Cutoff Frequencies for Impulse Response Tests of Existing Foundations

The impulse response test is a nondestructive evaluation technique commonly used for quality control of driven concrete piles and drilled shafts where the pile heads are accessible. When evaluating existing foundations, the presence of a pile cap or other structure makes the pile heads inaccessible and introduces uncertainties in the interpretation of impulse response results. A test section was constructed at the National Geotechnical Experimentation Site (NGES) at Northwestern University to examine the applicability of nondestructive testing methods in evaluating deep foundations under inaccessible-head conditions. This paper focuses on the results of impulse response tests conducted atop the three pile caps at the NGES. Based on field experimentation and numerical simulations, a frequency was determined below which the impulse response test could be used for inaccessible-head conditions. This cutoff frequency primarily depends upon the geometry of the pile cap and pile. A case study is presented that describes impulse response tests obtained on a number of drilled shafts both after the shaft was constructed and after grade beams and walls were built. The results of these tests also follow the trends observed in the NGES tests related to cutoff frequency.     

Coupled Response for Rigid Foundations

A matrix formulation to determine the coupled response of rigid foundations where soil properties are modeled by springs and dashpots is presented. Location and orientation of springs and dashpots can be arbitrary, thus a general solution is determined. The response is given in terms of translational and rotational displacements considering coupled effects. Physics of the problem presented here has been extensively studied and a broad range of useful formulae to determine springs and dashpots properties for soil-structure interaction is available. These published formulae for springs and dashpots properties are an input to the approach presented. Thus, the novelty of this approach is a matrix manipulation that leads to a simple expression allowing coupling all degrees of freedom even when springs and dashpots are not orthogonally oriented. For validation purposes, finite element solutions are compared with the approach presented.     

空间声场脉冲响应辨识及应用

介绍了采用伪随机信号相关辨识法获得空间声场脉冲响应的实验方法,并用多种实验对该方法作了验证．还将此方法应用于空间有源噪声控制,取得了满意的结果．     

Evaluation of Reliability of Platform Pile Foundations

A primary consideration and concern in development and implementation of risk assessment and management based hurricane and earthquake criteria for design and requalification of platforms in the Bay of Campeche criteria is the reliability characteristics of the pile foundations that support the platforms. Analyses of the ultimate limit state (ULS) performance characteristics of the platform pile foundations are summarized. These analyses indicated that in many cases when the capacities of the piles were based on American Petroleum Institute “static” pile capacity analysis methods, the piles were the most likely to fail elements in the platforms. Results from these analyses were in dramatic contrast with the performance of pile foundations supporting more than 250 platforms during the 100-year hurricane Roxanne (1995). Detailed analytical and field studies were performed to determine the ULS performance characteristics of pile foundations subjected to hurricane induced lateral and axial loadings. This study addressed all of the phases in the life cycle of the piles. The results of this work indicate that, in general, there can be large biases in the predicted capacities of piles that support the platforms.     

Theoretical Interpretation of Impulse Response Tests of Embedded Concrete Structures

For a concrete beam resting on a bed of sand, an analytical solution technique is derived by which the mobility can be identified. To achieve realistic predictions, significant damping in the bed needs to be introduced. The modest damping of the concrete has little effect on the mobility for small frequencies whereas it has a significant effect for higher frequencies. An imperfection in the bed in terms of a void increases the mobility dramatically for low frequencies whereas the mobility for higher frequencies is almost unchanged. An imperfection in the beam in terms of honeycombing of the concrete, on the other hand, manifests itself by increasing the mobility for high frequencies while leaving the mobility for small frequencies less influenced. These latter conclusions are in good agreement with field experience for concrete slabs resting on soil.     

Extensive Development of Leak Detection Algorithm by Impulse Response Method

The oscillatory flows in pipeline systems due to excitation by valve operation are efficiently analyzed by the impulse response method. The impact of leakage is incorporated into the transfer functions of the complex head and discharge. Frequency-dependent friction is used to consider the impact of unsteady friction for laminar condition. Extensive development of the impulse response method was made by considering the sources of friction associated with the local and convective acceleration of velocity for turbulent flow. The genetic algorithm was integrated into the impulse response method to calibrate the location and the quantity of leakage. The calibration function for leakage detection can be made using the pressure-head response at the valve, or the pressure-head and flow response at the section upstream from the valve. The proposed leak detection algorithm shows the potentials for being applied to a simple pipeline system with a single leak or multiple leaks.     

Multiple Resistance Factor Design for Shallow Transmission Line Structure Foundations

This paper presents the development of simplified reliability-based design (RBD) equations that are suitable for spread foundations subjected to uplift. Emphasis is placed on the loading and foundation characteristics relevant to the electric utility industry. A general reliability calibration procedure is used to derive robust resistance/deformation factors for load and resistance factor design (LRFD) and multiple resistance factor design (MRFD) formats. Two target reliability indices of 3.2 and 2.6 are proposed based on an extensive study of existing designs for ultimate and serviceability limit state, respectively. The main advantage of using these RBD factors is that a known level of reliability can be consistently achieved over a wide range of design conditions. Simple design calculations using the MRFD format are shown to demonstrate their ability to account for parametric uncertainties in a rational manner.     

