免震構造物における耐震安全性の確率論的評価に関する基礎検討

摘要

耐震設計とは建築構造物に関わる様々な不明確さに対して判断と決断を行い,対象とする建築構造物への要求性能を保証しようとする行為である．建築構造物の性能そのものにも未だ不明確な部分があり,またこれに対峠すべき地震動にも不明確さがある．前者に関しては過去の地震被害,構造実験や数値実験による成果,並びにそれらをまとめた設計基準・指針類が判断と決断の材料となる．後者に関しては,多数の条件下で観測された地震動,サイト波と呼ばれる模擬地震動の作成が判断と決断の材料となっている．%Seismic base isolation is a very effective way of controlling the seismic responses of building structures. However, compared with conventional seismic systems, the robustness of the base-isolated buildings is generally considered lower and thus the selection of input ground motions becomes a more crucial issue in assessing the seismic safety of base-isolated buildings. Extensive incremental dynamic analysis (IDA) is carried out to evaluate the ultimate state of base isolated buildings in terms of the fragility curves. The ground motion record set in FEMA P695 is adopted for analysis. The influences of the stiffness of isolation layer, the height of rubber bearings, clearance and hysteretic behavior of the retaining walls as well as the strength of the superstructures are examined. Based on the analysis results, the following can be concluded:(l)increasing the clearance of retaining walls is the most effective manner to enhance the seismic safety of base isolated buildings; (2)the seismic safety margin of base-isolated buildings might be significantly increased with stronger superstructures only if the retaining wall is appropriately proportioned so that the yielding of retaining wall is prior to that of the superstructure.