Impedance Function Research Articles

Factors associated with coronary heart disease in COPD patients and controls.

COPD and coronary heart disease (CHD) frequently co-occur, yet which COPD phenotypes are most prone to CHD is poorly understood. The aim of this study was to see whether COPD patients did have a true higher risk for CHD than subjects without COPD, and to examine a range of potential factors associated with CHD in COPD patients and controls. 347 COPD patients and 428 non-COPD controls, were invited for coronary computed tomography angiography (CCTA) and pulmonary CT. Arterial blood gas, bioelectrical impedance and lung function was measured, and a detailed medical history taken. The CCTA was evaluated for significant coronary stenosis and calcium score (CaSc), and emphysema defined as >10% of total area <-950 Hounsfield units. 12.6% of the COPD patients and 5.7% of the controls had coronary stenosis (p<0.01), whereas 55.9% of the COPD patients had a CaSc>100 compared to 31.6% of the controls (p<0.01). In a multivariable model adjusting for sex, age, body composition, pack-years, CRP, cholesterol/blood pressure lowering medication use and diabetes mellitus, the OR (95% CI) for having significant stenosis was 1.80 (0.86-3.78) in COPD patients compared with controls. In a similar model, the OR (95% CI) for having CaSc>100 was 1.68 (1.12-2.53) in COPD patients compared with controls. Examining the risk of significant stenosis and CaSc>100 among COPD patients, no variable was associated with significant stenosis, whereas male sex [OR 2.85 (1.56-5.21)], age [OR 3.74 (2.42-5.77)], statin use [OR 2.23 (1.23-4.50)] were associated with CaSc>100, after adjusting for body composition, pack-years, C-reactive protein, use of angiotensin converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs), diabetes, emphysema score, GOLD category, exacerbation frequency, eosinophilia, and hypoxemia. COPD patients were more likely to have CHD, but neither emphysema score, lung function, exacerbation frequency, nor hypoxemia predicted presence of either coronary stenosis or CaSc>100.

Investigation of Beraprost Sodium on Cardiac Function and Hemodynamics in Canine Models of Chronic Pulmonary Hypertension.

Pulmonary hypertension (PH) is a life-threatening disease in dogs characterized by increased pulmonary arterial pressure (PAP) and/or pulmonary vascular resistance. No study has evaluated the utility of Beraprost sodium (BPS) in dogs with PH. This study aimed to evaluate the effect of BPS on cardiac function and hemodynamics and examine the optimal dose of BPS in canine models of chronic embolic PH. In this prospective crossover study, three doses of BPS (5, 15, and 25 μg/kg, twice a day) were examined in eight canine models of chronic embolic PH. All model dogs underwent invasive PAP measurement, echocardiography, and non-invasive systemic blood pressure measurement before and after continuous administration of oral BPS for 1 week. No side effects of BPS were observed in any dog during the study. All doses of BPS significantly decreased systolic PAP and pulmonary vascular impedance. Additionally, systemic vascular impedance significantly decreased with 15 and 25 μg/kg of BPS. The right ventricular stroke volume and longitudinal strain significantly decreased with all doses of BPS. The left ventricular stroke volume and circumferential strain decreased with 15 μg/kg BPS. BPS was well-tolerated in this study. A dose-dependent vasodilating effect on pulmonary vessels was observed in canine models of chronic PH. Additionally, 15 μg/kg BPS showed a balanced vasodilating effect on systemic and pulmonary vessels. Furthermore, with a decrease in systemic and pulmonary vascular impedance, the left and right ventricular functions were significantly improved. Our results suggest that BPS may be useful in the treatment of canine PH.

Microstructural and mechanical characterization of HSLA X70 Welded joints using eddy current testing (ECT)

This paper proposes an approach for measuring the mechanical properties by non-destructive testing techniques using eddy current testing (ECT), for a welded and heat-treated microalloyed material (X70) dedicated to hydrocarbon transport. This study offers a new measurement method based on linear approach, to expect mechanical property in microstructures by using Non-destructive testing (ECT) that can apply to mechanical systems, materials, weldments, etc. The result show that hardness measurements are in broad have the same trend with impedance function which depend of heat treatment variable applied to weld joint.

