Probabilistic Seismic Design of Pile Foundations in Non Liquefiable Soil by Response Spectrum Approach

The behavior of pile foundations in non liquefiable soil under seismic loading is considerably influenced by the variability in the soil and seismic design parameters. Hence, probabilistic models for the assessment of seismic pile design are necessary. Deformation of pile foundation in non liquefiable soil is dominated by inertial force from superstructure. The present study considers a pseudo-static approach based on code specified design response spectra. The response of the pile is determined by equivalent cantilever approach. The soil medium is modeled as a one-dimensional random field along the depth. The variability associated with undrained shear strength, design response spectrum ordinate, and superstructure mass is taken into consideration. Monte Carlo simulation technique is adopted to determine the probability of failure and reliability indices based on pile failure modes, namely exceedance of lateral displacement limit and moment capacity. A reliability-based design approach for the free head pile under seismic force is suggested that enables a rational choice of pile design parameters.     

Experimental and Analytical Study of Seismic Soil-Pile-Structure Interaction in Layered Soil Half-Space

The dynamic interaction of pile foundations, embedded in a horizontally stratified soil profile, with superstructures under low to moderate earthquake excitation can be handled in different ways. In this article, the soil-pile-superstructure dynamic interaction problem has been investigated using the coupled finite element-boundary element method. Comparison with shaking table experiments of a small scale model pile shows a good correlation with the proposed method in terms of the kinematic response of pile foundations and the structural response. A parametric study of the proposed model has yielded important results essentially concerning the amplification factors of the pile foundation and the superstructure.     

Inelastic Demand Spectra of Bi-Linear Models for Capacity Spectrum Method and Response of Simple Structures under Near-Source Records

A very useful tool for the preliminary design of structures is the elastic demand spectrum that can be used in the capacity spectrum method. A pseudo-acceleration relationship has to be assumed when constructing a demand spectrum. This assumption results in large errors for long period structures with large damping ratios and the conventional demand spectra require a substitute elastic structure. In the present study, the conventional demand spectra are extended to bi-linear models. Pseudo-acceleration is still assumed but results in acceptably small errors, when a constant viscous damping coefficient for a single-degree-of-freedom (SDF) structure is calculated from the tangent stiffness and the damping ratio is set at 5% in both elastic and yield phases. For nonlinear structures, tangent stiffness dependency of damping force could be acceptable because energy absorption is primarily the result of structural nonlinear deformation. To extend the conventional demand spectra to a bi-linear model, effective period calculated from the secant stiffness has to be used. The use of effective period introduces no approximation because the peak displacement of the SDF structure is computed from nonlinear analysis in the time domain. The method presented in this study is also valid if damping coefficient proportional to initial elastic spectra is used. In this case, the pseudo-acceleration is defined as the base shear coefficient that is required to produce the peak displacement of the SDF structure in a static manner. We present demand spectra of bi-linear models for a number of near-source records from large earthquakes, and spectral ratios of two horizontal components. The effects of different types of ground motion on the response reduction factor due to inelastic deformation are investigated.     

Shock Response from Pseudodynamic Test Using Momentum Equations of Motion

The feasibility of using pseudodynamic techniques to yield shock responses from impulses is studied herein. A direct integration method is often used to solve the force equation of motion in performing a conventional pseudodynamic test. However, this technique might not be applicable to obtain an accurate shock response from an impulse as a load discontinuity occurs at the end of the impulse. This is because this discontinuity will lead to an extra amplitude distortion and the amount of this amplitude distortion is increased with the increase of time step. Hence, a small time step is needed to reduce the extra amplitude distortion and thus the displacement increment for each time step might be smaller than or as small as the resolution of the displacement transducer. As a result, the displacement increment cannot be accurately imposed upon the specimen and the responses will be contaminated by the noise. In addition, the test duration is drastically increased. Alternatively, this difficulty might be overcome if the momentum equation of motion instead of the force equation of motion is solved pseudodynamically. Hence, an external momentum is used in the solution of the momentum equation of motion. Since the external momentum is a resultant of the time integration of the external force the discontinuity problem will automatically disappear. Consequently, reliable shock responses can be obtained from pseudodynamic tests.     

Capacity Design of Coupled RC Walls

Capacity design aims to ensure controlled ductile response of structures when subjected to earthquakes. This article investigates the performance of existing capacity design equations for reinforced concrete coupled walls and then proposes a new simplified capacity design method based on state-of-the-art knowledge. The new method is verified through a case study in which a set of 15 coupled walls are subject to nonlinear time-history analyses. The article includes examination of the maximum shear force in individual walls in relation to the total maximum shear force in the coupled wall system, and subsequently provides recommendations for design.     

