Seismic behaviour of single-storey steel buildings connected to hybrid firewalls with fusible links

The seismic damage of non-structural components can lead tosignificant disruption in the functionality of buildings affected by fire and seismic events and result in considerable economic losses. Among non-structural elements, partition walls for several years were treated as architectural features and were not included in the structural design, although they can significantly influence the overall response of the structure. In recent decades, numerous seismic events have revealed the weakness of non-structural elements to relatively low-intensity forces and highlighted the severe economic and social impacts that their damage or collapse can cause. However, the seismic design of these systems is often treated as a secondary aspect compared to structural design, resulting in a lack of specific design guidelines. Partition walls may be designed with fire resistance requirements to prevent the spread of the fire to adjacent compartments. In this context, it is important that in single-storey buildings, for which collapse in case of fire is admitted after a fairly short amount of time, e.g. 15 min under the standard fire exposure, the stability of the firewall be satisfied during the fire to avoid fire propagation. Moreover, the stability of the firewall must be also guaranteed in case of a seismic event. In this context, the work presented in this thesis was developed in the framework of the FISHWALL project, funded by the European Research Fund for Coal and Steel (RFCS). This project investigates the behaviour of firewalls made of sandwich panels under both seismic and fire actions, acting individually, located between single-storey steel buildings devoted to industrial and commercial functions. In addition, the connection between the firewall and the unprotected steel structure is conceived with fire “fusible” elements made of aluminium bolts that quickly lose strength when exposed to fire, disconnecting the firewall from the steelwork; thus, preserving the building not affected by the fire. When subject to earthquake, the fusible links have to withstand the seismic forces without undermining the stability of the firewall. In order to analyse the proposed solution, several existing representative singlestorey buildings were selected with differences in terms of plan dimensions, steel sections, i.e. hot-rolled or welded, and seismicity level site, i.e. low or moderate. Preliminary linear dynamic analyses were performed on the case studies to estimate the force magnitude in the fusible links, and different specimens, including a part of the firewall made of sandwich panel, were designed to be tested at Materials and Structures Testing Laboratory (LPMS) of the University of Trento. As a result, six details were conceived with different size of aluminium bolts and different configurations to consider the firewall position parallel or perpendicular to the portal frames composing the steel structure. All bolts were tested under shear forces. Four tests were performed on each detail: two monotonic tests, and two cyclic tests, in accordance with the ECCS protocol. As expected, the brittle behaviour of the aluminium bolts governed the detail response. Moreover, the details conceived with larger diameter and, thus, characterized by a lower number of bolts, exhibited a better response because of their limited inherent imperfections and misalignments. Therefore, a more uniform loading of the fusible links was observed and the design force was reached, whilst the other components remained mostly in the elastic range. In addition, due to the hole-bolt clearance and an inherent different positioning of the bolts in the hole, details with higher number of bolts experienced progressive failure of the bolts and consequently exhibited lower maximum shear capacity. Eventually, it was possible to identify the detail that behaved most effectively. Then, a significant numerical part was developed. Firstly, numerical models of the details were developed in the OpenSees software through nonlinear springs calibrated against the experimental data. Secondly, global nonlinear numerical models incorporating the fusible link behaviour of selected case studies were developed, and parametric time histories analyses were performed. The structural elements were modelled as nonlinear elements with distributed plasticity while the fusible links were represented with the above-mentioned calibrated models. For each analysis and in accordance with EN 1998-1, at least three spectrum compatible accelerograms were selected, and the maximum force obtained in the fusible links was recorded. None of the details, in any configuration, exceeded the maximum capacity observed from the laboratory tests resulting in a satisfactory behaviour under seismic actions. Finally, fragility functions were derived by means of Multiple Stripes Analyses (MSAs) that provided a deeper insight into the seismic performance of the fusible links system.