Abstract
A limited gap between closely spaced structural parts may induce internal pounding in seismically isolated structures, because of notable displacement at the level of the isolation system under severe earthquakes. A gap between a fixed-base elevator shaft and the surrounding building is presented here with reference to a reinforced concrete building located in the Sicilian town of Augusta. The building, comprising a basement and three storeys above the ground level, is seismically isolated at the top of rigid columns in the basement with a hybrid isolation system including elastomeric and sliding bearings, while a steel framed elevator shaft crosses the isolation level. Despite the gap, internal pounding may occur at all levels of the superstructure when the elevator with maximum load stops at the upper floors. To reduce structural pounding effects, a magnetic damped link (MDL) between adjacent corners of the elevator and the surrounding building is proposed. This is obtained as an in parallel combination of an eddy current damped link (ECDL) and an elastic helicoidal spring, and occupies less space than traditional passive dampers and transmits considerably less forces compared to a rigid link configuration. Specifically, an ECDL consists of an outer cylindrical copper tube, as conductor, and an inner tube, equipped with an array of axially magnetized and ring-shaped permanent magnets separated by iron pole pieces, as mover. The relative motion between conductor and magnets, during seismic loading, induces an eddy current producing electromagnetic damping. Given that viscoelastic linear behaviour can be hypothesized for the MDL, a simplified iterative design procedure of the ECDL is proposed, with optimization of the thickness and radius of the magnets, thereby enhancing magnetic flux and energy dissipation. The directionality of the near-fault ground motions is investigated through nonlinear seismic analysis, comparing no connection with four configurations of the interconnection: i.e., flexible and rigid elastic links, viscous and magnetic damped links.
Highlights
Seismic pounding between two or more of adjacent buildings, of differing dynamic properties and built in close proximity to one other as is often the case in urban settings, has been observed in past earthquakes and investigated in many works (e.g. Efraimiadou et al 2013; Abdel Raheem et al 2019)
The most evident effects are found along the Y direction (Fig. 10), where mean values of gi for the original structure already fulfil the upper bound design gap imposed by EC8 (i.e. gd,EC8.2) considerably exceeding the corresponding lower bound (i.e. gd,EC8.1) while there are a few cases with values slightly above gd,NTC18 (Fig. 10b). These results suggest a revision of the EC8.1 threshold and the addition of a specific threshold on the collapse prevention (CP) seismic gap for base-isolated structures currently non-existent in the NTC18
An effective and reliable four-sweep-four-step iterative design procedure of the links is developed to evaluate their vertical distribution of stiffness and/or damping, with the aim of minimizing horizontal displacement of the elevator at all levels
Summary
Seismic pounding between two or more of adjacent buildings, of differing dynamic properties and built in close proximity to one other as is often the case in urban settings, has been observed in past earthquakes and investigated in many works (e.g. Efraimiadou et al 2013; Abdel Raheem et al 2019). FR (Bhaskararao and Jangid 2006) and YL (Ni et al 2001) dampers are characterized by a stable hysteretic behaviour independent of temperature and velocity of motion whose activation is triggered when preset stress levels are reached These devices are generally manufactured from traditional materials and require little maintenance, representing a low cost and reliable solution for energy dissipation. A seismic gap is designed to verify the possibility of displacement of the steel structure and deformation of the isolation system, without internal pounding of the unconnected r.c. superstructure To this end, the effectiveness of the Italian (NTC18 2018) and European (EC8 Eurocode 2004) seismic codes provisions is investigated, based on the assigned design gaps between independent units of a building expressed as function of maximum horizontal displacement of the two units at the same level. Traditional interconnection, assuming flexible and rigid elastic links and viscous damped links, and no connection are considered as a comparison
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