Ivan Balić

Target acceleration method for analysis of RC structures

Purpose – The purpose of this paper is to present a new modification of the multimodal pushover method, named the target acceleration method. The target acceleration is the minimum acceleration of the base that leads to the ultimate limit state of the structure, i.e., the lowest seismic resistance. Design/methodology/approach – A nonlinear numerical model is used to determine the target acceleration, which is achieved using the iterative procedure according to the envelope principle. Validation of the target acceleration method was conducted on the basis of the results obtained by incremental dynamic analysis. Findings – The influence of higher modes is highly significant. The general failure vector corresponding to the target acceleration differs from the first load vector and the form of the load with uniform acceleration according to the height of structure, as contained in the European Standard EN 1998-1. Comparison between the target acceleration, including the equivalent structural damping, and the ...

Multimodal Pushover Target Acceleration Method Versus Dynamic Response of R/C Frames

A new multimodal pushover target acceleration method for the nonlinear analysis of reinforced concrete (R/C) frames subjected to seismic action is presented in chapter. The aim of research shown in this chapter was to find the influence of multimodal combinations in assessing the bearing capacity of R/C frames based on linear (L) combination of modes, and to compare the target ground acceleration a gr,t (a gr,u ) of the multimodal pushover target acceleration method with the failure peak ground acceleration a gr,d obtained by a dynamic transient response of R/C frames. The target acceleration presents the lowest seismic resistance and it is the minimum acceleration of the base that leads to the ultimate limit state of structure, and it is reached by an iterative procedure. In accordance with the Eurocode 8 rules, application of the pushover method favors access utilizing the first mode. Examples of presented 5-storey spatial R/C frame show the significant influence of higher modes. A formulation for determining the equivalent structural system damping by equalizing the dissipated energy during one cycle of vibration of the nonlinear system and the equivalent linear system is presented in this chapter. Results of the dynamic response of R/C frames are also presented in this chapter. As seismic excitation eight real earthquake accelerograms are taken. On the basis of the results obtained by nonlinear dynamic time-history analysis validation of the procedure of searching the target ground acceleration was made. Taking into account the elastic spectrum with calculated equivalent structural damping and usability of the capacity curve up to 3/3 of a displacement, the comparison of the target acceleration a gr,u of the multimodal pushover method with the failure peak ground acceleration a gr,d , obtained by a dynamic response of the structure, shows a very good agreement between the target acceleration and the failure peak ground acceleration.

Extreme Modal Combinations for Pushover Analysis of RC Buildings

The pushover method is a practical procedure for comprehensive nonlinear analysis of structures subjected to seismic action. Application of this method, in accordance with the Eurocode 8 rules and due to engineering simplicity, favours application utilizing the first mode. The aim of the presented research in this paper was to find the influence of multi modal combinations in assessing the bearing capacity of reinforced concrete (RC) frames and walls. This paper presents a procedure in which the most extreme state is defined by the lowest ground acceleration caused by a predetermined shape of an elastic spectrum. The extreme bearing value is obtained by the envelope principle. Mode shapes and period sizes are determined on a linear elastic model while the limit state of the load bearing system is evaluated in a nonlinear state of structures. Results of the analysis show that influences of higher modes are significantly higher and that the safety/reliability, indicated by the criteria for the target displacement, in accordance with Eurocode 8 (Annex B), is not achieved. Inclusion of higher modes, in some presented examples, decreases the peak ground acceleration by more than two times, which is significantly less favourable than the target displacement criteria.