2nd Croatian Conference on Earthquake Engineering 2CroCEE

The prestandard of the Swiss Society of engineers and architects (SIA) 2018 for the verification of the seismic safety and risk-based retrofit of existing buildings (SIA, 2004) was published in 2004 and updated as the SIA269/8 building code (SIA, 2017) in December 2017. The Swiss standard for existing buildings is applied in practice since 2004 and shows an adequate balance between a consistent probabilistic risk-based framework and the necessary ease of use and flexibility for broad application. A large number of seismic assessments and retrofits of existing buildings have been completed in Switzerland using this approach. This contribution presents the central concepts of SIA 269/8 and focuses on the computation of the efficiency of retrofit measures using standardized risk curves for human-life and direct damage to the structure and its content.
The first central concept of SIA 269/8 is the compliance factor, which indicates the degree of compliance of an existing structure in comparison with the requirements for new structures. The second central concept is the definition of a minimum compliance factor under which the seismic safety is unacceptable because of the risk to individuals. If the seismic safety of an existing structure lies below this threshold, retrofitting is mandatory to reach this minimum threshold whatever the cost.
The third central concept of SIA 269/8 is a set of standardized curves linking the compliance factor with different risk values. These curves allow the computation of the risk reduction and the efficiency of retrofit measures if one knows the compliance factor before and after retrofitting. According to SIA 269/8 efficient measures, i.e. with an efficiency greater than 1, should be implemented. The efficiency is computed as the ratio between the risk reduction in Swiss francs (CHF) per year and the annualized cost of measures computed with a discounting factor of 2 % over the remaining time of use. To compute the risk reduction to human life, the casualty risk is computed before and after retrofit as the product of the individual risk, the occupancy rate and the value of statistical life set to 10,000,000 CHF. In this case, the risk curve has a unit of probability of casualty per year. The same principle applies to the computation of the risk reduction for direct damage to the structure and its content.
This elegant and relatively simple framework allows to focus on the buildings with an unacceptable risk and the ones for which efficient measures can be found and implemented.