ارائه‌ي رابطه‌ي بين پارامترهاي لرزه‌اي حركت زمين با تغييرمكان هدف

Relationship between seismic parameters of ground motion and target displacement of structure

حداكثر حركات زمين، مدت تداوم حركات شديد و محتواي‌ فركانسي از مهمترين خصوصيات زمين لرزه‌ها است كه در خسارت لرزه‌اي تاثير بسزائي دارد. در روش‌هاي متعارف به صورت مستقيم و غير‌ مستقيم از حداكثر شتاب زمين به عنوان يك پارامتر مهم براي طراحي لرزه اي استفاده مي‌شود. هدف از تحقيق حاضر بررسي رابطه ي بين تعدادي از پارامترهاي لرزه‌اي جنبش نيرومند زمين با تغيير مكان هدف سازه كه يك شاخص مهم در ارزيابي عملكرد لرزه اي است، مي باشد. به اين منظور، ابتدا تغييرمكان هاي هدف يك سازه فولادي با سيستم قاب خمشي 9 طبقه تحت 50 زلزله با استفاده از روش طيف ظرفيت تعيين مي‌شود. سپس از مفهوم آماري ضريب همبستگي براي پي بردن به رابطه ي بين تغيير مكان هدف و پارامترهاي لرزه‌اي مربوط به زلزله‌ها، استفاده مي‌شود. نتايج بدست آمده نشان مي‌دهد كه حداكثر شتاب زمين، رابطه‌ي ضعيفتري را نسبت به بقيه پارامترها از خود نشان مي‌دهد؛ در حالي كه پارامترهاي طيفي همبستگي بيشتري دارند

Performance-based seismic design is a relatively new concept in structural engineering and earthquake engineering and is rapidly becoming widely accepted in professional practice. The basic idea of performance-based design is to design a structure so that it will “perform” in a specified manner when subjected to various earthquake levels. Therefore this design method involves a set of procedures by which a building structure is designed in a controlled manner such that its behaviour is ensured at predefined performance levels under earthquake loading. A nonlinear analysis tool is required to evaluate earthquake demands at the various performance levels. Pushover analysis is widely adopted as the primary tool for such nonlinear analysis because of its simplicity compared with dynamic procedures. The essential feature of conventional static pushover analysis is that it is a nonlinear procedure in which monotonically increasing lateral loads along with constant gravity loads are applied to a framework until a control node (usually referred to the building roof) sways to a predefined ‘target’ lateral displacement, or to a 'target' base shear, which corresponds to a performance level. The target displacement is the maximum roof displacement likely to be experienced during the design earthquake. Different performance levels have different target displacements, which represent different seismic intensities. The accurate estimation of target displacement associated with specific performance objective affect the accuracy of seismic demand predictions of pushover analysis. The target displacement for a multi degree of freedom (MDOF) system is usually estimated as the displacement demand for the corresponding equivalent single degree of freedom (SDOF) system. The accuracy of approximate procedures utilized to estimate target displacement depend on the ground motion characteristics such as peak ground acceleration (PGA), peak ground velocities (PGV), acceleration response spectra (Sa) and pseudo velocity response spectra (Sv). Observing the damages of buildings after past earthquakes showed that the damage potential has a little dependence to maximum value of a single parameter such as ground acceleration or the total event duration. In the conventional methods, peak ground acceleration, as a parameter of the earthquake event, is directly/or indirectly used for force-based design and performance-based design. Peak ground motions, duration of strong motions and frequency content are important characteristics of earthquakes that have considerable effect on the earthquake damages. Main objective of the present research is to study the relationship between seismic parameters in the frequencydomain and peak ground acceleration, and the target displacement of the structure. For this purpose, the correlation coefficient between the seismic parameters and the target displacement is calculated and then the relationship between the target displacement and seismic parameters is evaluated based on it. The achieved results show that peak ground acceleration has weak correlation with the target displacement in comparison to the other parameters; whereas frequency domain parameters have better correlation with the target displacement