بررسي رفتار قاب خمشي فولادي ميان مرتبه با دهانه هاي بلند در برابر آتش و خرابي پيشرونده

در اين پژوهش ابتدا يك ساختمان نمونه 15 طبقه با سيستم سازهاي قاب خمشي فولادي با دهانههاي بلند و با ارتفاع طبقات 2 29/4 متر مربع با استفاده از مقررات ملي ساختمان ايران براي بارهاي ثقلي و جانبي مرسوم طراحي ميشود. در ادامه بحرانيترين قاب اين × 31/9 مدلسازي شده و سپس سناريوهاي مختلف آتشسوزي، خرابي پيشرونده ناشي از فروريزش OpenSees سيستم به صورت دو بعدي در نرمافزار تيرهاي اصلي و حذف ستونها بر اين قاب اعمال ميشود. براي انجام تحليلهاي حرارتي از منحني آتش استاندارد و تحليلهاي غيرخطي استفاده ميشود. نتايج نهايي پژوهش حاكي از آن است كه تيرها تحت اثر آتشسوزي تا دماي حدود 400 درجه سانتيگراد، تغييرشكل زيادي نداشتهاند. اما پس از اين دما با كاهش سختي و مقاومت فولاد، تغييرشكل تيرها به سرعت افزايش يافته است، به طوري كه در دماي حدود 500 تا 600 درجه از همان لحظات ابتدايي آتشسوزي (DCRnom) سانتيگراد، دچار تغييرشكل زيادي شده و خراب شدهاند. نسبت تقاضا به ظرفيت اسمي تيرها افزايش يافته كه بيشترين افزايش آن در دماي حدود 350 تا 400 درجه سانتيگراد اتفاق افتاده است. همچنين با فروريزش تير اصلي يك طبقه، ستونها در دهانه آتشسوزي تا دماي 500 درجه سانتيگراد مقاومت كردهاند. اما در دماهاي بالاتر، در محدوده 600 تا 800 درجه سانتيگراد، اين ستونها مقاومت خود را از دست دادهاند. در سناريوهاي حذف ستون در طبقات اول و هفتم، تيرها به ترتيب تحت اثر بارهاي ثقلي و در دماي 400 درجه سانتيگراد مقاومت خود را از دست داده كه نشاندهنده آن است كه خرابي ستونها نسبت به تيرهاي اصلي شرايط بحرانيتري ايجاد كرده است.

This paper investigates the behavior of a steel moment frame system with long spans subjected to compartment fires and progressive collapse scenarios due to floor drop and column removal. In this study, initially, a typical 15 – story building having moment frame system with long spans and story height 3.2 (m) is designed using relevant chapters of national building code of Iran for conventional gravity and lateral loads. The structure is designed based on the load and resistance factor method. The thermal analysis is carried out on a 2D internal frame because internal frames are more vulnerable to progressive collapse due to sustaining larger gravity loads compared to exterior frames. In order to perform thermal analyses, the most critical frame of this structure is modelled in finite element software OpenSees. The nonlinear behavior of the frame is studied at elevated temperatures under different fire and progressive collapse scenarios due to floor drop and column removal. In this analyses, the structure is subjected to both gravity and thermal loading simultaneously. Also for performing thermal analyses, standard fire curve (ISO 834) is used. Results of this study indicate that under compartment fire, beams do not deform significantly until approximately 400°C, but after that, vertical displacements of beams increase significantly due to degrading mechanical properties of steel. So beams deform and collapse at about 500°C to 600°C. Also heating the beams of structure, initially causes the axial force in the beams due to thermal expansion restraint. So Demand to Capacity Ratios of beams increase in the early stages of fire and the maximum value of DCRnom occurs at about 350°C to 400°C. In all scenarios, columns in lower stories are more critical because they carry more gravity loads. They fail at low temperatures, in which changes in strength and stiffness of steel are not remarkable. The reason is the great additional bending moment, which is produced due to the horizontal displacement and beam thermal expansion. Also Columns in middle span scenarios generally fail at 450 °C, which is less than the critical temperature for end bay scenarios. In the case of one story floor drop scenarios, columns survive up to 500°C. But at higher temperatures (about 600°C to 800°C), these heated columns lose their strength and buckle due to the reduction of steel properties. The middle bay floor drop produces more onerous situation compared to end bay floor drop. Also when lower floors drop, the failure of columns occurs earlier compared to when upper floor drop. In the case of column removal scenarios, the most significant effect is the increased length of beams and fire occurrence in these bays which may cause the heated beams lose their strength at lower temperatures. In these scenarios in first and 7th story, where beams have lost their strength under effect of gravity loads at about 400°C, more damage is observed compared to floor drop scenarios. Also when fire in the upper floors are considered, the building can survive longer because the beam, in which the span has been doubled due to column removal, is transferring less gravity load compared to lower stories.