ارزيابي احتمالاتي عملكرد قاب‌هاي خمشي فولادي طراحي‌شده به روش تنش مجاز و حالت حدي آيين‌نامه‌ ايران تحت زلزله نزديك به گسل با استفاده از تحليل ديناميكي افزاينده (ida)

Probabilistic Assessment of ASD and LRFD code criteria for steel moment frames under the near-fault events using incremental dynamic analysis (IDA

روش ضرايب بار و مقاومت (حالت حدي) كه چند سالي است در مبحث نهم مقررات ملي ساختمان (آيين نامه طراحي سازه هاي فولادي) به عنوان روش اصلي شناخته مي شود، با ويرايش چهارم آيين نامه 2800 به لحاظ ماهيت هماهنگ است. هدف از اين پژوهش، ارزيابي احتمالاتي ضوابط قاب هاي خمشي فولادي با شكل پذيري متوسط و ويژه، طراحي شده به روش حالت حدي آيين نامه ايران (مبحث دهم) و مقايسه آن با قاب هاي طراحي شده به روش تنش مجاز در مناطق حوزه نزديك به گسل با استفاده از تحليل ديناميكي افزاينده (ida) و منحني هاي شكنندگي است. از بهترين و جديدترين روش هاي ارزيابي عملكرد لرزه اي ساختمان ها، استفاده از تحليل ديناميكي افزاينده (ida) و استخراج منحني هاي شكنندگي است كه رفتار جامعي از سازه را در محدوده خطي تا غيرخطي نمايش مي دهد. نتايج حاصل از تحليل ها حاكي از اين است كه سازه هاي طراحي شده به روش حدي در ناحيه خطي داراي شيب بيشتري هستند كه نشان دهنده رفتار سخت و تردتر آن ها مي باشد. مقايسه مقادير ميانه توابع شكنندگي سازه هاي طراحي شده به دو روش نشان مي دهد كه در همه سازه ها در هيچ يك از سطوح عملكرد، روش حالت حدي وضعيت بهتري ندارد. به طور خلاصه، روش تنش مجاز منجر به افزايش (بهبود) ميانه تابع شكنندگي تا 11% براي سازه هاي مختلف در سطوح عملكرد مختلف شده است.

Recently, the Iranian code is established for the LRFD design of steel structures that consistent with the Iranian seismic design code (2800-4). This study is aimed to compare the performance of steel moment frames (SMF and IMF) in the Near-faults earthquakes designed with the Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) in a probabilistic framework. After the Static Push over (SPO), new Performance based earthquake engineering (PBEE) approach is incremental dynamic analysis (IDA) that can lead to the probabilistic judgment using fragility curves of the structure under the different types of ground motions at different levels of intensity. For the incremental dynamic analysis a large number of nonlinear time history analysis must be carried out. The evaluated steel moment frames are 4-story and 8-story frames. The nonlinear models of structures are constructed in the Perform-3D software to perform the nonlinear time history analysis. For the nonlinear modelling of beam element, the chord Rotation Model is used that proposed by FEMA and available in the Perform-3D software for the beam elements. This model predict the nonlinear behavior of element in the two end region that plastic hinge may be caused duo to the seismic load of earthquake. For the column elements, the fiber element method was employed. In this method, the cross-section of column element is subdivided into some spring elements. Each spring is subjected to axial load, given by the combination of axial force and bending moment acting on the section. This model sometimes is called multi-axial spring model (MS model). The fiber model represents a section at the structural member-end. This modelling can represent the axial-flexural interaction in the column element that their properties of nonlinear flexural bearing depends on its axial load in each time step. Near-field events due to their pulse-like effect are in the spotlight in the last decay. To evaluate their effects on the steel structures that located in the seismic areas of Iran, a number of near-field earthquakes are used in the probabilistic assessment. In the IDA curves, the roof drift is used as Damage Measure (DM) and the Spectral Pseudo-Acceleration of the first mode of the structure with 5% modal damping ( ) is used as Intensity Measure (IM). Also in the probabilistic fragility curves, the direct method is used. It means that the IM is used directly in the fragility curve. To predict the probabilistic function for the different level of performance of structures, the lognormal distribution was used. The study results show that the structures designed by the ASD method have a better seismic performance than the LRFD frames specially in the performance level of Life Safety (LS) and Collapse Prevention (CP). It can be concluded from comparison of the median of collapse functions. For example for the special moment frame (8-story structure), the use of ASD design (instead of LRFD design) leads to a 11% increase in the median of fragility function in the Life Safety (LS) level and 10% increase in the Collapse Prevention (CP) level.