## 2 Brief Historical Context of Chilean Construction Regulations
### Earthquake Building Resistance Code NCh 433 Of72  
Following the modification of OGCU-1949, during the 1950s, there were no changes in seismic regulations, although at least five earthquakes of magnitude Mw higher than 7.5 occurred, culminating with the mega-earthquake in Valdivia in 1960, which reached Mw 9.5. Nevertheless, by the end of the 1950s and prior to the great earthquake in 1960, the National Institute of Investigations, Inditecnor (which in 1973 became the National Institute of Standardization, INN), was requested in 1959 to start developing seismic design criteria independent from OGCU. A committee was created with professionals and academic experts. During the 1960s, different proposals were formulated; in particular, the work of [PERSON] and [PERSON] (19] had great influence. With respect to the ground effect, they had already indicated that the designer could modify the spectrum shape to consider selective amplifying cases for types of ground. Starting with models and spectrum analyses of real earthquakes, [PERSON] and [PERSON] (20] subsequently presented a theoretical basis to define relationships between the response spectrum and ground properties, directly expressing for the first time the need for seismic site classification analogous to the system employed today. An antecedent in the form of a 1968 preliminary regulation denoted NCh 433 Of68 ([PERSON] et al., 2011), which was not officially published but accepted the incorporation of the static method and dynamic method. The former is applicable to short structures with simple construction, while the latter is applicable to taller and more morphologically complex buildings. Starting from this basis, a seismic coefficient was defined, and an acceleration spectrum was categorized for different site classes to which the fundamental vibration period, T\({}_{0}\), was assigned. The equations applied for each method are listed as follows:  
Seismic coefficient for the static method:  
\[\text{C}=0.10\text{forT}<\text{T}_{0} \tag{1}\] \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ground Foundation** & **Structure Period, T\({}_{e}\), s** & **Foundation Rigidity** & **k\({}_{\text{h }}\)*1** & **Amplitude of Seismic Action, m** \\ \hline Rock & - & 0.08 & - \\ \hline Conglomerate or very & & - & 0.12 & - \\ dense ground & \textless{}0.4 & with raft foundation or & 0.10 & - \\ \cline{3-4} Loose soils or sand & & without raft foundation & 0.12 & - \\ \hline Rock & & - & 0.05 & - \\ \hline Conglomerate or very & & - & 0.10 & - \\ dense ground & \(0.4\leq\text{T}_{\text{e}}\leq 0.75\) & & with raft foundation or & 0.12 & - \\ \cline{3-4} Loose soils or sand & & & similar rigidity & 0.15 & - \\ \cline{3-4}  & & without raft foundation & - & 0.060 \\ \hline \hline \end{tabular}  
*1 The vertical component of the seismic wave will only be considered in cases where the nature of the work requires it. Its acceleration and amplitude will be equal to 50% of the horizontal seismic action. *2 Interpretation and limit of the period taken from the approximate method proposed by [PERSON], et al. (1977), which the OGCU authorizes can still be applied for the determination of seismic action.  
\end{table} 
Table 4: Interpretation of seismic classification criteria according to OGCU-1949. \[C=0.10\frac{2\text{TT}_{0}}{\text{T}^{2}+\text{T}_{0}^{2}}<\text{T}_{0} \tag{2}\]  
Acceleration spectrum for the dynamic method:  
\[\frac{\text{a}}{\text{g}}=0.10\text{K}_{1}\text{K}_{2}\text{for}\text{T}<\text{ T}_{0} \tag{3}\]  
\[\frac{\text{a}}{\text{g}}=0.10\text{K}_{1}\text{K}_{2}\frac{2\text{TT}_{0}}{ \text{T}^{2}+\text{T}_{0}^{2}}<\text{T}_{0} \tag{4}\]  
where C is the seismic coefficient; T is the period of the corresponding vibration mode; T\({}_{0}\) is the fundamental vibration soil period; K\({}_{1}\) is a coefficient relative to the intended use of the structure (factor between 0.8 and 1.2); and K\({}_{2}\) is a coefficient relative to the structural form (factor between 0.8 and 1.2).