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Nonlinear dynamic modeling of full-scale mid-and high-rise reinforced concrete structures through the use of the 3D detailed approach that foresees the use of hexahedral elements with embedded rebars is not yet feasible due to numerous reasons. The two main numerical problems that do not allow for this type of analysis to be performed, are the numerical instabilities that immerse when the opening and closing of cracks initiates during the dynamic analysis and the excessive computational demand that is required even when dealing with small numerical models. This work will present the computational response of a newly developed algorithm that is used herein to perform modal analysis of large-scale models. The under study algorithmic development is a part of a project that aims towards alleviating the prementioned numerical constraints in regard to performing nonlinear dynamic analysis of full-scale reinforced concrete structures. An extensive numerical investigation is presented that foresees the performance of modal analysis on different full-scale reinforced concrete structures that are discretized with the Hybrid Model (HYMOD) approach. Based on the numerical investigation findings, the developed algorithm was found to be computationally efficient offering a robust numerical tool for performing modal analysis of large-scale numerical models.

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The fundamental period of a structure is one of the key parameters utilized in the design phase to compute the seismic-resistant forces. Although the importance of seismic-resistant buildings is well understood it has been found that the current design code formulae, which are used to predict the fundamental period of reinforced concrete (RC) buildings are quite simplistic, failing to accurately predict the natural frequency, raising many concerns with regards to their reliability. The primary objective of this research project was to develop a formula that has the ability to compute the fundamental period of an RC structure, while taking into account the soil-structure interaction phenomenon. This was achieved by using a computationally efficient and robust 3D detailed modelling approach for modal analysis obtaining the numerically predicted fundamental period of 475 models, producing a dataset with numerical results. This dataset was then used to train a machine learning algorithm to formulate three fundamental period formulae using a higher-order, nonlinear regression modelling framework. The three newly proposed formulae were evaluated during the validation phase to investigate their performance using 60 new out-of-sample modal results, where, in this work, additional validation models are created and used to test the predictive abilities of the proposed fundamental period formulae. The findings of this research report suggest that the proposed fundamental period formulae exhibit exceptional predictive capabilities for the under-study RC multi-storey buildings, where they outperform all existing de-sign code fundamental period formulae currently in effect.