Seismic Qualification Research Articles

Sub‐assemblage testing and fragility analysis of connections of continuous‐plasterboard suspended ceiling systems

AbstractPast earthquake reconnaissance reports highlighted extensive seismic damages to suspended ceiling components and connections, including instances of complete ceiling failures. The seismic qualification of these nonstructural elements typically requires comprehensive evaluation through full‐scale shake table testing. However, such experimental evaluation is ordinarily not possible for every change made in various components and connections of ceiling systems due to the cost and effort involved. A feasible alternative is to obtain the behavior of components and connections from sub‐assemblage testing and incorporate them in appropriate numerical ceiling models to derive mechanical responses for developing alternative or new installation schemes. This paper considers critical connections of three continuous‐plasterboard suspended ceiling systems that were evaluated using shake table‐generated motions. The connections were classified according to their attachment to typical floors and walls of building structures, and sub‐assemblage testing was conducted using both monotonic and cyclic displacement loading. The observed failure modes in each connection were detailed, and appropriate damage states were assigned as the basis for constructing connection fragility curves. The results of the sub‐assemblage testing were presented in terms of the hysteresis responses, envelope curves, nonlinear backbone curves, and cumulative energy dissipation curves. Additionally, multilinear models of the connections were derived by approximating nonlinear backbone curves’ initial‐ and post‐yield behaviors. These multilinear models were further idealized to derive three equivalent linearized models for simplified yet reasonably accurate results for linear structural analyses. Finally, fragility curves were derived for all connections, considering their cyclic displacement failure capacities.

Seismic testing and fragility analysis considering material strength uncertainty of 1100 kV GFRP composite power transformer bushing

Nowadays glass-fibre reinforced polymer (GFRP) composite insulators are widely employed in electrical equipment in substations. However, there is a scarcity of reports on seismic qualification of transformer bushings that adopt GFRP insulators. Thereby, this case implements shaking table tests on a 1100 kV GFRP composite power transformer bushing. The study analyses the acceleration responses and dynamic amplification effects, as well as the strain responses. The combined stresses at the vulnerable positions are computed to authenticate the seismic performance. Using a simulation model, the seismic fragility of the transformer bushing considering the material strength uncertainty is assessed and discussed. The results indicate that the GFRP composite bushing meets the requirements of the relevant code and its seismic performance is qualified under an artificial ground motion with PGAs of 0.6 g and 0.8 g. For a given factor of safety of 1.67, the bushing can sustain 0.4 g earthquakes with a failure probability near 0 and 0.8 g earthquakes with a failure probability<50 %. Controlling coefficient of variation of the GFRP material strengths is beneficial to decreasing seismic fragility of the transformer bushing within a specific PGA range. Weibull and Normal distributions for fitting the strength distributions of the GFRP materials have different impacts on the seismic fragility of the bushing, which should be given more attention in seismic design.

Seismic Performance of Drop-In Anchors in Concrete under Shear and Tension

This paper presents an experimental study conducted on the behavior of drop-in anchors in uncracked concrete slabs. Both seismic (cyclic) load tests and static load tests to collapse are performed on drop-in anchors subjected to tension or shear forces. Three different anchor sizes are subjected to seismic qualification testing, followed by a static load test to collapse. The test results confirm the capability of the tested anchors to sustain simulated pulsating seismic tension and shear loading with frequency ranges between 0.1 and 2.0 Hz. It was observed that no tension failure occurred at the end of the cyclic load tests for all the tested anchors, and their residual inelastic maximum displacement at the end of the cyclic tension test was relatively small. Moreover, the experimental results show that the anchors’ ultimate capacities are higher than those specified by the anchor manufacturer. Finally, the anchors’ experimental pullout shear capacities are compared with the failure prediction equations in the literature and design codes. It is found that the theoretical models provide a conservative prediction of the concrete breakout of anchors in tension compared to the experimental ultimate loads. The coefficient for pry-out strength (kcp) equal to 2 or slightly smaller than 2 is likely to predict a better pry-out capacity with the experimental results compared to the application of the high conservative value of kcp equal to 1, as given in the code.

