台北盆地之地震力折減係數

Strength Reduction Factors in Taipei Basin

為了非彈性設計之需，可將彈性設計震譜以強度折減係數來降低地震力。建築物彈塑性振動時，地震力折減係數與韌性容量和自然振動週期有關。本文產生396組人造地表加速度相符於台北盆地的彈性設計震譜，以便推導預測地震力折減係數的經驗公式。即使這些人造地表加速度對應的震央距離、工址位置和回歸期有所差異，所得地震力折減係數的平均值隨自然振動週期變化的趨勢卻很相似。因此，可採用全部人造地表加速度所得的平均地震力折減係數，來建立經驗公式。此經驗公式在很大韌性容量時，會高估地震力折減係數；除此之外，將提供地震力折減係數的保守估計值。

For the inelastic design purpose, factors of strength reduction are usually used to reduce the seismic force from an elastic design spectrum. These factors depend on the ductility capacity and the natural vibration period of a building under elastoplastic vibration. A total of 396 artificial ground accelerograms compatible with the elastic design spectra provided for the Taipei basin are generated in order to derive an empirical formula in predicting the strength reduction factor. The variation of the mean strength reduction factor on the natural vibration period is very similar in spite of the differences in epicentral distances, site locations, and return periods. On the basis of all the mean strength reduction factors, an empirical formula is proposed. This empirical formula overestimates the strength reduction factors for pretty large ductility capacity, but generally offers conservative estimates elsewhere.