Predicting Settlement of Shallow Foundations using Neural Networks

Over the years, many methods have been developed to predict the settlement of shallow foundations on cohesionless soils. However, methods for making such predictions with the required degree of accuracy and consistency have not yet been developed. Accurate prediction of settlement is essential since settlement, rather than bearing capacity, generally controls foundation design. In this paper, artificial neural networks (ANNs) are used in an attempt to obtain more accurate settlement prediction. A large database of actual measured settlements is used to develop and verify the ANN model. The predicted settlements found by utilizing ANNs are compared with the values predicted by three of the most commonly used traditional methods. The results indicate that ANNs are a useful technique for predicting the settlement of shallow foundations on cohesionless soils, as they outperform the traditional methods.     

Simple Formulas for the Response of Shallow Foundations on Compressible Sands

The engineering design of shallow foundations on sand is almost universally based on one of the variants of the classical bearing capacity formula. However, this formula is suitable only where the sand exhibits dilative behavior and a clear rupture mechanism forms at failure. The main challenge then is choosing a suitable friction angle, taking into account the soil density and the high stresses beneath the footing. When other conditions apply, in particular when the footing is large or founded on compressible materials, alternative approaches need more focus on soil compressibility. Two simple semianalytical formulas are proposed and explored in this paper: (1) an analysis using a one-dimensional (1D) compression equation; and (2) an analysis using the concept of “bearing modulus.” It is argued that the bearing modulus approach may be used for conditions that reflect moderate design parameters (i.e., moderate foundation size and sand compressibility), but for very large foundations or highly compressible soils the 1D compression method is found more suitable. It is shown that the bearing modulus analysis can be approached in terms of the compression response of the soil, suggesting a possible route to link the bearing modulus directly to the compression model parameters of the soil.     

Numerical Modeling for Hydraulic Resonance in Hydropower Systems Using Impulse Response

An effective numerical method to compute hydraulic resonance in pressurized piping of hydropower systems is presented. For this purpose, the impulse response method is used, i.e., a unit pressure impulse is introduced at the downstream waterway as an exciter. The method of characteristics is used to solve the governing equations for the hydraulic transient and to get the pressure response of the system in the time domain. The discrete Fourier transform is used to compute the frequency response of the system which gives the resonance frequencies in the hydropower plant system. To increase the accuracy of the results, unsteady friction is incorporated into the methodology. The influence of the unsteady friction, wave speed, and power on the pressure response diagram is investigated for the waterway of a multiunit hydropower plant which has been recently installed. Computed results agree very well with those obtained from the standard method of the characteristics.     

Impulse Response of Elastic Half-Space in the Wave Number–Time Domain

This paper presents formulas for the response functions in the mixed wave number–time domain for a homogeneous, elastic half-space subjected to impulsive, spatially harmonic sources on its surface. These functions are useful when obtaining the wave field in a half space elicited by dynamic surface sources of arbitrary spatial distribution on the surface, in either two or three dimensions. The formulas in this paper are obtained by contour integration of the Green’s functions in the frequency–wave number domain. The correctness and accuracy of these solutions is then assessed by comparison with the results of the well-known transient response functions for suddenly applied loads in both two and three dimensions.     

Impulse Response Method for Pipeline Systems Equipped with Water Hammer Protection Devices

Surge protection devices, such as surge tanks and air chambers, have been modeled with the impulse response method for transient analysis of water distribution systems. The lumped inertia model and continuity equation are used to represent nonpipe hydraulic elements. Results of pressure or discharge variations obtained by using the impulse response method and the method of characteristics are in good agreement. The impulse response method provides total pressure and discharge along any pipeline segment by direct integration of the ratio of complex head or complex discharge to a complex downstream discharge, respectively. A modification is proposed so that transition between turbulent and laminar flows can be considered. The representation of hydraulic devices has been incorporated into the impedance matrix method, which was developed for heterogeneous and multilooped pipe network systems. The potential advantages of the proposed method over other conventional approaches were investigated by applying the proposed method to hypothetical pipe network systems.     

Response of Elastic Continuum Carrying Multiple Moving Oscillators

The problem of calculating the dynamic response of a one-dimensional distributed parameter system carrying multiple moving oscillators is examined. A solution procedure is suggested that reduces the problem to the integration of a system of linear ordinary differential equations governing the time-dependent coefficients of the series expansion of the response in terms of the eigenfunctions of the continuous structure. The program implementation of the solution procedure is discussed and numerical results are presented. Numerical illustrations clearly demonstrate that the incorporation of even one additional oscillator into the model makes the dynamics of the system vibration considerably more complicated.     

