بررسي زمان بقاي قاب فولادي با اتصالات نبشي در حرارت بالا

Endurance time of steel frames with angle connection Under fire

- با توجه به اهميت آتش سوزي و تاثير آن بر پايداري سازه هاي هاي فولادي، بررسي تاثير آتش سوزي بر اين سازه ها در سال‌هاي اخير مورد توجه قرار گرفته است. با توجه به حساسيت فولاد نسبت به حرارت و هزينه ي بالاي مقاوم سازي قاب هاي فولادي به كمك پوشش هاي حرارتي، لزوم بررسي رفتار دقيق قاب هاي فلزي در حرارت هاي بالا به منظور كاهش آثار نامطلوب افزايش درجه حرارت به خوبي احساس مي شود. به منظور نيل به اين هدف و با توجه به هزينه ي بالاي تست‏هاي آزمايشگاهي و محدوديت آن‌ها در بررسي پارامترهاي متعدد در هر آزمايش استفاده ازروش اجزا محدود به عنوان ابزاري توانمند در مدل كردن قاب هاي فولادي تحت شرايط حرارت ارزش و اهميت خود را نشان داده است. با عنايت به اين موارد در اين پژوهش به بررسي اثر آتش بر تير هاي فولادي با در نظر گرفتن نرم شدگي رفتار اتصال و افت مقاومت مصالح پرداخته شده است. سپس با استفاده از نرم افزار ANSYS بررسي زمان دوام سازه درآتش سوزي انجام شده است. اين قاب مورد آزمايش، يك قاب سه بعدي تك دهانه است كه در شرايط مختلف شامل تاثير نوع اتصال، بررسي تاثير انبساط طولي، تاثير نوع بارگذاري حرارتي و نوع آتش بررسي شده است. براي درستي آزمايي نتايج حاصل از تحليل عددي با نتايج به دست آمده از كار ساير پژوهشگران مقايسه شده است. در پژوهش انجام شده نتيجه گرفته شد كه استفاده از نمودار آتش استاندارد در طراحي سازه هاي فولادي، زمان دوام كمتري نسبت به آتش طبيعي دارد كه نشان از دست بالا بودن مقادير در طراحي است.

The importance of fire effect on stability of steel structures has become one of the major issues for structural engineering researchers. Since steel is very susceptible to high temperature and cost of strengthening the elements with use of fire proofing material or any other provision, the need for rigorous investigation of the behavior of steel frames at elevated temperature in order to reduce unfavorable effect is well understood. To achieve this goal, researchers have carried out both experimental and theoretical investigations. It is clear that the cost of experimental research is high and is not available in all the institutions. It is also know that due to the limitation of the parameters to be studied, finite element procedure can be used as a powerful method of investigating structural behavior under extreme loading. It has been approved that the modeling of structural elements can be performed by this procedure that will best represent the structural behavior under fire. In this research the effect of fire on steel beams considering the softening behavior of connection and reduction of strength of material is studied. ANSYS software is used for this purpose and beam sub-assembly that has been used by Bradford, with the same assumptions has been adopted considering different boundary conditions. The standard variation of temperature is imposed at the geometrical center while linear temperature gradient is considered though the beam cross section. Uniform gravity load representing service load is also imposed on the beam. Restrained ends are modeled by elastic springs with calculated stiffness. Bredford beam is first modeled with the same assumption to calibrate the modeling. In this research a four story steel building with concentrically bracing is designed and one of the exterior frames is selected for the study. ISO 834 as an standard fire is imposed on the beam. Cardington natural fire curve showing the variation of the temperature with time is also used on the beam. One of the beams of the frame is assumed to be in compartment and a uniform as well as partial fire are imposed on the beam. At the end of the beam both translational and rotational springs are provided. Nonlinear moment-rotation curve obtained from Yahyai and Saedi experiment is used. For linear spring behavior, lateral stiffness of the frame is considered. The results are obtained in the form of displacement and axial load at the center of the beam under partial and uniform fire loading. Endurance time of the beam at various temperature is obtained. As the results shows the displacement and axial loading at 500 degree increases considerably due to the reduction of the modulus of elasticity under both partial and uniform loading. This increase is more for uniform loading case. Endurance time is more in case of natural fire as compared to standard one. This is due to the steep slope of the standard fire curve.