طراحی لرزه ای و عملکرد سازه های پیش ساخته مونتاژ شده با اتصالات قابل انعطاف Seismic design and performance of dry-assembled precast structures with adaptable joints

1. Introduction Dry-assembled precast frame structures with hinged beams and cantilever columns restrained at their base are extensively used in Europe and in several other regions of the globe mainly for single-storey or low-rise multi-storey either industrial or commercial buildings. Wet-assembled partially precast structures are designed to emulate cast-in-situ concrete structures with rigid connections through in-situ concrete pouring of the joints, usually provided with rebars that protrude from the precast members. On the contrary, dry-assembled precast structures are connected by mechanical devices avoiding in-situ concrete pouring. Conventionally, dry-assembled joints also include semi-dry connections, which need in-situ casting of a small volume of special mortar for completion. Dry-assembled precast frame structures maximise the benefits of the prefabricated construction technique. Typical structural layouts and details of this type of structures are available in [1,2]. Over the last two decades an extensive research activity aimed at investigating the seismic behaviour of precast concrete frame structures [3] allowed a good knowledge of the seismic behaviour of precast systems to be consolidated and contributed to the achievement of outstanding realisations in terms of both quality and reliability [4]. The results from both analytical and full-scale experimental investigations showed that these precast systems (I) are characterised by an intrinsic large flexibility coming from their peculiar traditional static scheme with hinged beam-column joints [3,5–7]; (II) can provide comparable energy dissipation capacity/ seismic performance as cast-in-situ systems if the connections are properly designed and drift limitations and other minimum requirements provided by structural standards are respected [3,8]; however, (III) quite often the flexibility limitation requirements govern, resulting into larger column cross-sections than those strictly needed to resist the seismic forces for the assumed global ductility level [6]; in such case, (IV) minimum reinforcement requirements impose large over-strength in the columns, so that (V) while the structures possess adequate safety levels, they often behave elastically or in the range of low ductility even under the ultimate design seismic action, not fully exploiting the energy dissipation resources of the column [6,9]. This raises the problem of the capacity of the connections, in particular for multi-storey buildings [10], and the compatibility of displacements with possible interacting non-structural members, for instance the cladding panels [11–15], which caused quite extensive failures in the last strong earthquakes which hit Southern Europe [16–18]. A systematic framing of the design of precast structures including the in-plane effect of cladding panels supported by extensive experimental activity was addressed in the Safecladding project [19–25]. The seismic performance of these structures may also be influenced by the diaphragm effectiveness, since roofs often have spaced members and skylight openings. In this case, the diaphragm effect relies on the structural behaviour of the roof connections [26]. In the current European design practice [27], the key design parameters are often (i) the inter-storey drift sensitivity coefficient θ, de- fined as the ratio between vertical and horizontal loads at a storey divided by the inter-storey drift ratio (θ = Ptot dr / Vtot h), at Ultimate Limit State (ULS), or (ii) the drift limitation at Drift Limitation State (DLS), rather than (iii) the column base strength at ULS. To be noted that, in traditional multi-storey precast structures with hinged beamcolumn joints, the column strength is not influenced by the capacity design, since the beams are not part of the lateral load resisting system.