Effects of soil on the energy response of equipment–structure systems with different connection types between the equipment and structure

In equipment–structure systems, the soil–structure interaction and connection types between the equipment and structure significantly affect the seismic response. To understand this effect, in this study, the motion equation of an equipment–structure–soil system was derived, and energy balance equations for each part of the coupled system were obtained. Further, the effects of the soil on the energy response were analyzed based on the results of shaking table tests of an equipment–structure system and real‐time substructure shaking table tests of equipment–structure–soil systems with different connection types. The energy response of the equipment–structure system with a rigid ground was compared with that of the equipment–structure–soil systems. The analysis results showed that the energy response of the equipment–structure–soil system with different connections was quite different from that of the system with a rigid ground. The soil decreased the total energy input to the equipment and structure and significantly changed the time distribution characteristics of the input energy. Additionally, the soil weakened the energy consumption of the connections. Therefore, the influence of the soil should be considered in the design of equipment–structure systems with connections.     

Shaking table test and theoretical analysis of the pile–soil–structure interaction at a liquefiable site

Pile foundations are widely used to support high‐rise buildings, in which piles transmit foundation loads to soil strata with higher bearing capacity and stiffness. This process alters the dynamic characteristics of the pile–soil–structure system in seismically active areas, especially at a liquefiable site. A series of shaking table tests on liquefiable soils in pile group foundations of tall buildings were performed to evaluate the liquefaction process and dynamic responses of the pile, soil, and structure. The soil was composed of two layers: the upper layer was a clay layer and the lower layer was saturated sand. These layers were placed in a flexible container that was excited by El Centro earthquake events and Shanghai Bedrock waves at different levels. The test results indicate that the pore pressure ratio is gradually enhanced as the amplitude of the input acceleration increases. The liquefied sand has a filtering effect on the vibration with a high frequency and an amplified effect on the vibration with a low frequency. With increased excitation, contact pressure and strain amplitudes of the pile increase, whereas the peak acceleration magnification coefficient decreases. The seismic responses of a structure with pile–soil–structure interaction are generally smaller than those on a rigid foundation.     

Dynamic characteristics and seismic response of frame–core tube structures,considering soil–structure interactions

In order to study the dynamic characteristics and seismic response of high‐rise buildings with a frame–core tube structure, while considering the effect of soil–structure interactions (SSIs), a series of shaking table tests were conducted on test models with two foundation types: fixed‐base (FB), in which the superstructure was directly affixed to the shaking table, and SSI, consisting of a superstructure, pile foundation, and soil. To increase the applicability of the model to the dynamic characteristics of real‐world tall buildings, the superstructure of test models was built at a scale of 1/50. This simulated a 41‐floor high‐rise building with a frame–core tube structure. The mode shape, natural frequency, damping ratio, acceleration and displacement response, story shear, and dynamic strain were determined in each of the test models under the excitation of simulated minor, moderate, and large earthquakes. The SSI effect on frame–core tubes was analyzed by comparing the results of the two test models. The results show that the dynamic characteristics and seismic response of the two systems were significantly different. Finally, these results were verified by performing a numerical analysis on the differences in the seismic responses of the FB and SSI numerical models under various simulated seismic conditions.     

Nonlinear dynamic response of tall buildings considering structure–soil–structure effects

Numerous studies have shown that the interaction between adjacent buildings can result in changes in nonlinear dynamic response of structures, damage, and performance level, depending on the dynamic specifications of structures involved and the frequency content of the input motion. To study these effects, finite element method is used for the analytical investigations, and total soil–foundation–structure system is modeled all together. For modeling purposes and in order to realize the effects of the adjacent buildings on the dynamic response, two buildings, namely, 15‐story and 30‐story tall buildings, which were separated by distances of 1/4 and 1/8 of the width of the foundation and were located on hard and soft soil profiles, were considered. It was concluded that in the case where the soil and structure's periods were near to each other, the interaction of adjacent structures on increasing nonlinear responses (displacement and interstory drift) and structural damage indexes was noticeable and therefore was not negligible. Whereas in the case where periods are distant from each other, the interaction of adjacent buildings has a decreasing effect on damage indexes and nonlinear responses and therefore was negligible. Copyright © 2011 John Wiley & Sons, Ltd.     

