ارزیابی قابلیت اعتماد ترکیب بارهای شامل بار باد در آیین‌نامه بارگذاری ایران

بار باد از جمله بارهای با عدم قطعیت زیاد و وابسته به موقعیت ساختگاه می باشد. بنابراین ضروری است که در تدوین و تنظیم آیین‌نامه‌ها، شرایط اقلیمی و پارامترهای آماری مربوط به مناطق مختلف به‌صورت جداگانه مورد بررسی قرار گیرند. در این پژوهش بر اساس یک مطالعه پارامتریک به ارزیابی سطح قابلیت اعتماد ترکیب‌های بار باد در مبحث ششم (ویرایش 1392) پرداخته شده است. برای دستیابی به این امر، پارامترهای آماری بار باد برای نواحی مختلف ایران بررسی گردید و سپس شاخص قابلیت اعتماد تیرهای فولادی در مود خمشی برای آن نواحی محاسبه و با مقدار شاخص قابلیت اعتماد هدف مقایسه شده است. نتایج نشان می‌دهند که شاخص قابلیت اعتماد در مبحث ششم نسبت به شاخص قابلیت اعتماد هدف، پایین‌تر است. یکی از دلایل پایین بودن شاخص قابلیت اطمینان تا حد زیادی ناشی از تخمین دست پایین سرعت مبنای باد نسبت به مقدار تجربی آن می‌باشد.

Reliability assessment of wind load combinations based on Iranian National Building Code

The new generation of design codes is established by reliabilitybased calibration methods. The overall aims of these methods are to achieve consistent levels of safety or structural reliability under different type of uncertainties. According to these methods, the acceptable reliability level of structures is obtained based on statistical descriptions of loads and resistance and also consideration of different types of uncertainties such as the physical uncertainty, the statistical uncertainty and the model uncertainty. In the last decades, based on reliabilitybased calibration approaches, load and resistance factor design (LRFD) method has been developed for steel buildings design. In this method desired level of safety is obtained by a set of partial load and resistance factor.The design load combinations for steel structures in Iranian National Building Code (INBC), Part 6, are generally based on other codes such as ASCE/SEI 710 standard and National Building Code of Canada (for wind load factors), while the effect of Iranian statistical data for load and resistance has not been considered. In comparison to other loads, such as gravity loads, wind load has a high degree of uncertainty and also it is completely site dependent. Therefore it is important to estimate a suitable statistical model for wind load and also investigate the reliability level of structures subjected to wind load combinations. This paper is a parametric study to assessment the reliability level of wind load combinations for steel beams based on INBC. For this purpose, wind load statistical data are provided for whole of Iran by the climatology data of wind speeds. Based on the FOSM method explicit formulation for reliability index of beams is calculated. The reliability indices for a range of practical load ratios are obtained and compared to the target reliability index. The results indicated that reliability level of wind load combinations in INBC is lower than target reliability index. One of the main reasons for the low level of reliability index for wind load combinations is related to underestimation of reference speed. The results show that by considering the reference speeds based on statistical data, reliability index approaches to target reliability index.