## 2 Brief Historical Context of Chilean Construction Regulations
### Earthquake Resistance of Buildings NCh 433--Modified Proposal 2018-2021  
After the validation of DS61-2011, members of the code committee continued their meetings with the objective of evaluating the large amount of data obtained and the new data generated by the practical application of DS61-2011. Additionally, two important earthquakes occurred in the northern part of the country with magnitudes of 8.2 and 8.3 in 2014 (Iquique) and 2015 (Illapel), respectively, which provided interesting complementary information for the seismic classification system. The most recent proposal to code NCh 433 was presented in December 2018, and currently, it is in the final revision stage, which includes a period of public inquiry, where suitable professionals will deliver their observations. This process will enable the consolidation of the final version of the document, the application of which is expected to start during the next few years. [PERSON] [5], based on a rigorous analysis of the seismic site classification procedures identified from available data, including seismic records of earthquakes with a magnitude Mw greater than 8.0, discovered that the H/V spectral ratio (HVR) obtained via ambient vibrations reproduces the predominant periods shown by the response spectra and suggests its use as a complementary parameter for seismic site classification. Centring the analysis of the proposal only on those revisions related to seismic site classification, one of the main changes proposed by the code committee, taking into account all the antecedents, consists of the incorporation, as a classification parameter of the fundamental period of vibration of the ground, T\({}_{\rm g}\), measured in situ using the HVR method, which is directly linked with the seismic response of the ground and is complementary to Vs30. This criterion allows differentiation of the cases that may present an equal Vs30 but may have stiffness variations with depth that condition the seismic response at the surface; the incorporation of T\({}_{\rm g}\) will allow each case to be distinguished. This proposal is innovative in global design codes; although the Japanese code [44] also uses the period as a classification parameter, it is estimated starting with the shear-wave velocity to the named engineering bedrock, and its direct in situ measurement is not required. Considering the fundamentals of the previous code, the proposed code replaces properties and/or indices associated with the resistance of the ground formerly applied for seismic classification (its density q\({}_{\rm u}\) or N-value SPT) for parameters that represent the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Site Class** & **Description** & **Vs30, m/s** & **RQD** & **q\({}_{\rm u}\) (MPa)** & \begin{tabular}{c} **(N\({}_{\rm l}\))\({}_{\rm 60}\)** \\ **(For Sands)** \\ \end{tabular} & \begin{tabular}{c} **s\({}_{\rm u}\) (MPa)** \\ **(For Fines Soils)** \\ \end{tabular} \\ \hline A & \begin{tabular}{c} Rock or cemented soil (not \\ soluble in water) \\ \end{tabular} & \(\geq\)900 & \(\geq\)50\% & \(\geq\)10 (\(\epsilon_{\rm qu}\leq 2\%\)) & \\ \hline B & \begin{tabular}{c} Soft rock or very dense soil \\ or very firm soil \\ \end{tabular} & \(\geq\)500 & \(\geq\)0.4 (\(\epsilon_{\rm qu}\leq 2\%\)) & \(\geq\)50 & \\ \hline C & \begin{tabular}{c} Dense or firm soils \\ \end{tabular} & \(\geq\)350 & \(\geq\)0.3 (\(\epsilon_{\rm qu}\leq 2\%\)) & \(\geq\)40 & \\ \hline D & \begin{tabular}{c} Medium dense or firm soils \\ \end{tabular} & \(\geq\)180 & & \(\geq\)30 & \(\geq\)0.05 \\ \hline E & \
\begin{tabular}{c} Medium compactness or \\ medium consistence soil \\ \end{tabular} & \(<\)180 & & \(\geq\)20 & \(<\)0.05 \\ \hline F &
\begin{tabular}{c} Specials soils \\ \end{tabular} & - & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Seismic site classification—DS61-2011. dynamic stiffness properties of a location, which are correlated with the phenomenon of ground amplification (Vs30 and T\({}_{\text{g}}\)).