Seismic-Derived Rock True Resistivity (R) Revisited. Part I: Reformulation Of Combined Gassmann – Shaly Sand Models

The last decade has observed frantic efforts by geoscientists to extract as much information as possible from seismic data. From the traditional role of establishing subsurface structural geometry, seismic processing and interpretations have evolved into an ever increasing role in providing rock physical properties such as acoustic impedance (AI) and porosity (0). The more common use of 3-D seismic surveys, in both exploration and development stages, have fur- ther underlined the role of seismic data as provider of inter-well rock property data. Further developments in the petrophysics-related seismic interpretation have also shown efforts to ex- tract information related to contents of formation rocks. From the widely acknowledged brightspot analysis for detecting presence of gas-bearing porous rocks in the last decades of the 20 century to the later efforts to extract information regarding fluid saturation in reservoir. Actually, as early as in mid- 1960s have scientists started to investigate the relations between acoustic signals and fluid saturation (e.g King, 1966; Domenico, 1976; Gregory, 1976). However, due to the fact that the then commonly used of 2-D seismic was considered as having insuf- ficient resolution for any practical uses in the field, the efforts remained mainly for academic purposes only. Rapid developments in technology of 3-D seis- mic survey and processing, as well as its more com- mon use at present have prompted attentions back to the investigations aimed at extracting fluid saturation information from seismie data. In 1990s have Widarsono and Saptono (1997) started a series of investigation through laboratory measurements and modeling using core samples. This was followed by more works not only at laboratory level but at larger levels of well and field scales (e.g. Widarsono Saptono, 2000a, 2000b, and 2001; and Widarsono et al, 2002a, 2000b). Other investigators (e.g. Furre Brevik, 2000; Wu, 2000; Zhu et al, 2000; and more recently Wu et al, 2005) have also devoted some works to achieving the same goal. Other paths of development have incorporated other supporting tech- niques such as non-linear regression (e.g.Balch et al, 1998) and artificial neural network (e.g. Poupon Ingram, 1999; Oldenziel et al, 2000).From various investigations using seismic waves as the sole data for fluid saturation extraction, short- comings were soon felt in the form of 'narrow bands' of acoustic signals (ie P-wave velocity, V, and acoustic impedance, AI) that are influenced by varia- tions in fluid saturation. In other words, V, and AI are not too influenced by variation in fluid saturation. This reduces the effectiveness of seismic-derived V and Al as fluid saturation indicators. Efforts were then devoted to link V, and AI to other parameters such as rock true resistivity (R), a parameter known to be very sensitive to variation in fluid saturation. Widarsono and Saptono (2003, 2004) provide laboratory verifications and first field trial with some degree of succes. However, certain assump- tions (i.e. constant/uniform porosity) in the theoreti- cal formulation were still adopted in the above works, which in turn reduced the validity of the resulting formula produced and used. In this paper, the first part of a three-part work, is devoted to reformulating the combination of Gassmann theory and shaly sand water saturation models of Poupon and Hossin. These are to replace the shale-free Archie model used in the above works, which is considered as invalid for most field uses. With this reformulation, it is hoped that a more robust model/formula of resistivity as a function of acoustic impedance (R, = fAI)) is achieved, hence a more reliable resistivity could be extracted from seismic- derived acoustic impedance. Summarily, the objectives of the works a part of them presented in this paper are - To establish a model/method to obtain formation rock true resistivity (R) from seismic-derived acoustic impedance (AI), and To provide correction/modification onto previous works reported in Widarsono Saptono (2003, 2004).

