Evaluation of concrete models for local damage in reinforced concrete structures under impact loadings

Kang, Z.; Nishida, Akemi; Tsubota, Haruji; Itoi, Tatsuya

Mechanical Engineering Journal (Internet), 13(4-S1), p.25-00386_1 - 25-00386_18, 2026/08

https://doi.org/10.1299/mej.25-00386

no abstracts in English

Analysis of vibration characteristics using a multi-point observation system for high temperature engineering test reactor; Effects of irregular shapes and differences in structural type on building response

Yamakawa, Koki*; Nishida, Akemi; 3 of others*

Nihon Kenchiku Gakkai Kozokei Rombunshu, 91(839), p.121 - 132, 2026/01

https://doi.org/10.3130/aijs.91.121

We proposed a multi-point observation system for nuclear power plants that involving the installation of numerous accelerometers on each floor. To demonstrate its effectiveness in buildings with irregular shapes, such as those featuring an atrium or setback, we installed this system at the High-Temperature Engineering Test Reactor (HTTR) of the Japan Atomic Energy Agency. Analysis of the seismic records acquired by the system successfully identified seismic behavior believed to be caused by the building's irregular shape. Furthermore, the study demonstrated the effectiveness of a multi-point observation system in understanding the three-dimensional behavior of irregularly shaped buildings.

Foundation uplift nonlinearity in 3D finite element seismic response analysis; Sensitivity and validation study

Ota, Akira*; Ito, Sho*; Sonobe, Hideaki*; Ino, Susumu*; Choi, B.; Nishida, Akemi; Shiomi, Tadahiko

Nuclear Engineering and Design, 444, p.114403_1 - 114403_7, 2025/12

https://doi.org/10.1016/j.nucengdes.2025.114403

no abstracts in English

Applicability of equivalent linear three-dimensional FEM analysis of reactor buildings to the seismic response of a soil-structure interaction system

Ichihara, Yoshitaka*; Nakamura, Naohiro*; Nabeshima, Kunihiko*; Choi, B.; Nishida, Akemi

Nuclear Engineering and Design, 441, p.114160_1 - 114160_10, 2025/09

https://doi.org/10.1016/j.nucengdes.2025.114160

This paper evaluates the applicability of equivalent linear analysis of reinforced concrete model, which uses frequency-independent complex damping with a small computational load, to the seismic design of nuclear power plant reactor buildings. To this end, a three-dimensional finite element method analysis of the soil-structure interaction focusing on nonlinear and equivalent linear seismic behavior of the building embedded in an ideally uniform soil condition was performed for the Kashiwazaki-Kariwa Nuclear Power Plant Unit 7 reactor building. The equivalent linear analysis results correlated well with the nonlinear analysis results of the shear strain, acceleration, displacement, and acceleration response spectrum, demonstrating the effectiveness of the equivalent linear analysis method. Moreover, the equivalent linear analysis results were more conservative than those of nonlinear analysis using the material constitutive law in evaluating the shear strain of the external wall of the reactor building. From this result, equivalent linear analysis method tended to obtain a lower building stiffness than nonlinear analysis under the analysis conditions used in this paper.

Analysis of acceleration mode of HTTR building considering phase differences among accelerometers based on multiple seismic observation records

Yamakawa, Koki*; Hiramatsu, Masako*; Moritani, Hiroshi*; Iiba, Masanori*; Nishida, Akemi; Shiomi, Tadahiko; Choi, B.; Iigaki, Kazuhiko

Transactions of the 28th International Conference on Structural Mechanics in Reactor Technology (SMiRT28) (Internet), 9 Pages, 2025/08

To understand the three-dimensional seismic behavior of nuclear buildings, the Nuclear Regulation Authority (NRA) and the Japan Atomic Energy Agency (JAEA) have been conducting research and development aimed at improving the accuracy of three-dimensional seismic analysis methods for nuclear buildings. A large-scale observation system was constructed in the High Temperature engineering Test Reactor (HTTR) building of JAEA, enabling seismic observation and artificial wave excitation. Using this system, vibration characteristics of the HTTR building have been analyzed based on both seismic observation records and analyses using a three-dimensional seismic analysis model of the HTTR building. This paper reports on the vibration characteristics of the HTTR building obtained from the analysis of seismic observation records, as well as on the improvements made to the three-dimensional finite element model and the results of simulation analyses using artificial waves. In particular, this paper presents the results of an analysis of the Fourier spectral ratio at the first floor of the HTTR building based on seismic records, and discusses the dominant frequencies and acceleration modes of the superstructure.

