تخمين الگوي بار ناشي از ريزش آوار زلزله بر تيرهاي فولادي

Estimation of the Pattern of Debris Load Caused By an

آنچه كه در طول يك زلزله و اثر آن بر روي ساختمانها رخ مي دهد آسيب و تخريب موضعي ساختمان مي باشد . در اين شرايط سازه در حين وقوع زلزله و در طي لحظاتي، تحت يك بار شوك و ضربه ناشي از آوار و خرابي اجزا سازه قرار گرفته و علاوه بر حذف اجزا و المان هايي از سازه بار ديناميكي ناشي از زلزله با آوار تركيب شده و بار افزايش يافته جديدي را ايجاد مي كند. مدلسازي ضربه كف طبقات بالايي به طبقه پايين تر با استفاده از برنامه هاي كامپيوتري حرفه اي امروزي كه قادر به انجام تحليل ديناميكي غير خطي مي باشند امكان پذير مي باشد. اما حجم كاري مدلسازي كامپيوتري بويژه اگر سازه بزرگ و پيچيده باشد و تمام جزييات نيز در نظر گرفته شوند، بسيار زياد مي شود. در اين تحقيق سعي شده است كه با محاسبه ي ضريب بار ديناميكي براي نمونه اي از تيرهاي فولادي (در دو ناحيه الاستيك و پلاستيك) به استخراج الگوي بار ضربه ي آوار تحت شرايط مختلف و درنتيجه ارايه ي يك الگوي بار گذاري معادل به منظور تخمين رفتار سازه در مقابل ريزش طبقات فوقاني گام برداشت . در طول تحقيق مشخص مي شود كه پارامترهايي نظير زمان تناوب تير، ارتفاع سقوط و جرم آوار اثر بسزايي بر نتايج بدست آمده دارند؛ بطوريكه با افزايش زمان تناوب و جرم آوار مقدار ضريب بار ديناميكي كاهش و با افزايش ارتفاع سقوط اين مقدار افزايش مي يابد.

Abstract: During an earthquake and its effect, what happens on buildings is local damage and destruction which is different due to different characteristics such as diverse earthquake and geotechnical characteristic, different methods of analysis for earthquake-resistant structures, etc. Therefore, in addition to removing structural elements and their falling as debris, a shock and impulsive load could be added to the dynamic load imposed by the earthquake. Impact loads with extreme severity are loads with very low frequency of occurrence, but with extraordinary consequences. Undoubtedly appearing greater success in designing buildings resistant against extraordinary loads is required to forecast the real pattern and the impact of the impulsive loads. In other words, identify all possible failure modes of structures under extraordinary loads is necessary, which requires nonlinear analysis of structures under various loading conditions. Detailed modeling of the impact of an upper floor onto the floor below is feasible using current sophisticated nonlinear dynamic analysis software. Yet the computational effort in the case of large and complex structural systems can be excessive, especially if a detailed model of the whole structure is considered. Moreover, such analysis requires structural engineers with considerable expertise in nonlinear structural dynamics. Due to these limitations, detailed impact modeling is not practical for design applications. Hence, there is an evident need for simple, yet sufficiently accurate methodologies that can be used to establish whether the strength, ductility supply and energy absorption capacity of the lower impacted floor are adequate to withstand the imposed dynamic loads from the falling floor(s). This paper proposes a new design methodology for progressive collapse assessment of floor within multi-storey buildings subjected to impact from an above failed floor. The amount of debris and its scattering and distribution on different floors of a building play an important role in the operation of the impact load. The important point is that how these parameters should be considered in the dynamic analysis and how much the structure’s respond is sensitive to the impact characteristics such as impact velocity, the mass of debris and its distribution. This study has attempted to calculate the dynamic load factor (DLF) for samples of steel beams with an elastic-plastic behavior, and to extract the pattern of debris impact load in order to provide an equivalent loading pattern to estimate the performance ofstructures subjected to above failed floor(s). The study of dynamic load factors for the samples illustrates that the period of beam, the height of falling and the mass of debris have a noticeable effect on the result so that the DLF decreases by increasing in amount of the period of beams or the mass of debris, as well as it increases by increasing the height of falling. About the achieved patters for the impact load what is important is providing multi-line graphs in order to estimate the dynamic effect of the debris impact.