PSI - Issue 44

Emanuele Renzi et al. / Procedia Structural Integrity 44 (2023) 737–744 Emanuele Renzi et al./Structural Integrity Procedia 00 (2022) 000–000

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5. Operating Instructions and seismic Attention Class The definition of the seismic AC considers the main parameters that influence the response to seismic actions of the bridges and road networks to which they belong. The approach used for the determination of the seismic AC is, once again, an approach for classes and logical operators, for which logical flows must be followed that allow to move from the classification of primary and secondary parameters to the classification of hazard, vulnerability and exposure factors and, finally, to the determination of the seismic AC. The OI become a valid support for infrastructure managers as they clarify and better specify the methodology described in the Bridge Guidelines that often could lead to significant excess evaluations, with consequent classification of the bridges in high class. In fact, the definition of parameters and criteria that define the correct prioritization of the interventions to be performed. In the determination of overall AC, high vulnerability classes are associated with high attention classes regardless of the value of the other two quantities. The OI at § 4.3.5.1 reports «[...] For example, priority can be given to structures falling within the major vulnerability class: if Bridge 1 is characterized by high hazard class and medium vulnerability class, while Bridge 2 by medium hazard class and high vulnerability class, …, both bridges fall under the high Attention Class, it may be appropriate to assign, on the part of the manager, the priority of intervention to Bridge 2 compared to Bridge 1. ». For the seismic AC, the hazard is determined by the peak ground acceleration (a g ) and topographic category (T i ), and by the subsoil category. If it is possible to identify the subsoil category (as defined by NTC) the classification of the seismic hazard is corrected to consider the amplification of seismic acceleration as a function of the construction site. When this parameter is not deductible from the available information, the worst of the subsurface categories In the presence of additional parameters of seismic vulnerability that specifically characterize the bridges structural scheme, such as rough or curved decks, single-colon piles or with a very irregular height or in any case situations that determine concentrations of stress, rotational motions or anything else constitutes an aggravating factor to the bridge vulnerability, the classification based on static scheme or, light and material, is modified in the following way: • the vulnerability class of static scheme, light and material increases by one level if there are vulnerability elements (from Low to Medium-low, from Medium-low to Medium, etc.); • the vulnerability class of static scheme, light and material remains unchanged if the vulnerability elements are absent or little influential on the behavior of the system. The OI at § 4.3.3.1 defines and specifies when the scheme can be considered "isostatic" or "hyperstatic". In the case of static schemes not covered, the choice of classification based on static scheme, light and material is made and left to the evaluator. In the case of structural types that provide for the presence of different materials, such as bridges in a mixed steel-concrete system, we proceed in analogy with the considerations made for one of the two materials considered, by the evaluator, prevalent in the determination of structural and seismic behavior. The seismic vulnerability classification based on the static, light and material scheme is correct according to the design standard, distinguishing the so-called "seismic" codes from the "non-seismic" ones. In fact, it is necessary to consider the possibility that the seismic action has not been considered at all in the draft of the structures. In this regard, the OI at § 4.3.3.2 specifies «with reference to the design regulations, bridges built in accordance with the Regulations adopted since 2003 can be considered made according to anti-seismic design criteria. Structures reasonably foreseeable for that site shall be assumed. The vulnerability classification is carried out based on: • deck material; • static scheme; • spans number; • spans light.