SEISMIC-DERIVED ROCK TRUE RESISTIVITY (Rt ) REVISITED. PART II: REFORMULATION USING WYLLIE’S TIME-AVERAGE MODEL

This paper is the second of a three-part presentation. As highlighted in the previous paper (Part I, Widarsono Mendrofa, 2006), the main objective of the study is to re-evaluate the potential of acoustic impedance as a source of resistivity data. This essentially came from the very idea of extracting information of resistivity (Rt ), data that plays a very important role in the determination of water saturation in reservoir, from seismic-derived acoustic impedance (AI).As observed in the past view years, there have been a lot of efforts devoted to the extraction of water saturation information from seismic. However, as Widarsono Mendrofa (2006) put it, most of the efforts were mainly based on pattern recognition activities with little attention was given to the theoretical aspects of relationships between seismic signals and water saturation. The work reported in this threepart presentation is concentrated more as re-establishing (a reformulation of works reported in Widarsono Saptono, 2003; 2004) the theoretical relationship between resistivity and acoustic impedance.In the Part I (Widarsono Mendrofa, 2006), a reformulation between the classical Gassmann acoustic velocity model and shally sand models of Modified Simandoux and Hossin is presented. In the reformulation, a new resistivity function of acoustic impedance has been established. In principle, whenever acoustic impedance data from seismic has been made available resistivity data for the determination of fluid saturation can be estimated.Despite the theoretical correctness of the resistivity function presented in the Part I, practicallity is not the function’s best aspect. In other words, the resistivity function is not an easy one to be used practically. Various parameters (e.g. matrix moduli) have to be assumed, since the data cannot easily measured even in the laboratory. This is indeed the main reason why gassmann model, and others such as Biot, has not been used much in day-to-day practices such as log interpretation for porosity determination.Being aware of such difficulties, in 1954 M.R.J. Wyllie et al proposed their “time average” model (named after its proportional averaging of pore fluid, rock matrix, and shale transit time values to represent transit time of a fluid-filled porous medium) for any practical uses related to P-wave velocity in porous media. Due to its simplicity, the model, as well as its subsequent modifications, has been used extensively since then in some areas especially in log analysis for porosity determination. Considering this simplicity aspect, this three-part study also adopted Wyllie “time average” model into its reformulation works. This Part II paper presents the formulation using Wyllie and the two shally sand models following the same manner that was adopted and presented in the Part I paper.Summarily, the objectives of the works presented in this paper are:- To establish a model/method to obtain formation rock true resistivity (Rt ) from seismic-derived acoustic impedance (AI),- To provide correction/modification onto previous works reported in Widarsono Saptono (2003, 2004), and- To provide a simpler alternative to the resistivity function yielded from the reformulation works presented in Part I paper (Widarsono Mendrofa, 2006)

Impedance functions of rigid rectangular foundations bonded on layered transversely isotropic elastic/poroelastic half-space

This paper explores a complete set of impedance functions including vertical, torsional, horizontal, rocking, and coupling impedances for a surface rigid rectangular foundation bonded on a multilayered transversely isotropic half-space composed of alternatively elastic and poroelastic layers with the use of boundary element method. The slow convergence issue of Green's functions in numerical evaluation is remedied by a simple asymptotic approach presented in this paper. To evaluate the impedances accurately and efficiently, new finite elements namely exponentially-gradient elements are employed to consider the singular behavior of tractions that happens at the edges and corners of the foundation. The validity and accuracy of the solutions are verified by comparing the results with some simpler existing cases. The impedance functions are presented for different aspect ratios of the foundation to be applicable for a variety of structures. The effects of anisotropy, inhomogeneity, layer thickness, underground water level, and soil damping are assessed.

