Acceleration-sensitive Equipment Research Articles

Study on a conical bearing for acceleration-sensitive equipment

Study on a conical bearing for acceleration-sensitive equipment

Seismic response mitigation of prefabricated industrial equipment structural frames through a hybrid isolation system

For reducing the excessive acceleration response of the critical equipment in a type of industrial equipment structural frames under strong earthquakes, a hybrid isolation system (HIS), incorporating linear natural rubber (LNR) and lead rubber (LR) bearings, is proposed. The efficiency and reliability of the HIS are verified through a series of full-scale shaking table tests on two prefabricated industrial moment resisting frames (MRFs), with respect to a corresponding fixed-base system (FBS). The structural dynamic characteristic and seismic performance of the two types of MRFs, i.e., reinforced concrete structure (RCS) and composite concrete-filled steel tube structure (CCFSTS), are examined both with the HIS and FBS subjected to a range of seismic intensities. The test results demonstrate the superior performance of the HIS in controlling transient acceleration response under near-fault, far-field and artificial ground motions. The average peak acceleration reduction coefficients (PARCs) of the HIS range between 75.60% and 79.51% in the two considered MRFs along two horizontal directions, with a strict satisfaction on the peak acceleration response for normal operation of the critical equipment. Also, the hysteretic behavior of the HIS is stable and controllable during the earthquake excitations. The peak transient displacement of the HIS is satisfactory, and the residual deformation is negligible even under the maximum considered earthquake (MCE) excitations. Besides, components of the two MRFs are undamaged during the whole test. Thus, with the capacity of protecting the acceleration-sensitive equipment and structural components, the proposed HIS possesses a great prospect in enhancing the structural resilience.

Study on a Novel Variable-Frequency Rolling Pendulum Bearing

Seismic isolation is a technique that has been widely used around the world to decouple the superstructure from the ground motions during earthquakes. However, the attention of seismic isolation is mostly focused on the protection of the building structures. Acceleration-sensitive devices or equipment, which are in desperate need of seismic protection, are still not fully emphasized. Meanwhile, the stiffness and frequencies of the conventional rolling- and sliding-type isolation bearings demonstrate an upward trend as the isolation layer displacement increases, which may bring self-centering and resonance issues. Thus, a novel variable-frequency rolling pendulum bearing is developed for the protection of acceleration-sensitive equipment. The rolling-type isolation bearing is selected to enhance the self-centering capacity, and additional viscous dampers are incorporated to improve the system damping. Moreover, the theoretical formulas of several typical variable-frequency rolling pendulum bearings are derived and presented to figure out the dynamic characterization of the device. The isolation efficiency of the proposed device under different parameters is also validated using shake table tests. Test results demonstrate that the newly proposed devices show excellent isolation performance at reducing both acceleration and displacement responses. Finally, the numerical model of this isolation system is proposed in detail. The simulated results, including relative acceleration responses, relative displacement responses and movement locus of the upper plates, are consistent with test results, which demonstrates this simplified model could be used for further studies.

Combination of decentralized sliding mode control and online wavelet analysis for control of equipment with isolation system

Critical equipment and machinery, especially the one in the technology facilities, hospitals, data centers, etc., are vulnerable to strong earthquakes, and most importantly, the failure of the critical equipment will result in countless economic loss, directly and indirectly. For decades, passive control devices in the form of isolation systems between the equipment and its supporting structure have been acknowledged as an effective measure to promote the earthquake resistibility on the basis of theoretical and experimental results. Alternative to passive isolation systems, researchers found that adding a semiactive device to the isolation system can increase damping and lessen the isolator displacement. In order to mitigate the acceleration of equipment, various control algorithms have been developed to reduce the response produced by the internal forces. However, little attention has been given to the balance between the response of equipment and the structural response. In this study, an innovative control algorithm is developed based on decentralized control architecture and wavelet analysis, and this control algorithm is proposed for semiactive isolation systems to protect acceleration-sensitive equipment against strong earthquakes. Subsequently, a full-scale specimen is constructed; moreover, a curved surface slider (CSS) system with a magneto-rheological (MR) damper are placed between the equipment and the floor to suppress the vibration. The experiment is conducted with a feedback control system to validate the performance and efficiency of proposed control algorithm. In the last place, the discussion and some conclusion are also drawn in this paper.

