## 2 Methodology for Detailed Seismic Performance Assessments
The experimental testing of structural dynamic parameters may be very useful for this purpose, as it provides building eigen-frequencies, damping ratios, and modal shapes under ambient excitation. Although the data obtained using this experimental method are valid only for the stiffness levels corresponding to the vertical load combination, they are extremely valuable for the calibration of initial numerical models of irregular buildings (in plan and in height). A system of accelerometers needs to be installed relatively densely on each story, as the number and position of measurement points affects the accuracy level of the overall modal parameters. If the building geometry is irregular, it is necessary to check the potential independent vibration of structural parts in plan or in height. Particular attention should be given to the entrance spaces, which are usually extended from the floor plan, are often high and pass through several stories. Furthermore, in irregular buildings, critical elements are often located at the edge of the structure. It is recommended to install accelerometers in such places in order to determine the center of stiffness as accurately as possible as well as the locations of maximum displacements at the edges. After the input data have been determined, numerical modeling strategies, demand parameters and acceptance criteria have to be adopted. Usually, development of the numerical model starts with the adoption of linear strategies, which may be calibrated with the results of OMA measurements. After this state of an uncracked structure and low amplitude excitations is verified and linear seismic methods applied, nonlinear models may be created. These detailed models, which include material nonlinearities of structural elements, are necessary to apply more complex nonlinear static and dynamic methods capable of capturing the performance of the structure during severe seismic shaking. For the nonlinear static (pushover) method, it is necessary to choose an appropriate model of material nonlinearity which can predict the real behavior of an element, appropriate displacement patterns and control points, while representative earthquake scenarios and corresponding accelerograms, artificial or real ones, have to be determined for the nonlinear dynamic time-history analysis using the most complex piecewise exact step by step method. The seismic performance is assessed on the basis of the obtained results, including the building collapse mechanisms and identification of critical structural elements. Following detailed seismic assessments, specific measures for target retrofit may be proposed. A nonlinear time-history analysis is very demanding, both computationally and in terms of time, and the input data should be as accurate as possible to obtain a reliable output. Moreover, the interpretation of the results is challenging, even for regular and simple buildings, and it is necessary to be an experienced professional/researcher in order to draw the right conclusions. Additional problem lie in the fact that older reinforced concrete buildings often have brittle failure mechanisms of critical structural elements which need to be accurately assumed and defined. The described procedure is directed at limiting the area of key parameters scattering in structural performance assessments. The results of the previous, simpler analysis step are validated in each subsequent, and more complex, step, and the conclusions that have been drawn need to be confirmed or refuted. Much attention needs to be given to input data on seismic hazards, geometry, material properties and knowledge of construction methods in order to reduce the uncertainties to a minimum. Ultimately, a complex numerical model formed and validated by the proposed methodology requires much less computational analysis, which is very important for complex and irregular structures.