Abstract
New precast frame industrial structures are seismically designed according to reliable modern criteria. However, most of the existing built stock hosting many workers and both regular and strategic industrial activities was designed and detailed neglecting the earthquake load or according to outdated seismic design criteria and regulations. Its seismic retrofit is a main challenge for the Engineering Community and a critical objective for institutional and private bodies. Among the envisaged solutions, the introduction of dissipative braces appears to be promising, although mostly inapplicable for these buildings, due to the brace lengths required by their typical large dimensions and the related proportioning against buckling. In this paper, an innovative seismic retrofitting technique based on monolateral dissipative bracing is investigated. The device proposed in this paper, yet in phase of preliminary design and testing, dissipates energy through friction in tension only while freely deforming in compression, which makes the issue related to compressive buckling irrelevant. A numerical analysis is carried out to investigate the efficiency of the proposed device in seismic retrofitting of precast industrial frame buildings with the aim to explore its feasibility and to better orient the definition of the slip threshold load range and the future development of the physical device. The simplified Capacity Spectrum Method (CSM) is employed for the global framing of the structural behaviour of the highly nonlinear retrofitted structures under seismic actions. A numerical tool is set to automatically apply the CSM based on the definition of few main parameters governing the seismic response of precast frame structures. The efficacy of the CSM is critically analysed through the comparison with the results of a set of nonlinear dynamic analyses. A smart simplified design process aimed at framing the most efficient threshold slip/yield load of the device given an existing structural configuration is presented with the application of the CSM through the identification of the most efficient performance indicator related to either displacement, shear force, equivalent dissipation of energy or a combination of them.
Highlights
Recent earthquakes occurred in Southern Europe highlighted a satisfactory behaviour of existing precast concrete industrial frame structures, whether they were originally conceived and designed for seismic strength
A novel retrofitting technique for precast concrete industrial frame buildings based upon tension-only monolateral dissipative devices mounted on steel braces was proposed
A novel device is proposed based on friction mechanism, allowing for (a) higher specific energy dissipation thanks to rectangular-like cyclic shape and large triggering stiffness, (b) no influence of ratchet mechanisms, (c) limited damage even under heavy engagement, and (d) theoretically infinite cumulated displacement as far as the hysteresis occurs within the maximum and minimum displacements allowed by the external tube length
Summary
Recent earthquakes occurred in Southern Europe highlighted a satisfactory behaviour of existing precast concrete industrial frame structures, whether they were originally conceived and designed for seismic strength. As effectively summarised in a well-known paper on the subject (Pall and Marsh 1982): «the motion of vibrating buildings is slowed down by braking rather than breaking» Following this approach, it is natural to wonder whether the non-symmetrical behaviour of braces might be overcome by exploiting the properties of FDDs. by installing a device with a determined slip threshold connected to a brace, it is possible to predetermine the maximum load transferred to the connected frame (with a safety margin) and avoid the yielding of brace at the same time; in this manner FDD acts as a mechanical-fuse, limiting the force exerted, and like a damper, controlling the displacement. A smart simplified design process aimed at framing the most efficient threshold slip/yield load of the device given an existing structural configuration is presented with the application of the CSM through the identification of the most efficient performance indicator related to either displacement, shear force, equivalent dissipation of energy or a combination of them
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