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41.
42.
Behavior of braced excavation in sand under a seismic condition: experimental and numerical studies 总被引:1,自引:1,他引:0
The behavior of braced excavation in dry sand under a seismic condition is investigated in this paper. A series of shake table tests on a reduced scale model of a retaining wall with one level of bracing were conducted to study the effect of different design parameters such as excavation depth, acceleration amplitude and wall stiffness. Numerical analyses using FLAC 2D were also performed considering one level of bracing. The strut forces, lateral displacements and bending moments in the wall at the end of earthquake motion were compared with experimental results. The study showed that in a post-seismic condition, when other factors were constant, lateral displacement, bending moment, strut forces and maximum ground surface displacement increased with excavation depth and the amplitude of base acceleration. The study also showed that as wall stiffness decreased, the lateral displacement of the wall and ground surface displacement increased, but the bending moment of the wall and strut forces decreased. The net earth pressure behind the walls was influenced by excavation depth and the peak acceleration amplitude, but did not change significantly with wall stiffness. Strut force was the least affected parameter when compared with others under a seismic condition. 相似文献
43.
In the aggressive marine environment over a long-term service period,coastal bridges inevitably sustain corrosion-induced damage due to high sea salt and humidity.This paper investigates the strength reduction of coastal bridges,especially focusing on the effects of non-uniform corrosion along the height of bridge piers.First,the corrosion initiation time and the degradation of reinforcement and concrete are analyzed for bridge piers in marine environments.To investigate the various damage modes of the concrete cover,a discretization method with fiber cells is used for calculating time-dependent interaction diagrams of cross-sections of the bridge piers at the atmospheric zone and the splash and tidal zone under a combination of axial force and bending moment.Second,the shear strength of these aging structures is analyzed.Numerical simulation indicates that the strength of a concrete pier experiences dramatic reduction from corrosion initiation to the spalling of the concrete cover.Strength loss in the splash and tidal zone is more significant than in the atmospheric zone when structures’service time is assumed to be the same. 相似文献
44.
Piled embankments, which offer many advantages, are increasingly popular in construction of high-speed railways in China. Although the performance of piled embankment under static loading is well-known, the behavior under the dynamic train load of a high-speed railway is not yet understood. In light of this, a heavily instrumented piled embankment model was set up, and a model test was carried out, in which a servo-hydraulic actuator outputting M-shaped waves was adopted to simulate the process of a running train. Earth pressure, settlement, strain in the geogrid and pile and excess pore water pressure were measured. The results show that the soil arching height under the dynamic train load of a high-speed railway is shorter than under static loading. The growth trend for accumulated settlement slowed down after long-term vibration although there was still a tendency for it to increase. Accumulated geogrid strain has an increasing tendency after long-term vibration. The closer the embankment edge, the greater the geogrid strain over the subsoil. Strains in the pile were smaller under dynamic train loads, and their distribution was different from that under static loading. At the same elevation, excess pore water pressure under the track slab was greater than that under the embankment shoulder. 相似文献
45.
Viscoelastic (VE) dampers, with their stiffness and energy dissipation capabilities, have been widely used in civil engineering for mitigating wind-induced vibration and seismic responses of structures, thus enhancing the comfort of residents and serviceability of equipment inside. In past relevant research, most analytical models for characterizing the mechanical behavior of VE dampers were verified by comparing their predictions with performance test results from small-scale specimens, which might not adequately or conservatively represent the actual behavior of full-scale dampers, especially with regard to the ambient temperature, temperature rise, and heat convection effects. Thus, in this study, by using a high-performance testing facility with a temperature control system, full-scale VE dampers were dynamically tested with different displacement amplitudes, excitation frequencies, and ambient temperatures. By comparing the analytical predictions with the experimental results, it is demonstrated that adopting the fractional derivative method together with considering the effects of excitation frequencies, ambient temperatures, temperature rises, softening, and hardening, can reproduce the design performance of full-scale VE dampers very well. 相似文献
46.
This study constructs a 3D velocity structure model of the Ludian region in the Yunnan province, southwestern China, and simulates ground motion propagation of the 2014 Ludian Ms 6.5 earthquake. It aims to construct the local velocity structure of the Ludian region in three dimensions and with high precision. The simulation, using the spectral element method, is validated by field data from the Ludian earthquake records. Thus, it demonstrates that the adopted key parameters, such as the seismic source mechanism, propagation medium and geographical features of the engineering site, are appropriated for the simulation. Meanwhile, the simulation generates the ground motion distribution of the study region with an earthquakeinduced landslide in Ludian earthquake. 相似文献
47.
