Moat Wall Research Articles

Comprehensive evaluation of seismic fragility in base isolated buildings enhanced with supplemental dampers considering pounding phenomenon

Base isolation is a proven technology effective in minimizing earthquake-induced damage to both the structural and non-structural elements (especially freestanding contents) of buildings. Nevertheless, it has been shown that allowing the flexible isolation layer to endure substantial demand displacements might subject the isolated structure to failure. This vulnerability arises from potential superstructure yielding due to increased deformations and reduced stiffness, such as those occurring through pounding against a moat wall during intense earthquakes. One alternative to controlling these large deformations is to use supplementary dampers at the isolation plane of the building, creating a hybrid base isolation system (HIS), which adds damping to the isolation system. In this research, the seismic performance of base isolated structures with lead-rubber bearing (LRB) are studied in conjunction with linear viscous dampers (LVD). Due to this aim, a base-isolated steel frame with 2 story X-type braces and a surrounding moat wall is modeled with two various configurations: LRB, and LRB with LVD. Fragility curves are developed for an ensemble of near-field (NF) pulse-like ground motions and for the maximum inter-story drift ratio (MIDR) as an engineering demand parameter (EDP). The incremental dynamic analysis (IDA) is conducted to calculate the damage probability of the structure by assuming different threshold values for damage states. It was found that adding viscous dampers to the isolation system balanced the behavior of the structure under near field pulse-type records; resulting in reduced isolation displacement while its adverse effects are insignificant and can be ignored. Moreover, the median spectral acceleration, namely the spectral acceleration associated to a probability of exceedance (POE) equal to 50 %, at the specific limit state, in the hybrid base isolation system is higher compared to the conventional one.

Overturning Resistance of Structure Base-Isolated by Laminated Rubber Bearing Considering Pounding Against the Moat Wall

The overturning resistance of structures base-isolated using laminated rubber bearings (LRBs) is crucial to the stability of the structure due to the poor tensile performance of LRBs. Pounding occurs between the structure and moat wall, which can result in a reduction of the overturning resistance of the structure. This study aims to investigate the overturning resistance of a structure base-isolated by LRBs considering pounding against the moat wall based on numerical simulations. The influence of the gap size, pounding stiffness, and the horizontal stiffness of the isolation storey on the overturning resistance of the isolated structure was evaluated through parameter studies. The results indicate that poundings between the structure and moat wall result in short but large pounding forces. Pounding forces would amplify acceleration, inter-storey drifts of the isolated structure, and significantly increase the risk of overturning of the isolated structure. Increasing the gap ratio can improve the seismic performance of the structure. Larger pounding stiffness leads to greater pounding forces generated by the structure, and the risk of overturning of the structure increases. The coefficient of overturning resistance initially decreases, and then increases, as the stiffness of the isolation storey increases.

Data on the dynamics of the fortification construction of the Baitovo culture in the context of the traditions of defensive architecture among the population of the early iron age of the forest-steppe Trans-Ural

The article analyses remains of the Baitovo Culture fortifications in the focus of archaeological research. These remains are dated back to the early Iron Agee and situated in the Transural forest-steppes. Previously determined stages of the culture development (early to late 7th – 6th century BC and late 5th – 3rd century BC) are relied on to analyse and restore the walls that used to fringe the settlements. The study reviews the shapes, depth and water levels in the moats at Maray5, Likhachevskoe and Borovushka2 hillforts. The research suggests that the settlements used to be fortified with a stakewall at the bottom of the moat. Likhachevskoe hillfort had its gate towers restored. The researched Baitovskoe, Staro-LybaevoII and Bochanetskoe hillforts allow to conclude that even later on the early Iron Age settlements preserved the tradition of erecting compact stakewalls or wattles at the bottom of moats. However, this as when the Baitovo culture people started to erect fortifications with deeper moats and higher walls (Bolshoy Imbiryay3, Maray1 hillforts). This is evident of increasingly more substantial defence of the settlements. The studied Sargat and Gorokhovo culture fortifications peaked in 3rd – 2nd century BC. The comparison reveals that on one hand people kept opting for simple stakewall structures or wattlers to fortify their settlements, but they tended to change the properties and made their moats and walls more substantial. On the other, they reinforced fortifications through gate towers, wooden supports for moat walls, log frameworks and cages on the walls. Such structures are typical for fortresses that bore the function of social and economic centers. It is concluded that the defense works the Baitovo people erected illustrate the early defensive architecture of the early Iron Age Transurals.