Seismic Assessment of Electrical Equipment in Power Substations: A Case Study for Circuit Breakers

Electric power is essential in post-earthquake periods for the continuous functionality of disaster management and emergency services. In addition, interruption of electric power can cause significant economic losses due to downtime of critical facilities. Therefore, it is very important to maintain seismic safety of electric power systems and components. There are existing seismic regulations and standards regarding electric power systems, especially in the United States of America (USA) and Europe. A similar regulation has been prepared recently in Türkiye, which is a country in a seismically active region. This study focuses on the current state of practice regarding the seismic assessment of electrical equipment in power stations and implementation of the regulations on seismic qualification of these systems. Among many electrical equipment, circuit breakers have been selected for case study. The seismic assessment of the selected high voltage equipment has been performed according to the new regulation under the seismic hazard specifically defined for Türkiye. The case study experiment presents the new methodology in evaluating and classifying the seismic response of high voltage electrical equipment and provides insight to the expected behaviour of circuit breakers under earthquake induced action.

Shaking table tests of power distribution cabinets: Physical damage, post-earthquake functionality and seismic response evaluation

Power distribution cabinets (PDCs) are widely used in several critical facilities to ensure the power supply and distribution. Although these cabinets may suffer seismic damage during earthquakes, and result in cascading functionality, economic and life losses, they have received little attention in terms of seismic performance. Therefore, there is an increasing need to understand the performance of such equipment during earthquakes. This study conducted shaking table tests on three PDCs to investigate the seismic performance and dynamic responses. Both physical damage and post-earthquake functionality evaluation were conducted during shaking table tests. The seismic responses of tested cabinets including natural frequency, stress, acceleration, and deformation responses were presented in detail. Seismic qualification levels of tested cabinets based on existing rules were also introduced. This paper not only enlarges the experimental database of the seismic performance of PDCs, but also provides basic data and rules for performance-based seismic evaluation of such nonstructural components.

Four-Bolt Unstiffened End-Plate Moment Connections with 36-in.-Deep Beams for Intermediate Moment Frames

Previous testing has shown that four-bolt extended, unstiffened end-plate moment connections can be used with beams up to 24 in. deep and develop sufficiently ductile performance to satisfy seismic qualification. It is desirable to extend this depth limit to allow deeper beams for intermediate moment frames. Three moment connection specimens using built-up 36-in.-deep beams with a four-bolt extended, unstiffened end-plate moment connection were tested to determine if they satisfy the intermediate moment frame qualification criteria given in AISC 341-16 (2016a). The web of the specimens satisfied the moderately ductile section criteria for web slenderness, which is required for intermediate moment frames, but not highly ductile section criteria required for special moment frames.&#x0D; All three specimens passed qualification criteria for intermediate moment frames (IMFs) as given in AISC 341-16 by retaining at least 80% of the nominal plastic moment strength through 2% story drift. The observed failure modes included lateral torsional buckling of the specimen, flange local buckling, net section fracture of the beam flange, and failure of the beam flange-to-beam web fillet weld. The results of these tests support current moderately ductile limits associated with lateral bracing and cross-section slenderness, given that the associated specimens reached IMF qualification criteria but not special moment frame (SMF) criteria. It was concluded that four-bolt extended, unstiffened end-plate moment connections satisfy intermediate moment frame requirements with a beam depth of 36 in. and that a modification is needed for beam web to flange welds in built-up sections near the moment connection.

