بررسي تاثير خصوصيت جهت پذيري پيش رونده و جابجايي ماندگار ركوردهاي نزديك گسل بر رفتار لرزه اي قاب هاي خمشي فولادي بلند مرتبه با مهار بند برون محور و تعيين نقطه آستانه فرو ريزشيب

هدف از اين مقاله، و به دست آوردن نقطه آستانه فروريزش(CP) از روي منحني هاي IDA تحت تاثير ركوردهاي نزديك گسل وتعيين تاثيرات خصوصيت ركوردهاي نزديك گسل(خصوصيت جهت پذيري پيش رونده، خصوصيت جهت پذيري پس رونده و خصوصيت جابجايي ماندگار) در زمان تشكيل نقطه آستانه فروريزش و تعيين مقدار شتاب طيفي ركوردها در زمان واردشدن به محدوده آستانه فروريزش مي باشد. براي رسيدن به اين اهداف، 6 قاب 20،15 و 25 طبقه با دو مقدار برون محوري 1 و 2 متر طراحي شدند. سپس 22ركورد نزديك گسل براي تعيين نقطه آستانه فروريزش به دو روش تحليل IDA و روش طيف ظرفيت انتخاب شدند. براي بررسي تاثير خصوصيات ركوردهاي نزديك گسل بر تعيين نقطه آستانه فروريزش، ركوردهاي نزديك گسل به 2 دسته شامل داراي خصوصيت جهت پذيري پيش رونده و جابجايي ماندگار، تقسيم گرديد. سپس براي قاب هاي طراحي شده تحليل IDA انجام گرديد و نتايج براي هردسته از اين ركوردها به صورت جداگانه مورد بررسي قرار داده شده اند. نتايج نشان مي دهد مود شكست ساختمان هاي بلند مرتبه با تجمع فزاينده تغيير مكان نسبي در يك يا دو طبقه همراه است. همچنين به منظور تعيين خصوصيات ركوردهاي نزديك گسل برروي پاسخ سازه، مشخص گرديد كه ركوردهاي نزديك گسل با خصوصيت جابجايي ماندگار، در شتاب طيفي پايين تري نسبت به ركوردهاي نزديك گسل با خصوصيت جهت پذيري پيش رونده به نقطه آستانه فروريزش مي رسند.

The aims of this research are mainly to obtain the collapse preventation (CP) seismic capacity of eccentric braced high rise steel moment resisting frames under near field earthquakes and evaluate the effects of fling step and forward directivity, using incremental dynamic analysis and capacity spectrum methods. Drift ratio θ and mode acceleration specrum with 5% damping Sa(T1,5%) are selected as the appropriate damage measue (DM) and intensity measure (IM) respectively. Six eccentric steel braced frames with 3 bays and 15, 20 and 25 storeys with eccentricity of 1m and 2m are designed according to standards. The frames systems are equivalently taken as shear-flexural beams with representative lateral stiffness ratios. Twenty two earthquakes according to FEMAP695 are chosen which are divided in two groups, one with fling step effects and the other with forward directivity effects. Incremental dynamic analysis (IDA) is one of the efficient tools for estimating seismic capacity of frames. In this research, IDA is implemented for each of 6 frames using all 22 earthquakes. The frames are once subjected to multiple scaled 11 selected earthquakes with forward directivity effects, and once more they are subjected to other multiple scaled 11 selected earthquakes with fling step effects, and response parameters of frames are calculated using nonlinear dynamic analysis. Each IDA curve is ploted for Sa(T1,5%) versus maximum inter story drift ratio θ. Spectral acceleration Sa(T1,5%) is scaled until the collapse of the frames occure. IDA analysis results show that fling step and forward directivity characteristics of near field earthquakes have less effects on drift ratios compared with their effects on spectrum acceleration values. Frames under earthquakes with fling step reach CP point with lower acceleration spectrum values compared to those under earthquakes with forward directivity characteristics. Also IO and LS limit state spectrum values for frames under earthquakes with forward directivity effects are more than those values for frames under erthquakes with fling step effects. Collapse prevention (CP) points are also calculated using the capacity spectrum method. For this purpose, at first, mode shapes for elastic vibration of frames are calculated. Then each scaled shear force is distributed using the first mode of vibration and capacity curves are drawn using nonlinear static push over analysis. The push over capacity curve is then converted to capacity spectrum using first mode of vibration. Acceleration resposnse spectrum are drawn for each of 22 earthquakes using 5% damping and then converted to demand spectra. The demand spectra are then corrected for effective damping which is a value more than 5%. The corrected demand spectra for 11 near field earthquakes with fling step effects and those for other 11 earthquakes with forward directivity effects are then averaged. CP point for each frame is then obtaind from the point of conjuction of the averaged corrected demand spectra with those corresponding capacity spectra of frames. Results from capacity spectrum method show that CP values are less than thos CP values calculated from IDA analysis.