## 2 Brief Historical Context of Chilean Construction Regulations
### Earthquake Resistance of Buildings, Codes NCh 433 Of93 and NCh 433 Of96  
The earthquake on 3 March 1985, in the central zone of Chile represented a great test for the prevailing seismic laws in Chile, and as usual after a great seismic event, the National Institute of Standardization, INN, in 1986 constituted the Seismic Resistance Coordinator Code Committees. With input from these committees, the first modification to the code was proposed and then approved in 1993, and as a result, code NCh 433 Of93 [28] was cancelled and replaced by NCh 433 Of72 [23]. The 1993 version included different changes in the design code, highlighting the incorporation of a new system for the seismic \begin{table}
\begin{tabular}{c c} \hline
**Site Class** & \(\text{T}_{0}\)**, s** \\ \hline Rock, dense gravel, dense sandy gravel & 0.20 \\ Dense sand, stiff cohesive soils & 0.30 \\ Loose granular soils, soft or medium soft cohesive soils & 0.90 \\ \hline \end{tabular}
\end{table} 
Table 5: First seismic site classification from NCh 433 Of72. site classification of the ground and directly defining parameters to characterize the ground as well as the shear-wave velocity, Vs; SPT N-value; undrained strength, s\({}_{\text{u}}\); and the resistance to simple compression, q\({}_{\text{u}}\). Considering the lessons learned after the earthquake of 1985 and great seismic events recorded around this time at an international level (Los Angeles, CA, USA, Mexico, and Peru), these regulations clarified for the first time the application range with respect to the geomorphological conditions of the ground. Parameters for seismic characterization and solicitation were defined considering horizontal surfaces and horizontal stratification and acknowledged that some structures are built on foundations that reveal geomorphological and topographic singularities. This last point was motivated by the effects of topographic amplification observed and categorized for structures built in the area of the Beagle channel, Vina del Mar, during the earthquake and aftershocks in 1985 [29]. Importantly, note that by the publication date of the document that contained the modifications to NCN 433 Of72, 1989, no national or international codes had defined specific criteria for the identification of ground types that may suffer topographic effects, leaving their identification to engineers' judgement. This procedure was generally applied in such cases as in regular professional practice, and topographic effects were not considered. Even today, most design codes at the international level still lack criteria for the consideration of topographic effects. Exceptions, such as the French code [30], Eurocode [31], and Italian code [32], share the same criteria. In relation to special cases, for the first time, associated criteria were introduced for ground types with the potential to liquefy, identifying this phenomenon for sand (N\({}_{1}\))\({}_{60}\) less than 20 blows/feet, which had been excluded from ground classification systems. In 1989, this point also represented a development, as different design codes, such as the USA [33], Japan [34] and New Zealand codes [35], did not include criteria that conditioned the seismic site classification in cases with possible liquefaction. As a part of the revision process for the practical use of code NCh 433 Of93, the Chilean Association of Seismology and Antiseismic Engineering (ACHISINA) proposed adjustments to the code based on the main conclusions registered by the professional community; in 1996, a second update became official, denoted NCh 433 Of96 [36]. With respect to ground and seismic classification, the 1996 version did not involve any changes. The seismic classification contents for NCh 433 Of93 and NCh 433 Of96 are presented in Table 6. In this case, four site classes were defined; they were identified starting with defined parameters and helped to reduce subjectivity, which could affect the characterization of ground types.