Increasing the seismic resilience of existing reinforced concrete framed structures by using timber-based panels

In this Thesis, an intervention system applicable to existing reinforced concrete (RC) framed buildings is presented. The main purpose of the intervention is to improve the seismic behaviour of such buildings using cross-laminated timber (CLT) panels without modifying the original concrete structural system. The intervention proposed can also be implemented to reduce the energy consumption of buildings. Furthermore, the intervention can be partially prefabricated in order to speed up execution and reducing labour costs. The need to intervene on existing concrete buildings to improve their seismic behaviour arises from: a) this type of buildings represents a large percentage of the built heritage of many Countries; b) most of these structures were designed considering vertical loads only. Specifically, the built heritage is often characterised by details and construction techniques that deviate significantly from those required for new structures. Recent earthquakes have shown that, under seismic actions of smaller magnitude than those considered for new buildings, past construction practices frequently involve unforeseen damage or the activation of brittle mechanisms of collapse. The main critical issues of this kind of structure are: the detachment and the collapse of the infills; the failure of the beam-column joints; the shear failure of the concrete elements due to the interaction with the infills; the activation of soft-storey mechanisms as a consequence of torsional motions or concentration of lateral deformations in a single storey. To reduce the seismic vulnerability of these structures, a retrofit solution was proposed. The solution sees the connection of structural CLT panels to the existing RC elements through dissipative steel fasteners. The main goal of this intervention is to improve the in-plane response of the frames. Specifically, two alternative intervention configurations with different levels of invasiveness were studied. With reference to an isolated one-storey one-bay frame, the most invasive configuration, named RC–TP (Reinforced Concrete–Timber Panels), consists in the removal of one or more original masonry wythes and in their replacement with a CLT panel. The panel is inserted into the space originally occupied by the infill and is connected to the RC frame through a timber subframe and metal dowel-type fasteners. The least invasive configuration, named RC–TPext (Reinforced Concrete–external Timber Panels), consists in the arrangement of the CLT panel from the outside without removing the original infill. Vertical cuts at the lateral edges of the masonry infill prevent the infill, which in case of in-plane actions responds as a strut, from transferring additional forces to the columns and causing their collapse in shear. Because in RC–TP the panel is inserted inside the frame, it can also contribute to resist vertical actions if the seismic action damages the structural elements or increases the vertical loads. In addition, the replacement of the masonry infills with CLT panels results in reduced seismic masses and wall thickness. RC–TP has therefore a greater impact on the building and requires more burdensome labours, while RC–TPext leads to shorter execution time and less disturbance to the occupants. In the work presented here, numerical analyses and experimental tests were conducted to investigate the seismic response of existing RC frames both in pre- and post-intervention configurations. Specifically, finite element models comprising bare, masonry-infilled and retrofitted one-storey one-bay frames were subjected to nonlinear static and dynamic analyses by adopting numerical strategies with different levels of refinement. An extensive numerical campaign was devoted to investigating and optimising the behaviour of the intervention system on the basis of several aspects, such as the state of load, the mechanical and geometrical properties of the original system, the presence of weak beam-column joints, and the influence of openings. These analyses enabled the definition of a few “design rules” to guide the implementation of the retrofit system. Through the definition of “simplified” modelling approaches, the intervention system was also applied to entire case-study structures which were subjected to nonlinear static analyses. An analytical approach able to reproduce the response of isolated frames retrofitted with the RC-TP solution was developed to facilitate the pre-design of the intervention. This approach does not require the use of numerical analyses and permits to analyse multiple configurations with small amounts of output data. In the final phase, a set of four full-scale one-storey one-bay frames representative of the Italian heritage were tested by applying in-plane quasi-static cyclic loading up to a collapse condition. The four specimens comprised: a) masonry infilled frame; b) RC–TP retrofit 1; c) RC–TP retrofit 2; d) RC–TPext retrofit 3. The results obtained indicate that the proposed retrofit intervention can be used to significantly improve the seismic behaviour of existing RC frames by increasing both displacement and resistance capacities and by favouring the development of ductile mechanisms of collapse.