Masayoshi Nakashima’s research while affiliated with Kyoto University and other places

In Japan, structural health monitoring (SHM) for building structures has received increased attention since the 2011 Tohoku earthquake, particularly regarding business continuity and functional recovery. Most applications are market-driven, and building owners deploy an SHM system at their own expense so that they can promptly assess the condition of their buildings as one of the tags denoted “Green,”“Yellow,” or “Red.” In 2015, the authors’ group began installing such an SHM system, and as of December 2023, 553 privately owned buildings have been equipped with this SHM system. This opinion paper presents the authors’ experiences to date on monitoring, utility of data, and interaction with building owners. It conveys three opinions that the authors believe to be most interesting, particularly in the context of the benefits of the “Caution (Yellow)” tag. The article briefly introduces SHM, followed by the specific system in question and the data accumulated over the years in Japan. The first opinion is that the damage criteria, that is, the boundaries among the three tags, should not be considered permanent but subject to updates based on experience and newly acquired data. The second opinion is that SHM is one of the best means to characterize the damage (and fragility) of major nonstructural elements, while “Caution (Yellow)” has inherent relevance to the nonstructural damage, particularly for initial damage states. The third opinion is that “Caution (Yellow)” is a good message for building owners and managers with a solid contingency plan that includes readily available building maintenance workers and engineers. On the contrary, it tends to frighten building owners and managers when such a contingency plan is absent, discouraging them from installing SHM. Overall, it presents some valuable insights of interest to those wishing to further such SHM systems on a broader scale.

The role of non‐structural elements (NSEs) in the seismic performance of buildings has been highlighted in past years. Research studies following state‐of‐the‐art methodologies generally find that when the structural collapse is not of significant concern, NSEs tend to dominate the repair costs and financial investment required in a building. This paper examines field observations from interviews and data collected from commercial buildings via structural health monitoring (SHM) following the 2018 Osaka earthquake in Japan. It suggests that fragility functions used in current methodologies for estimating NSE damage may not be entirely representative of the in‐situ reality and possibly underestimate actual damage. Additionally, interviews with building owners/managers indicate that the alarm and financial impact of NSEs was not as critical as anticipated, with much of the observed NSE damage not of serious concern and tolerable in many cases. This article provides discussion and insight into possible causes for these differences before discussing how current methodologies can benefit from these observations. It is believed that many of the fragility functions currently used to estimate NSE damage may not be representative because of differences in installation conditions and loading protocols used in experimental testing, possible interaction with other elements, variability in the quality of workmanship during installation and possible wear, tear and degradation during service. On the other hand, it is seen how SHM data recorded during seismic events may provide valuable data for an alternative means to develop fragility functions. Furthermore, it is seen that when building recovery states (RSs) other than the implicitly assumed ‘full recovery’ state used in guidelines like FEMA P‐58 are explored, the role of NSEs in direct monetary losses significantly reduces. This coincides with the field observations in Japan regarding the impact of NSEs and supports the recent developments in functional recovery on what building owners and occupants are prepared to tolerate post‐earthquake. It indicates that when discussing the relative importance of different building performance groups, it is vital that the expected RS is also stated, as for most decision‐makers following major events, functionality rather than full recovery remains the primary goal; hence, repairs and proactive measures should bear this in mind for more effective use of resources.

Despite recent international efforts toward a risk‐conscious approach to advancing seismic design, Japan, a country very prone to earthquakes, remains conservative regarding the consideration of risk‐based seismic hazards in developing seismic designs. This article attempts to interpret the reasons. First, the evolution of Japanese seismic design codes and specifications is introduced, noting that the design seismic loads have remained essentially the same since the revisions implemented in 1981. Second, Japanese efforts in providing and renewing seismic hazard maps are briefly introduced and discussed. The maps have been developed using the data recorded by Japanese strong motion networks. Some unique challenges in preparing seismic hazard maps for the territory of Japan are presented, in light of the observations of actual earthquakes in the past three decades. On many occasions, the damage disclosed in modern buildings remained limited even when the recorded ground motions significantly exceeded the design earthquake level. An inherent conservatism is embedded in the performance criteria imposed on structural systems and elements. Third, political‐legal and sociocultural aspects are discussed. Centralized operation is commonly exercised in Japan for post‐disaster relief efforts, and the general public also values “equality” for such support. The factors naturally encourage cautiousness for the revision of relevant seismic codes and specifications.