Non-Dimensional Probabilistic Analysis of Seismic Pounding Between Flexible Structures and Rigid Boundaries

ABSTRACT Pounding between adjacent structures subjected to earthquake actions can cause significant damage. Due to the many uncertainties inherent to the seismic input and the impact phenomenon, a probabilistic assessment of the occurrence of seismic pounding and of its consequences on the structural performance is necessary. This work analyzes the problem of pounding by considering a single-degree-of-freedom benchmark system surrounded by rigid boundaries and subjected to a stochastic earthquake input. Although simplified, the model is representative of several realistic configurations, such as base-isolated systems surrounded by moat walls or bridge decks near the bridge abutments. The problem is cast in non-dimensional form and a parametric study is carried out to evaluate the influence of the identified non-dimensional input parameters on the statistics of the response. A probabilistic demand model is developed for the impact forces via non-linear regression, with the demand expressed as a function of the identified non-dimensional parameters. This model provides an estimate of median pounding force and of its dispersion given the seismic intensity of the input. Finally, global sensitivity analysis is used to rank the model parameters in terms of their influence on the system performance.

Formation mechanism of drift-moat contourite systems revealed by in-situ observations in the South China Sea

Contourite drifts are ubiquitous sedimentary features in the world′s oceans, and their formation are usually ascribed to sedimentation from contour currents. However, in-situ observations of contour currents and their associated sedimentary processes were inadequate. Here, we present mooring observation results from a drift-moat contourite system in the South China Sea to gain insight into its sediment dynamics and formation mechanism. We found that quasi-persistent contour currents develop in the moat, on and below the drift, yet subject to diverse physical oceanographic processes. High suspended sediment concentrations observed on the drift mainly occur in winter, induced by high levels of sediments transported by contour currents from Taiwan. In the moat, however, high suspended sediment concentrations are mostly caused by erosion of the moat wall during periods of strong tidal currents and low shear variance. Such results underscore the complexity of current patterns and sedimentary processes across various topographic units in the drift-moat contourite system. A novel formation mechanism of the contourite drift is thereby proposed, wherein sediment transport by contour currents brings materials for constructing the drift, while its formation is driven by near-critical reflection and upslope transport of sediments by tidal currents.

Moats in the residual south polar cap of Mars: Ages, formation, and evolution

The residual south polar cap of Mars (RSPC) is a region comprising tabular deposits of solid CO2 averaging a few m thick resting on layered, water-ice rich deposits a few km thick. Estimates of the ages of morphologic features in the RSPC have relied on average rates of change obtained from geographically limited samples. In this work we apply image coverage of the whole RSPC at 6 m/pixel spanning up to seven Mars years to determine how well ages of RSPC forms can be modelled from recent observations, and to explore implications of the calculated chronology. Topographic troughs separating CO2 layers of different ages and thickness, termed moats, are widespread, changing, and dateable features of the RSPC. Although they all develop by the same phenomena, we divide moats into two groups based on their linear dimensions and the span of relevant spacecraft imaging: 1) moats separating large areas of CO2 ice of different age and thickness, termed sinuous boundary moats, that imaging data show are related to post-1972 CO2 deposition. 2) moats separating the walls of pits from thinner, younger, interior mounds, termed pit moats; these are not resolved in early spacecraft images. Our survey of the entire summer RSPC spanning seven Mars years yields modelled formation times for both kinds of moats that cluster largely about Mars Years (MY) 11–13. A population of smaller moats is calculated to have formed near MY 26–28. These times closely follow planet-encircling dust events in MY 9, MY 12, and MY 28 (Earth years 1971, 1977, 2007). High-resolution images show that both pit and sinuous moats are initiated by slow downward erosion of debris left by retreating scarps of CO2 ice that are flanked by brighter, accumulating CO2 ice. We find that the most probable role of the dust events in moat formation is to clear thin CO2 layers from underlying water-ice-rich materials through thermal effects of the atmospheric dust content. The age results severely limit the magnitude of variations in moat scarp retreat rates during the last 25 MY. Pit moats have limited lifetimes because scarp retreat eventually removes the interior mound that defines the inner moat wall. Pits can reach much greater ages than moats; some pits formed prior to MY -100 (Earth year 1765).

Pounding responses of a base-isolated liquid storage tank under bidirectional earthquakes

ABSTRACT The probability of seismic pounding of sliding isolation liquid storage tank (LST) is high. Considering normal impact force and tangential friction force at the location of pounding under bidirectional earthquake, a 3-D calculation model of sliding isolation LST is established based on finite element method. Pounding dynamic responses under unidirectional and bidirectional earthquakes are comparatively investigated, and frequency domain analysis is also conducted. Finally, mitigation measure for adverse effect caused by the pounding is proposed. Results show that the influence of concrete nonlinearity on the pounding is very significant. Dynamic responses are significantly increased due to the pounding, and the maximum tensile stress of tank wall reaches to 2.510MPa under ChiChi earthquake, which exceeds the concrete tensile strength. Besides, pounding causes the high frequency response. Reasonable rubber cushion design can ensure that the maximum displacement of the sliding isolation LST is limited by the moat wall and the failure probability caused by the pounding is decreased.

Lumped mass models for use in predicting collapse of an isolated building

Development of fragility functions is an importantstep in the seismic evaluation of isolated buildings. To develop collapse fragility curves, a series of detailed dynamic ground motion analysis with multiple intensity measure levels must be conducted, often requiring considerable computational time. In addition, including more modeling detail to enable the prediction of multiple failure modes can make numerical convergence more challenging. Therefore, the possibility of simplified models to predict the collapse probabilityof isolated buildings is attractive. The collapse fragility curves derived from two lumped mass models with differing degrees of simplification are compared to a full nonlinear model. This is done for both sliding and rubber bearings with varying moat wall or restraining rim (for sliding bearings). The comparison demonstrates that the simplified model proposed in this study can be a potential alternative for estimating fragility functions of isolated buildings, as they can significantly improve the computational efficiency withrelatively smallcompromises on accuracy. This will allow for faster validation of system design when incorporating performance targets under large earthquakes.

On the Influence of Unexpected Earthquake Severity and Dampers Placement on Isolated Structures Subjected to Pounding Using the Modified Endurance Time Method

The aim of this study is to investigate the performance of isolated structures by considering the possibility of impact under severe earthquakes. In the design of isolated structures, the required displacement capacity is determined based on the considered earthquake hazard level. However, there is a possibility of an impact caused by moat walls or adjacent structures under severe earthquakes. Dampers are used in this study to improve the performance of structural and nonstructural components. In this regard, three isolated structures (6, 9, and 12 stories) equipped with Triple Friction Pendulum Isolator (TFPI) are designed under earthquake hazard levels of BSE-1 with return periods of 475 years. Based on the different positions of these three structures relative to each other, four scenarios are defined to investigate the effect of impact. Modified endurance time (MET) method, as a cost-efficient nonlinear time history analysis method, is employed for structural evaluation under variable earthquake hazard levels. The placement of dampers is also taken into account in evaluating the effect of dampers. Therefore, the structures have been retrofitted once by adding damping and stiffness devices (ADAS) on the stories and once by adding fluid viscous dampers (FVD) at the isolated level. Results indicate that structures might collapse under earthquake hazard levels of BSE-2 with return periods of 2475 years. This matter is influenced by the adjacency of two isolated structures next to each other, and the severity of this fact depends on the height of the structures and the displacement capacity of the isolators so that the tall, isolated structures have decreased the performance of the adjacent shorter isolated structure. Moreover, the placement of dampers has a significant influence on the performance of structural and nonstructural components, depending on the reason for the impact.

Earthquake‐induced impact of base‐isolated buildings: theory, numerical modeling, and design solutions

AbstractEarthquake shaking more intense than that used to size the horizontal clearance between a base‐isolated building and near‐rigid perimeter moat wall will result in hard impact, producing high‐frequency, high‐amplitude acceleration response in the structure and supported equipment. This paper provides a design solution for the damaging effects of hard impact by installing a compliant engineered element in the load path between the base‐isolated building and the moat wall, resulting in soft impact and a much smaller acceleration response. The engineered element assumed herein is a commercial‐off‐the‐shelf marine fender with mechanical properties determined by physical testing. The attachment of a flexible engineered element, with well‐defined stiffness and damping, to a near‐rigid moat wall, simplifies the numerical modeling of the building‐moat wall system and eliminates the need to bound the lateral stiffness of the wall for impact calculations. The simple model of the engineered element can be implemented in commercial finite element codes. Theory is developed for two‐sided impact of a single‐degree‐of‐freedom oscillator. Analytical solutions are derived for the shifted first‐mode frequency of the impacted oscillator and for its free‐vibration response. The shifted first‐mode frequency is a function of the composite lateral stiffness of the isolator‐engineered element assembly and its earthquake‐induced displacement. Local peaks in the spectral response of the impacted oscillator form at odd integer multiples of the shifted first‐mode frequency. The analytical solutions can be used to verify, in part, the numerical model used for impact analysis.

Effect of modeling of inherent damping on the response and collapse performance of seismically isolated buildings

AbstractThis paper investigates the effect of inherent damping modeling on the computed seismic response and collapse performance of selected seismically isolated buildings. The analyzed seismically isolated buildings were designed by the procedures of the ASCE/SEI 7–16 standard. The structure is a six‐story perimeter frame building designed with special moment resisting frames or with special concentrically braced frames (SCBF) for a location in California. Three different seismic isolation systems are considered: (i) triple friction pendulum (TFP) bearings without moat walls, (ii) TFP bearings with moat walls (double concave [DC] friction pendulum bearings with moat walls have effectively the same ultimate behavior), and (iii) DC friction pendulum bearings without moat wall. The superstructure inherent damping schemes considered are (i) zero damping, (ii) modal damping, (iii) Zareian‐Medina damping, (iv) added virtual viscous dampers with and without force‐caps, and (v) added virtual viscous dampers with the same damping constant value. The response parameters computed are peak floor accelerations, peak story drift ratios, peak residual story drift ratios, peak isolator horizontal displacement, and floor acceleration spectra. Also, the probability of collapse in the maximum considered earthquake (MCER) is computed. It is shown that the modeling approach for the inherent damping has minor effects on the computed responses and the collapse probability of the studied seismically isolated buildings, except for the peak floor acceleration and the floor response spectra for periods below one second. It is suggested that a convenient way to model inherent damping is to use virtual viscous dampers with all having the same damping constant.

Inspection and repair considerations for downtime assessment of seismically isolated buildings

This paper investigates the seismic downtime of seismically isolated buildings with steel moment or braced frames designed by the procedures of ASCE/SEI 7–16. The seismic isolation systems considered in this study are comprised of triple or double friction pendulum isolation bearings with and without moat walls. The seismic downtime is calculated from the damage to structural components and non-structural components, demolition and collapse of buildings. The downtime components (repair and inspection) are defined, and mathematical expressions are provided for the computation of downtime fragility curves, expected annual downtime, and economic losses due to the expected annual downtime. The procedure is then implemented using the results of nonlinear response history analysis from previous studies by the first author. The study demonstrates that the expected annual downtime of seismically isolated buildings is less than that of the comparable non-isolated buildings regardless of the seismic isolation systems used. Among the cases of seismically isolated buildings studied in this paper, it is found that the most effective structural system to mitigate long downtime is the seismically isolated building with seismic isolators with enhanced sizes and with braced frames that are designed to be minimally compliant with the seismic design requirements of ASCE/SEI 7–16.

Study on Seismic Performance of TID-LRB Hybrid Control System under Multi-Level Earthquakes

The seismic response characteristics of a lead-rubber-bearing(LRB) base-isolated structure under rare and very rare earthquakes were investigated. The acceleration, ductility coefficient, and shear strain of the LRB increase significantly under very rare earthquakes in comparison to rare earthquakes; in particular, the shear strain of the LRB may exceed the ultimate shear strain and cause damage to the base-isolated structure. The criterion selected for the optimum tuned inerter damper (TID) of the TID–LRB hybrid control system is the minimization of the mean value of the maximum shear strain of the LRB. For each inertance mass ratio of the TID, there exists an optimum tuning frequency ratio and damping ratio of the TID to minimize the shear strain of the LRB, and the effectiveness is increased with a higher inertance mass ratio. By equipping the TID with appropriate parameters, the safety of the LRB during rare and very rare earthquakes can be ensured. Finally, the pounding response of the base-isolated structure collision with the moat wall under very rare earthquakes was analyzed. It was observed that under very rare earthquakes, the ductility coefficients of the superstructure by equipping with the suitable TID were improved, and the shear strain of the LRB was reduced. In addition, equipping the TID can reduce the required width of the isolation joint to avoid collision between the isolation layer and the moat wall, and with an increase in the inertance mass ratio, the required width of the isolation joint is smaller.

Seismic collapse probability and life cycle cost assessment of isolated structures subjected to pounding with smart hybrid isolation system using a modified fuzzy based controller

The possibility of pounding on isolated structures with surrounding moat walls is one of the concerns in the design of isolation systems, especially in pulse-type near-field earthquakes. This paper puts forward the seismic probability assessment of structures equipped with passive and smart hybrid isolation systems by considering pounding possibilities. This investigation is performed on isolated structures equipped with a high damping rubber bearing (HDRB) considering stiff moat walls around the structure. In the Hybrid isolation system, magnetorheological dampers (MR) are considered an adaptive dissipation energy device along with isolators using an optimized novel interval Type-2 fuzzy logic controller with adaptive red-zone function (IT2FS + RZF) to reduce pounding possibilities. The fragility curves of the building for various cases are determined using incremental dynamic analysis (IDA), and possible damage costs are evaluated by using exceedance probability in each damage level. This study concludes that the collapse probability of the isolated structures with restrains at the code-based distance is over the acceptable limit of ASCE 7–22. The smart additional damping system with the proposed controller reduces the possible damage cost of the building by about 64 % compared to the uncontrolled system and puts the collapse probability of the structure in the acceptable range.

Experimental seismic performance of a base-isolated building with displacement limiters

The base displacement of isolated structures may exceed its design limit, resulting in pounding with moat walls or adjacent buildings that leads to structural and nonstructural damage. This study investigates the feasibility and effectiveness of applying displacement limiting devices (i.e., limiters), with soft or hard impact mechanisms, to base-isolated buildings. Extensive shake table tests are performed on a base-isolated three-dimensional building with eight types of soft and rigid limiters. The test setup consists of a one-sixth scale three-story steel moment-resisting frame isolated at the base with laminated rubber bearings. The main input parameters are limiter stiffness, reserved gap size, and ground motion (GM) characteristics, while the main output parameters are the base displacement and dynamic response of the superstructure. The test results indicate that a proper combination of a soft limiter stiffness and the reserved gap can effectively limit the isolated base displacement response, without substantial adverse reactions to the superstructure. Rigid limiters shall be avoided since they can significantly increase inter-story drifts and superstructure accelerations under the maximum considered earthquakes.

Influence of the gap size on the response of a single-degree-of-freedom vibro-impact system with two-sided constraints: Experimental tests and numerical modeling

The large displacements caused by strong earthquakes in base-isolated structures (buildings, bridges, strategic facilities, equipment, ⋯) can excessively deform or damage the isolation system or lead to pounding with surrounding moat walls or adjacent structures, if the available seismic gap is not sufficient. The acceleration spikes caused by the impact can damage the structure itself as well as sensitive equipment housed in it. A possible mitigation measure consists in the interposition of deformable shock absorbers (bumpers). In this paper, the influence of the gap amplitude on the experimental response of a single-degree-of-freedom oscillator, excited by a harmonic base acceleration and symmetrically constrained by two unilateral deformable and dissipative bumpers, is investigated. The parametric investigation considered both positive, null, and negative gaps. Particular attention is paid to the study of the effect, on the system response, of the transition from positive to small negative gaps and of excessive negative gaps. Secondary resonances in the low frequency range, associated with the occurrence of multiple impacts, were experimentally observed for small positive gaps. Finally, the experimental results were reproduced, in a sufficiently accurate manner, using a suitable numerical model, whose parameters were identified based on the experimental data.

Considerations for modeling of base isolated nuclear power plants subjected to beyond design basis shaking

Modeling of the seismic isolation system plays a key role in simulating the seismic response of base isolated nuclear power plants (NPP), including estimation of peak isolator displacement and potential to exceed the clearance to stop. Under beyond design basis shaking, seismic isolators such as lead rubber bearings (LRB) may undergo large displacements and exhibit complex behavior including cyclic heating causing strength degradation of the lead and strain hardening in the rubber at large strains. Displacement demands on the isolation system can be limited by a stop such as a moat wall to prevent bearing failure, with the impact to the moat having potential to increase the transfer of forces to the superstructure. Towards a more accurate seismic assessment of NPP subjected to beyond design basis shaking, a model of a seismically isolated NPP is developed with advanced modeling capabilities for LRB isolators and simulation of moat wall impact. The bearing models capture the cyclic behavior of LRB as observed in measured experimental data from full-scale bearings designed for NPP applications. The moat wall model considers a concrete retaining wall with backfill soil and contact elements developed based on experimental data and high-fidelity Finite Element Method (FEM) simulations. The simulation tools are used to evaluate the consequences of bearing modelling and impact to moat walls on the seismic performance of NPP and further develop effective mitigation measures for the seismic protection of NPP. As a result of the studies presented here, a simplified design methodology to estimate impact response parameters including moat wall deformation due to impact, impact velocities, and forces is proposed.

Seismic poundings of multi-story buildings isolated byTFPB against moat walls

The gap provided between adjacent structures in the metropolitan cities is mostly narrow due to architectural and financial issues. Consequently, structural pounding occurs between adjacent structures during earthquakes. It causes damages, ranging from minor local to more severe ones, especially in the case of seismically isolated buildings, due to their higher displacements. However, due to the increased flexibility of isolated buildings, the problem could become more detrimental to such structures. The effect of the seismic pounding of moat walls on the response of buildings isolated by Triple Friction Pendulum Bearing (TFPB) is investigated in this paper. To this propose, two symmetric three-dimensional models, including single-story and five-story buildings, are modeled in Opensees. Nonlinear Time History Analyses (NTHA) are performed for seismic evaluation. Also, five different sizes with four different sets of friction coefficients are considered for base isolators to cover a whole range of base isolation systems with various geometry configurations and fundamental period. The results are investigated in terms of base shear, buildings' drift, and roof acceleration. Results indicated a profound effect of poundings against moat walls. In situations of potential pounding, in some cases, the influence of impact on seismic responses of multi-story buildings was more remarkable.

Evaluation of Clearance to Stop Requirements in A Seismically Isolated Nuclear Power Plant

Seismically isolated nuclear power plants (NPPs) can provide substantial benefits towards reducing the failure probability of NPPs, especially for beyond design basis earthquake shaking. One risk posed by seismic isolation is the potential for pounding to a stop or moat wall, with currently little guidance provided by design standards on how to address this concern. In this paper, a structural model of an isolated NPP based on the Advanced Power Reactor 1400 MW is enveloped with moat walls and advanced bearing models. The bearing models account for large strain behavior through failure based on full-scale experiments with lead rubber bearings (LRBs). Using these analytical models and a measured ultimate property diagram from LRB failure tests, the range of clearance to the stop considering the performance criteria for the NPP is investigated. Although the analysis results are dependent on the particular models, ground motions, and criteria employed, this research provides an overview of the seismic response and performance criteria of an isolated NPP considering the clearance to the stop.

Real-Time Hybrid Simulation Analysis of Moat Impacts in a Base-Isolated Structure

Base isolation is a well-known technique used to reduce accelerations and inertial forces in structures during earthquakes. However, excessive displacements of the structure due to flexibility of the isolation layer bearings may contribute to moat wall impact events. These impact events have the potential to cause substantial structural damage. This impact behavior and the effects on structural dynamics have been shown to be highly complex and difficult to model by means of pure numerical simulation. In this paper, the cyber-physical technique called Real-Time Hybrid Simulation (RTHS) is employed to capture the uncertainties in force profiles of moat wall impacts and to analyze the complex interactions between impacts and the dynamics of base-isolated structures during earthquake excitations. It is shown that RTHS is capable of accurately capturing the interactions between the isolation layer and moat wall during impact events induced by ground motions. In addition, the RTHS technique is used to analyze the role played by moat wall material nonlinearities in reducing the inertial demand on the structure during impact events. Finally, possible extensions of the research to larger scales as well as consideration of additional moat wall variants are proposed.