A Probabilistic Methodology for Assessing Post-Earthquake Fire Ignition Vulnerability in Residential Buildings

Post-earthquake fire (PEF) ignition events constitute a secondary consequence of an earthquake and may result in the loss of life and substantial property damage, especially in urban areas where the potential for fire spread and conflagration exists. These secondary hazards can cause severe structural and non-structural damage, potentially more significant damage than the direct damage caused by the initial earthquake, and can lead to catastrophic structural failures, devastating economic losses, and casualties. To manage the impact of PEF in urban areas, it is important to identify the potential ignition sources and quantify the vulnerabilities of these ignition sources as a result of earthquake-induced structural damage. The results of such analyses can be used to offer resiliency improvement investments and mitigation strategies in urban areas located in seismically active regions. Most of the previous PEF studies are data-driven, utilizing ignitions reported following recent earthquakes. However, in areas for which historical PEF data are not available, such as the Midwestern United States, a different methodology for developing a PEF model is needed. This paper describes an analytical model for quantifying the vulnerabilities of residential buildings to PEF by estimating the failure of ignition sources upon a probable seismic event. The underlying concept in developing the method is that (1) ignition sources in residential buildings remain unchanged before and after an earthquake, and (2) the total probability of PEF occurrence can be estimated by adjusting the probabilistic fire occurrence data for normal conditions (everyday operation of ignition sources) to account for the effect of the earthquake. This paper’s contribution to state of the art is in developing a new framework for estimating the probability of PEF for areas in which historical PEF data is unavailable. The developed framework uses the likelihood of ignition occurrence during normal condition as a baseline; this baseline is then adjusted using certain key parameters to capture spatial characteristics, ignitability, and potential seismic intensity of the study area to estimate the probability of PEF as a function of projected earthquake characteristics. The model was tested for St. Louis City as a populated area with potential future earthquake hazard because of its proximity to the New Madrid Fault zone. Using the National Fire Incident Reporting System dataset, the frequency of normal condition ignitions was determined as 1.97E−03 ignition per unit per year. Using the proposed PEF model considering PEFs caused by damage to drift and acceleration sensitive equipment and human actions, the projected frequency of PEF was estimated between 2.79E−06 and 2.81E−06 ignitions per household per year. Using this model, and the average number of households between 2010 to 2015, 175,854 households, it was estimated that in the next 50 years, approximately 25 households would experience fires related to probable earthquake events in St. Louis City.

Code-oriented floor acceleration spectra for building structures

In the seismic design and assessment of acceleration-sensitive equipment installed in buildings, floor acceleration spectra, which are based on an uncoupled analysis of the structure and the equipment, are usually used. However, in order to obtain an “accurate” determination of floor spectra, a complex and quite demanding dynamic response-history analysis is needed. Recently a method for the direct generation of floor acceleration spectra from ground motion spectra, taking into account the dynamic properties of the structure, has been developed and validated. It is based on the theory of structural dynamics, in combination with empirically determined values for the amplification factors in the resonance region. The method can be used for both elastic and inelastic multi-degree-of-freedom structures and equipment modelled as an elastic or inelastic single-degree-of-freedom oscillator. In the case of inelastic primary structures, the method is coupled with the pushover-based N2 method. The variant of the method which is presented in this note is intended for practical applications, e.g. for implementation in guidelines and codes, and it represents a simplified version of the original method. In addition to some simplifications, the option of taking into account the inelastic response of the equipment was added. In the note, the method is summarized, and all the formulae needed for the calculation of floor acceleration spectra are provided. A description of all steps of the analysis, together with all the relevant numerical data, is presented in a test example.

Influence of isolation hysteresis on the seismic performance of isolated buildings

Seismic isolation is one of the most used protective technologies for earthquake engineering. Many innovative devices have been researched and implemented. Selection of the different isolation devices is commonly based on engineering experience, where no detailed and systematic comparison has been provided. In this paper, the seismic performances of a three-story office building isolated using five different devices were evaluated. The isolation devices included the bilinear, flag-shaped I, flag-shaped II, crystallizing rubber (CR), and rigid-perfectly plastic (RPP) models. These isolation devices were designed to achieve an equivalent performance at the seismic shaking intensity of 2% probability of exceedance in 50 years. The evaluation was conducted using the state-of-the-art assessment procedure at 2%, 10%, and 50% probability of exceedance in 50 years (50/50, 10/50, and 2/50) hazard levels. The results indicated that all the isolators performed well by limiting the financial losses. At both the 50/50 and 10/50 shaking intensities, the building with CR isolators experienced no repair costs, while those with bilinear, flag-shaped I, flag-shaped II, and RPP isolators had minor repair costs contributed from the content performance groups. At the 2/50 shaking intensity, all isolators incurred minor damages in the content performance groups. The building with CR isolators had additional repair costs because of the interior drift sensitive nonstructural components, while the building with the RPP isolators had repair costs contributed from the interior drift and acceleration sensitive nonstructural components and the acceleration sensitive equipment at the roof. Copyright © 2014 John Wiley & Sons, Ltd.

