Precast Structures with Adaptable Restraints

In Southern Europe the heritage and the everyday practice of precast structures consists mostly in dry or semidry assembled frames with hinged beams and cantilever columns restrained at their base. Such structures can provide a large energy dissipation, but they are intrinsically very flexible, especially if moving from industrial mono-storey buildings to commercial or residential multi-storey buildings. Their design becomes influenced by the need of reducing such flexibility, thus largely over-dimensioning the members and obtaining a complex seismic behaviour due to the influence of higher modes. Two solutions are envisaged to solve this problem by keeping the key feature of precast structures, that is the speed of erection: the introduction of dry or semi-dry assembled precast walls (particularly favourable in case of residential buildings) or the stiffening of the classical statically determined frame by means of introduction of moment resisting beam-column connections made with mechanical devices (mostly envisaged in the case of commercial buildings). In such enhanced frames, the realization of clamped joints can be performed in a second stage of the erection process, adapting the existing hinged connection realized at the first stage to clamped by activating the mechanical devices. Precast structures with adaptable restraints (PRESAR) are therefore presented in their essential features, among them the possibility of mix of a high quality dry precast construction technology with high speed of erection and the stiffness and redundancy provided by a traditional cast-in-situ frame. If properly designed, the horizontal members can fully be exploited by transferring the dead loads in a simply supported scheme before being adapted to clamped, with improvements in the behaviour and design of the whole building. A design comparison among 3 precast frames with similar geometries but different static schemes shows how this structural flexibility can be exploited to optimize the structure. A further comparison made on the real nonlinear dynamic behaviour of 4 different structures, ranging from flexible hinged frame to stiff coupled wallframe