Reactive transport and thermo‐hydro‐mechanical coupling in deep sedimentary basins affected by glaciation cycles: model development,verification, and illustrative example

Deep sedimentary basins are complex systems that over long time scales may be affected by numerous interacting processes including groundwater flow, heat and mass transport, water–rock interactions, and mechanical loads induced by ice sheets. Understanding the interactions among these processes is important for the evaluation of the hydrodynamic and geochemical stability of geological CO₂ disposal sites and is equally relevant to the safety evaluation of deep geologic repositories for nuclear waste. We present a reactive transport formulation coupled to thermo‐hydrodynamic and simplified mechanical processes. The formulation determines solution density and ion activities for ionic strengths ranging from freshwater to dense brines based on solution composition and simultaneously accounts for the hydro‐mechanical effects caused by long‐term surface loading during a glaciation cycle. The formulation was implemented into the existing MIN3P reactive transport code (MIN3P‐THCm) and was used to illustrate the processes occurring in a two‐dimensional cross section of a sedimentary basin subjected to a simplified glaciation scenario consisting of a single cycle of ice‐sheet advance and retreat over a time period of 32 500 years. Although the sedimentary basin simulation is illustrative in nature, it captures the key geological features of deep Paleozoic sedimentary basins in North America, including interbedded sandstones, shales, evaporites, and carbonates in the presence of dense brines. Simulated fluid pressures are shown to increase in low hydraulic conductivity units during ice‐sheet advance due to hydro‐mechanical coupling. During the period of deglaciation, Darcy velocities increase in the shallow aquifers and to a lesser extent in deeper high‐hydraulic conductivity units (e.g., sandstones) as a result of the infiltration of glacial meltwater below the warm‐based ice sheet. Dedolomitization is predicted to be the most widespread geochemical process, focused near the freshwater/brine interface. For the illustrative sedimentary basin, the results suggest a high degree of hydrodynamic and geochemical stability.     

Seismic Response of Bridges Subjected to Different Earthquake Types Using IDA

Incremental Dynamic Analysis (IDA) was used to evaluate the seismic response of straight, continuous 4-span bridges with different sub-structure configurations. Three different record sets were chosen to represent three different earthquake types which can occur for a site such as Vancouver (i.e., crustal, subduction interface, and subduction inslab earthquakes). Seventy eight records were considered in each set (i.e., a total of 234 records) and the capacities of the bridges were evaluated using a fast IDA algorithm. A simplified method to account for the effects of spectral shapes was used. Different subsets of the records with specific characteristics were also used in the IDA. The bridges were designed and evaluated for two different design force modification factors and bridges with different degrees of irregularity were studied. Comparisons of the IDA results obtained indicated that in most of the cases the interface record sets resulted in lower median collapse capacities and hence were the most critical of the ground motions studied.     

Effectiveness of FRCM Reinforcement Applied to Masonry Walls Subject to Axial Force and Out-Of-Plane Loads Evaluated by Experimental and Numerical Studies

ABSTRACT

An experimental campaign and a numerical analysis devoted to the investigation of the out-of-plane behavior of masonry walls reinforced with Fiber Reinforced Cementitious Matrix (FRCM) are presented here. The main goal of this study is to analyze and evaluate the effectiveness of the strengthening system, by discussing failure modes and capacity of strengthened masonry walls, in order to assess their behavior under out-of-plane horizontal actions, such as, for example, seismic actions. A purposely designed experimental set-up, able to separately and independently apply an axial force and out-of-plane horizontal actions on masonry walls, was used. Experimental results are discussed and compared with the outcomes of nonlinear analyses performed on simplified finite element models of the walls. A proper evaluation of the flexural capacity of FRCM strengthened walls is the first step of the ongoing process of drawing reliable code guidelines leading to a safe design of strengthened masonry structures.     

A Simplified Procedure to Select a Suitable Retrofit Strategy for Existing RC Buildings Using Pushover Analysis

Over the past ten years, the development of analytical procedures to accurately evaluate the seismic performance of existing buildings has gathered the attention of researchers. This has resulted in the publication of several standards, which, however, inadequately cover the issue of retrofit strategy selection. In the present article, a procedure that allows a comparison of available strategies in order to select the optimum solution for an existing deficient building is proposed. The procedure is based on calculating the pushover curve for the unstrengthened structure. A capacity spectrum is then estimated assuming different retrofit scenarios, which is then used for the evaluation of the strategies. The latter is based on criteria that assess the main structural system characteristics and how each solution benefits them. The final step of the procedure introduces simplified rules that allow the approximate design of each retrofit solution, which allows the evaluation of their applicability. The proposed procedure was applied to two idealized buildings with different structural systems. Results obtained indicate that less effective or inapplicable rehabilitation strategies were properly detected. Thus, the results were considered acceptable in terms of identifying the possible optimum strategy, which, however, should be verified with a detailed design of the retrofit system.     