Vertical Dynamic Impedance of a Viscoelastic Pile in Arbitrarily Layered Soil Based on the Fictitious Soil Pile Model

The vertical vibration of a viscoelastic pile immersed in arbitrarily layered soil is investigated by taking the interaction among pile, pile surrounding soil (PSS) and pile end soil (PES) into account. Firstly, considering both the stratification and stress wave effect of soil, a mathematical model of the pile–soil system is established based on the fictitious soil pile (FSP) model. Then, utilizing the impedance function transfer method and Laplace transform technique, the analytical solutions of the vertical dynamic impedance of pile are derived in the frequency domain. The analytical solutions are validated by comparing them with other existing solutions. Finally, a parametric study is put forward to investigate the properties of PES on the vertical dynamic impedance of pile. The results reveal that the properties of PES have a significant effect on the vertical dynamic impedance of pile, but there is a critical influence thickness for this effect. For the cases of the PES thickness exceeding the critical influence thickness, further increase of PES thickness will not affect the dynamic behavior of the pile–soil system.

Optimization of train timetables in high-speed railway corridors considering passenger departure time and seat-class preferences

ABSTRACT There are a number of trains running in high-speed railway (HSR) corridors throughout the day, which results in diverse passenger travel choices. Passengers can select their preferred train according to their favorite departure time and seat class based on a fixed train timetable, that is, the selection differences from departure time to seat classes are dramatic. The train timetable determines the distribution of passenger flow on trains, and the requirement distribution in turn affects the train timetable. To address this game relationship, this paper develops a method to optimize uneven running train timetables with a given total number of trains considering passenger departure time and seat-class preferences. We analyze the impact of departure time and seat class on passenger choice behaviors and build a time-space-seat 3-dimensional network to account for these choices. Then, the impedance function of each type of arc in the network is designed. In addition, a bi-level programming model is constructed to optimize the train timetable in the HSR corridor; the upper-level model calculates train departure time, arrival time and dwelling time at each station and determines the assignment of car lists with different seat classes for each train. The lower-level model is used to distribute the passenger flow to each train. Combined with the user equilibrium principle, a complex genetic algorithm embedded with the Frank–Wolfe method is designed to reasonably distribute passenger flows to each train. Finally, we take the Lanzhou-Xi’an HSR as an example to test both the model and the algorithm. The results show that the optimal train timetable can meet the requirements of both seat class and departure time with appropriate solution time limitations.

On the modeling of frequency- and intensity-dependent impedance functions at the head of horizontally loaded single piles

This paper proposes a phenomenological model that represents both the frequency and intensity dependencies of the dynamic force–displacement relationship at the head of horizontally loaded piles. The present model consists of a bilinear spring unit with hysteretic characteristics and a so-called “Gyro-Lumped Parameter Model (GLPM)” unit arranged in series. The former unit represents the static yielding procedure of soil-pile systems, while the latter represents the frequency-dependent characteristics of impedance functions. Firstly, this study validates the general behavior of the proposed model and then verifies it by simulating the experimentally obtained pile head impedance functions of a single pile for small to large amplitudes of loading in a wide range of frequencies. The results show that the proposed model can sufficiently reproduce variations in the frequency-dependent characteristics of impedance functions for a wide range of loading amplitudes.

Freight distribution analysis and modelling of inland waterway transport using big data

The Yangtze River economic belt (YREB) serves as the main east–west axis of China to promote economic development and environmental protection along the Yangtze River (YR). The factors that affect the freight distribution of major types of cargo transported through the YR were analysed using automatic identification system data and ship visa data. First, a set of freight impedance functions was developed for different types of links of the waterway network, considering a number of factors (e.g. cargo type, delays at ship locks, water levels and flows at different waterway segments and upstream and downstream shipping speeds). Distance-based (DB) and time-based (TB) impedance matrices of different types of cargo were computed. A gravity model (GM) and an intervening opportunity model were then used to simulate the distributions of different types of cargo based on the computed impedance matrices. A trip length distribution method was applied to validate the estimated distribution models. The results showed that the GM with a power term outperformed the other models and that the TB models were superior to the DB models for the prediction of freight distributions over large geographies like the YREB. This work offers an in-depth understanding of the freight characteristics of inland waterways and therefore should be helpful for relevant authorities in formulating port and inland waterway plans and policies.