Dynamic and Seismic Analysis of Foundations based on Free Field B-Spline Characteristic Response Histories

A direct time domain formulation for the analysis of unbounded media and foundations is developed that treats dynamic excitations and ground motion in a uniform manner. The method uses the boundary element method with higher order B-Spline fundamental solutions to compute the characteristic responses of the surface of the elastodynamic domain. Subsequently, time histories of the system response to general excitations are computed by a mere superposition scheme that accommodates in a uniform manner arbitrary time histories of external loads and/or ground motion. The characteristic responses are computed in the form of time dependent flexibility matrices of the medium that are sparse due to the finite duration of the B-Spline excitation signal and the characteristics of the wave propagation. The duration of the B-Spline impulse response is limited to only a few time steps. Consequently, significant savings in computing time and storage requirements are achieved. Furthermore, the characteristic responses do not depend on the type or wave form of the actual external excitations and the presence of rigid foundations. This is a significant advantage when the response of a system to excitations of long duration is to be computed. In addition, the proposed approach significantly reduces the size of the problems under consideration and yet fully considers the effects of the free field. The significance of nonrelaxed boundary conditions and correct representation of the free field is established. The method is demonstrated and validated through applications pertaining to the analysis of foundations and inclusions subjected to transient loads and seismic excitations.     

Multiple Hypnotizabilities: Differentiating the Building Blocks of Hypnotic Response.

Although hypnotizability can be conceptualized as involving component subskills, standard measures do not differentiate them from a more general unitary trait, partly because the measures include limited sets of dichotomous items. To overcome this, the authors applied full-information factor analysis, a sophisticated analytic approach for dichotomous items, to a large data set from 2 hypnotizability scales. This analysis yielded 4 subscales (Direct Motor, Motor Challenge, Perceptual-Cognitive, Posthypnotic Amnesia) that point to the building blocks of hypnotic response. The authors then used the subscales as simultaneous predictors of hypnotic responses in 4 experiments to distinguish the contribution of each component from general hypnotizability. This analysis raises interesting questions about how best to conceptualize and advance measurement of the ability to experience hypnosis. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Earthquake Response and Energy Evaluation of Inelastic Structures

A computational method is derived to characterize the energy in inelastic structures and the transfer among various energy forms over the duration of an earthquake. This computational method is based on the force analogy method, which uses a change in displacement field to represent the inelastic behavior of structure instead of the traditional method of changing stiffness. The evaluation of plastic energy due to inelastic deformation in the structure becomes very simple using the force analogy method, where the accumulation of plastic energy due to plastic rotations is exactly equal to the elastic moment multiplied by the change in plastic rotations. In addition, this plastic energy formula can be used for any material with predefined stress–strain relationship, and therefore the transfer of energy among various forms can be calculated at any specific time. Once the energy equation is derived, numerical analyses are performed on a single degree of freedom system to study the characteristics of energy transfer. This is then extended to study the transfer of energy among various forms in a multidegree of freedom system. These two studies show that the analytically derived equation for plastic energy is accurate in studying the structural energy response due to earthquake excitations.     

Economic Design Optimization of Foundations

A geotechnical foundation design should address at least three basic requirements: ultimate limit state (ULS), serviceability limit state (SLS), and economics. Most conventional design approaches focus on ULS and/or SLS optimization, with economics being evaluated afterwards. As an alternative, this paper develops a design approach that explicitly considers the construction economics and results in a foundation that has the minimum construction cost. This design approach is expressed as an optimization process, in which the objective is to minimize construction cost, with the design parameters and design requirements as the optimization variables and constraints, respectively. This design approach is illustrated using a spread footing example. Because construction costs vary by locale, the economically optimized designs differ regionally. Sensitivity studies on soil properties and design requirements show that, for typical spread footing designs in cohesionless soils, Young’s modulus (E) and the effective friction angle (?′) are the key parameters. A quantitative assessment illustrates the importance of soil property variability on cost. It is also found that, for typical spread footing designs, a relatively stringent ULS requirement generally ensures fulfillment of the SLS requirement.     

Lateral Response Evaluation of Single Piles Using Inclinometer Data

In an effort to develop an efficient method for interpretation of lateral pile load test results via measured inclinometer data only, an analytical model is proposed based on energy conservation of a pile-soil system. A Fourier series function is used to represent deflection behavior of the pile-soil system. In order to obtain shear, moment, and soil reaction along the pile shaft, convergence of the series after differentiation is guaranteed by applying the Cesaro sum technique. The concrete cracking effect is also incorporated into the pile model to account for yielding of the pile itself. Three full-scale pile load cases are then used to verify the feasibility of the developed methodology as well as make comparison to other methods.