Seismic qualification of crossover piping systems on a sodium-cooled fast reactor with a seismic isolation system

Seismic qualification of crossover piping systems on a sodium-cooled fast reactor with a seismic isolation system

Seismic assessment and qualification of acceleration-sensitive nonstructural elements through shake table testing: Reliability of testing protocols and reliability-targeted safety factors

Shake table testing represents the best option for assessing and qualifying acceleration-sensitive nonstructural elements (NEs). Several testing protocols are defined in regulations and codes and implemented in the literature, but no commentaries or literature studies provide information regarding the expected reliability or recommend applicative safety factors. The study investigates the seismic reliability of shake table testing protocols for seismic assessment and qualification of acceleration-sensitive NEs. Numerical analyses are carried out considering an incremental procedure and modeling NEs as inelastic single degree of freedom (SDOF) systems, over a wide range of frequencies of interest and NE structural properties. Real floor motions recorded in instrumented reinforced concrete (RC) buildings are considered as a reference. The reliability is estimated considering several damage states and various floor motion sets. In particular, the seismic capacities associated with the real (shake table protocol) floor motions are considered as actual-demand (nominal-capacity) measures, and the demand to capacity margin is meant as the protocol overestimation of the NE capacity. In the light of the estimated protocol reliability, reliabiliy-targeted capacity safety factors are assessed, and applicative factor abaci and closed-form criteria are proposed.

An analytical model for seismic response analysis of electrical equipment-steel support structure

Electrical equipment mounted on supporting structures is typical in a substation but vulnerable to earthquakes from previous field investigations. To evaluate the seismic performance of such an electrical equipment-support coupling system, a modal space analytical model was developed using the vibration partial differential equation (PDE) with distributed parameters and joint nodes. In the analytical model, resonant frequencies and mode shapes were obtained through numerical algorithms based on the boundary condition of each segmented beam. Then seismic responses of the coupling system can be attained by the modal decomposition method and superposition principle. A full-scale shaking table test was conducted on a 550 kV bushing-support coupling structure, and the accuracy of the analytical model was verified by the experimental results. Furthermore, a ceramic breakage inside the connecting flange at the base cross-section of the bushing specimen was observed after the seismic qualification test, which means its seismic performance can not meet the demand in the IEC seismic design specifications. To figure out this problem, parametric analysis was carried out using the analytical model to study the influence of parameters of the supporting structure and flange connection on the seismic responses of the equipment. In light of the analytical results, halving the original flange's flexural rigidity can reduce the bottom stress of the bushing by about 20% without over-magnifying its top displacement for this bushing-support coupling system. Application of the presented analytical modelling method can assist in the seismic optimization design of other equipment-support coupling systems in substations.

Prototype Test of Resilient Friction Materials for Seismic Dampers

In recent decades, low-yielding seismic devices based on the use of friction dampers have emerged as an excellent solution for the development of building structures with improved reparability and resilience. Achieving an optimal design for such low-yielding seismic devices requires precise control of bolt preloading levels and predictability of the friction coefficient (CoF) between the damper interfaces. While various types of friction devices exist that are capable of providing significant energy dissipation, ongoing research is focused on the development of novel friction materials that exhibit a stable hysteretic response, high CoF values, minimal differences between static and dynamic CoF, and predictable slip resistance. In this context, an experimental campaign was conducted at the STRENGTH Laboratory of the University of Salerno to evaluate the behaviour of new friction shims employing specially developed metal alloys. Specifically, the influence of the characteristics of the contact surfaces in the sliding area on the behaviour and performance of the friction device was analysed. The tests followed the loading protocol recommended by EN12159 for seismic device qualification. Monitored parameters included preloading force values and the evolution of slip resistance. The friction value was determined, along with its degradation over time. Finally, the material’s performance in terms of hysteretic behaviour was assessed, providing a comparison of the tested specimens in terms of slip force degradation and energy dissipation capacity.