Seismic performance of reinforced concrete shear wall frames considering soil–foundation–structure interaction

A practical application of ‘beam on nonlinear Winkler foundation’ approach has been utilized in this paper for a case study on seismic performance of concrete shear wall frames to assess the soil–foundation–structure interaction effects. A set of 3‐, 6‐, 10‐ and 15‐story concrete shear wall frames located on hypothetically soft, medium and hard soils were designed and modeled using the OpenSees platform. The numerical model of each frame was constructed employing the distributed and lumped plasticity elements as well as the flexure–shear interaction displacement‐based beam–column elements incorporating the soil–footing interface. Pushover analysis was performed, and the results were studied through two code‐based viewpoints: (a) force‐based design and (b) performance‐based design. A comparison was made afterwards between the frame behaviors in the fixed‐/flexible‐base conditions. The results indicate some degree of inaccuracy in the fixed‐base assumption, which is regularly applied in analysis and design practice. The study emphasizes on how the fixed‐base assumption overestimates the design of the wall element and underestimates the design of the connected moment frame. Copyright © 2012 John Wiley & Sons, Ltd.     

A theoretical solution for analysis of tunnels below groundwater considering the hydraulic–mechanical coupling

An analytical solution for the analysis of tunnels below groundwater table in plane strain axisymmetric condition is presented. Seepage body force and secondary permeability of the rock mass due to the mechanical–hydraulic coupling are taken into account. The strain-softening behavior model and Hoek–Brown empirical strength criterion for the rock mass are used in the analysis. As the derived analytical equations do not have closed form solutions, a computer program has been prepared for solving the corresponding equations numerically and examining the analysis. It is shown that the tunnel stability depends on the seepage and the pore water pressure particularly in the case of high pore pressure gradient.     

Substructure shake table test for equipment‐adjacent structure–soil interaction based on the branch mode method

A substructure shake table test (SSTT) based on the branch mode method was performed to reveal the mechanism and rules of equipment‐adjacent structure–soil interaction (EASSI) under a seismic effect. EASSI system was divided into three substructures, namely, equipment‐single structure, foundation soil, and adjacent structure. The coupling terms of interaction among the substructures were proposed. The branch mode method was effectively applied to the SSTT by decomposing and transforming the dynamic equation of the entire system and utilizing the coupling terms of interaction for data exchange among substructures. The degree of freedom was reduced for the linear substructures. Experiments indicated that in EASSI, the presence of soil magnified the flexibility and equivalent damping of the entire system. The overall effect was presented as a reduction in the dynamic response of the system. The dynamic feedback of the equipment inhibited the dynamic response of the main structure, which intensified the rate of vibration attenuation of the system. The seismic response analysis was also performed for the system when the mass ratio and frequency ratio between the equipment and the main structure and the position of the equipment in the main structure varied.     

Responses of calcareous sand foundations to variations of groundwater table and applied loads

The long-term settlement of calcareous sand foundations caused by daily periodic fluctuations has become a significant geological hazard, but effective monitoring tools to capture the deformation profiles are still rarely reported. In this study, a laboratory model test and an in situ monitoring test were conducted. An optical frequency domain reflectometer (OFDR) with high spatial resolution (1 mm) and high accuracy (±10^-6) was used to record the soil strain responses to groundwater table and varied loads. The results indicated that the fiber-optic measurements can accurately locate the swelling and compressive zones. During the loading process, the interlock between calcareous sand particles was detected, which increased the internal friction angle of soil. The foundation deformation above the sliding surface was dominated by compression, and the soil was continuously compressed beneath the sliding surface. After 26–48 h, calcareous sand swelling occurred gradually above the water table, which was primarily dependent on capillary water. The swelling of the soil beneath the groundwater table was completed rapidly within less than 2 h. When the groundwater table and load remain constant, the compression creep behavior can be described by the Yasong-Wang model with R² = 0.993. The daily periodically varying in situ deformation of calcareous sand primarily occurs between the highest and lowest groundwater tables, i.e. 4.2–6.2 m deep. The tuff interlayers with poor water absorption capacity do not swell or compress, but they produce compressive strain under the influence of deformed calcareous sand layers.     