Hydrodynamics and membrane seal capacity

The impact of hydrodynamic groundwater movement on the capacity of seals is currently in debate. There is an extensive record of publication on seals analysis and a similar history on petroleum hydrodynamics yet little work addresses the links between the two. Understanding and quantifying the effects of hydrodynamic flow has important implications for calibrating commonly used seal capacity estimation techniques. These are often based on measurements such as shale gouge, clay smear or mercury porosimitry where membrane sealing is thought to occur. For standard membrane seal analysis, seal capacity is estimated by quantifying capillary pressure‐related measurements and calibrating them with a large observational database of hydrocarbon column heights and measured buoyancy pressures. The seal capacity estimation process has historically been adjusted to account for a number of different generic trapping geometries. We define the characteristics of these geometries from a hydrodynamics viewpoint in order to fine‐tune the seal capacity calibration process. From theoretical analyses of several simplified trapping geometries, it can be concluded that generally, the high pressure side of the seal should be used as the water pressure gradient with which to calculate buoyancy pressure. Secondly, trap geometries where hydrocarbon is reservoired on both sides of a fault are not useful for estimating across fault seal capacity.     

MODELLING OF STRUCTURAL ACCELERATIONS AND APPLICATION TO CONTROLLER DESIGN

In this paper a method for the identification of simplified linear models for building structures is applied to the case when acceleration, rather than displacement, is measured. A frame from benchmark structural controller studies is simulated, and from the input-output data of these simulations, simplified models for the acceleration response of the frame are obtained that have far fewer degrees of freedom. One of these simplified models is used to design a controller, which is tested using an evaluation model from the benchmark controller studies and found to be effective.     

Displacement Ductility Limits for Pile Foundations in Cohesionless Soils

The displacement ductility limit for seismic design of concrete piles is determined for the range of cohesionless soils expected in practice. The curvature ductility capacity associated with specified performance limit states, namely, the “serviceability” and “damage-control” limits, is determined based on the current provisions for confining steel. An analytical model is applied to assess the displacement ductility factor at the specified curvature ductility level. The investigated parameters include the soil stiffness, pile diameter, longitudinal reinforcement ratio, axial force level, and pile above-ground height. A set of design displacement ductility factors is recommended and verified to ensure the satisfactory seismic performance.     

SIMPLE EXPRESSION OF THE DYNAMIC STIFFNESS OF GROUPED PILES IN SWAY MOTION

A simplified expression is derived for the dynamic stiffness of grouped piles subjected to lateral loading. A computer program, based on the Thin Layered Element Method, is used for this purpose. The results of the program are compared with rigorous solutions. The simplified expression of the dynamic stiffness of grouped piles, where the mass, damping and stiffness parameters are frequency invariant, will be particularly useful for an equivalent linear analysis of the interaction between nonlinear soil and grouped piles in real time.     

Natural Frequency of Single Pier Bridges Considering Soil-Structure Interaction

Because of the crucial role of free vibration frequency of a structure (e.g., a bridge) in design procedure, more realistic estimation of the frequency ends up in safer and more optimized design. As obtaining the free vibration frequencies of a bridge, considering soil-pile group-structure interaction, provide more realistic values, development of an analytical model to obtain such free vibration frequencies is studied in this research work. Most researchers have studied models with a single pile foundation. The purpose of this study is to assess soil-structure interaction (SSI) effects on dynamic performance of pile group supported bridges. A new analytical model is proposed to predict seismic analysis of these bridges. Applying the dynamic equations of motion for the system, SSI effects have been estimated. Based on the suggested analytical model, a new approximate equation is proposed for calculating natural frequency of pile group supported bridges. Equation accuracy has been investigated by comparing the results with those achieved by previous studies. Most periods calculated by the approximate equation are similar to those given for other case studies, indicating that the model could be applicable to other projects. Since the proposed model is very similar to real soil-pile-pier systems, this approximate equation can be used in preliminary seismic design of bridges.     

Effect on the Structure in Elevation of Wood Deterioration on Small-Pile Foundation: Numerical Analyses

Wooden pile foundations are quite common in Venice historical building. Very short, small-diameter piles are embedded into the soft shallowest soil layer under the groundwater level, but wood degradation is not prevented as anaerobic bacteria can flourish even in anoxic condition. This study couples the behavior of the masonry structure and the foundation as result of pile degradation. The numerical analyses, relative to the ancient piling, consider wood decay and secondary settlement of soil. The effect of deterioration as a function of the pile spacing and the possible presence of a stiff layer under pile tips are also investigated. The geotechnical results are then used as input for the structural model. The behavior of the masonry as result of different states of wood conservation along the wall is studied numerically.     