Choosing friction pendulum isolators to avoid problems with device displacement capacity or boundary wall collision is a critical part of the seismic design of base isolation systems. This paper developed a probabilistic method to quantify the risk of failure in single friction pendulum bearing (FPB) devices. This was done by calibrating a demand‐intensity model for FPBs from numerical analysis for several device property combinations. These properties encompassed the dynamic friction coefficient and effective radius of curvature—key parameters impacting these FPB devices’ seismic response. Through this demand‐intensity model, a closed‐form and relatively simple approach to quantify the mean annual frequency of exceeding a given device displacement threshold, or risk of failure, was proposed. By applying this simplified method to several case study structures and comparing it with a more extensive assessment involving multiple stripe analysis (MSA) using hazard‐consistent ground motions, the proposed simplified approach could provide very accurate estimates of displacement‐based failure risk in FPB devices under the assumption of no non‐linear behaviour in the superstructure. This proposed approach implies that structural engineers can now quickly assess the actual failure rates of different FPB device combinations used in practice to give a more uniform level of safety or reliability when designing and assessing FPB‐isolated systems. It is also arguably much simpler, direct and accurate than currently available code‐based approaches.

This is an extension of the companion paper (Part 1) that presents the details of the two buildings located in Tokyo and designed using the US Performance Based Seismic Design (PBSD) methodology for one building (US Prototype Building) and the Japanese PBSD methodology for the other (Japan Prototype Building). The paper presents the IDA results and associated discussion on the collapse safety margin and the damage progress of the major structural elements. The Japan Prototype Building made of steel moment-resisting frames equipped with oil dampers is 1.5 times stronger in terms of the collapse safety margin than the US Prototype Building made of an reinforced concrete (RC) core shear wall system. The difference between the two buildings is believed to occur because of the larger redundancy and resultant larger over-strength achieved by the Japan Prototype Building. It is noteworthy that the construction cost of the Japan Prototype Building is about 30% higher than the US Prototype Building as indicated in Part 1. To explain the behavioral difference, sequences, and progresses of damage to major structural members are characterized in reference to the fragility curves of respective members. The benefits of oil dampers are found that the addition of the oil dampers would increase the collapse safety margin by 29% with an extra construction cost of 3%.

Performance-based seismic analysis and design of tall buildings have become increasingly common in the United States and have been the standard practice in Japan for a long time. The methodologies for seismic hazard analyses, selection of earthquake records for design, and acceptance criteria, however, are widely different in the two countries. Japan experiences large earthquakes and strong shakings of buildings significantly more frequently than the United States, and the data on actual seismic performance are richer in Japan than in the United States. Comparison of performance and calibration between the two design procedures can help to improve the quantitative assessment of seismic performance of buildings in the United States. This article presents the application of US and Japanese procedures to a tall building archetype once proportioned, analyzed, and designed according to US practices and once per Japanese practices. The structural systems selected are very different; for the United States, the archetype is designed using a core reinforced concrete shear wall building with composite floor systems and steel columns as the gravity system, whereas for Japan, the archetype is designed using a steel moment frame made of square concrete-filled steel tube (CFT) columns and composite beams and installed with oil dampers. Although the design procedures followed are vastly different, the seismic performances of the systems are rather similar during serviceability, design basis, and maximum considered events. As will be presented in the second part of this article, the collapse safety margins of the two designs are significantly different.

In Japan, structural health monitoring (SHM) of building structures began in the 1950s, but, until recently, its widespread use was not realized. A new trend arrived a few years ago, and currently over 850 buildings have SHM systems installed. The most recent SHM systems have been installed voluntarily by owners in the private sector; that is, the major development of recent Japanese SHM has been based on market forces. This article reports on why SHM was not accepted widely in the past, what were the keys for change of the atmosphere, how the building owners evaluate SHM after it is deployed, and what tangible benefits the building owners realize by experience on SHM implementation. To investigate those, an SHM system named q-NAVI is introduced as an example. The system has been deployed for 450 buildings, and they experienced a few significant shakings from recent earthquakes. SHM is also found effective for acquiring information on the quantification of fragility curves for various nonstructural components, using the data samples collected in recent earthquakes.

This historical note reports on the early days of the development of an experimental method called “hybrid simulation.” As background, the seeds of this concept, initiated in the early 1970s by Japanese researchers, are presented first, followed by initial efforts (regarded as Stage I) to realize the concept of hybrid simulation and its first applications to explore the seismic performance of structures. The initial research in this now‐seminal field of earthquake engineering began in the early 1970s by Koichi Takanashi and his coworkers at the Institute of Industrial Science, the University of Tokyo. Their highly notable efforts in laying the groundwork for hybrid simulation occurred in the mid‐1970s through the early 1980s by Takanashi (for steel structures) and Tsuneo Okada (for RC structures). These two men and their coworkers first applied hybrid simulation to explore the seismic behavior, performance, and design of various types of building structures. In Stage I, this method was called “the on‐line computer‐controlled test” or “pseudo dynamic test” because the unique feature of the method was the combined test and simulation and the intentional slow loading in the test. Extension of the scope and application of hybrid simulation occurred largely between the early 1980s and the early 1990s (regarded as Stage II) in conjunction with the United States–Japan joint research project. A few notable efforts made around that period are touched upon briefly, including error propagation and suppression in multi‐degree‐of‐freedom hybrid simulation, application of the substructure methodology to hybrid simulation, and real‐time hybrid simulation.