How groundwater seepage and transport modelling software can be useful for studying gaseous transport in an unsaturated soil

The aim of this paper is to show how standard hydrogeologic software, usually used to model contaminant transport in groundwater under unsaturated conditions, can also be used to model gas transport in unsaturated porous media. Physical processes involved in the interaction between the atmosphere and the unsaturated soils are considered: transport by diffusion through the air and the groundwater, exchange between the liquid and gas phases and consumption. These physical processes are incorporated into the governing equations of a groundwater numerical code; by considering air, contained in the unsaturated soil, as water in the seepage numerical model, the air effectively becomes fluid within the numerical code. Then, the investigated gas is defined as the contaminant in the transport model, which is transported by -the air for the modeller-, and -water for the numerical code-. The over-riding assumption is that the air profiles and, therefore, water profiles of volume contents remain constant. The approach is illustrated using two examples, which consider the transport of oxygen. The first deals with oxygen distribution through a laboratory-cell diffusion containing reactive mining tailings. The second deals with the oxygen fluxes through the vadose zone, between the atmosphere and an unconfined aquifer's water table. Both examples consider different cases of oxygen consumption.     

遥感反演地下水位方法的比较研究

本文回顾了国内外利用遥感技术反演地下水位的研究成果,依据地下水与地表参数的作用原理,归纳为四种地下水位反演方法,即分别从土壤水分、地表温度、植被指数和地下水流系统的角度进行地下水位反演.详细论述了各种方法的原理和模型,并从模型反演精度、前提条件和适用范围对四种方法进行比较分析.最后进行了总结和展望,对今后的工作重点和研...     

Impacts of climate and land‐cover changes on water resources in a humid subtropical watershed: a case study from East Texas,USA

This study investigates the response of water resources regarding the climate and land‐cover changes in a humid subtropical watershed during the period 1970–2009. A 0.7°C increase in temperature and a 16.3% increase in precipitation were observed. Temperature had a lower increase trend, and precipitation showed definite increasing trend compared to previous studies. The main trend of land‐cover change was conversion of vegetation and barren lands to developed and crop lands affected by human intervention, and forest and grass to bush/shrub which considered to be caused by natural climate system. Hydrologic responses to climate and land‐cover changes resulted in increases of surface run‐off (15.0%), soil water content (2.7%), evapotranspiration (20.1%) and a decrease in groundwater discharge (9.2%). We found that surface run‐off is relatively stable with precipitation, whereas groundwater discharge and soil water content are sensitive to changes in land cover, especially land cover brought about by human intervention.     

Influence of uncertainty in the initial groundwater table on long-term stability of reservoir landslides

The seepage calculation of water fluctuation is the crucial precondition for landslide stability evaluation. It is controlled by the initial condition, boundary condition, and soil or rock structure as well as geotechnical parameters. Although the geotechnical parameters and boundary condition have greater influence on seepage and the stability of landslides, the role of the initial groundwater position cannot be ignored. To investigate the influence of an initial groundwater table on landslides, especially the influence of cyclical reservoir water fluctuation on their long-term stability, seven different methods to determine the initial groundwater table were proposed in the presented study, among which one is the actual groundwater table, and the other six are obtained with simplified methods. According to the results of the calculations, at the beginning, there is some difference in the factor of safety among different initial groundwater tables. The maximum value of relative tolerance reduces to less than 1.0 % 160 days later. After a filling–drawdown cycle, the relative tolerance of all methods reduces to less than 0.8 %, which could be ignored. It is also observed that, the closer the hydraulic gradient of the proposed initial groundwater table is to the actual condition, the smaller is the relative tolerance. Therefore, to study the influence of cyclical reservoir water fluctuation on landslide long-term stability, the initial groundwater table could be proposed as (1) equal to reservoir water level and (2) on a straight line with the hydraulic gradient equal to the weighted average of actual groundwater table for simplification.     

