由生命週期成本分析探討鋼筋混凝土消防廳舍耐震設計及補強基準

Abstract

有鑑於近年來地震造成嚴重的建築物損壞與巨大的經濟衝擊，位於地震帶的國家無不擬定相關方案，投入防災的規劃與整備工作，期能防止此類廣域性災害的發生。其中，為了減低建築物於地震中所受到的損害，現行耐震設計規範已日趨嚴謹，並以提高建築物耐震能力為目標，政府部門亦逐步推動既有建築物的耐震補強工作。而耐震設計基準與耐震補強基準的選擇，將牽動著初期建造成本或補強成本與未來地震損失的消長。為了在建造成本、補強成本與地震損失之間獲得最大的效益，生命週期成本分析便是評估各種耐震設計方案或補強方案的經濟可行性最合適的工具。 本文以生命週期成本分析法，探討鋼筋混凝土消防廳舍最適宜的耐震設計基準（用途係數）與耐震補強基準。此建築物生命週期成本分析的架構，除了建築物使用年限的設定及生命週期期間各項成本支出的估算外，並結合地震危害度曲線與結構物易損性曲線的或然率模式。該生命週期成本則包括建造成本、補強成本與地震造成的各種損失，如建築物修復費用、內部財產損失、消防車輛損失、人員傷亡損失、建築廢棄物處理費、搬遷損失及建築物服務功能中斷引致的損失（僅保守包括額外火災損失及額外人命損失）。 研究結果發現，當建築物耐震設計基準或補強基準提升後，其地震損失與生命週期成本均可大幅降低；當新建消防廳舍採用途係數I = 2.1設計時，建築物生命週期成本最低，為最佳的耐震設計基準，惟與I = 1.5時之生命週期成本間的差異極低，因此，建築物耐震設計規範選用I = 1.5，可視為恰當的耐震設計基準；對於已使用30年，耐震能力僅為規範規定值（已考量用途係數）1/2之消防廳舍，如預計再使用10年至40年時，其最佳補強基準約為規範規定值（已考量用途係數）的1.1倍至1.5倍，惟最佳補強基準與僅補強至規範規定值時之生命週期成本的差異亦在可容許接受之範圍內，因此在補強經費有限下，建議將補強基準訂為規範規定值（1.0倍）即可；此外，如能注重建築物的使用維護，延長其生命週期，並適當給予補強，將可提高整體國家社會之效益。

Earthquakes are unpredictable, causing not only human casualties but also tremendous economic impact on the affected area. To reduce any loss by earthquakes, most countries in high-seismicity regions have proposed their disaster mitigation strategies and emergency counter-measures. In these countries, the seismic design codes were revised several times according to the state of the art in earthquake engineering, in order to prevent buildings from severe damage during earthquakes. Programs of strengthening or replacing old buildings have been created and carried out, notably for pre-earthquake code buildings. For buildings, different seismic design criterions or seismic retrofit criterions could induce different building construction cost, retrofit cost, and earthquake loss. Life-cycle cost analysis could be employed to evaluate the trade-offs between construction cost, retrofit cost, and earthquake loss, helping engineers make an optimal decision. With the support of life-cycle cost analysis, this study determines a seismic design criterion, the importance factor I, and seismic retrofit criterions of a reinforced concrete fire department building. The framework of such life-cycle cost analysis comprises the estimates of building service period and costs data, seismic hazard curve, and structure fragility curves. The costs are considered to be composed of the initial construction cost, retrofit cost, and the earthquake loss. The earthquake loss includes the building repair cost, contents loss, loss from fire truck damages, wage loss from casualties, debris disposal cost, relocation expenses, and loss caused by interruption of building functions. The study shows the earthquake loss and the life-cycle cost may be reduced by raising the seismic design or seismic retrofit criterions of buildings. For a new fire department building, the life-cycle cost reaches its minimum when the importance factor I is 2.1, which is considered an optimal criterion of seismic design. But while the difference between the life-cycle cost of I = 1.5 (suggested by the current seismic design code) and that of I = 2.1 is quite small, one may select I = 1.5 to design the fire department building; especially when the building construction budget is the planners’ concern. For a 30-year-old fire department building whose seismic capacity is half what the code suggests, the optimal seismic retrofit criterions are 1.1 to 1.5 times what the code suggests if the service life of the building remains 10 to 40 years. There is also quite small difference between the life-cycle cost with the optimal seismic retrofit criterions and that with the criterions the code suggests. The latter appears acceptable for the planners, especially for those who concern about the retrofit budget.