不同方法之震譜相符強地動歷時對結構地震需求之變異性探討

Abstract

在結構設計中，隨科技資訊之進步，對於重要結構物或設施而言，設計中必須考慮動力-歷時分析，以確保整體與細部結構的安全性，在進行動力-歷時分析中，必須具備設計地震紀錄，經常要求輸入之加速度歷時必須與該工址之規範設計反應譜相符合(spectrum-compatible)，以符合該工址之地質狀況與預期之地震強度。設計反應譜係由眾多真實地震紀錄經過統計分析後歸納而得，是故對於單一測站之單一地震紀錄是不具代表性，其反應譜也不像規範中如此平整，而容易導致設計不足或過於保守，無法滿足規範之需求。所以，對於真實地震紀錄必須經過適當的修正，才能符合設計規範之需求，提供工程師作為結構設計參考之依據。於工程應用方面，有許多方法可用以製作與設計反應譜相符合之人造地震紀錄，概分為頻率域分析和時間域分析兩種，EX：SIMQUAKE程式即屬於頻率域分析方法之一。 本文致力於探討不同震譜相符合方法之最具潛勢人造加速度歷時的產生，及其所引致結構動力分析之影響與變異性。探討方法包含：方法一為利用小波分析方法來處理，將起始所輸入之真實地震紀錄進行分解(decompose)，再依據設計反應譜與其反應譜之比值，將各頻率之分量進行調整，如此不斷修正致人造地震反應譜與設計反應譜滿足符合之需求；方法二為在各個既定頻率計算其反應譜值，根據其與目標反應譜之誤差，在時間域原地震紀錄發生反應極值之時間點附近加減一個微小地震紀錄改變量，對每個既定頻率均進行微小地震紀錄改變量之修正，如此重複迭代致人造地震之反應譜與設計反應譜滿足符合需求；方法三為頻譜修正法；方法四為相位模擬並配合頻譜修正；方法五則為多振態調整法，考量結構物高振態之參與，並利用最小誤差平方法調整原始地震之反應譜強度(將原始地震歷時作線性縮放)，使與規範之設計反應譜強度更加相符。

The goal of earthquake-resistant design is to produce a structure or facility that can withstand a certain level of shaking without excessive damage. This level of shaking is described by a design ground motion, which can be characterized by design ground motion parameters. Generally, spectrum-compatible seismic excitations are required when dynamic time-history analysis is used for determining the response of a building structure. This paper describes the simulation methods to generate the artificial ground motion time history compatible with a target acceleration response spectrum. Five different approaches to generate spectrum compatible ground motion are presented. First, a wavelet-based procedure has been used to decompose a recorded accelerogram into a desired number of time-histories with non-overlapping frequency contents, and then each of the time-histories has been suitably scaled for matching of the response spectrum of the revised accelerogram with a specified design spectrum. The key idea behind this iterative procedure is to modify a recorded accelerogram such that the temporal variations in its frequency content are retained in the synthesized accelerogram. Second, a method based on the modification of original wave form by using a series of modification functions which is in relating to a damped single-degree-of-freedom is used. Third, method from manipulate the amplitude of Fourier transform for overall matching is also discussed. Fourth, spectrum compatible ground motion using generated phase spectrum is also discussed. Finally, the multi-mode ground motion scaling (MMS) method takes into account the structural modal characteristics and aims to minimize the difference between the response spectrum of a selected ground motion and the design spectrum at the first few modes. Discussion on the overall shape, duration and frequency contents, and peak acceleration and phase contents of the artifical ground motion time history are made. Finally, this report also presents results from a study on the effects of different types of spectrum-compatible excitations on the response of building with different height. A SDOF nonlinear system, a 6-story and a 12-story building are used in this study. Nonlinear time-history analyses were conducted by using the spectrum-compatible excitations (i.e., accelerograms) generated from different approach.