Efficient structural analysis of wall–frame structures

In this study, an efficient analytical model for the dynamic analysis of tall buildings with a shear wall–frame structural system has been proposed. A shear wall–frame structural system usually consists of a core wall showing flexural behavior and a frame presenting shear behavior. Therefore, the deformed shape of the shear wall–frame structural system is shown by the combination of flexural mode and shear mode. To consider this characteristic in developing an efficient analytical model, the effect of shear wall and frame on the dynamic behavior of a tall building with a dual system has been separately investigated. In order to consider the effect of the shear wall in the frame model without shear wall, a rigid body was used instead of the shear wall. Each equivalent model for the separated shear wall part and frame part has been independently developed, and two equivalent models were then combined to create an efficient analytical model for tall buildings with a shear wall–frame structural system. In order to verify the efficiency and accuracy of the proposed method, time history analyses of tall buildings with a shear wall–frame system were performed. With analytical results, it has been confirmed that the proposed method can provide accurate results with significantly reduced computational time and memory. Copyright © 2013 John Wiley & Sons, Ltd.     

Enhanced Hilbert–Huang transform and its application to modal identification

The well‐known Hilbert–Huang transform (HHT) consists of empirical mode decomposition to extract intrinsic mode functions (IMFs) and Hilbert spectral analysis to obtain time–frequency characteristics of IMFs through the Hilbert transform. There are two mathematical requirements that limit application of the Hilbert transform. Moreover, noise effects caused by the empirical mode decomposition procedure add a scatter to derivative‐based instantaneous frequency determined by the Hilbert transform. In this paper, a new enhanced HHT is proposed in which by avoiding mathematical limitations of the Hilbert spectral analysis, an additional parameter is employed to reduce the noise effects on the instantaneous frequencies of IMFs. To demonstrate the efficacy of the proposed method, two case studies associated with structural modal identification are selected. In the first case, through identification of a typical 3‐DOF structural model subjected to a random excitation, accuracy of the enhanced method is verified. In the second case, ambient response data recorded from a real 15‐story building are analyzed, and nine modal frequencies of the building are identified. The case studies indicate that the enhanced HHT provides more accurate and physically meaningful results than HHT and is capable to be an efficient tool in structural engineering applications. Copyright © 2012 John Wiley & Sons, Ltd.     

Nonlinear interactive analysis of cooling tower–foundation–soil interaction under unsymmetrical wind load

This paper deals with physical and material modelling of a cooling tower–foundation–soil system. The physical modelling has been carried out using solid 20-noded isoparametric element to model the cooling tower, annular raft foundation and soil media. The cooling tower–foundation–soil system was analysed under vertical and lateral load generated due to self-weight and wind loads. The soil nonlinearity has been taken into consideration using hyperbolic nonlinear elastic constitutive law. The response of the structure has been investigated with respect to displacement and stresses. Moreover, an attempt has been made to study the effect of the linear and nonlinear interactive analyses compared with conventional analysis. It was seen that the interactive analysis of the cooling tower–foundation–soil media plays a major role in releasing the stresses in the cooling tower, particularly at the bottom ring beam.     

Alternative solution for kinematic interaction problem of soil–structure systems with embedded foundation

An effective procedure to incorporate kinematic interaction (KI) aspects in seismic analysis of soil–structures systems was presented. In this regard, first, the effect of KI on the structural response was investigated with special focus on the role of rocking component of foundation input motion (FIM). This was performed parametrically for a wide range of selected nondimensional parameters, which well define the introduced simplified soil–structure model. It was observed that ignoring the effect of rocking input motion may introduce errors, which can be on the unsafe side especially for slender structures with large embedment ratios. On the other hand, it was known that introducing the rocking input motion makes the problem too complicated to be addressed by simplified guidelines suitable for seismic codes or practicing engineers. As an alternative solution, a modified translational input motion was introduced, which can replace both translational and rotational components of FIM. This modified input motion, which was referred to as the net horizontal (NH) FIM in this article, was generated such that the roof displacement of the soil–structure system to this motion is identical to that of the same model subject to the multicomponent FIM resulted by KI. The applicability of the proposed procedure was then examined for a wide range of soil–structure systems subjected to a couple of real ground motions. Copyright © 2010 John Wiley & Sons, Ltd.     