Emergency medical response in mass casualty incidents considering the traffic congestions in proximity on-site and hospital delays

The response efficiency of Emergency Medical Service (EMS) is related to the survival rate of patients in Mass Casualty Incidents (MCIs). This study aims to reduce delays caused by congestions in the transportation network and also that in hospitals. A patient transportation and assignment model is proposed considering routing of ambulances and operational conditions of hospitals for an efficient MCI response. The proposed model is composed of a Cell Transmission Model (CTM) and a nonlinear treatment impedance function. A Lagrangian heuristic is utilized to expedite model solution by decomposing the problem into two relatively tractable sub-problems: one linear and the other nonlinear. The linear sub-problem is solved by the simplex algorithm, and is related to ambulance routing, while the nonlinear sub-problem is solved by a gradient projection algorithm for the optimization of patients’ hospital assignment. A case study and several benchmark networks were tested, and the proposed methodology outperformed a typical Lagrangian Relaxation approach. This work has the potential to enable a more efficient patient assignment in MCIs.

Integrating fuzzy set function into floating catchment area measures: a determination of spatial accessibility of service points

ABSTRACT The use of discrete (binary or zonal/step-wise) instead of continuous travel impedance functions in floating catchment area (FCA) measures generates inconsistent and less reliable accessibility indices that might be overestimating or underestimating actual accessibility. Thus, this study illustrates the limitations of discrete travel impedance function in the enhanced two-step (E2S) FCA method; and develops a fuzzy-based continuous travel impedance function by combining the degree to which travel distances belong to different sub-zones of catchment areas and their respective E2SFCA zonal weights using the weighted mean fuzzy-set operation. With the developed fuzzy-based travel impedance function, a new accessibility measure, fuzzy-based two-step (F2S) FCA is defined. The E2SFCA and F2SFCA measures were implemented in the hypothetical set up and Rural Wards of Dodoma Urban District in Tanzania to determine spatial accessibility of service points and water points, respectively. The resulting E2SFCA and F2SFCA accessibility indices portrayed a slightly similar pattern. However, F2SFCA accessibility indices varied among households within and across sub-zones, while E2SFCA accessibility indices were identical within but changed abruptly across sub-zones. The variations in F2SFCA accessibility indices reflect the reality and to a greater extent Tobler’s first law of Geography. Thus, the developed F2SFCA measure could be used to measure spatial accessibility of other services or commodities as it properly models travel impedance. The F2SFCA measure could further be improved to generate much more realistic accessibility indices by capturing local instead of total potential demand, commonly modelled in FCA measures.

Control Framework for Sloped Walking With a Powered Transfemoral Prosthesis.

User customization of a lower-limb powered Prosthesis controller remains a challenge to this date. Controllers adopting impedance control strategies mandate tedious tuning for every joint, terrain condition, and user. Moreover, no relationship is known to exist between the joint control parameters and the slope condition. We present a control framework composed of impedance control and trajectory tracking, with the transitioning between the two strategies facilitated by Bezier curves. The impedance (stiffness and damping) functions vary as polynomials during the stance phase for both the knee and ankle. These functions were derived through least squares optimization with healthy human sloped walking data. The functions derived for each slope condition were simplified using principal component analysis. The weights of the resulting basis functions were found to obey monotonic trends within upslope and downslope walking, proving the existence of a relationship between the joint parameter functions and the slope angle. Using these trends, one can now design a controller for any given slope angle. Amputee and able-bodied walking trials with a powered transfemoral prosthesis revealed the controller to generate a healthy human gait. The observed kinematic and kinetic trends with the slope angle were similar to those found in healthy walking.

Applications of the Carathéodory’s Inequality for Driving Point Impedance Functions

In this study, the Carathéodory’s Inequality, which is a highly popular topic of complex analysis theory, has been applied to electrical engineering to obtain novel driving point impedance functions. In electrical engineering, driving point impedance functions correspond to positive real functions and they are used for representation of the spectral characteristics of a particular circuit. Accordingly, boundary version of the Carathéodory’s inequality has been considered here assuming that the driving point empedance function, Z(s) has a fractional function structure with 0 =0 and it is analytic in the right half plane. At the end of the analyses, new driving point impedance functions have been obtained and they have been presented with their spectral characteristics. According to simulation results, it is possible to say that the frequency responses of the obtained generic driving point impedance functions have spiky filter structures where the number of the spikes in the frequency response of these filters depend on a pre-defined parameter, n.