Performance of 420 kV Instrument Transformers under Earthquake

Seismic performance of substation equipment during past earthquakes is poor. In addition, requirement of electric power during post-earthquake led to an increased focus on the earthquake survivability of substation equipment. Equipment installed in the substation is vulnerable to earthquakes by virtue of its geometry and configuration. Interconnection between equipment further aggravates the problem. Failure of substation equipment results in financial loss and considerable time is required to repair/replace the equipment to restore power supply. Earthquake vibration level can be evaluated based on site/location but predicting time of occurrence of earthquake is not possible. High voltage substation equipment shall be designed for seismic loading. Foundation, anchoring, installation and interconnection between equipment also need to be engineered according to the seismic load based on location of installation of equipment. Seismic qualification of various substation equipment as per National and International standard is performed using tri-axial shake table facility of Earthquake Engineering and Vibration Research Centre. Performance of 420kV instrument transformers under earthquake environment is presented in this paper.

Earthquake response of head-mounted equipment in advanced nuclear reactors

The seismic response of safety-related equipment mounted on the head of an advanced reactor, including pumps, control rod drive mechanisms, and reactor monitoring devices, will affect the design and layout of many advanced reactors. High earthquake-induced accelerations in such equipment may challenge their seismic qualification and trigger the need for additional support framing on the reactor head. Base isolation is a design solution that can drastically reduce seismic demands on equipment. This article describes a set of earthquake-simulator experiments conducted on a scale-model of a base-isolated reactor vessel including four representations of head-mounted equipment, with frequencies spanning from 4.5 to 27 Hz. Dynamic responses of the head-mounted equipment, including displacements, accelerations, and strains, were measured in the experiments for three support conditions: conventional, and seismically isolated using single concave Friction Pendulum (SFP) bearings and triple Friction Pendulum (TFP) bearings. Seismic isolation was effective at reducing equipment responses (accelerations, displacements, and strains) with respect to those in the conventionally supported vessel across a range of seismic inputs. Companion numerical studies highlight the accuracy to be expected in the calculation of different response quantities for lightly damped equipment. The importance of characterizing damping in head-mounted, safety-related equipment through physical experiments to support design and risk assessment is made clear through the numerical simulations.

Seismic Standards Qualification of Instrument Transformers by Shake Table Test and Linear FEM Analysis

Power grids are widely recognized as vital components of modern infrastructure. In today's society, nearly every aspect of daily life comes to a halt without electricity. Therefore, it is crucial to design power grids and their components to withstand various natural disasters. In this context, special attention is given to earthquakes and the need to enhance equipment resilience against this highly unpredictable event, which significantly impacts power grids.&#x0D; Instrument transformers could be the critical component in substations due to their slender design. Having this in mind, it is necessary to carry out further research and encourage development in order to reduce the impact of earthquakes on substations. Seismic qualifications are one of the means to achieve that goal.&#x0D; The primary objective of this paper is to share insights and experiences related to the preparation, implementation, and supervision of seismic tests conducted on a shake table. The FEM analysis, conducted as a crucial step in transformer design preparation and shake table testing, strictly adhered to the standards set by IEEE 693. IEEE 693 is widely recognized as the most demanding seismic standard regarding the substation equipment. Furthermore, the latest version of IEEE 693 was compared to other relevant standards, including IEC 61869 (current draft 38/652/CD), ETGI 1.020, and IEC 62271-300, to gain a comprehensive perspective.&#x0D; FEM analysis is characterized not only as a tool for design preparation and pre-test analysis but also as a valid tool for performing seismic qualifications, based on the comparison with the actual shake table tests. The paper extensively utilized the findings from seismic tests conducted on two distinct transformers, each employing different materials for critical components, in various test laboratories. These findings formed the basis for detailed analysis and conclusive insights, aligning with different versions of IEEE 693.&#x0D; This article aims to offer a comprehensive understanding of the seismic qualification process, providing valuable insights for substation designers, seismic specialists, and end users.

