توسعه‌ي روش طيف ظرفيت و تعيين تغييرمكان هدف در الگوي بار متناسب با جرم براي پل‌هاي داراي انحنا در پلان

Development of Capacity Spectrum Method and Determining Target Displacement for Mass Proportional Load Pattern in Horizontally Curved Bridges

در نظرگرفتن الگوهاي بار جانبي و نحوه‌ي تعيين تغييرمكان هدف در مورد پل‌ها در تحليل‌هاي استاتيكي غيرخطي فزاينده، همواره يكي از مسايل مورد توجه پژوهشگران و الگوهاي بار متداول آيين‌نامه‌يي، الگوي بار متناسب با شكل مود اصلي سازه و الگوي بار جانبي متناسب با جرم بوده است. در همين زمينه، يكي از روش‌هاي مورد استفاده جهت تعيين تغييرمكان هدف براي الگوي بار جانبي متناسب با شكل مود اصلي سازه، استفاده از روش طيف ظرفيت است. در اين نوشتار، يك روش پيشنهادي درخصوص رسم طيف ظرفيت در الگوي بار متناسب با جرم ارايه شده است. نتايج استفاده از روش پيشنهادي در تحليل استاتيكي غيرخطي فزاينده جهت رسم طيف ظرفيت و تعيين تغييرمكان هدف در يك پل داراي انحنا در پلان، نشان‌دهنده‌ي كارايي روش پيشنهادي در تعيين تغييرمكان هدف در نقاط كنترل در مقايسه با نتايج تحليل ديناميكي فزاينده است.

Considering appropriate lateral load pattern and determining target displacement for bridges in nonlinear static pushover (NSP) analysis have been important issues for researchers. There are two conventional load patterns in the standard codes of practices which are called:1- Mode shape proportional load pattern, 2-Mass proportional load pattern. Capacity Spectrum Method (CSM) is one of the approaches applied in determining target displacement of structures usually using the first mode proportional load pattern. In conventional (CSM) method for determining the spectrum capacity curve during NSP analysis, the base shear of the multi degrees of freedom system versus displacement of the control point is drawn using the first mode proportional load pattern, and then is converted to spectral displacement and spectral acceleration format. Converting base shears and control point displacements into spectral accelerations and displacements is based on the assumption that the dynamic behavior is dominated with only one of the natural vibrational mode shapes which is assumed to be constant and not changing during the (NSP) analysis. Assessing this assumption for horizontally curved bridges shows that the lateral load patterns are effected with combination of modes and are not proportional to one especial mode shape. In this context, mass proportional load pattern is usually used to consider the effects of some special pre-specified mode shapes in (NSP) analysis. In this research, the (CSM) method is further developed and a method is proposed for determining target displacements using the mass proportional load pattern. In the proposed method, the capacity spectrum is drawn using mass proportional load pattern based on the displacement vector in the linear range instead of some specified mode shapes. Therefore, the developed proposed method is not dependent on some pre-specified modes of vibration and the effects of all modes of vibration are considered simultaneously. The efficiency of the proposed method is proved by determining target displacements of control points on a horizontally curved bridge using the proposed method and then comparing the results with the results of incremental dynamic analysis (IDA).