Isolation Systems

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74. INTRODUCTION TO SHOCK AND VIBRATION ISOLATION AND DAMPING SYSTEMS

This paper presents an introduction to shock and vibration isolation of complex structures and mechanisms. It provides an outline of various ways to provide isolation, shock absorbing and damping within a wide array of dynamic systems and structures. This paper presents key definitions that are widely used within the shock and vibration community. Additionally, useful formulae are presented that provide the user with an approach to typical problems. Finally, a comparison of different types of shock isolators, shock absorbers and dampers compares their advantages and disadvantages for use in the commercial, military, and aerospace sectors.

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69. ANALYSIS, OPTIMIZATION, AND DEVELOPMENT OF A SPECIALIZED PASSIVE SHOCK ISOLATION SYSTEM FOR HIGH SPEED PLANING BOAT SEATS

The Mk V Special Operations Craft (SOC) is used to carry Special Operations Forces (SOF) into and out of combat operations. During operation, particularly during extended training missions, the passengers and crew have reported numerous cases of musculoskeletal injuries from operation in high sea states. This paper describes the analysis, development and operational testing of a highly specialized, non-linear, passive shock isolated seat for this craft. Initial sea trial testing of the isolation system resulted in positive operator feedback that correlated well with earlier field measurements and also validated the analytical predictions.

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63. VIRTUAL BASE ISOLATION BY BUILDING SOFTENING WITH DRIFT CONTROL PROVIDED BY FLUID VISCOUS DAMPERS

The paper describes “virtual isolation” for buildings with one or more soft stories. Using the 1999 SEAOC Blue Book (SEAOC, 1999) recommendations for passive energy dissipation, the building’s Lateral Force Resisting System (LFRS) is designed for strength requirements only, resulting in a relatively flexible LFRS, while Fluid Viscous Dampers (FVD) are incorporated to limit story drifts to acceptable levels. There are many benefits to this “virtual isolation” system. With the elimination of the maximum drift requirements, the moment frames are substantially lighter than a traditionally framed building, thus lowering the structural steel cost of the LFRS. The long period structure also produces significantly reduced forces in the foundation elements. Velocity and displacement are reduced significantly through the use of the FVDs, which protects the sensitive contents of the building. These benefits lead to a reduced response resulting in an enhanced performance level during a major seismic event.

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61. A NUMERICAL INVESTIGATION OF COMBINED SHOCK AND VIBRATION ISOLATION THROUGH THE SEMI-ACTIVE CONTROL OF MAGNETORHEOLOGICAL FLUID DAMPER IN PARALLEL WITH AN AIR SPRING

Combining shock and vibration isolation into a single isolation mount is investigated numerically through the use of the Bouc-Wen model of a magnetorheological fluid damper in parallel with an air spring. The stability and dissipative capabilities of the Bouc-Wen model are proven mathematically. The response characteristic of this hybrid isolator to shock and vibration inputs is explored. The advantages of combining shock and vibration isolation into a single package is discussed. It is possible, using this technique, for a single device to perform equally well as a shock and a vibration isolator.

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55. EXPERIMENTAL STUDY OF BRIDGE ELASTOMER AND OTHER ISOLATION AND ENERGY DISSIPATION SYSTEMS WITH EMPHASIS ON UPLIFT PREVENTION AND HIGH VELOCITY NEAR-SOURCE SEISMIC EXCITATION

A series of shake table tests on an isolated bridge model included low and high damping elastomeric isolation systems, and low damping elastomeric systems with added linear and nonlinear viscous dampers. Each of these configurations could withstand much stronger seismic excitations than the non-isolated configurations. A set of low intensity tests was conducted to form a basis for comparison with the non-isolated configurations and also to test the effectiveness of these systems under low intensity excitation. The results of these tests are presented, followed by a discussion of the effects of scragging, the benefits of seismic isolation, and the significance of damping, the importance of added damping in near source seismic excitation and on the benefits and drawbacks of using nonlinear viscous damping.

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51. APPLICATIONS OF HERMETICALLY SEALED FLUID DAMPERS FOR LOW LEVEL, WIDE BANDWIDTH VIBRATION ISOLATION

Vibration isolation of sensitive components like high resolution cameras requires extremely low friction in the isolator system. Hydraulic dampers for these systems must be leak-free, which equates to relatively high friction seals. There is always a trade-off between allowable leakage and allowable friction in this type of application. This paper describes the isolation performance of a new hermetically sealed damper with essentially zero friction. It contains both an analytical representation of damper performance and dynamic test results.

Case Study

48. Arrowhead Regional Medical Center

This article describes every aspect of the design and construction of the Arrowhead Regional Medical Center, including the use of base isolators and viscous dampers to insure continuous operation even after a major seismic event. The article even includes many of the financial aspects of this huge project.

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47. Development and Testing of an Electronically Controlled Shock and Vibration Damper Having an Electrorheological Fluid Medium

An electrorheological (ER) fluid has been developed comprised of zeolite particles suspended in silicone oil. Testing of this fluid in a damper of 1,000 lbs. nominal output force rating has demonstrated the ability to control damper output with internal pressures above 500 psi and control power requirements of less than one watt. The damper control valve is a simple ER duct, with no moving parts, requiring only that a voltage potential exists across the duct’s cross section to activate the ER material, and thus cause the material to exhibit plastic behavior. All tests were successful, with no degradation of the damper or ER material occurring over large numbers of activation cycles or with time.

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44. Fluid Viscous Damping as an Alternative to Base Isolation

Base isolation of large structures has proven to be an effective way to attenuate seismic excitation. However it can be costly, and can also involve major building modification. It is now possible to attain a comparable degree of earthquake mitigation with fluid viscous dampers located throughout a structure, without having to isolate the building. This paper describes several techniques for doing this, provides analytical back-up and describes several applications of this technology.

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36. Viscous Damping for Base Isolated Structures

Seismic Base Isolation can use elastomeric pads, sliding plates or inverted pendulums. Each method can include an energy dissipation means, but only as some kind of hysteretic damping. Hysteretic damping has limitations in terms of energy absorption and may tend to excite higher modes in some cases. It’s possible to avoid these problems with viscous dampers. Viscous damping adds energy dissipation through loads that are 90o out of phase with bending and shear loads so even with damping levels as high as 40% of critical adverse side effects tend to be minimal. This paper presents basic theory of viscous damping and also describes a sample project. Viscous dampers being built for the new San Bernardino Medical Center reduce both deflections and loads by 50% compared with high damping elastomer base isolation bearings by themselves.