تحليل اتصال مركزگراي پاي ستون با استفاده از ميلگرد پس كشيده در سازه هاي فولادي

Parametric Study of Seismic Performance of Post-Tensioned Self-Centering Column Base Connections

هدف از اين مطالعه، بررسي تاثير پارامترهاي مختلف بر روي رفتار لرزه‏اي اتصال پاي ستون با استفاده از ميلگرد پس‏كشيده در سازه‏هاي فولادي مي‏باشد. استفاده از اين نوع اتصال در پاي ستون سازههاي فولادي باعث ميشود، مفصل پلاستيك پاي ستون كه عامل مخربي براي سازهي فولادي ميباشد، حذف گرديده و تغييرشكلهاي پلاستيك در ميراگر انرژي بوجود آيد. ميلگردهاي مورد استفاده در اين اتصال داراي مقاومت بالا بوده و نيروي پس‏كشيدگي اوليه در ميلگردها مي‏تواند سازه را به حالت قبل از زلزله بازگرداند. ‏به منظور به‏دست آوردن رفتار اتصال، اتصال مورد نظر با استفاده از نرم‏افزار مهندسي زلزله OpenSees مدل شده است. مدل ساختهشده در اين نرمافزار تحت اثر بارگذاري چرخهاي ذكرشده در آييننامهي AISC قرار گرفته است. نتايج تحليلها نشان ميدهد،كه اتصال علاوه بر استهلاك مناسب انرژي زلزله، هيچگونه تغييرمكان نسبي باقيمانده در سازه بهجاي نميگذارد. در اين تحقيق تاثير قطر ميلگرد پس‏كشيده، ميزان نيروي پسكشيدگي ميلگردها و همچنين ابعاد مختلف ميراگر انرژي بر عملكرد اتصال مد نظر مي‏باشد. نتايج به دست آمده نشان ميدهد كه افزايش قطر و نيروي پسكشيدگي ميلگرد باعث افزايش نيروي مورد نياز براي ايجاد بازشدگي ميگردد. افزايش ابعاد ميراكننده انرژي باعث افزايش نيروي مورد نياز براي ايجاد بازشدگي و افزايش ميزان استهلاك انرژي توسط اتصال ميگردد. همچنين به منظور جلوگيري از ايجاد مفصل پلاستيك پاي ستون و خرابي ايجاد شده در اعضاي اصلي سازه، ميتوان اين نوع اتصال را جايگزين اتصالات متعارف پاي ستون نمود.

The main objective of this study is to introduce and evaluate the seismic behavior of post tensioned column base connection in self centering steel moment resisting frames (SCMRFs). The components of the PT column base connection include PT high strength bars for clamping and restoring forces, buckling restrained steel plates for energy dissipation and damping, reinforcing plates for increasing shear bearing and shear yielding resistance, and keeper plates for increasing shear resistance. Buckling restrained plates and keeper plates are welded together. The PT bars are anchored between half-height of the first story column and that of basement column. Structural damage at column bases in the SC-MRF is likely to occur due to yielding of the columns. Columns in the SC-MRF are continuous and thus the bottoms of the columns are high demand locations in terms of moment, shear and axial force. Therefore, the column bases in the SC-MRF may suffer damage (e.g., plastic hinging) under a major earthquake. This damage results in permanent residual deformations of columns affecting the self-centering capability of the SCMRF system. To eliminate the structural damage in columns as well as to enhance the self-centering behavior of the SC-MRF, a PT column base connection is introduced for use at column bases. By using this connection, the plastic hinge in column bases is removed and plastic deformation is shifted into energy dissipation device. A posttensioned column base connection is modeled in OpenSees software (i.e., Open System for Earthquake Engineering Simulation) to evaluate the seismic behavior of this connection. The materials used in the model of connection are: Steel01 for beam and column, Elastic perfectly plastic for Post tensioned bar, Elastic-no-tension for modeling gap opening and Steel02 for energy dissipation device. Nonlinear beam column element was used for beam and column element. