Summary
The paper presents a lumped parameter model for the approximation of the frequency‐dependent dynamic stiffness of pile group foundations. The model can be implemented in commercial software ...to perform linear or nonlinear dynamic analyses of structures founded on piles taking into account the frequency‐dependent coupled roto‐translational, vertical, and torsional behaviour of the soil‐foundation system. Closed‐form formulas for estimating parameters of the model are proposed with reference to pile groups embedded in homogeneous soil deposits. These are calibrated with a nonlinear least square procedure, based on data provided by an extensive non‐dimensional parametric analysis performed with a model previously developed by the authors. Pile groups with square layout and different number of piles embedded in soft and stiff soils are considered. Formulas are overall well capable to reproduce parameters of the proposed lumped system that can be straightforwardly incorporated into inertial structural analyses to account for the dynamic behaviour of the soil‐foundation system. Some applications on typical bridge piers are finally presented to show examples of practical use of the proposed model. Results demonstrate the capability of the proposed lumped system as well as the formulas efficiency in approximating impedances of pile groups and the relevant effect on the response of the superstructure.
The material point method (MPM) is often applied to large deformation problems that involve sharp gradients in the solution field. Representative examples in geomechanics are interactions between ...soils and various “structures” such as foundations, penetrometers, and machines, where the displacement fields exhibit sharp gradients around the soil‐structure interfaces. Such sharp gradients should be captured properly in the MPM discretization to ensure that the numerical solution is sufficiently accurate. In the MPM literature, several types of locally refined discretizations have been developed and used for this purpose. However, these local refinement schemes are not only quite complicated but also restricted to certain types of basis functions or update schemes. In this work, we propose a new MPM formulation, called the mapped MPM, that can efficiently capture sharp gradients with a uniform background grid compatible with every standard MPM basis function and scheme. The mapped MPM is built on the method of auxiliary mapping that reparameterizes the given problem in a different domain whereby sharp gradients become much smoother. Because the reparameterized problem is free of undesirably sharp gradients, it can be well solved with the standard MPM ingredients including a uniform background grid. We verify and demonstrate the mapped MPM through several numerical examples, with particular attention to soil‐structure interaction problems.
With the soil constitutive model developed for the dynamic large deformation of soft soil, a finite element model was constructed to simulate the soil-underground structure static and dynamic ...coupling interaction system, in which the dynamic contact between soil and underground structure and the dynamic damage of reinforced concrete were considered. The seismic performance and damage mechanism of the large underground subway station in different soil foundations were investigated and evaluated in detail. It was proven that the interaction mechanism between the soil and underground structure would change with the type of soil foundation and the peak acceleration of input ground motion, which can be evaluated by the interaction coefficient between the soils and underground structure. At the same time, some regulations on the seismic performance of underground structure in China code are already unsuitable for evaluating the seismic performance of a large underground subway station in the soft soil foundation or subjected to a strong earthquake, which should be investigate by the specialized time-history analysis method instead. In this study, five seismic performance levels were firstly defined by the inter-storey drift angle of underground structure and corresponding earthquake damage states.
•Shaking table tests were conducted on a series of soil-structure and soil-structure group systems, where the influence of structure height, the number of structures, the composition of structure ...group and seismic records was considered.•As compared with the SSI, the SSGI effect on accelerations at the top of the superstructures ranged from −40% to 27% in tests.•Parametric analyses of the dynamic soil-structure group interaction (SSGI) effect on structural responses were conducted based on the three-dimensional FEM method.•In general, the SSGI effect is significant for soft soils, and increases with larger number of structures and smaller structure spacing.
Dynamic interaction between soil and structure group (SSGI) has received increasing attention due to higher requirements for urban seismic hazard prevention in recent years, especially in densely built areas. However, investigations on SSGI effects are still limited so far. To develop a better understanding on the SSGI effect, shaking table tests were conducted on a series of soil-structure and soil-structure group systems, where the influence of structure height, the number of structures, the composition of structure group and seismic records was considered. Besides, a three-dimensional numerical model was proposed to account for the SSGI effect and was verified through experimental results, based on which parametric analyses were conducted. It is found from the test results that as compared with the soil-structure interaction (SSI), the SSGI effect on accelerations at the top of the superstructures ranged from −40% to 27%. In general, the SSGI effect is significant for soft soils, and increases with larger number of structures and smaller structure spacing. In addition, the SSGI can change the response spectra of ground acceleration and significantly alter the maximum Fourier amplitude of acceleration responses of structures.
A comprehensive study is performed on the dynamic behavior of offshore wind turbine (OWT) structure supported on monopile foundation in clay. The system is modeled using a beam on nonlinear Winkler ...foundation model. Soil resistance is modeled using American Petroleum Institute based cyclic p–y and t–z curves. Dynamic analysis is carried out in time domain using finite element method considering wind and wave loads. Several parameters, such as soil–monopile–tower interaction, rotor and wave frequencies, wind and wave loading parameters, and length, diameter and thickness of monopile affecting the dynamic characteristics of OWT system and the responses are investigated. The study shows soil–monopile–tower interaction increases response of tower and monopile. Soil nonlinearity increases the system response at higher wind speed. Rotor frequency is found to have dominant role than blade passing frequency and wave frequency. Magnitude of wave load is important for design rather than resonance from wave frequency.
•Soil stiffness degradation is more at higher wind speed, which increases responses.•Static p–y curves in offshore monopile design leads to underestimation in design.•Non-consideration of dynamic analysis may lead to unplanned resonance condition.•Rotor frequency has dominant role than blade passing frequency.•Magnitude of wave load has vital role in design than resonance from wave frequency.
More and more projects of underground structures undercrossing adjacent surface structures have emerged in recent years. The existence of underground structure influences the propagation of seismic ...wave, and the inertia effect of the surface structure affects both the seismic response of surrounding site and that of underground structures. Thus, the underground structure and near surface structure can be deem as an interaction system. In this paper, in order to investigate the law of seismic response of underground structure-soil-surface structure interaction system, a shaking table model test was designed and implemented. Based on the experimental results, the law of seismic response of such complex system was analyzed. Note that to overcome the limitation of the bearing capacity of shaking table and eliminate the distortion of soil-structure rigidity ratio, the sawdust soil was chosen to model the surrounding site soil. Test results showed that using sawdust soil as a model soil is feasible. Regarding the law of seismic response of interaction system, it was found that the existence of the tunnel structure has weakened the rigidity of the whole model, and therefore, the seismic response of surrounding soil was amplified. However, it was observed that the existence of the tunnel impeded the propagation of seismic wave to some extent and thus reduced the seismic response of the surface structure, especially for lower and medium floors. The existence of surface structure suppressed the seismic response of soil and underground structures. By comparing the test results under different earthquake excitations, it was shown that the seismic response of the system was affected by the type of seismic wave significantly.
Monopile foundations have been commonly used to support offshore wind turbine generators (WTGs), but this type of foundation encounters economic and technical limitations for larger WTGs in water ...depths exceeding 30m. Offshore wind farm projects are increasingly turning to alternative multipod foundations (for example tetrapod, jacket and tripods) supported on shallow foundations to reduce the environmental effects of piling noise. However the characteristics of these foundations under dynamic loading or long term cyclic wind turbine loading are not fully understood. This paper summarises the results from a series of small scaled tests (1:100, 1:150 and 1:200) of a complete National Renewable Energy Laboratory (NREL) wind turbine model on three types of foundations: monopiles, symmetric tetrapod and asymmetric tripod. The test bed used consists of either kaolin clay or sand and up to 1.4 million loading cycles were applied. The results showed that the multipod foundations (symmetric or asymmetric) exhibit two closely spaced natural frequencies corresponding to the rocking modes of vibration in two principle axes. Furthermore, the corresponding two spectral peaks change with repeated cycles of loading and they converge for symmetric tetrapods but not for asymmetric tripods. From the fatigue design point of view, the two spectral peaks for multipod foundations broaden the range of frequencies that can be excited by the broadband nature of the environmental loading (wind and wave) thereby impacting the extent of motions. Thus the system lifespan (number of cycles to failure) may effectively increase for symmetric foundations as the two peaks will tend to converge. However, for asymmetric foundations the system life may continue to be affected adversely as the two peaks will not converge. In this sense, designers should prefer symmetric foundations to asymmetric foundations.
•Dynamic soil–structure interaction of offshore wind turbines on three types of foundations are explored through scaled model tests.•Scaling laws for multi-pod foundations are derived.•Behaviour of symmetric and asymmetric multipod foundations are presented. Practical aspects are highlighted.
Previous computational studies of short‐period buildings have shown increasing collapse probabilities with decreasing building period; a trend that has not been observed in past earthquakes. In this ...study, collapse performances of twelve short‐period steel special concentrically braced frame (SCBF) building archetypes were examined using dynamic analysis and the methods described in FEMA P‐695 to investigate this gap between analytically predicted and historically observed collapse rates. The archetypes varied key design and modeling parameters that could influence the behavior. These parameters included number of stories, level of design seismicity, redundancy, the inclusion of soil‐structure interaction (SSI) and foundation flexibility, and the removal of reserve moment frame resistance in modeling. Practicing engineers designed the archetypes resulting in realistic designs, including overstrength values representative of this building type. The calculated overstrength values were much higher than those found in previous studies, largely a result of the archetype building layouts, brace width‐to‐thickness ratio limitations, and the large differences between tension and compressive brace strength that occur when design demands are low. The higher overstrength values found in this study resulted in lower probabilities of collapse, relative to previous collapse studies and showed that probabilities of collapse decreased with decreasing period. Additionally, the probability of collapse increased significantly for the nonredundant archetype, but changed insignificantly for the archetype without reserve moment frame capacity. Inclusion of SSI and foundation flexibility resulted in a complete change in behavior where braces remained elastic and the braced frames rocked on their foundations, resulting in decreased collapse probabilities.
Background and aims Afforestation and thinning management are effective ways to mitigate global warming. Soil carbon reconstruction mechanisms can be effectively explored by linking soil aggregates ...and isotopic .sup.13C. Methods Soil samples were collected from agricultural land (AL) and larch plantations (established in 1965 and thinned in1995, UT: 2500 treeâ§ha.sup.-1, MT: 1867 treeâ§ha.sup.-1, and ST: 1283 treeâ§ha.sup.-1). The soil was separated into three aggregate sizes (LMAC: > 2 mm, SMAC: 2-0.25 mm, MIC: 0.25-0.053), minerals associated with organic matter (MAOM: < 0.053 mm), and carbon fractions within macroaggregates. Results We found that afforestation on agricultural land significantly increased the mean weight diameter (MWD). However, intensifying thinning decreased MWD by increasing SMAC. Moreover, after afforestation, the carbon concentration in soil aggregates and MAOM significantly decreased, and the carbon stability of macroaggregates weakened but could be strengthened after thinning. Thinning decreased the C/N in the soil aggregates and MAOM when no change in carbon concentration. The effect of thinning intensity on C/N was obvious with decreasing of particle size. The delta.sup.13C, mainly controlled by soil aggregates, significantly decreased in each soil aggregate after afforestation but increased after thinning. Additionally, the carbon concentrations, C/N and delta.sup.13C of small-size particles (< 0.25 mm) and the distribution of SMAC are important for SOC, C/N, CO.sub.2 fluxes and delta.sup.13C in bulk soil. Conclusion We conclude that soil aggregate distribution is conducive to soil carbon renewal, suggesting that increasing thinning intensity is beneficial for accumulating older carbon and acquiring nitrogen in more stable fractions.
This paper investigates the dynamic structure-soil-structure interaction (SSSI) between two adjacent sway frames, replicating closely spaced residential buildings in an urban setting subjected to ...seismic input motions. The structures are considered with and without external damping mechanisms by means of tuned mass damper (TMD) configurations. Geotechnical centrifuge tests were conducted to address the current lack of experimental case studies into SSSI and uniquely explore the influences from the presence of TMDs and variations in their configurations on SSSI. SSSI was found to significantly alter the response of proximally located urban structures, as was evident from significantly larger rocking amplitudes and changes in frequency response spectra. TMD effects under SSSI were mostly found to amplify an adjacent structure's peak roof acceleration response and inter-storey drift – this was most aggravated when the damper was de-tuned. TMD effects were found to be relatively more pronounced under smaller earthquakes and could certainly cause occupational inconvenience or even damage in adjacent buildings.
•SSSI causes significantly larger rocking amplitudes in an adjacent structure.•SSSI alters the frequency response spectra of an adjacent structure.•TMD effects under SSSI mostly amplify an adjacent structure's peak roof acceleration.•TMD effects under SSSI amplify an adjacent structure's peak inter-storey drift.•TMD effects under SSSI are most aggravated when the damper is de-tuned.
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