Ho-Young Son

Bellows expansion joints accommodate the large ground displacements imposed on buried pipelines crossing active faults, but their tension–shear interaction at the leakage limit state has not been quantified. This study characterizes the component-level tension–shear behavior of a thin-walled stainless steel bellows expansion joint and derives a leakage-based interaction curve, idealizing the demand as prescribed axial and transverse displacements between fixed ends. Monotonic combined loading tests were performed on a 100A 2-ply joint under five loading conditions, and a finite element model was validated against the measured force–displacement curves, leakage forces, and failure locations. Pre-loading in one direction reduced the displacement capacity in the other by approximately 15%, and simultaneous loading reduced it by approximately 40% relative to single-mode loading, with leakage at the pipe–bellows weld connection in all cases. The leakage limit state was defined by the critical principal strain at this connection, calibrated from the tests, and the resulting interaction curve was nonlinear, lying outside the linear sum of the two normalized displacements. The four coefficients of the power-law equation were expressed as closed-form functions of the number of convolutions n (3 to 10), giving a predictive equation within that range. Because the shear leakage displacement grows more steeply with n, adding convolutions is most effective where shear dominates. Compared with the EJMA allowable displacements, the margin to leakage under monotonic loading was approximately 56% in pure tension and 78 to 93% under shear and combined loading.

The statistical parameters of a fragility curve estimated from a finite number of samples are subject to sampling uncertainty, which depends on both the sampling method and the sample size. However, clear guidelines for quantifying this uncertainty have not been established. In this study, the sampling uncertainty in the internal pressure fragility assessment of a 1:4 scale prestressed concrete containment vessel was quantified for Monte Carlo Simulation (MCS), Latin hypercube sampling (LHS), and the scrambled Sobol sequence (SSS). An artificial neural network-based surrogate model was utilized to evaluate the structural responses, and an independent replication procedure was employed to estimate the variability of the fragility parameters, in which the capacity of each realization was obtained through closed-form maximum likelihood estimation. Three damage states preceding excessive leakage were considered, and the sample size was increased from 16 to 2,048. The empirical convergence rate of the coefficient of variation (C.o.V.) of the dispersion estimate was 0.50-0.51 for MCS, in agreement with the theoretical rate, whereas SSS achieved 1.00-1.08. The LHS method accelerated only the estimation of the median capacity. Consequently, SSS reduced the C.o.V. of the estimated dispersion by a factor of 2.5-3.5 at a sample size of 128 and that of the high confidence of low probability of failure (HCLPF) capacity by a factor of 3-4 compared with MCS. Practical guidelines for the required sample size were derived, indicating that SSS reduces the number of nonlinear finite element analyses required for a target dispersion accuracy by a factor of approximately 4-14.

Bellows expansion joints are used to accommodate the large permanent ground displacements imposed on buried pipelines at active fault crossings, but their leakage displacement capacity at the buried system level and its sensitivity to parameter uncertainty have not been quantified. This study quantifies the leakage displacement capacity for a strike-slip fault crossing under parameter uncertainty and identifies its governing factors. The first full-scale split-box test of a buried dual-bellows pipeline system measured the deformation under monotonic fault displacement, and a three-dimensional finite element model predicted the measured deformation angle conservatively within 11.8%. An analysis database of 200 Latin hypercube samples was constructed for nine uncertain soil, steel, manufacturing, and interaction parameters. A Gaussian process surrogate trained on the strain–displacement relationship (cross-validation R2 = 0.998) yielded the leakage displacement as the displacement at which the predicted strain reaches the critical weld strain. The leakage displacement was narrowly distributed, with a median of 451 mm and a coefficient of variation of 2.5%, and remained stable under ±15% perturbation of the critical strain. Sobol analysis showed that the soil friction angle governs the capacity variance (total-order index 0.71) and that 35% of the variance stems from parameter interactions, all significant pairs involving the friction angle. The dominant parameter shifted with fault displacement from the steel yield stress to the friction angle. The results indicate that friction angle characterization is the first priority for buried pipelines with bellows joints at fault crossings, with the bellows wall thickness as the key controllable resistance variable.