Performance‐based seismic classification of acceleration‐sensitive non‐structural elements

AbstractThis paper presents a general seismic classification procedure for acceleration‐sensitive non‐structural elements (NSEs) based on seismic qualification shake table testing and provides an application example for electrical cabinets in Italy. The proposed seismic classification procedure requires the definition of a set of required response spectra (RRS) for qualification shake table testing defined at different seismic hazard levels and the definition of a formal procedure to generate a test input motion. Each required response spectrum at a given seismic hazard level is then linked to different class levels and the requirements that an NSE should present to achieve each class level are defined. Using this framework, a seismic classification procedure for NSEs is developed for Italy, in which, the RRS are defined according to a peak floor spectral acceleration seismic hazard map of the territory so as to predefine regions in which an NSE belonging to a given class can be installed or not according to either a life safety performance objective or a continuous operation performance objective. Also, the limitations of the proposed seismic classification procedure are outlined. Finally, a nominal application example is presented in which four electrical cabinet specimens were tested on a shake table and their class levels were defined. The results of the application example are used to nominally show how the proposed classification procedure could be applied to discriminate between different types of the same NSE depending on the specific desired application.

Finite-Element Modeling for Seismic Damage Estimation of Suspended Ceiling Systems

The seismic qualification and seismic performance assessment of suspended ceiling systems have been traditionally conducted through large-scale shake table testing. This experimentally-based approach not only involves significant financial and time resources but is also not flexible to evaluate several configurations of suspended ceiling systems. As an alternate approach, this paper discusses the development of a general finite-element (FE) model of suspended ceiling systems that accurately captures the propagation of damage observed during full-scale shake table testing using the commercial finite-element analysis software, Abaqus/Explicit. The results of the large-scale shake table testing on suspended ceiling systems conducted at the University at Buffalo as part of the NEES nonstructural project were used to validate the FE model. The major components of suspended ceiling systems such as grid runners, ceiling tiles, and hanger wires were modeled and assembled with significant detail in the FE model. The properties of the connections between the grid runners, which are vital for capturing the seismic behavior of suspended ceiling systems were derived numerically. A user-defined subroutine was incorporated in the Abaqus/Explicit software framework to capture the multidirectional pinched hysteretic behavior of the connections between grid runners. An illustrative example of the application of the proposed FE model to develop robust numerically generated fragility curves for suspended ceiling systems is presented.

A shake table protocol for seismic assessment and qualification of acceleration‐sensitive nonstructural elements

AbstractA shake table protocol for seismic assessment and qualification of acceleration‐sensitive nonstructural elements (NEs) is developed. The paper critically reviews existing protocols and highlights their criticalities, pointing out the need for the development of novel assessment and qualification approaches and protocols. The protocol is developed in light of these criticalities, considering the most recent advances in the field and the specific expertise of the research team. The most significant and contributing parts of the developed protocol consist of the definition of novel required response spectra and the generation of signals for seismic performance evaluation tests. The reliability and robustness of the protocol are evidenced in the paper considering real‐floor motions as a reference, also proving the superiority of the developed protocol with respect to the reference alternatives. The defined approach and procedures are generally applicable and easily extendable to different case studies, as the process is highly versatile and modifiable. The implementation of the developed approach and protocol in the literature and in practice will significantly enhance seismic assessment and qualification of acceleration‐sensitive NEs. This will possibly have a strong impact on public safety and economy.

Experimental Seismic Assessment of Nonstructural Elements: Testing Protocols and Novel Perspectives

Nonstructural elements (NEs) are generally defined as elements typically housed within buildings/facilities that are not part of the structural system. Nonstructural elements are often classified as architectural elements, mechanical/electrical/hydraulic systems, and building contents. Nonstructural elements are often associated with critical seismic risk, due to their high vulnerability and exposure to seismic actions, especially for critical facilities such as hospitals and nuclear plant facilities. Accordingly, the combination of major exposure and vulnerability makes NEs extremely critical in terms of seismic risk even for low to moderate seismicity. The paper reviews and evaluates the main international testing approaches and protocols for the seismic assessment of NEs by means of experimental methods, which are referred to for seismic qualification. Existing test protocols are technically analyzed considering quasi-static, single-floor dynamic, and multi-floor dynamic procedures, supplying technical and operative guidance for their implementation, according to the latest advances in the field. The study proposes novel perspectives and a unified approach for the seismic assessment and qualification of NEs. The technical recommendations lay the groundwork for a more robust and standardized testing and qualification framework. In particular, the provided data might represent the first step for developing code and regulation criteria for the experimental seismic assessment and qualification of NEs.