Nonlinear Static Methods vs. Experimental Shaking Table Test Results

Three different Nonlinear Static Methods (NSM's), based on pushover analysis, are applied to a 3-story, 2-bay, RC frame. They are (i) the Capacity Spectrum Method (CSM), described in ATC-40, (ii) the Displacement Coefficient Method (DCM), presented in FEMA-273 and further developed in FEMA 356, and (iii) the N2 Method, implemented in the Eurocode 8. Pushover analyses are conducted with DRAIN-3DX by using four different lateral force distributions, according to the acceleration profile assumed along the height of the structure: uniform, triangular, modal-proportional, and multimodal fully adaptive. In the numerical model, RC members are modeled as fiber elements.

The numerical predictions of each method are compared to the experimental results of the shaking table tests carried out on two similar 1:3.3-scale structural models, with and without infilled masonry panels, respectively. The comparison is made in terms of maximum story displacements, interstory drifts, and shear forces. All the NSM's are found to predict with adequate accuracy the maximum seismic response of the structure, provided that the associated parameters are properly estimated. The lateral load pattern, instead, is found to little affect the accuracy of the results for the three-story model considered, even if collapse occurs with a soft story mechanism.     

A SIMPLE METHOD FOR ANALYSIS OF SLIDING STRUCTURES CONSIDERING VARIATIONS OF FRICTION COEFFICIENT

In this paper, the responses of multidegree of freedom (MDOF) structures on sliding foundations, subjected to harmonic or random base motions, are investigated taking into consideration the variations of friction forces. The variation of friction force is a consequence of variation of friction coefficient, which depends on such parameters as relative velocity and the existing pressure. Modelling of the friction force under the foundation raft is accomplished by using a fictitious rigid link with a rigid-perfectly plastic material. This results in identical equations of motion for the sliding structure, both in the sliding and nonsliding (stick) phases and considerably decreases the required number of time steps for the nonlinear analysis. Since the force in the link is of constant value, to consider the varying friction force, a compensatory force, which is the difference between the exact friction force and the constant force in the rigid link, is applied to the foundation raft. A model of variable friction coefficient for Teflon-steel interfaces is used for the assessment of the method and the results are compared with existing literature, through which, the capability of the method is illustrated. It is shown that by using exact model of friction lower values for the superstructure responses are predicted compared with those obtained by using Coulomb friction model. Furthermore the effect of the stiffness of the structure on the differences between the results of the two models is also studied.     

A SIMPLIFIED METHOD FOR ELASTIC SEISMIC ANALYSIS OF HILL BUILDINGS

Abstract

Buildings on hill slope are highly irregular and asymmetric in plan and elevation. They are subjected to severe torsion in addition to lateral forces under the action of earthquakes. In the present study, simplified 3D dynamic analysis of hill buildings based on transformation of stiffness and mass of various components about a common arbitrarily-chosen reference axis is presented. Few actual hill building problems have been analysed with the simplifed method and the rigorous method of analysis. The results of the two methods of analysis advocate the use of the simplified method in the Code of Practices of different countries. With the use of the simplified method, the configurations for buildings on hill slope can be decided by taking various trial configurations so as to obtain the most economical and safe design from the seismic point of view.     

COUPLED P-Y T-Z ANALYSIS OF SINGLE PILES IN COHESIONLESS SOIL UNDER VERTICAL AND/OR HORIZONTAL GROUND MOTION

Various approaches are currently used for the analysis of piles under vertical and lateral loading. Among these, the beam-on-a-nonlinear Winkler foundation (BNWF) approach using published P-y, T-z and Q-z curves is widely used in practice. In this approach, the P-y and T-z responses are generally uncoupled from each other. The objective of this paper is to investigate the influence that the coupling of the P-y and T-z responses has.on the cyclic and dynamic response of piles in cohesionless soil. A cyclic model is first developed and a parametric study is conducted to investigate the effect the initial confining pressure, angle of wall friction and effective vertical stiffness have on the lateral cyclic hysteretic response. A dynamic model is then developed, and used to study the response of a single pile in cohesionless soil under horizontal and/or vertical ground motion. Results from the parametric study showed that the three parameters did not have a significant influence on the lateral cyclic hysteretic response. Under horizontal and/or vertical ground motion, the horizontal ground motion was observed to dominate the inertial interaction response, and significantly affected both the horizontal and vertical displacement response, mainly due to second-order P-Δ and gapping effects.     

Integration of use-wear with protein residue analysis – a study of tool use and function in the south Scandinavian Early Neolithic

This study presents research on how an integration of use-wear analysis with protein residue analysis can produce new results on prehistoric tool use and function. Thirty flint artefacts from an Early Neolithic TRB site in south Sweden were analysed for both use-wear and protein residues. The results show a positive correlation between the two methods. The Early Neolithic tools analysed were primarily used to process fish.