Identifying boundary between near field and far field in ground vibration caused by surface loading

The boundary between the near and far fields is generally defined as the distance from the vibration source beyond which ground vibrations are mainly dominated by Rayleigh waves. It is closely related to the type of vibration source and the soil properties. Based on the solutions of the Lamb＇s problem, the boundary at the surface between the near and far fields of ground vibration was investigated for a harmonic vertical concentrated load and an infinite line load at the surface of a visco-elastic half-space. Particularly, the variation of the boundary with the material damping was investigated for both cases. The results indicate that the material damping slightly contributes to the attenuation of vibrations in the near-source region, but significantly reduces the vibrations in the region that is at some distance away from the source. When taking the material damping into consideration, the boundary between the near and far fields tends to move towards the vibration source. Compared with the vibrations caused by a concentrated load, the vibrations induced by an infinite line load can affect a larger range of the surrounding environment, and they attenuate more slowly. This means the boundary between the near field and far field should move fitrther away from the source. Finally, the boundaries are defined in terms of R-wave length （2R） and Poisson ratio of the ground （o）. For the case of a point load, the boundary is located at the distance of （5.0-6.0）2R for v≤0.30 and at the distance of （2.0--3.0）2R for v≥0.35. For the case of an infinite line load, the boundary is located at the distance （5.5-6.5）2rt for v≤0.30 and at the distance （2.5--3.5）2R for v≥0.35.

Recent progress and application on seismic isolation energy dissipation and control for structures in China

China is a country where 100% of the territory is located in a seismic zone. Most of the strong earthquakes are over prediction. Most fatalities are caused by structural collapse. Earthquakes not only cause severe damage to structures, but can also damage non-structural elements on and inside of facilities. This can halt city life, and disrupt hospitals, airports, bridges, power plants, and other infrastructure. Designers need to use new techniques to protect structures and facilities inside. Isolation, energy dissipation and, control systems are more and more widely used in recent years in China. Currently, there are nearly 6,500 structures with isolation and about 3,000 structures with passive energy dissipation or hybrid control in China. The mitigation techniques are applied to structures like residential buildings, large or complex structures, bridges, underwater tunnels, historical or cultural relic sites, and industrial facilities, and are used for retrofitting of existed structures. This paper introduces design rules and some new and innovative devices for seismic isolation, energy dissipation and hybrid control for civil and industrial structures. This paper also discusses the development trends for seismic resistance, seismic isolation, passive and active control techniques for the future in China and in the world.

Design Methodology of a Passive Vibration Isolation System for an Optical System With Sensitive Line-of-Sight

The performance of an optical system with sensitive line-of-sight(LOS)is influenced by rotational vibration.In view of this,a design methodology is proposed for a passive vibration isolation system in an optical system with sensitive LOS.Rotational vibration is attributed to two sources:transmitted from the mounting base and generated by modal coupling.Therefore,the elimination of the rotational vibration caused by coupling becomes an important part of the design of the isolation system.Additionally,the decoupling conditions of the system can be obtained.When the system is totally decoupled,the vibration on each degree of freedom(DOF)can be analyzed independently.Therefore,the stiffness and damping coefficient on each DOF could be obtained by limiting the vibration transmissibility,in accordance to actual requirements.The design of a vibration isolation system must be restricted by the size and shape of the payload and the installation space,and the layout constrains are thus also discussed.

Bio-inspired polygonal skeleton structure for vibration isolation:Design,modelling,and experiment

Inspired by the safe landing of a cat falling from a high altitude,a bio-inspired polygonal skeleton(BIPS) structure is proposed,and its nonlinear characteristics are systematically studied to explore its potential application in the suppression of vibration. The polygon is formed by the skeleton structure of the cat’s entire body and the ground. The BIPS system consists of two symmetrical bionic legs with three robs(as skeleton) and four horizontal springs(as muscle). Two bionic legs are connected through the bearing platform(as spine),which could adjust the distance between the two bionic legs. A theoretical model is developed to characterize its stiffness nonlinearity through geometrical and mechanical analysis. Parameter analysis reveals that the BIPS structure has diverse stiffness,including nonlinear positive stiffness and negative stiffness. By imitating adjustment of leg posture and telescopic function of the spine(control the distance between legs),these flexible stiffness properties can be adjusted by structure parameters. In addition,the load capacity and working range can also be designed by the length of the bars,the initial angle,the mounting position,and the spring stiffness. The experimental setup is established,and the vibration isolation performance under various excitation is tested. The experimental results verify the accuracy of the dynamic model and also show that the proposed BIPS structure can suppress the vibration effectively under a variety of excitations. These peculiarities may provide potential possibility of an innovative approach to passive vibration control and isolation.