Probabilistic evaluation of the internal pressure capacity before excessive leakage is essential for ensuring preemptive response in case of a severe accident in a prestressed concrete containment vessel (PCCV) subjected to internal pressure in a nuclear power plant. Therefore, in this study, we conducted a fragility analysis to evaluate the probabilistic internal pressure capacity for damage states before excessive leakage. Three damage states were classified, and a finite element (FE) model was developed based on the experimental test results of a 1 : 4 scale PCCV under the overpressurization condition at Sandia National Laboratory. The three damage states were determined as the onset of inelastic behavior (DS1), onset of leakage (DS2), and excessive leakage (DS3). Fragility analysis was performed using a validated FE model to consider uncertainties in material properties and prestressing force. The Latin hypercube sampling method was used to perform uncertainty quantification, and normality tests verified that the samples exhibited normal distributions. The internal pressure capacity variability in DS1 and DS2 was higher than that in DS3. DS1 is the point at which the compressive deformation caused by the prestressing force was converted into tensile deformation because of the increase in the internal pressure, which resulted in tensile concrete cracking. Furthermore, DS2 is the state in which localized damage of the liner occurred and cracks in the concrete spread. Thus, the internal pressure capacity for DS1 and DS2 exhibited high variability because of the uncertainty of the prestressing force and material properties. Moreover, the median capabilities of DS1, DS2, and DS3 were ~1.85, 2.61, and 3.45 times the design pressure, respectively. Therefore, material and structural uncertainties in nuclear power plants should be appropriately considered, and a conservative assessment of the damage state before excessive leakage should be performed for preemptive response in the case of severe accidents.

Bellows expansion joints are known to have a large displacement capacity and can thus be potentially used to improve the seismic performance of buried piping systems. However, there are no guidelines on the installation of bellows expansion joints for the seismic performance improvement of buried piping systems. Furthermore, there are very few studies on the seismic performance of buried piping systems with bellows expansion joints. In this study, therefore, we performed seismic fragility analysis according to the installation conditions to obtain basic data for the installation guidelines of bellows expansion joints. Therefore, in this study, an experimental test was performed on bellows expansion joints considering the characteristics of earthquake loading conditions, and a 3D finite element (FE) model using the ABAQUS platform was developed and validated based on the experimental results. This model was verified by comparing the force-displacement relationship and energy dissipation. Leakage occurred at a displacement of 113.6 mm in the experiment, and the FE analysis result was also applied up to the same displacement. In the case of energy dissipation, an error between the FE model and experimental result was determined not to be significant. However, the appearance of such physical performance errors is due to the manufacturing errors resulting from the bellows forming process and the variability of material properties. Finally, seismic fragility analysis of buried pipeline systems with bellows expansion joints was performed. In addition, the following cases were used for analysis according to whether bellows were applied or not: (1) without a bellows expansion joints; (2) with a single bellows expansion joint; and (3) with two bellows expansion joints. In conclusion, it was found that the seismic performance of the buried pipeline system was improved when bellows were applied. However, the effect of the seismic fragility curve according to the increase in the number of bellows was insignificant.

Piping systems are a representative social infrastructure to provide oil, gas, and water. Damage to piping systems may cause serious consequences, such as fire, water outage, and environmental pollution. Therefore, piping systems need to be protected from natural disasters, such as earthquakes. Earthquakes may cause deformation that exceeds piping design criteria. For example, large relative displacements and liquefaction of the ground resulting in loss of strength and ground subsidence, and the side-sway of primary structures subjected to a strong ground motion may cause critical damage to piping systems. Therefore, expansion joints to maintain flexibility can be applied to locations where excessive deformation is expected to improve the seismic performance of piping systems. Metal bellows, a type of expansion joints, are flexible, so they are highly durable against deformation and fatigue loads. This indicates that metal bellows can be used as seismic separation joints for piping. In this study, experimental research was conducted to analyze the seismic performance of multi-ply bellows type expansion joints, a type of metal bellows. Monotonic loading tests and cyclic loading tests were conducted on 2-ply bellows and 3-ply bellows, and the results were compared. In the cyclic loading tests, multi-step increasing amplitude cyclic loading, which used the displacement history amplified in stages, and constant amplitude cycling loading with various magnitudes were considered. The test results showed no significant difference in bending performance for monotonic loading between the two types of multi-ply bellows. The 3-ply bellows, however, showed higher performance for low-cycle fatigue than 2-ply bellows.