Using a new low-temperature dynamic triaxial apparatus, the influence law of freezing-thawing cycles on clay shear strength is studied. In this research, the concept of correction coefficients of freezing-thawing cycles on clay static strength, cohesion and internal friction angles is proposed, and the change patterns, correction curves and regressive formulae of clay static strength, cohesion and internal friction angles under freezing-thawing cycles are given. The test results indicate that with increasing numbers of freezing-thawing cycles, the clay static strength and cohesion decrease exponentially but the internal friction angle increases exponentially. The performance of static strength, cohesion and internal friction angles are different with increasing numbers of freezing-thawing cycles, i.e., the static strength decreases constantly until about 30% of the initial static strength prior to the freezing-thawing cycling and then stays basically stable. After 5–7 freezing-thawing cycles, the cohesion decreases gradually to about 70% of the initial cohesion. The internal friction angle increases about 20% after the first freezing-thawing cycle, then increases gradually close to a stable value which is an increase of about 40% of the internal friction angle. The freezing-thawing process can increase the variation of the density of the soil samples; therefore, strict density discreteness standards of frozen soil sample preparation should be established to ensure the reliability of the test results. 相似文献
48.
Based on performance-based seismic engineering, this paper proposes an optimal seismic retrofit model for steel moment resisting frames (SMRFs) to generate a retrofit scheme at minimal cost. To satisfy the acceptance criteria for the Basic Safety Objective (BSO) specified in FEMA 356, the minimum number of upgraded connections and their locations in an SMRF with brittle connections are determined by evolutionary computation. The performance of the proposed optimal retrofitting model is evaluated on the basis of the energy dissipation capacities, peak roof drift ratios, and maximum interstory drift ratios of structures before and after retrofitting. In addition, a retrofit efficiency index, which is defined as the ratio of the increment in seismic performance to the required retrofitting cost, is proposed to examine the efficiencies of the retrofit schemes derived from the model. The optimal seismic retrofit model is applied to the SAC benchmark examples for threestory and nine-story SMRFs with brittle connections. Using the retrofit efficiency index proposed in this study, the optimal retrofit schemes obtained from the model are found to be efficient for both examples in terms of energy dissipation capacity, roof drift ratio, and maximum inter-story drift ratio. 相似文献
49.
Preliminary design of offshore wind turbines requires high precision simplified methods for the analysis of the system fundamental frequency. Based on the Rayleigh method and Lagrange's Equation, this study establishes a simple formula for the analysis of system fundamental frequency in the preliminary design of an offshore wind turbine with a monopile foundation. This method takes into consideration the variation of cross-section geometry of the wind turbine tower along its length, with the inertia moment and distributed mass both changing with diameter. Also the rotational flexibility of the monopile foundation is mainly considered. The rigid pile and elastic middle long pile are calculated separately. The method is validated against both FEM analysis cases and field measurements, showing good agreement. The method is then used in a parametric study, showing that the tower length Lt, tower base diameter d0, tower wall thickness δt, pile diameter db and pile length Lb are the major factors influencing the fundamental frequency of the offshore wind turbine system. In the design of offshore wind turbine systems, these five parameters should be adjusted comprehensively. The seabed soil condition also needs to be carefully considered for soft clay and loose sand. 相似文献
50.
In the field of seismic exploration, ground roll seriously affects the deep effective reflections from subsurface deep structures. Traditional curvelet transform cannot provide an adaptive basis function to achieve a suboptimal denoised result. In this paper, we propose a method based on empirical curvelet transform (ECT) for ground roll attenuation. Unlike the traditional curvelet transform, this method not only decomposes seismic data into multiscale and multi-directional components, but also provides an adaptive filter bank according to frequency content of seismic data itself. So, ground roll can be separated by using this method. However, as the frequency of reflection and ground roll components are close, we apply singular value decomposition (SVD) in the curvelet domain to differentiate the ground roll and reflection better. Examples of synthetic and field seismic data reveal that the proposed method based ECT performs better than the traditional curvelet method in terms of the suppression of ground roll. 相似文献