Combined primary–secondary system approach to the design of an equipment isolation system with High-Damping Rubber Bearings

Isolating acceleration-sensitive equipment from the motion of the supporting structure represents an effective protection from earthquake damage. In this paper, a passive equipment isolation system composed of High-Damping Rubber Bearings (HDRB) is designed by adopting a coupled approach in which the supporting structure and the isolated equipment are considered as parts of a combined primary–secondary system and analyzed together. This allows for taking into account their dynamic interaction when significant and non-negligible according to the mass ratio and to the frequency ratio. The design methodology is developed by resorting to a reduced-order 2-DOF model of the combined system, a linear visco-elastic constitutive model of the isolation system and to a modal damping constraint depending upon the damping properties of the HDRB and their rubber compound. A 1:5 scale experimental model, consisting of a two-storey steel frame and a heavy block-type mass isolated from the second floor, is subsequently used to exemplify the design methodology and to perform shaking table tests. The dynamic properties of the experimental model are identified and the seismic performance of the equipment isolation system is discussed under a wide selection of seismic inputs, both artificial and natural.

Absolute-energy-based active control strategies for linear seismic isolation systems

The general purpose of seismic isolation systems is to reduce the ground acceleration transmitted onto the isolated object down to a minimum possible level within an allowable isolator displacement. Therefore, the efficiency of an isolation system can usually be quantified by the acceleration level transmitted onto the isolated object. In order to improve the performance of active isolation systems (AISs) for acceleration-sensitive equipment or structures, two optimal control laws that utilize performance indices associated with system absolute energy are proposed in this study. The first control law, which requires the measurement of the ground velocity in deciding the control force, is derived based on the concept of instantaneous optimal control. The second one is based on the concept of discrete-time optimal control, in which the feedback gain can be obtained by solving the discrete-time algebraic Riccati equation. The numerical simulation results of this study demonstrate that the isolation performance of an AIS system using the proposed absolute-energy-based control laws is much superior to the conventional optimal control in reducing the super-structural acceleration response for either near-fault or far-field earthquake. In addition, a comparison between the two proposed control laws reveals that the first control law has a lower acceleration response, whereas the second one results in less isolator displacement. Copyright © 2009 John Wiley & Sons, Ltd.

Unavailability of wind turbines due to wind-induced accelerations

The malfunctioning of acceleration-sensitive equipment in wind turbines has the potential to affect their annual failure rates during normal operating conditions. Current protective measures for wind turbines are triggered by wind speed. However, this option neglects the structural response of the wind turbines, and limits the possibility of effectively controlling accelerations at the top of the wind towers. In this study, a dynamic model of the wind tower/rotor blade system is used to estimate the acceleration response at the top of the towers. This model is coupled with a probabilistic description of the dynamic properties of the wind turbines, and an annual distribution of the wind hazard to obtain a distribution of the acceleration response for various levels of wind speed. This distribution is used to develop wind-induced acceleration fragility curves for typical wind turbines. Acceleration thresholds of 10, 15, and 20 m/s 2 are likely to be exceeded at the onset of protective cut out wind speeds of 25 m/s, with probabilities as high as 64%, 48%, and 37%, respectively. Several wind turbine subcomponents are sensitive to these levels of acceleration. Some examples, in decreasing order of acceleration susceptibility, include generators, brakes, yaw systems, inverters, mechanical controls, electrical controls, hydraulic systems, gearboxes, shafts, and physical/electrical connections to the power grid. By weighting the fragility curves with the likelihood of observing specific wind speeds during any given year, this study provides unconditional probabilities of failure that represent annual unavailability rates for wind turbines. Exceeding any of the acceleration levels between 10 m/s 2 and 20 m/s 2 has probabilities of 47% and 49% at two locations with different wind regimens: Western Texas, USA, and Northern Ireland. This annual unavailability rate indicates that almost half the units of a wind farm at either location are likely to experience abnormal operation owing to the demands on acceleration-sensitive equipment. The use of fragility curves and the risk of unavailability for wind turbines will continue to grow because they allow performance assessment of future systems which favor larger, more flexible, more massive, and more complex wind turbine units. These new generation units will be able to operate at higher wind speeds and more demanding environmental conditions. A short- term option to reduce acceleration-induced failures in built systems is to establish shutdown criteria based on acceleration, rather than just wind speed. However, long-term handling of acceleration-sensitive equipment requires fragility and reliability analysis at the design stage, and the use of actively controlled protective devices.