A macroscopic approach for seismic damage transition within reinforced concrete wall–frame structures

The damage models proposed to date fail to identify the damage transition within one typical dual structural system, that is, wall–frame structures, under strong earthquakes. It's conceptually promising to combine a macroscopic global damage model physically with a simplified structural model that can characterize the interaction mechanism between the frame part and wall part of the structures. Thus, a macroscopic approach, with the integration of a macroscopic global damage model and a generalized shear–flexure coupled model, is proposed to have a closer insight into the damage transition within the wall–frame structures. The relation between the stiffness ratio and the ratio of vibration periods is established by introducing two stiffness‐based damage indicators and modal damage indexes. By this relation and the stiffness degradation in the frame part and wall part, the correlations between the modal damages, global damage, and the damages of the two parts are clarified. The combination of the damage indicators of the two parts with global damage is proved to be capable of capturing the damage transition during the dynamic redistribution of internal forces. The case study indicates that the damage predictions of the wall–frame structures are consistent with the interstory drift and proportions of failed components. The comparative study implies the efficiency of strengthening the link beams.     

Cost–benefit analysis of constructing a filtration plant for the National Water Carrier in Israel

The paper presents an economic cost–benefit analysis (CBA) of the construction of a filtration plant for the Israeli National Water Carrier (NWC). Its main contribution lies in the comparison between the costs and the benefits of filtration in the context of a concrete policy choice. The first part of the paper presents a cost analysis of two alternative engineering systems: central filtration and localized filtration. The analysis shows that the costs of constructing and operating a central filtration plant are significantly lower than those of a system of local plants. The second part of the paper presents a two‐stage method for assessing the benefits of filtration. First, we valuate the damages caused by consumption of unfiltered water; then we estimate consumers' willingness to pay (WTP) for improved water quality, taking into account households' potential risk aversion. The main result is that total WTP significantly outweighs the costs of constructing and operating the plant.     

Coupled‐two‐beam discrete model for dynamic analysis of tall buildings with tuned mass dampers including soil–structure interaction

An equivalent coupled‐two‐beam discrete model is developed for time‐domain dynamic analysis of high‐rise buildings with flexible base and carrying any number of tuned mass dampers (TMDs). The equivalent model consists of a flexural cantilever beam and a shear cantilever beam connected in parallel by a finite number of axially rigid members that allows the consideration of intermediate modes of lateral deformation. The equivalent model is applied to a shear wall–frame building located in the Valley of Mexico, where the effects of soil–structure interaction (SSI) are important. The effects of SSI and TMDs on the dynamic properties of the shear wall–frame building are shown considering four types of soil (hard rock, dense soil, stiff soil, and soft soil) and two passive damping systems: a single TMD on its top (1‐TMD) and five uniformly distributed TMDs (5‐TMD). The results showed a great effectiveness of the TMDs to reduce the lateral seismic response and along‐wind response of the shear wall–frame building for all types of soils. Generally speaking, the dynamic response increases as the flexibility of the foundation increases.     

Numerical modeling of fluid–structure interaction between sewage water flow and bar screen to improve the screening process

A self‐cleaning bar screen is often used in the first filtration level of a wastewater treatment plant to screen out solid debris from sewage water. The screening process of the bar screen was numerically investigated using the arbitrary Lagrangian–Eulerian method to model the fluid–structure interaction between the sewage water and bar screen. The bar screen design is improved by installing a rotating subscreen under the bar screen to prevent solid waste from passing through. The effects of bar screen on the water flow were examined using hydrodynamic properties. The structural properties were estimated to observe the effects of water flow on the parts of the bar screen. Various inlet flow rates and positions of subscreen and rake were used to examine their effects on velocity, pressure coefficient, structural deformation and von Mises stress. The screening process decreased with the increase in sewage flow rates. The water flow significantly affected the bar screen when the subscreen and rake are not in contact.