## 3R method

The building codes assume that structures designed according to simple design rules are safe against collapse due to earthquakes. It is clear that such a principal assumption should be checked in the design.

In order to simply check the adequacy of the seismic collapse risk, the concept of intensity-based assessment for risk-based decision making is introduced. This is realized by means of the so-called 3R method (Response analysis, Record selection, Risk-based decision making), where only a few ground motions scaled to a single intensity $S_{a,ct}$ are used to estimate the seismic demand. If collapse is observed for less than half of ground motions, it can be decided that the design of the structure is appropriate and vice versa.

The decision making is based on results of few nonlinear dynamic analyses. The accuracy of the 3R method is sufficient even in the case of taller reinforced concrete frame buildings.

The collapse ratio $r_C$ for multiple levels of the target collapse risk $\lambda_t$ with indicated maximum possible error obtained by the 3R method for example of 15-storey reinforced concrete frame building.

More details regarding this study are available in the paper.