Seismic Assessment and Retrofit of Historical Masonry Structures
Cuzzilla, Roberto (2009) Seismic Assessment and Retrofit of Historical Masonry Structures. [Tesi di dottorato] (Inedito)
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The Italian building heritage is composed mainly of masonry structures, which over the years have acquired historic significance and artistic values in the national culture. These buildings are particularly vulnerable to the seismic actions, because they were design for gravitational loads without considering seismic actions applied on them. Thus, the constructive details are not compliant with the present design code provisions (e.g. in plan or elevation structural regularity) and to avoid collapse or wide crack patterns, different strengthening interventions should be proposed or were done during the time (orthogonal wall connections, steel ties applications). In recent years, conservation and restoration of existing constructions assumed very important roles to reach an appropriate structural safety level, especially for artistic and monumental constructions, taking into account the benefit achieved by building restorations reducing the new constructions. The large number of destructive earthquakes occurred in Italy during the last century and the beginning of the new century has highlighted the need to redefine the design strategies and requirements especially in high seismic risk regions. Therefore, different seismic design provisions have occured during the time both in national and regional levels; the most recent is the Italian provisions (named, DM2008 ). The fundamental innovative aspect of the aforementioned DM2008  is connected to the existing constructions and in particular it takes into account strategic buildings or structures playing very important rules for civil protection, introducing the research seismic engineering results into the professional world. The new seismic design provision introduced by the DM2008  give more emphasis on the management and maintenance of structures, which are often a relevant fraction of the total cost of the rehabilitation work. The new seismic design code have been inspired by the following general principals: 1) avoid structural damage and minimize non-structural damages for moderate intensity seismic events and seismic action characterized by return period comparable with the building operating life. Moreover non-structural damages for seismic events characterized by return period less than the building life should be avoided. These criteria respect the damage limit state condition (DLS); 2) The construction elements must have structural characteristics such as ductility to dissipate the energy released by earthquakes during high intensity seismic events without reaching collapse mechanisms; the safety of the people is, thus, ensured in case of events characterized by a lower probability of occurrence (return period of 475 years) which, to be endured without plastic deformation, would require an uneconomical design of the structure. A structural element damage are accepted in order to make the dissipative mechanisms possible. These criteria respect the ultimate limit state condition (ULS). As stated in the seismic design code, "…the purpose is to ensure the human life protection in case of earthquakes, the damages should be limited and the important structures in terms of civil protection should be working…”. Materials and strengthening techniques characterized by high performances and minimal impact on the structures play a crucial role on rehabilitation and restoration of existing buildings. The masonry structures, in a lot of cases having historic and architectural values, represent an important part of the Italian structural heritage. Many of the structural defects related to the masonry buildings are due to inadequate techniques and materials, earthquake and wind actions (horizontal loads), foundation settlements, atmospheric agents deterioration. In addition, higher loads acting on the structure due to different structural use and more stringent standards for seismic design provision lead to have need of appropriate strengthening interventions. The methods traditionally used for masonry structure rehabilitation are: - Fill cracks and voids with grout injections; - stitching large cracks or weak areas with metal parts or concrete elements; - reinforced perforations injected with mortar, in order to increase the masonry tensile strength; - jacketing on one or both masonry panel sides with reinforced concrete by using an electro welded steel grid. The use of fiber-reinforced composite materials represents an alternative to traditional intervention techniques: these materials, usually made of carbon fiber (CFRP), glass (GFRP) or aramid (AFRP) held together by a polymer matrix, provide unique combination of mechanical performances, including high strength and stiffness in fiber direction, corrosion resistance, light weight. They are available in form of sheets or rolls characterized by virtually unlimited length. All these properties allow to realize the structural seismic reinforcement without increasing the seismic mass. These types of composites are characterized by a high compatibility with the substrate in terms of geometric, chemical and mechanical properties. The research activities described in the following were partially carried out in Barcelona (Spain) and several ecclesiastic structures were selected in order to evaluate their seismic behavior applying European and Spanish design codes (EuroCode 8  and NCSE-02 , respectively). For that reason the earthquake phenomena effects were studied on three Spanish gothic structures by using limit analysis criteria in order to compute their seismic performances in terms of safety level, according to the Italian guide line for evaluation and reduction of the seismic risk of the cultural heritage (2006) . The structures were separated into macro-elements subjected to in-plane and out-of-plane actions, applying the Capacity Spectrum Method (CSM) and the safety levels were computed by means of seismic demand and structural capacity ratio. Then, the attention was focused on in-plane structural behavior and, using available data from experimental tests on masonry panels carried out by other authors, a numerical simulations were implemented by using the finite element software DIANA TNO rel.9.2 . Three experimental program on panels made by different materials were examined in order to be sure that the results obtained using numerical simulations were not strongly connected to the relative substrate. Thus, the analyzed models were made by hollow bricks, solid bricks and tuff bricks respectively and different mortars for each masonry panel were also used. Two aspects, considered of primary importance for masonry structures were analyzed: - the global performances by changing mechanical properties of structural elements; - the global performances reached by using FRP strengthening system. With reference to the first point, the masonry panel behaviour was analyzed by changing the compression (Gfc) and tensile fracture energy (Gft) for both bricks and mortar. The knowledge of the previous mentioned physical parameters is very complex to establish with traditional laboratory tests, but it is important to have a reliable masonry panel numerical simulation. In particular a specific range of feasible values of the fracture energy was considered, calculating them by using formulas introduced in the literature. Within that range the two extreme values and the average value or, in alternative, the value calculated according to the material mechanical strength were selected; then three different analyses were performed for each masonry panel. The second point has been performed by means of two different panel strengthening systems using Carbon Fiber reinforced (CFRP): one by using diagonal stripes and the other composed by horizontal stripes (Grid layout). These numerical simulations, the global strengthened panel response was highlighted, changing several geometric characteristic of the strengthening system; width of the strip (for both diagonal and horizontal reinforcement) and the spacing between each strip (only for horizontal reinforcement) were modeled. The scope of the present research work was to compute the panel shear capacity and the initial stiffness of cracked panels subjected to in plane compression and shear loads by changing material mechanical properties, both for tuff and mortar, and considering different arrangement and amount of FRP reinforcement. Particular attention was also paid to the comparison between the “as built” panels and the strengthened ones by means of failure modes and crack patterns. Therefore, the present work was arranged in order to specify the structural typology where the introduced strengthening system could be applied, describing the different masonry structures and their main performances under seismic actions, such as shear capacity and failure mode. The work goes on describing the characterization of materials used in the simulation phase, the model by using DIANA software  considering both “as built” and strengthened masonry panels, the comparison between the numerical analyses results and experimental ones in order to validate the modelling and, at the end, to implement the parametric analysis by changing the geometrical and mechanical properties.
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