## 2 Brief Historical Context of Chilean Construction Regulations
### Emergency Supreme Decree 117, Ds17-2011  
Although code NCh 433 Of96 mod2009 had recently been approved, damage generated by the Mw 8.8 earthquake on 27 February 2010, in Maule, provided evidence for the urgent need to evaluate and adapt the code. The derived changes and immediate adjustments were included in Emergency Supreme Decree number 117-2011 issued by the Urbanism and Buildings Ministry, which modified NCh 433 Of96 mod2009 [40]. One of the points of special interest at the design code committee level was the seismic site classification, which at the time had undergone no changes for 17 years, as was evidenced from the damage recorded that was considered greater than acceptable; accordingly, the necessity of modifying the classification to incorporate the seismic requirements of the country according to its ground typologies was recognized. Among the main changes in DS117-2011, new criteria were included for the seismic classification of ground types and associated new design spectra. Therefore, considering the international validation implied by the incorporation of parameter Vs30 proposed by [PERSON] [1,2], the parameter Vs, which included the previous version of the code for certain site classes, was modified to Vs15 and Vs30, considering the lesser value of the two velocities as a parameter for the classification of each type of ground. Here, Vsi represents the velocity of shear waves that travel through the uppermost i metres of the ground. Because of the changes to the regulations, measuring the shear-wave velocity in the field became a professional procedure. At this time, there was not enough equipment to adequately measure the shear-wave velocity. Thus, for a temporary period of three years, the estimation of shear-wave velocity using indirect methods, data analysis or correlations \begin{table}
\begin{tabular}{c c} \hline \hline
**Site Class** & **Description** \\ \hline I & Rock: Natural material with in-situ Vs \(\geq\) 900 m/s, or qu\({}_{\text{u}}\) intact rock \(\geq\) 10 MPa and RQD \(\geq\) 50\% \\ \hline II & (a) Soil, Vs \(\geq\) 400 m/s in the upper 10 m, and increasing with depth; or well, (b) Dense gravel, \(\gamma_{\text{d}}\geq\) 20 kN/m\({}^{3}\), or relative density \(\geq\) 75\%, maximum dry density \(\geq\) 95\%; or well, (c) Dense sand, relative density \(\geq\) 75\%, (N\({}_{1}\))\({}_{60}\) \(>\) 40, maximum dry density \(\geq\) 95\%; or well, (d) Stiff cohesive soil, \(\mathbf{s}_{\text{u}}\geq\) 0.10 MPa (unfissured samples). \\ \multicolumn{2}{c}{Minimum thickness of the horizon layer on the rock, 20 m. If the thickness of the stratum on the rock is less than 20 m, the site class will be classified as type I.} \\ \hline III & (a) Sand permanently unsaturated, 55 \(<\) relative density \(\leq\) 75\%, N\({}_{60}\) \(>\) 20; or well, (b) Unsaturated gravel or sand, maximum dry density \(\geq\) 95\%; or well, (c) Cohesive soil with 0.025 \(<\) s\({}_{\text{u}}\leq\) 0.10 MPa, regardless of the water table; or well, (d) Saturated sand with 20 \(<\) (N\({}_{1}\))\({}_{60}\)\(\leq\) 40. \\ \multicolumn{2}{c}{Minimum horizon layer thickness: 10 m. If the thickness of the stratum on the rock or on the site class corresponding to type II is less than 10 m, the site class will be classified as type II.} \\ \hline IV & Minimum horizon layer thickness: 10 m. If the thickness of the horizon layer corresponding to certain types I, II or III is less than 10 m, the site class will be classified as type III. \\ \hline \hline \end{tabular}  
* Vs: shear wave velocity; qu\({}_{\text{u}}\): unconfined compressive strength; RQD: rock quality designation; s\({}_{\text{u}}\): undrained shear strength; \(\gamma_{\text{d}}\): dry unit weight; (N\({}_{1}\))\({}_{60}\): normalized SPT N-value.  
\end{table} 
Table 6: Seismic site classification NCh 433 Of93/NCh 433 Of96. with geotechnical exploration was authorized. Addressing this same point, the parameter Vs15 would represent a conservative criterion of classification in some cases, which could simplify exploration in the short term.