The electrical cabinet systems in power plants are critical non-structural components to maintaining sustainable operation and preventing unexpected accidents during extreme events. This system consists of various electrical equipment such as relays, circuit breakers, and switches enclosed by a steel cabinet for the protection of the equipment. The cabinet systems are installed in and protected by structures so that the cabinet’s behavior is totally dependent on the behavior of the structures when subjected to an earthquake. Therefore, it is essential to qualify the seismic performance of the cabinet system considering the effect of the primary structure where the electrical cabinet system is mounted. In addition, with the implementation of ASCE-43 design standards for nuclear facilities, facility design allowing nonlinear behavior has gained greater attention in nuclear power plants, and research on how the response of the cabinet varies according to allowable damage levels of structures is needed. In this study, Finite Element (FE) models of a single-door electrical cabinet and concrete shear wall structure validated through experimental data are used for a decoupled analysis to estimate the seismic demands of the electrical cabinet. Three different earthquake loadings, referred to as EQ#13, #17, and #19, used in the SMART-2013 project are selected to obtain floor responses of the concrete structure, and the loadings lead to different levels of damage (minor, moderate, and major damage, respectively) to the structure. Finally, the floor responses based on levels of the damage to the primary structure are applied to the electrical cabinet system as input loadings for the decoupled analysis. Thus, this study presents the effects of the cabinet elevation and performance limit-state for concrete shear wall structures on the response of the electrical cabinet, and it shows that while the difference in seismic demands is not significant in the minor and moderate damage states, a meaningful difference occurs in the degree of the major damage state.

Earthquakes with magnitudes over 5.0 occurred near Gyeongju and Pohang in southern Korea in 2016 and 2017, respectively. These earthquakes had both low- and high-frequency components. Due to earthquakes with high-frequency motions, damage to nonstructural systems has been observed to be relatively more than that in structural systems. Consequently, the seismic design or performance evaluation of nonstructural components in critical facilities has emerged as a key research area in Korea. This study presents the results of experimental and numerical analyses using a high-fidelity finite element (FE) simulation in the ABAQUS platform for a press-to-connect piping system as a nonstructural component. Press-to-connect piping systems based on NFPA-13 with two elbows, a flexible coupling, and a T-joint were used. In addition, a cyclic loading protocol was applied using the KBC 2016 and IBC 2015. Based on the component-level experimental test, an FE model of the press-to-connect elbow was developed, and the high-fidelity large-scale piping system with an elbow was validated in this study. In both the experimental and analytical results, no leakage or plastic deformation of the piping system was observed under cyclic loading conditions. The results of the high-fidelity simulation model of the large-scale piping system were identical to those of the experimental test. More specifically, the error of the von-Mises stress at the upper and lower elbows was less than 9%, and the angle between the elbows was less than 2%, corresponding to the limit state of the drift ratio of the building system. Therefore, the high-fidelity simulation model of a large-scale piping system can have high application value. In addition, the design requirements and engineering demands of the piping system, such as the condition of ASME B and PV section III for service level D, were satisfied.

Most nuclear and nonnuclear power plants have been designed in the frequency range of 2 to 10 Hz, but now, the design guidelines for structural and nonstructural components such as electrical cabinets must be improved by including high frequency greater than 10 Hz for sustainable energy. The electrical cabinet is the essential piece of equipment for safety functions and the uncertainty of seismic capability in power plants. Consequently, the attention of this study focused on evaluating the seismic demands of the electrical cabinet under high frequency earthquakes and also, seismic qualification of the electrical cabinet using the identification of experimental tests and numerical models. An experimental test based on ICC-ES AC 156 and IEEE std.344 was conducted for seismic qualification of the cabinet and then, a high-fidelity finite element model to capture the significant deformation was developed in this study. It is observed that the fundamental frequencies were 16 and 24 Hz from the experimental tests, respectively. In order to verify the proposed high-fidelity simulation model, the target fundamental frequencies of the cabinet were evaluated in the ABAQUS platform. It was interesting to note that the reconciliation of experimental and analytical results was extremely identical. Furthermore, in order to evaluate seismic response characteristics of the cabinet subjected to high and low frequency earthquakes, time history analysis was conducted in this study, using the ABAQUS platform. As a result, the observation showed that the seismic response of the cabinet system under a high frequency earthquake was relatively higher than that of low frequency. It can be very important to note that the cabinet system was sensitive to high frequency vibration.