توليد شتاب‌نگاشت‌هاي مصنوعي براي يك ناحيه‌ي خاص با تبديل موجك

GENERATION OF ARTIFICIAL ACCELOGRAMS FOR A SPECIFIC AREA USING WAVELET TRANSFORM

كليد موفقيت در تحليل‌هاي تاريخچه‌‌ي زماني تا اندازه‌ي زيادي به ميزان دسترسي به شتاب‌نگاشت‌هاي متناسب با شرايط خاك محلي بستگي دارد. اما در بيشتر موارد، نگاشت‌هاي ثبت‌شده در يك ناحيه‌ي خاص بسيار كم است. در اين نوشتار، روشي جديد برمبناي تبديل ‌موجك براي توليد شتاب‌نگاشت‌هاي مصنوعي يك ناحيه‌ي‌ خاص پيشنهاد شده ‌است. تجزيه‌ي نگاشت‌هاي حقيقي ثبت‌شده در يك ناحيه به سطوح مختلف، سيگنال‌هاي پايه با خصوصيات فيزيكي معني‌داري ارايه مي‌كند؛ كه تركيب مجدد آنها ولي متفاوت، نگاشت‌هاي جديدي متاثر از مشخصه‌هاي منطقه به دست مي‌دهد. بدين‌ترتيب مي‌توان تعداد بسيار زيادي نگاشت مصنوعي از تعداد محدودي نگاشت ثبت‌شده به‌دست ‌آورد. براي نشان‌دادن كارايي روش از نگاشت‌هاي ثبت‌شده در ايستگاه‌هاي شبكه‌ي ‌لرزه‌نگاري Smart1 كشور تايوان استفاده شده‌ است. شتاب‎‌نگاشت توليدشده از لحاظ مشخصه‌هاي آماري، طيفي، و تاريخچه‌ي زماني با ‌نگاشت‌هاي حقيقي مقايسه و با استفاده از آن رفتار لرزه‌يي يك سازه‌ي فولادي 10 طبقه بررسي شده ‌است. نتايج ارزيابي، كارايي روش پيشنهادي را تاييد مي‌كند.

The successful time history analysis mostly depends on the accessibility of the adequate ground acceleration records given for a specific site soil condition. But, except for a few regions in the world, in most cases, limited numbers of seismic records are available. Designers overcome this limitation by using artificial records that may be constructed by combining sine waves with different phase and amplitudes, or matching spectra with a specific design spectrum. Although, these procedures are acceptable from a mathematical overview, they are not satisfactory procedures for the designer to obtain the best or even probable acceleration records for the considered region. So, these procedures, by generating a lot of artificial records, solve the designer’s problem. But, using these acceleration records to evaluate the earthquake response of a structure in order to design, rehabilitate or strengthen may lead the designers or clients to make a wrong decision. In this paper, a new innovative method, based on the wavelet transform to generate artificial acceleration records for a specific area, is presented. First, each seismic record is decomposed to its basis signals. Each basis signal, besides the specific limited frequency domain, includes the unique physical phenomenon of the area. In other words, each basis signal inherently defines specific characteristics of the region. This is based on the number, type, and mechanical, geotechnical and seismic properties of the soil layers in the region, and also probable scenarios of the passing seismic waves. Hence, as a second step, by randomly combining these basis signals, we obtain other possible seismic records of that area. Therefore, we can generate many artificial seismic records using only limited site specific recorded ground motion. In order to explain the method, the acceleration earthquakes of the Smart1 earthquake network station of Taiwan, recorded during a 5.8 magnitude earthquake on the Richter scale, in 1985, are used. Similar and different features of the artificial records are compared with that of the real seismic records. Finally, the performance of a 10-story steel frame against artificial and real acceleration records is compared together. Artificial records may or may not be stronger than real accelerograms, but they, specifically, are the probable acceleration records of that particular area.