Circuit Applications of Schwarz-Pick Lemma

In this brief, the relation between Schwarz-Pick lemma and driving point impedance function is investigated. Schwarz-Pick lemma is given as the generalized version of Schwarz lemma. Here, a generic driving point impedance function is obtained by applying Schwarz-Pick lemma to positive real functions. At first, a novel inequality is obtained for the modulus of the derivative of the driving point impedance function and in the next step, an extremal function is found by applying sharpness analysis to this inequality. At the end, obtained extremal function is considered as a generic driving point impedance function and corresponding circuit schematics are presented with their spectral analyses. Obtained driving point impedance function is not an arbitrary function but it is the intuitive result of the considered problem in this study. Accordingly, it is guaranteed to obtain a practically realizable driving point impedance function. Also, novel upper bounds are determined for the magnitude of driving point impedance function and for its derivative.

Modelling the Impedance Response of Graded LiFePO4 Cathodes for Li-Ion Batteries

Graded electrodes for Li-ion batteries aim to exploit controlled variations in local electrode microstructure to improve overall battery performance, including reduced degradation rates and increased capacity at high discharge rates. However, the mechanisms by which grading might deliver performance benefit, and under what conditions, are not yet fully understood. A Li-ion battery electrochemical model (a modified Doyle-Fuller-Newman type model capable of generating impedance functions) is developed in which local microstructural changes are captured in order to understand why and when graded electrodes can offer performance benefits. Model predictions are evaluated against experimental electrochemical impedance data obtained from electrodes with micro-scale, controlled variations in microstructure. A region locally enriched with carbon at the electrode/current collector interface is shown to significantly reduce the overpotential distribution across the thickness of a LiFePO4-based Li-ion battery cathode, resulting in a lower charge transfer resistance and impedance. The insights gained from the LiFePO4-based electrodes are generalised to wider design principles for both uniform and graded Li-ion battery electrodes.

Analyzing spatiotemporal accessibility patterns to tertiary healthcare services by integrating total travel cost into an improved E3SFCA method in Changsha, China

Accessibility is an essential indicator for measuring the effectiveness and equity of transport and public health services. However, previous studies neglect the effects of monetary costs for evaluating access to healthcare services. Therefore, this paper attempts to analyze the spatial accessibility patterns to healthcare services by integrating the total travel costs into an improved enhanced three-step floating catchment area (E3SFCA) method. Firstly, we adopt the explanatory factor analysis to compute the attractiveness coefficient by considering healthcare service capacities and qualities. The total travel cost under the private car and public transport modes are then determined by the integration of both real-time travel times and monetary costs during the journey, expressed in time. Notably, the real-time travel times under two travel modes are collected from the open-source route planning application program interface. Following that, the healthcare service catchment area thresholds are determined based on the results of frequency distribution of travel time under each travel mode, and the corresponding Gaussian impedance functions are produced. Based on the improved enhanced three-step floating catchment area (E3SFCA) method, Changsha, China's empirical case study is conducted to verify and investigate the accessibility between 189 subdistricts and 25 tertiary hospital locations at six different time periods in a working day under both the private car and public transport modes. The results suggest that the accessibility distribution shape reflects a circular diffusion centered on the central urban areas. The accessibility to tertiary hospitals is inadequate and regionally unbalanced in Changsha with respect to population allocations, especially during peak hours. Moreover, the private car mode is superior to public transport in most areas of the city. These findings may provide new insights for city planners and policymakers in terms of equity evaluations of medical resource accessibility and site allocations.

