Flavia De Luca

2012 Emilia earthquake, Italy: reinforced concrete buildings response

Data of the Italian National Institute of Statistics are collected aimed at characterizing Reinforced Concrete (RC) building stock of the area struck by the 2012 Emilia earthquake (number of storeys, age of construction, structural typology). Damage observations, collected right after the event in reconnaissance reports, are shown and analyzed emphasizing typical weaknesses of RC buildings in the area. The evolution of seismic classification for Emilia region and RC buildings’ main characteristics represent the input data for the assessment of non-structural damage of infilled RC buildings, through a simplified approach (FAST method), based on EMS-98 damage scale. Peak Ground Acceleration (PGA) capacities for the first three damage states of EMS-98 are compared with registered PGA in the epicentral area. Observed damage and damage states evaluated for the PGA of the event, in the epicentral area, are finally compared. The comparison led to a fair agreement between observed and numerical data.

Analytical versus observational fragilities: the case of Pettino (L’Aquila) damage data database

A damage data database of 131 reinforced concrete (RC) buildings, collected after 2009 L’Aquila (Italy) earthquake, is employed for the evaluation of observational fragility curves. The specific interpretation of damage data allowed carrying out fragility curves for slight, moderate, and heavy damage, (i.e., DS1, DS2, and DS3), defined according to EMS 98 macroseismic scale. Observational fragility curves are then employed for the calibration of FAST analytical methodology. FAST method is a spectral based approach, meant for the estimate of fragility curves of infilled RC buildings up to DS3, evaluated, again, according to EMS98. Kullback–Leibler divergence is employed to check the matching between analytical and observational fragilities. FAST input variables can vary in quite large ranges and the calibration provides a valuable suggestion for the application of the method in other cases in which field damage data are not available. Results showed that optimizing values, for the input variables calibrated, are in good agreement with typical values assumed in literature. Analytical results showed a very satisfactory agreement with observational data for DS2 and DS3, while systematical underestimation was found for the case of DS1.

Ductility of wide-beam RC frames as lateral resisting system

Some Mediterranean seismic codes consider wide-beam reinforced concrete moment resisting frames (WBF) as horizontal load carrying systems that cannot guarantee high ductility performances. Conversely, Eurocode 8 allows High Ductility Class (DCH) design for such structural systems. Code prescriptions related to WBF are systematically investigated. In particular, lesson learnt for previous earthquakes, historical reasons, and experimental and numerical studies underpinning specific prescriptions on wide beams in worldwide seismic codes are discussed. Local and global ductility of WBF are then analytically investigated through (1) a parametric study on chord rotations of wide beams with respect to that of deep beams, and (2) a spectral-based comparison of WBF with conventional reinforced concrete moment resisting frames (i.e. with deep beams). Results show that the set of prescriptions given by modern seismic codes provides sufficient ductility to WBF designed in DCH. In fact, global capacity of WBF relies more on the lateral stiffness of the frames and on the overstrength of columns rather than on the local ductility of wide beams, which is systematically lower with respect to that of deep beams.

RC infilled building performance against the evidence of the 2016 EEFIT Central Italy post-earthquake reconnaissance mission: empirical fragilities and comparison with the FAST method

Damage data on low-to-mid-rise Reinforced Concrete (RC) buildings, collected during the UK Earthquake Engineering Field Investigation Team post-earthquake reconnaissance mission on the August 24 Central Italy earthquake, are employed to derive empirical fragility relationships. Given the small dataset, the new data distributions are used for the Bayesian update of fragility functions derived for the L’Aquila earthquake (same seismic region and similar construction typologies). Other properties such as number of storeys, age of construction and shape in plan of the buildings are also analyzed. This information is employed to assess the ability of the FAST method to predict damage states in non-regular infilled RC buildings for the municipalities of Amatrice, Accumoli, Arquata del Tronto and Norcia, all severely affected by the 2016 Central Italy sequence. FAST is a spectral-based method to derive capacity curves and peak ground acceleration damage state thresholds for buildings. It is a dedicated methodology for regular RC frame buildings with masonry infills, first calibrated on damage data from the 2009 L’Aquila earthquake and applied to the 2011 Lorca (Spain), the 2012 Emilia (Italy) events for damage back-analyses. The new data from the August 2016 Central Italy earthquake provide a test-bed for FAST further employments in case of less homogenous building samples. The application of FAST presented here accounts for different shake-maps produced by both the United States Geological Survey and the Italian National Institute of Geophysics and Volcanology which are significantly different and representative of different refinements of the demand scenario. For the area of Amatrice, where the two shake-maps provide similar estimates and the buildings considered match reasonably well the typology for which FAST is calibrated, the comparison between damage level observed and as provided by FAST is very satisfactory. For other structural typologies like RC industrial structures and dwellings with non-hollow-clay-bricks as infills, FAST needs further calibration.

Improving Static Pushover Analysis by Optimal Bilinear Fitting of Capacity Curves

An improvement of codes’ bilinear fit for static pushover (SPO) curves is put forward aimed at decreasing the error introduced in the conventional SPO analysis by the piecewise linear fitting of the capacity curve. In the approach proposed herein, the error introduced by the bilinear fit of the force-deformation relationship is quantified by studying it at the single degree of freedom (SDOF) system level, away from any interference from multiple degree of freedom (MDOF) effects. Incremental Dynamic Analysis (IDA) is employed to enable a direct comparison of the actual curved backbones versus their piecewise linear approximations in terms of the spectral acceleration capacity for a continuum of limit-states, allowing an accurate interpretation of the results in terms of performance. A near-optimal elastic-plastic bilinear fit can be an enhanced solution to decrease systematically the error introduced in the SPO analysis if compared to the fit approaches provided by most codes. The main differences are (a) closely fitting the initial stiffness of the capacity curve and (b) matching the maximum strength value, rather than disregarding them in favor of balancing areas or energies. Employed together with selective area discrepancy minimization, this approach reduces the conservative bias observed for systems with highly curved force-deformation backbones.

Real, Scaled, Adjusted and Artificial Records: A Displacement and Cyclic Response Assessment

Different procedures to obtain sets of spectrum compatible records for nonlinear dynamic analysis of structures are compared in terms of post-elastic structural response. Six typologies of records are considered: un-scaled real records, real moderately linearly scaled, real significantly linearly scaled, real adjusted by wavelets, artificial generated by two different procedures. The study is spectral shape-based, that is, all sets of records considered, either generated or selected, match individually (artificial and adjusted) or on average (real records), the same design spectrum of a case-study site in Italy. Bilinear non-degrading single degree of freedom systems were used to evaluate the nonlinear response for the compared sets at different non-linearity levels; i.e., demand spectra in terms of peak and cyclic response were derived for two strength reduction factors. Results show that artificial or adjusted records may underestimate, at high non-linearity levels, the displacement-related non-linear response if compared to real records, which are considered as a benchmark. Conversely, if the cyclic response is considered, artificial record sets show a (more evident) overestimation of the demand, while wavelet-adjusted do not display a significant bias. Finally the two groups of linearly scaled records seem to show no systematic bias for both types of response considered, suggesting that scaling does not impair estimation of seismic response if the spectral shape is controlled.