PSI - Issue 11

Antonio Bossio et al. / Procedia Structural Integrity 11 (2018) 394–401 Author name / Structural Integrity Procedia 00 (2018) 000–000

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5. Push-Over analyses and main results Push-Over analyses were performed by using the commercial software SAP2000®. The 3D model of the simulated structure is characterized by three degrees of freedom (D.O.F.) for each floor. Each floor is considered as rigid and it was simulated by imposing a “diaphragm” restraint. A “Master node” was defined and placed at the geometrical center of gravity of each floor and was considered coincident to the center of the masses. The collapse of the structure was considered when the Limit State of Life Safeguard (LSV) occurs (i.e. plastic rotation is ¾ of ultimate rotation for the plastic hinges). Detailed description of Push-Over analyses is out of the scope of present work, hence significant results only are outlined. 5.1. Push-Over analyses of non-corroded structure Results obtained by P-O analyses performed for non-corroded structure show that the collapse of the structure corresponds to a PGA of 4.015 m/s 2 and 3.760 m/s 2 considering X-direction and Y-direction, respectively; while the return periods are 968 years and 759 years, respectively. 5.2. Push-Over analyses at the first level of corrosion Considering the first corrosion configuration, global behavior of the structure depends on the verse (“+”or “-”) of action of the seismic forces, because of the asymmetry of concrete sections due to corrosion. Anyway, the global seismic capacity is comparable to the non-corroded structure. Considering the second configuration, the seismic behavior is comparable to the previous cases; in fact, in the ADRS (Acceleration Displacement Response Spectrum) format both the structural capacity (bilinearized pushover) curve and the demand spectra are plotted in spectral acceleration versus spectral-displacement coordinates and are overlapping with respect to previous cases. The first and the second corrosion configurations present a ductile failure occurring at the bottom of two columns, positioned into the middle of the frames. Considering the third configuration, according to Biskinis et al. (2004) shear failure capacity model, a brittle failure occurs due to shear failure of beams loaded by the slabs, however those elements are verified if considering the Italian building code NTC (2008). 5.3. Push-Over analyses performed considering the third level of corrosion This level of corrosion is a very hard condition for the structure. In fact the first configuration presents an ADRS curve very different with respect to previous cases (figure 8a). The second and the third configuration present shear failures of slab-loaded beams both considering NTC (2008) and Biskinis et al. (2004). 5.4. Push-Over analyses of HPC retrofitted structures The retrofit of the structure was repeated on each of the three different levels of corrosion and P-O analyses were performed. In order to understand the contribution offered by the HPC jacket, it is analyzed the retrofit of the corroded structure at first level: in this case the capacity in terms of PGA is 7.267 m/s 2 with a return period 2,275 years. Figure 8b shows the ADRS capacity and bi-linear curves of non-corroded structure and retrofitted structure, compared to ADRS demand. The retrofit cannot completely restore the capacity of the structure, but is able to increase the global ductility of the structure; in fact, figure 9a shows the moment rotation of the plastic hinge after HPC retrofit, compared to non-corroded and corroded condition. The bending capacity of the retrofitted hinge is about 20% higher than the corroded one, while the ductility is even much higher than the ductility of non-corroded hinge, proving the high benefits of the HPC retrofit both in terms of strength as of ductility. This benefit is obviously relevant for seismic vulnerability, while under static loads, there is no full restoration of bending capacity, hence the thickness of the HPC jacket should be increased in such a case. Moreover, the use of HPC as retrofit of corroded structure is very useful in terms of shear, too. To evaluate the benefits in terms of shear, the HPC wrap is evaluated, as in the case of confinement, by means of its thickness and tensile strength.