Multifrequency inverse obstacle scattering with unknown impedance boundary conditions using recursive linearization

In this paper, we consider the reconstruction of the shape and the impedance function of an obstacle from measurements of the scattered field at a collection of receivers outside the object. The data is assumed to be generated by plane waves impinging on the unknown obstacle from multiple directions and at multiple frequencies. This inverse problem can be reformulated as an optimization problem: that of finding band-limited shape and impedance functions which minimize the L2 distance between the computed value of the scattered field at the receivers and the given measurement data. The optimization problem is highly non-linear, non-convex, and ill-posed. Moreover, the objective function is computationally expensive to evaluate (since a large number of Helmholtz boundary value problems need to be solved at every iteration in the optimization loop). The recursive linearization approach (RLA) proposed by Chen has been successful in addressing these issues in the context of recovering the sound speed of an inhomogeneous object or the shape of a sound-soft obstacle. We present an extension of the RLA for the recovery of both the shape and impedance functions of the object. The RLA is, in essence, a continuation method in frequency where a sequence of single frequency inverse problems is solved. At each higher frequency, one attempts to recover incrementally higher resolution features using a step assumed to be small enough to ensure that the initial guess obtained at the preceding frequency lies in the basin of attraction for Newton’s method at the new frequency. We demonstrate the effectiveness of this approach with several numerical examples. Surprisingly, we find that one can recover the shape with high accuracy even when the measurements are generated by sound-hard or sound-soft objects, eliminating the need to know the precise boundary conditions appropriate for modeling the object under consideration. While the method is effective in obtaining high-quality reconstructions for many complicated geometries and impedance functions, a number of interesting open questions remain regarding the convergence behavior of the approach. We present numerical experiments that suggest underlying mechanisms of success and failure, pointing out areas where improvements could help lead to robust and automatic tools for the solution of inverse obstacle scattering problems.

A single-step recursive representation of foundation flexibility functions to soil-structure interaction using first-order IIR filters

The rational approximation of the dynamic impedance function of the foundation plays a vital role in the soil-structure interaction (SSI) analysis in the time domain. The filter representation of the rational approximation proposed by Safak provides a multi-step recursive parameter model facilitating incorporation into the time history analysis. However, it encounters both the uncertainty of the parameter identification and numerical instability. According to the linear time-invariant system theory in signal processing, this paper presents an improved single-step recursive representation of the foundation flexibility function using first-order infinite impulse response (IIR) filters. Based on the discrete responses of the foundation to the triangular unit impulse, the well-developed technologies of the digital filter can efficiently identify the stable filter parameters. It is easy to incorporate the filters into the motion equations of the structure in the forms of additional stiffness and single-step recursive variables. A computational scheme founded on the Newmark integration method is proposed to obtain the structural responses. According to the spectral radius theory, combining the filters with the Newmark integration method produces only a single operation matrix, thus effectively making the numerical stability guaranteed. Finally, a multi-degree-of-freedom model subjected to earthquake ground motion is employed to approve the proposed scheme.

SHARPENED FORMS FOR DRIVING POINT IMPEDANCE FUNCTIONS AT BOUNDARY OF RIGHT HALF PLANE

Driving point impedance functions (DPIFs) are frequently used in electrical engineering, and they represent characteristic properties of various types of circuits such as RL, RC, LC and RLC networks. In this paper, boundary analysis of driving point impedance functions are investigated using Schwarz lemma. Assuming that the driving point impedance function, Z(s), is given as Z(s)=A/2+c_p (s-1)^p+c_(p+1) (s-1)^(p+1)+... and it is analytic in the right half of the s-plane, novel boundaries are obtained for |Z^' (0)|. Accordingly, it is aimed to obtain novel inequalities which presents higher boundaries for |Z'(0)| and derive novel generic driving point impedace functions by performing extremal analysis of these obtained inequalities. It is also aimed to investigate how |Z'(s)| can be interpreted when it is considered at the boundary. According to simulation results, frequency characteristics of obtained driving point impedance functions can be used to design of multi-notch filters which are localized at certain frequency values.