Risk-Based Strategies for Upgrading Existing Nonconforming Wooden Houses and Risk Information for Promoting Upgrading

Abstract

There is widespread awareness in seismic-prone countries about the necessity to upgrade the seismic performance of old buildings and wooden houses for disaster mitigation; however, upgrading occurs at a very slow pace. At present, Japan has approximately 11 million nonconforming old wooden houses, but most of them remain without upgrade because of the large expenditure that would be incurred by both house owners and the local government. An owner would need to spend approximately US$11,000 on average to upgrade his/her house such that it satisfies the current design requirements. In Aichi prefecture alone, upgrading would cost approximately US$10 billion in total, which is approximately 40% of the annual budget. This study, therefore, investigates alternative strategies for upgrading existing nonconforming wooden houses more efficiently and cost-effectively. The seismic risks of wooden houses as a whole in Aichi prefecture are estimated, and then the effective target level for upgrading is discussed from the viewpoint of both economic loss and the number of fatalities. It is shown here that it is worthwhile from the viewpoint of life safety to consider a target upgrading level that is lower than the current design requirements. Although the analytical models discussed in this paper might be specific to Japan, the framework of the risk assessment is universal and the conclusions drawn from this analysis are applicable to other seismic-prone regions and countries.