تعيين طول مفصل خميري ستون‌هاي بتن‌آرمه تحت اثر زمين‌لرزه‌هاي نزديك گسل حاوي اثر پرتابي

A Proposed Equation for the Estimation of Plastic Hinge Length of RC Columns subjected to Near-Fault Ground Motions having Fling Step

در زلزله‌هاي قوي، اعضاي بتن‌آرمه دچار تغييرشكل‌هاي خميري در نواحي معروف به نواحي مفصل خميري مي‌شوند. مفصل‌هاي خميري اساساً عاملي جهت اتلاف انرژي هستند كه انرژي ناشي از زلزله را از طريق دوران خميري تلف مي‌كنند. از اين رو دوران و طول مفصل خميري يك عضو بتن‌آرمه، پارامتري مهم در ارزيابي پاسخ و خسارت سازه‌ي تحت زلزله محسوب مي‌شود. اين موضوع خصوصاً براي زلزله‌هاي حوزه‌ي نزديك در پژوهش‌هاي پيشين كمتر مورد توجه قرار گرفته است. بدين منظور، در اين نوشتار به‌كمك تحليل‌هاي ديناميكي غيرخطي، تاثير متغيرهاي مختلف اعم از مشخصات زلزله، سطح بار محوري، نسبت ارتفاع به پهناي مقطع و درصد فولاد طولي بر رفتار غيرخطي ستون‌ها مورد بررسي قرار گرفته است. نتايج نشان داده‌اند كه بار محوري ستون نقش مهمي در تعيين طول مفصل خميري دارد. بر اساس نتايج به‌دست‌آمده، رابطه‌يي ساده جهت تعيين طول مفصل خميري ستون‌هاي بتن‌آرمه تحت اثر زلزله‌هاي حوزه‌ي نزديك حاوي اثر پرتابي پيشنهاد شده است.

In a strong earthquake, a standard reinforced concrete column may develop plastic deformations in regions often termed as plastic hinge regions. The formation of a plastic hinge in an RC column in regions that experience inelastic actions depends on the characteristics of the earthquakes as well as the column details. Recent earthquakes in Turkey (1999), Taiwan (1999), India (2001), Iran (2003), China (2008) and Japan (2011) have caused considerable loss of life and extensive damage to structures. The damage observed during the recent earthquakes proved the engineer’s hypothesis that structures located within the near-fault area suffered more severe damage than structures located outside of this zone. The near-fault of an earthquake can be defined as the area in the close vicinity of the fault rupture surface. Besides strong shaking, the characteristics of near-fault ground motions are linked to the fault geometry and the orientation of the travelling seismic. Vertical strike-slip faults can produce a directivity effect, and dip-slip faults can produce directivity effects as well as hanging wall effects. Although experimental tests of RC members provide valuable information about their behavior; these are normally expensive, time-consuming and require considerable human and physical resources. By using dynamic nonlinear finite element analysis, it is possible, at comparatively lower cost and effort, to predict the response of RC structures and members, e.g. plastic hinge length. Using the wealth of recent ground motion data, in the present paper, 462 inelastic time-history analyses have been performed to predict the nonlinear behavior of RC columns under the near-fault earthquakes having fling step. The effects of axial load and height over depth ratio, as well as different characteristics of earthquakes, are evaluated by finite element methods and results are compared with corresponding experimental data. Analytical models for columns analyzed under high axial loads exhibit longer plastic hinges than those analyzed under low axial loads. Based on the results, simple expression is proposed that can be used to estimate plastic hinge length of RC columns subjected to earthquakes. The results also show that potential lp specified by ICC is not satisfactory for columns supporting high axial loads and can even be non-conservative in some cases. It is suggested that the length of the region in which closely-spaced ties are used is increased from 1.0h to 1.5h from the joint face.