Risk - based seismic design of buildings - An example of an eight storey reinforced concrete building

Experiences from recent earthquakes around the world show that the risk of life and property loss due to seismic hazard is too high if buildings are not properly designed. This finding is not new, since there are many different standards for the design of structures in seismic areas. Those standards were developed to reduce seismic risk, but they only prescribe the capacity design procedure for a given design earthquake. Therefore we cannot claim that the current standards for seismic resistant design of structures control seismic risk to such an extent that would be acceptable for all types of structures and for all investors. In order to overcome this shortcoming we developed a procedure for design of buildings based on seismic safety, which is defined with acceptable seismic risk. The process is iterative and requires the use of nonlinear methods of analysis. Firstly, the structure should be preliminarily designed, which can be achieved by utilising standards for seismic resistant design of buildings. Then we estimate the seismic risk and compare it to an acceptable or tolerated risk. If the seismic risk is too high, we take measures to reduce it and re-evaluate it for a new and improved structure. The proposed procedure was demonstrated by means of an example of an eight-storey reinforced concrete building. For the base structure that was designed under current standards, we found, that the risk of collapse due to earthquakes was too high. We also proved that the capacity design method does not provide the assumed collapse mechanism, which is a weakness of current standards for seismic resistant design. Finally, we showed that the adequate seismic safety can be provided by a small increase of the columns' reinforcement, which does not substantially affect the cost of the investment, but reduces the seismic risk by a factor of two.