Sophistication of seismic performance evaluation for plant structures utilizing finite element analysis of assembly modelling

Satoda, Akira*; Matsukawa, Keisuke*; Guo, Z. H.*; Choi, B.; Nishida, Akemi

Transactions of the 28th International Conference on Structural Mechanics in Reactor Technology (SMiRT28) (Internet), 10 Pages, 2025/08

In order to contribute to the seismic safety assessment of nuclear power plants and other industrial facilities against large earthquakes, it is an important issue to realistically capture the seismic behavior of plant structures and enable practical response analyses. Among the structural features that significantly influence seismic behavior, joints play a particularly critical role. In the case of steel pipe support structures (hereafter referred to as pipe racks), the joints connecting members (hereafter referred to as joints) have conventionally been modeled by empirical approaches, often idealized as pinned or rigid, leading to conservative evaluations. The final objective of this study is to develop a joint modeling method that reproduces more realistic behavior by utilizing three-dimensional detailed models of joints. To date, using the developed three-dimensional detailed analysis techniques, we have evaluated realistic semi-rigid stiffness of joints in both elastic and elasto-plastic ranges, proposed simplified joint models, and reported the obtained results. Furthermore, three-dimensional detailed analyses of an entire pipe rack including joints have been conducted, demonstrating the possibility of simultaneously capturing both the global seismic response of the pipe rack and the local joint behavior. In this paper, the simplified joint model proposed in previous studies is applied to the pipe rack joints modeled with conventional beam elements used in design practice, and the analysis results are compared with those of the detailed three-dimensional analysis. The findings are organized to clarify the differences and applicability. This enables improvement in the accuracy of practical seismic response analysis methods using beam elements. By applying the obtained results to the modeling of joints in steel structures of plant facilities, more rational seismic safety assessments of nuclear facilities and other plant structures can be expected.

How buildings collapse: Collisions; The Faster the impact speed, the more localized the damage

Nishida, Akemi

Kenchiku Chishiki, 67(5), p.74 - 75, 2025/05

Building damage caused by impacts ranges widely, from collisions with natural objects such as volcanic projectiles to large-scale collisions with aircraft or ships. These impacts are classified into low-speed, medium-speed, and high-speed categories based on the impact velocity. Furthermore, the extent of building damage varies depending on the hardness of the impacting object. Furthermore, if critical columns or walls of a building are damaged, progressive collapse can occur, posing a risk of the entire building collapsing in a chain reaction. Therefore, it is essential to consider the structural stability of the entire building. This explanation provides a clear, illustrated overview of these concepts.

Proposal of a nonlinear spring model on piping support structures for an elastoplastic response analysis method

Takito, Kiyotaka; Okuda, Yukihiko; Nishida, Akemi; Li, Y.

Journal of Pressure Vessel Technology, 146(6), p.061301_1 - 061301_12, 2024/12

https://doi.org/10.1115/1.4066278

Development of a probabilistic risk assessment method for combined hazards; A Classification and modeling framework for multihazard events

Choi, B.; Nishida, Akemi; Tsutsumi, Hideaki*; Takada, Tsuyoshi

Proceedings of Probabilistic Safety Assessment and Management & Asian Symposium on Risk Assessment and Management (PSAM17 & ASRAM2024) (Internet), 8 Pages, 2024/10

The Fukushima Daiichi Nuclear Power Plant accident highlighted the importance of developing safety assessment methods that consider multihazard events involving numerous simultaneously occurring events such as earthquakes (shaking) and tsunamis (submersion). When addressing such multihazard events, traditional methods often focus on assessing the load combinations of general structures in their structural designs and adopt simple selection criteria. However, these methods fall short when evaluating, countering, and screening external events, such as earthquakes, tsunamis, strong winds, and concentrated heavy rainfall, that occur simultaneously or in a chain. To address this, we reviewed existing literature on multihazard assessment methods, focusing particularly on scenarios involving earthquake and tsunami events. Based on concepts and basic theories, we examined various methods for addressing multihazard scenarios and classified their characteristics. Specifically, several multihazard scenarios were surveyed, and the relationships between multiple hazards were organized. In addition, common causes leading to combined events, their mutual influences, and potential cascading effects were analyzed.

Investigation on damage evaluation index with ductility factor based on simulation analysis for loading test of piping support structure

Okuda, Yukihiko; Takito, Kiyotaka; Nishida, Akemi; Li, Y.

Mechanical Engineering Journal (Internet), 11(2), p.23-00405_1 - 23-00405_12, 2024/04

https://doi.org/10.1299/mej.23-00405

After the Great East Japan earthquake and the accident at the TEPCO's Fukushima Daiichi Nuclear Power Stations in March 2011, the regulation for nuclear power plants (NPPs) has been enhanced to take countermeasures against beyond-design-basis events. To improve the seismic safety of nuclear facilities against earthquakes that exceed the design input ground motion, the importance of seismic probabilistic risk assessment (PRA) has drawn much attention. It is essential to evaluate the realistic seismic response of the equipment and piping in NPPs for fragility assessment in seismic PRA. In particular, since piping systems have plant-specific complex route geometries, it is known that the arrangement and stiffness of piping support structures have a significant impact on seismic response characteristics of the entire piping system. To construct a realistic seismic response analysis method for excessive input ground motion exceeding the elastic response, it is desired to develop an elastic-plastic response analysis method that can estimate the realistic response of piping systems including pipe support structures. In this study, the applicability of the method is confirmed by the simulation analysis of the elasto-plastic response for the piping support structure loading test previously reported. Moreover, based on the good correlation between the ductility factor and the damage status obtained from the test results and simulation analysis results, it is shown that the ductility factor is effective as a damage evaluation index for piping support structures.