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Evaluation of the uncertainty related to concrete compressive strength is one of the important aspects in the seismic assessment of an existing RC building. This works aims to characterize the compressive concrete strength based on both in-situ destructive and non-destructive test measurements. A large database of the (destructive) core tests and SONREB (hammer rebound and ultrasonic pulse measurements) non-destructive test results for the same structural elements has been gathered from different existing RC buildings mainly located in Campania region, Italy and constructed in the last century between the decades 30’ and 90’. A regression model is employed in order to derive predictive expressions for calculating the compressive concrete strength based on the ultrasonic test measurements. The paper further investigates the quantification in terms of relative weights of the concrete strength based on ultrasonic test results for seismic performance assessment of existing RC buildings. The calculated relative weights for ultrasonic tests are compared with the NTC 2018 code recommended relative weights of 1 and 1/3 for destructive and non-destructive tests, relatively.

The original N2 method for reasonably regular buildings oscillating predominantly in a single mode envisioned a two-stage analysis where the first stage involved the non-linear static analysis of the MDOF system using alternative monotonically increasing lateral load patterns and the second stage involved the non-linear time-history analysis of the equivalent SDOF system. Today, the N2 method constitutes the backbone of Eurocode 8 and the Italian building Code (NTC2018) as far as it regards the seismic design and assessment of buildings. In the codebased approach, the equivalent SDOF system is defined as an elastic-perfectly-plastic model and the structural analysis is performed through the inelastic design spectrum. On the other hand, non-linear dynamic time-history analysis of the MDOF system subjected to un-scaled ground motion records, better known as the Cloud Analysis, is noteworthy due to its simplicity, relatively small number of records employed and its preservation of the original frequency content. A modified version of the Cloud Analysis can effectively consider the cases of structural collapse and/or numerical non-convergence. This modified version of the Cloud analysis proves ideal for implementation in the context of the N2 method and the code-based provisions since it can be applied as a dynamic analysis procedure directly to the code-based equivalent elasticperfectly- plastic SDOF model. This method can be applied for seismic design and assessment both for the serviceability and the ultimate limit states. The proposed non-linear dynamic analysis procedure is applied to a frame belonging to a pre-seismic code school building in Avellino (Campania), located in southern Italy. The frame is modelled by considering the interaction between the shear, axial force and flexure and the rigid-end rotation due to bar slip.