كليدواژه :
نسبت نياز به ظرفيت خمشي , ضريب بزرگنمايي تغيير مكان , ستون بتن مسلح , سطوح عملكرد
چكيده فارسي :
در اين مقاله به بررسي ضوابط طراحي براساس استاندارد 2800 و متناسب نمودن طراحي با سطح عملكرد انتخابي پرداخته شده است. براي انجام اين بررسي 6 قاب خمشي بتن مسلح سه دهانه با شكل پذيري متوسط و تعداد طبقات 4، 5، 6، 8، 10 و 12 در نرم افزار IDARC مدلسازي و تحليل استاتيكي غيرخطي و ديناميكي غيرخطي با استفاده از 7 شتاب نگاشت بر روي آنها انجام شده است. نسبت نياز به ظرفيت خمشي هريك از ستون ها و سطح عملكرد سازه محاسبه شده و سپس با استفاده از نتايج حاصل شده معيار كنترل تغييرمكان نسبي و همچنين ضريب بزرگنمايي تغييرمكان نسبي در ويرايش سوم و چهارم استاندارد 2800 مورد نقد و بررسي قرار گرفته است. بدين منظور نسبت تغييرمكان غيرارتجاعي حاصل از تحليلهاي غيرخطي به تغييرمكان ارتجاعي تعيين و ضريب (Cd) براي هريك از قابها محاسبه شد. سپس نتايج حاصل با ضريب بزرگنمايي استاندارد 2800 (0/7R) مقايسه و به مقدار 0/5R اصلاح گرديد. در نهايت نظر به اينكه ويرايش چهارم استاندارد 2800 در حال ابلاغ ميباشد، به منظور بررسي تغييرات ايجاد شده، يك نمونه از قابها با ضوابط پيش نويس نهايي استاندارد 2800 و همچنين با استفاده از ضريب بزرگنمايي كاهش يافته (0/5R) مجددا طراحي و تغييرات ايجاد شده در پاسخ سازه مورد مطالعه قرار گرفت. نتايج بدست آمده حاكي از آن است كه كاهش محدوديت تغييرمكان نسبي موجب كاهش ابعاد مقاطع، كاهش تمركز آسيب در طبقه اي خاص از سازه، طراحي واقع بينانه تر سازه و در نهايت نزديك شدن سطح عملكرد سازه به هدف عملكردي استاندارد 2800 ميشود.
چكيده لاتين :
In this study, the design criteria of Iranian Seismic Standard (2800) is investigated in order to make the design results more proportional with the performance levels. To have a better proportionality between structure performance level and performance targets of Iranian Standard, the parameters of inelastic drift of structure are regulated. In seismic design of structures, estimating maximum inelastic lateral displacement occurring in the sever earthquake is of great importance. In most seismic design provisions, maximum
inelastic displacement of structure is estimated by amplifying the lateral displacement computed using an elastic analysis with a displacement amplification factor (Cd). Reviewing several seismic design provisions indicates that in most cases, Cd only depends on the seismic force resisting system. To embark on the aim of this study, 6 RC frames with intermediate ductility and 4, 5, 6, 8, 10 and 12 stories are adopted for numerical modeling. For determination of real drift occurring in major earthquake (inelastic drift), nonlinear time history analysis and nonlinear static analysis (pushover analysis) utilizing IDARC computing program are performed. For linear analysis, equivalent static procedure is employed using ETABS program. In nonlinear time history analysis, seven earthquake ground motions -consistent with soil type-II of Iranian Standard- are used. These records are scaled according to the same standard requirements. In addition, four types of lateral loading patterns are used in pushover analysis consisting of triangular distribution, generalized power distribution, uniform distribution and modal distribution. Performance level of structure elements is obtained based on the mentioned analysis. Bending forces of each column are calculated and compared with the strengths calculated based on the Iranian concrete code of practice (ABA). The inelastic displacements that are computed by nonlinear analysis are then divided by elastic displacements in order to determine the Cd parameter for each story of the 6 frames. The results are compared with magnification factor of 3rd edition of Iranian Standard (0.7R), resulting in a proposed improved value of 0.5R. In investigated frames, most of the columns are in IO performance level, while only a small percentage of them have reached the level of LS, under the pushover and time history analysis. The results indicate that lower criteria limitations are needed to control the drifts and their effects on the structural performance level. In this research, Cd is considered as a function of R (structural behavior factor), like most researches and provisions.
Finally, in order to investigate the changes in 4th edition of Iranian Standard, mentioned frames are designed based on the 4th edition criteria and then the structural responses are evaluated. The results indicate that in the frames design by 4th edition of the Iranian Standard, demand capacity ratio of bending in the upper stories of frame increases while there is no significant change in the lower stories. Furthermore, the reduction in drift criteria limitations leads to reduction in section dimensions, damage concentration in a specified story, total structure damage index and also more contribution of section capacity. The latter will result in convergence of structure performance level and Iranian Standard performance target.