Seismic analyses for power transformers

Reliability and security of power systems, especially in areas prone to earthquakes, depends on the seismic withstand of its components and interaction of these components with other elements. All relevant power products and components should be designed and tested to guarantee high seismic performance. Option which is strongly recommended for seismic qualification is shake table test. This way is very expensive and in some cases like power transformers impossible due to its weight and size. Because of this the numerical analyses can be very helpful to determine the dynamic characteristic of the system. This way is more and more used during evaluation of seismic performance of power products, especially in the design phase. In the paper a different numerical approaches for seismic analyses of the power transformers have been presented. In the first part of the article focus was put on typical simulation methods defined by IEEE and IEC standards. This approach is dedicated only for transformer’s components. Due to fact that standards do not provide clear information about fluid influence on power equipment during seismic events, some investigations related with oil filled transformers were done and summarized. Three different numerical methods were investigated. First one is built based on the Fluid-Structure Interaction (FSI) methodology. In this approach combination of different software (CFD, structural, and coupling code) is used to cover phonemes related with fluid dynamics and structural analyses. FSI methodology gives a wide possibility but, it’s very complex however, is very complex which can be a disadvantage for very complex objects. Next one uses acoustic elements, where the fluid is modeled as acoustic medium. This is method which allows to take into account fluid during seismic simulations in simplified way. The last one uses Lagrange and Euler element formulations (CEL) in which sloshing effect of the oil in power products can be considered. All this approaches can be very helpful to determine the dynamic characteristic of the transformers and its equipment including fluid.

Numerical and experimental study of the BUEEP connection using an inner box

In this paper, a prefabricated part called Inner Box is proposed to be embedded in box-columns at the connection zones to solve the well-known problems of the box-column connections, namely the inaccessibility of the interior side of the box-columns for both continuity plates welding and bolt tightening. The inner box contains four side-plates and two continuity plates assembled like a cube. Nuts are encased with specially designed gadgets welded to inner box side-plates preparing for a direct bolted connection. Fabrication, nondestructive tests, and repairs of typical inner boxes are practically and economically justified. For optimization study on IB dimensions, non-linear finite element modeling is used. Then a full-scale specimen of the optimized connection is tested under cyclic loading. Test results, according to AISC 341–22, approve the connection's seismic qualification in the range of the tested dimensions. The test specimen and FEM models show good agreement. Finally, some design criteria are proposed by numerical sensitivity studies.

Loading protocols for seismic qualification of steel plate shear walls

As a new type of lateral force resisting member, steel plate shear walls (SPSWs) have the advantages of excellent energy dissipation, high lateral stiffness, and ductility. Owing to the lack of loading characteristics of an actual earthquake in the existing loading protocol of SPSWs, the interpretation of the seismic performance of SPSWs is still challenging. A set of standardized loading protocols for SPSWs under far-field and near-fault earthquakes were proposed, to replicate the real loading histories in severe earthquakes. By using the rainflow counting method, the seismic parameters (such as damage cycle, interstory drift range, and residual interstory drift) from four frame–SPSW structures subjected to a series of representative ground motions, were analyzed to develop the loading protocols. Moreover, the rationality of the proposed loading protocols was discussed and verified by demonstrating the cumulative distribution function and energy dissipation of these protocols. It is well illustrated that the proposed protocols can truly simulate the loading history of members and accurately reflect the seismic damage demands in the inelastic stage.