تاثير جداساز لرزه يي لاستيكي با هسته ي سربي بر پتانسيل خرابي پيش رونده ي قاب هاي خمشي فولادي تحت بار لرزه يي

THE EFFECT OF LEAD RUBBER BEARING SEISMIC ISOLATOR ON PROGRESSIVE COLLAPSE POTENTIAL OF STEEL MOMENT FRAMES

«خرابي پيش رونده» به خرابي موضعي كه بر اثر عوامل مختلف در اعضاي باربر سازه ايجاد و سپس به اعضاي مجاور سرايت مي كند و در كل سازه گسترش مي يابد اطلاق مي شود. يكي از عوامل موجد خرابي در سازه ها «زلزله» است. بر اثر زلزله هاي شديد ممكن است در اجزاي سازه نيروهاي مضاعف ايجاد شود كه باعث گسيختگي عضو باربر و در نتيجه پديده خرابي پيش رونده مي شود. بدين منظور در اين مطالعه، به مقايسه ي رفتار قاب هاي خمشي فولادي با شكل پذيري متوسط، در دو حالت پايه ثابت و داراي جداسازي لرزه يي لاستيكي با هسته ي سربي، تحت خرابي پيش رونده و با استفاده از تحليل هاي استاتيكي و ديناميكي غيرخطي در موقعيت هاي مختلف حذف ستون پرداخته ايم. افزودن سيستم جداساز لرزه يي به سازه باعث كاهش پاسخ سازه ها تحت زلزله مي شود.همچنين نتايج تحليل هاي خرابي پيش رونده نشان مي دهد كه افزايش تعداد طبقات سازه موجب كاهش پتانسيل خرابي پيش رونده مي شود و استفاده از سيستم جداساز لرزه يي تاثير قابل توجهي در موضعي كردن خرابي ها تحت بارهاي لرزه يي و جلوگيري از گسترش آنها در سازه دارد.

Progressive collapse is defined as a local failure that may occur due to various factors in structural members; then, it can spread to adjacent members and ultimately result in the total collapse of the structure or a large portion of it. Though the abnormal loads could cause progressive collapse, many structures have experienced progressive collapse due to seismic actions in our modern history. Recently, some code specifications and guideline requirements such as Unified Facilities Criteria (UFC) have introduced different analysis methods for the assessment of progressive collapse in buildings based on increasing strength, ductility, and continuity. Many research works have been conducted in relation to the phenomenon of progressive collapse almost considering the gravity loads and, in recent years, seismic progressive collapse has attracted much attention and is an open research area for researchers. Lead Rubber Bearing (LRB) is considered as one of the most conventional isolation systems that has been studied and examined theoretically and developed widely in practice.This study investigates the potential of LRB base isolation under progressive collapse. For this purpose, the behavior of intermediate steel moment frames in the two cases of fixed and with the LRB seismic isolator with 4, 8, and 12 number of stories under progressive collapse is compared using nonlinear static and dynamic analysis in different situations of the column removal. At first, two fixed and isolated 3D structures were designed by SAP2000 software according to Iranian codes; then, analysis was performed under gravity load (consist of nonlinear static and dynamic analysis) according to UFC guild-lines and seismic loading (by nonlinear time history analysis) using Perform-3D software. The base isolation is modeled with an isolator element in the Perform-3D software, and these separators provide hysteresis damping through the sink of lead core.The addition of seismic base isolation system to structures averagely reduces the response of the frames under earthquakes by 61%. The progressive collapse potential of fixed and base isolated structures in the middle and corner column removal conditions is the same as the results of nonlinear static and dynamic analysis according to loading UFC instructions. Furthermore, the results of the progressive collapse analysis show that increasing the number of structural members leads to a reduction in progressive failure potential. It is observed that the use of base isolation system has a significant impact on the localization of the failures under seismic loads and prevention of their expansion in the structure.