抄録
In recent years, global environmental issues has been one of the main concerns among the society of building construction, and buildings with longer service lives than ordinary ones is considered as one of the solutions. On the other hand, the social needs of buildings with "limited service life" have been increasing for the purpose of efficient use of land leasehold, and experimental verification of new construction methods. However, according to the current Japan's Building Standards Act, a building with service-life longer than a year must be designed as an ordinary building. Because the possibility that a building with a service-life shorter than that of an ordinary building meets a rare event during its service-life is smaller, the design load could be reduced. In order to meet such social needs, “KIGEN-TSUKI Buildings” has been proposed and Recommendations for Design of KIGEN-TSUKI Buildings was published by AIJ. According to the recommendations, the design load on a KIGEN-TSUKI building against wind and snow can be reduced provided that preliminary preventive care such as installation of temporary support is executed.
With preliminary preventive care conducted based on the weather forecast level (hereinafter, the trigger level), safety level of a building against an extremely rare events such as strong wind and heavy snow should be kept equivalent to that of ordinary building even if its service life is short. On the other hand, the construction cost can be reduced by reducing the design load. Although the possibility of loss of functionality due to damage caused by a rare event would increase, it can be basically converted into monetary loss and can be compensated by such as insurance. The degree of such risk depends on the design load and the trigger value; however, it is difficult for a structure engineer to evaluate the risk. In order to implement practically the design of a KIGEN-TSUKI building, a simply guidelines for the design load and trigger level are expected.
This paper first presents the evaluation method of the limit state probability of components in a KIGEN-TSUKI building against a load combination of dead, live, and either wind load or snow load in an ordinary area. The method takes into account the accuracy of the forecast of the wind speed and ground snow weight, and the posterior distribution of the daily maximum wind speed and ground snow load as a function of the accuracy and level of forecast. It also takes into account the possibility of the failure of conducting the preliminary preventive care due to the uncertainty in the forecast. Then, this paper discusses the optimum design load and trigger value on a KIGEN-TSUKI building that minimizes the total expected cost, which is the sum of the construction cost, expected cost associated with preliminary preventive care, and the expected cost of failure in serviceability. Here, it is assumed that safety level can be kept unchanged by an appropriate trigger level and the preliminary preventive care. Base on the discussion, it proposes simple formulae to determine the optimum wind and snow design load and the optimum trigger value in a form of return period as a function of variability in annual maximum, service life of buildings, cost conducting preliminary preventive care, cost of failure in serviceability and the slope of the construction cost. Finally, the applicability of the simple formulae and cost effectiveness of the reduction of design load is investigated using a building model.
