurred. Thus, an extensive database of accelerations, pore pressures and load histories have been recorded. In particular, this paper focuses on the bridge response observed during two moderated intensity events, the 2004, 6.3 M_w, Guerrero Coast and the 1999, 7.0 M_w, Tehuacan earthquakes. Finite element models were developed to reproduce the measured responses and to assess the soil–foundation-support performance for long-term conditions, including the effects of potential changes in the dynamic soil properties due to regional subsidence. The computed dynamic responses obtained with the simulation for the free field and structure compares fairly well with the recordings.     

Efficient 3D nonlinear Winkler model for shallow foundations

This paper presents a new practical modeling approach, based on the beam-on-a-nonlinear Winkler foundation (BNWF) model, to simulate the 3D rocking, vertical and horizontal responses of shallow foundations using structural elements that are readily available in the element library of commercially available structural analysis programs. An assemblage of a moment-rotation hinge, shear hinge connected in series with an elastic frame member attached to the bottom end of ground story columns was proposed to model the response of the footing under combined action of vertical, horizontal and moment loading. To couple the responses of these hinges, two bounding surfaces equations were introduced and derived mathematically: a surface that defines the interaction between the rocking and vertical capacities of the footing along its width and length; and a surface that defines the interaction between the horizontal capacities of the footing along its width and length. Simple calculation steps to evaluate the geometric and mechanical properties of the proposed assemblage of structural elements are provided. The proposed modeling approach was verified using experimental results from large scale model foundations subjected to cyclic loading. Based on this study, it was found that the proposed assemblage can be reliably used in modeling the rocking and horizontal responses of shallow foundations under cyclic loading.     

SSI效应对粘弹性阻尼结构减震效果的影响分析

本文以一单跨7层框架结构为研究对象,对不同场地和地震波输入条件下的粘弹性阻尼结构进行了二维有限元时程分析,探讨了SSI（土-结构动力相互作用）效应对粘弹性阻尼结构减震效果的影响。分析结果表明：①在硬土和稍硬土地基条件下,SSI效应明显降低了结构的楼层位移峰值,若在抗震设计中对客观存在的SSI效应加以考虑,设置较少数量的阻尼器（与刚性地基假定条件下确定的阻尼器数量相比）就能使结构的实际地震位移反应满足基于刚性地基假定的地震位移控制目标;②粘弹性阻尼结构的减震效果与场地条件、输入地震动特性密切相关;③与刚性地基相比,SSI效应使粘弹性阻尼结构的减震效果明显降低,且地基越软,降低幅度越大。因此,在实际的工程设计中,应当充分考虑SSI效应,对粘弹性阻尼结构的减震控制效果进行合理的评价,并针对不同的场地条件选用合适的阻尼器类型和性能参数,才有可能达到预期的减震控制效果。     

Seismic response of single-column bent on pile: evidence of beneficial role of pile and soil inelasticity

While seismic codes do not allow plastic deformation of piles, the Kobe earthquake has shown that limited structural yielding and cracking of piles may not be always detrimental. As a first attempt to investigate the consequences of pile yielding in the response of a pile-column supported bridge structure, this paper explores the soil–pile-bridge pier interaction to seismic loading, with emphasis on structural nonlinearity. The pile–soil interaction is modeled through distributed nonlinear Winkler-type springs and dashpots. Numerical analysis is performed with a constitutive model (Gerolymos and Gazetas 2005a, Soils Found 45(3):147–159, Gerolymos and Gazetas 2005b, Soils Found 45(4):119–132, Gerolymos and Gazetas 2006a, Soil Dyn Earthq Eng 26(5):363–376) materialized in the OpenSees finite element code (Mazzoni et al. 2005, OpenSees command language manual, p 375) which can simulate: the nonlinear behaviour of both pile and soil; the possible separation and gapping between pile and soil; radiation damping; loss of stiffness and strength in pile and soil. The model is applied to the analysis of pile-column supported bridge structures, focusing on the influence of soil compliance, intensity of seismic excitation, pile diameter, above-ground height of the pile, and above or below ground development of plastic hinge, on key performance measures of the pier as is: the displacement (global) and curvature (local) ductility demands and the maximum drift ratio. It is shown that kinematic expressions for performance measure parameters may lead to erroneous results when soil-structure interaction is considered.     

可液化土中地铁结构的地震响应

在饱和土耦合作用与土和结构相互作用理论基础上,以地铁车站为例,用有限元法研究地下结构在地震液化作用下的响应。所采用的软件为动力两相体非线性有限元软件Dyna-Swandyne-II,该软件可以应用先进的Pastor-Zienkiewicz III广义塑性模型模拟可液化土的动力特性,应用u-p形式的Biot方程,在有限元分析中充分考虑孔隙水与土之间的耦合,同时考虑地下结构与饱和土在动力作用下的非线性相互作用。分析了地铁车站的动力响应,包括地铁内力、加速度以及地铁位移。研究结果表明,地铁结构在地震液化作用下会产生较大的上浮,从而对结构造成比较严重的破坏;地铁结构在地震作用下的最大内力位于结构的交接处。因此,结构交接处的配筋应该格外小心。     

地下工程中支架和围岩相互作用的突变模型

建立了支架和围岩相互作用的突变模型,利用支架刚度和围岩的弱化刚度模量研究了围岩的突变失稳。研究结果从理论上解释了新奥法的支护原理。利用突变模型定量地给出了突变失稳的关键点。当围岩的变形量达到变形关键点以后,围岩就进入了散体区。变形关键点的大小是围岩－支架系统内部因素决定的,与外界条件无关。     

软土地基中PHC管桩水平受荷性状的试验研究

桩在水平荷载作用下的受力性状是一个复杂的桩土相互作用过程。针对软土地集中预应力管桩受水平荷载的问题,结合工程实例,通过水平静载试验,实测得到了水平荷载作用下PHC（超长预应力）桩与土共同作用时的工作性状,分析了PHC桩受水平力作用时的内力、变形和临界承载力以及地基土的水平抗力比例系数。试验结果可为今后同类地区水平受荷桩的设计与研究提供参考。     

南秦岭张家坝岩体和新院岩体地球化学特征及其形成环境

南秦岭中生代张家坝岩体和新院岩体的LILE相对富集、HFSE贫化,高钾、富Sr（（498—623）×10^-6）、低Y（（5．57～24）×10^-6）和Yb（（0．52～2．13）×10^-6）,Eu异常微弱,显示了地壳增厚背景下下部地壳物质部分熔融所形成的高钾钙碱性花岗岩特征。岩体内的包体中出现长宽比很高的针状磷灰石,且边缘发育冷凝边结构,包体与寄主岩间存在明显的成分间断,并构成了两个不同的岩浆演化趋势和它们所具有的不同稀土特征,证明它们为来自两个不同源区岩浆混合的产物,同时指示秦岭地区中生代时期发生了一次壳幔混合作用和重要的地壳垂向增生事件。