Seismic capacity

Acceptance criteria for load combinations, including the effects of SL-2 with LI or L2 loads, or L3 loads, should be the same as those adopted in related practices for L3 loads acting without an earthquake. 

In most States some acceptance criteria are lowered in the case of an extreme earthqnake for some leaktight stmctures (e.g. containment and fuel pool). In this case integrity is only required in an extreme earthquake, but restoration of operation afterwards is conditional on a structural evaluation of the earthquake s effects on the leaktightness of such structures. 

Material properties should be selected according to characteristic values supported by appropriate quality assurance procedures. Appropriate ageing evaluation should be carried out to guarantee the long term safe performance of materials and SSCs (Ref. [l],para. 5.47). 

Specific evaluations should be carried out concerning the acceleration of degradation mechanisms by seismic events. If such mechanisms are responsible for any reduction in seismic capadty over the lifetime of the plant, additional safety margins should be adopted to guarantee the required safety level in the design after any seismic event. 

To ensure adequate seismic safety, ductile design should be effected and gradual and detectable failure modes should be incorporated. The following measures are a sample, indicative of what should be considered at the design 

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Failure of power or controls to the valve (generally related to the seismic capacity of the cable trays, control room, and emergency power). These failure modes are analyzed as failures of separate systems linked to the equipment since they are not related to the specific piece of equipment (i.e., a motor-operated valve) and are common to all active equipment. 

Table 4 shows the results of the seismic margin evaluations and the seismic capacity of KALIMER reactor internal structures including the reactor vessel and containment vessel. From the results, the containment vessel, reactor vessel, inlet plenum, and core support have large seismic stress margins but the reactor vessel liner, support barrel, separation plate, and baffle plate have small margins. The maximum stress occurs in reactor vessel liner parts coimected with the separation plate due to the vertical seismic loads. 

The experimental tests clearly pointed out the potential of the recommended FRP solution to increase the seismic performances of beam-column joints designed according to non-seismic code provisions. The experimental validation of this local strengthening technique may strongly encourage the use of local interventions based on the use of FRP reinforcement to significantly increase both the local and global seismic capacity of RC structures. 

The lower trigger level (alert) should be close to SL-1 (usually associated with operational limits), at which significant damage to safety related items is not expected. If the overall seismic capacity of the plant is lower than SL-1 (e.g. during the seismic re-evaluation), the lower trigger level should be referred to the actual seismic capacity of the plant. 

Currently, the assessment of seismic capacity is being carried out to comply with the relevant IAEA recommendations. In addition, a probabilistic seismic hazard analysis is included in the Temelin PSA Project scope in order to address the contribution from earthquake induced accident sequences to the overall CDF of the plant. The seismic hazard curves for the Temelin site has been developed and seismic fragility analysis has been performed for the structures and components. Based on the preliminary results (annual frequency of O.lg PGA (SSE) earthquake is lE-6/year), it is expected that the contribution of seismic events and the consequential accident sequences to the overall CDF will be negligible (i.e. less then 1% of overall CDF). The independent review of this PSA task is to be carried out in the framework of the 2nd IAEA IPERS (Level 1 - external initiating events) in August/September 1995. 

Return period can also be used for the assessment of structural capacity and vulnerability of a structure in order to design proper mitigation measures. In other words, it can be used to quantify the safety levels and structural deficiencies in a more straightforward manner and to assess the improvement of structural performance after certain retrofitting interventions. For this purpose, Raffaele and Fiore (2013) presented a simple methodology (based on the reverse application of the well-known N2 method) that allows the assessment of the return period when the elastic period and the capacity of the examined structure for a given limit state are available. Subsequently, this so-called capacitive return period can be used to determine the seismic capacity and the reference life for any limit state considered. 

The choice of an appropriate and reliable analytical model for the study and assessment of a mascMiry structure seismic capacity prerequisites a thorough knowledge of its characteristics and behavior, as well as its pathology and degree of degradadcMi. 

The nonstructural elements, having no role in the seismic capacity, are generally considered as additional weight to be included in the mass evaluation, neglecting the structural interaction between them and the structural resisting system. 

Damage probability matrix Fragility curve Fragility surface Limit state Numerical simulation Seismic capacity Seismic demand Uncertainty... 

Basically, seismic fragility analysis is the comparison of seismic capacity and demand and to estimate whether the seismic capacity is exceeded for a well-defined performance level when the structural system is subjected to specified levels of ground motion intensity. Due to the probabilistic nature of seismic fragility analysis, both seismic capacity and demand are defined by probability functions in terms of certain random variables to quantify all the uncertainties involved in the process. 

I Compaiison of seismic capacity and demand"] -f Attainment of limit states... 

FEMA-273 document (Federal Emergency Management Agency 1997), three limit states were defined as Immediate Occupancy, Life Safety, and Collapse Prevention in terms of interstory drift. The limit states are generally considered as deterministic parameters. However, there are cases in literature in which limit states are also considered in a probabilistic manner to account for the variability in seismic capacity. 

In consequence the nuclear industry and regulatory authorities are frequently faced with the question whether these changes can be accommodated within the seismic capacity of the original design or whether modifications are necessary to maintain an adequate level of safety (see ASCE 2000, p. 41). 

The nondeterministic character of both load demand and seismic capacity is typically modeled by random variables, as illustrated in Fig. 2. 

The most widely used model for the seismic capacity is given by... 

The main goal of an SMA is to identify the seismic capacity of the plant, i.e., the maximum level of seismic ground motion for which the plant can still be safely shut down. In addition, it is the goal of any SMA to identify the weaker components, i.e., the components limiting the seismic capacity of the plant. 

As discussed in section Log-Normal Capacity Model, the uncertainty associated with the seismic capacity of SSC is characterized by the uncertainty variability parameter Pu. Similarly as for the hazard curve, the uncertainty expressed by Pu can be shown graphically in terms of the fi agility curves associated with the various confidence levels, as shown in the lower left... 

Such system improves the seismic capacity of structures by ... 

A summary of the methods reviewed above is presented in Table 1. While this is by no means an exhaustive list of the many applications of seismic vulnerability assessment for masonry structures available in literature, it concentrates on procedures that have been specifically developed for vulnerability assessment at territorial scale. For this reason procedures aimed at the assessment of single buildings are not included. The choice of the most suitable procedure is highly dependent on the resources available for the data collection, the computational expertise available, and ultimately the scale and aim of the study. Empirical procedures can be used for fairly large-scale studies to define damage scenarios however, if the purpose of the study is to identify within a district or urban center specific buildings in need of strengthening, so as to increase their seismic capacity, then a suitable analytical procedure should be preferred. 

In the case of design or assessment purposes, the seismic capacity of the structural system should be computed for different seismic hazard levels by means of linear or ncmlinear time history seismic analyses using a number of properly selected ground motions. The selection of the ground motions plays an important role in the efficiency and accuracy of the design or assessment procedure. In this entry, we provide an overview of the time history seismic analysis applied for design and assessment purposes. 

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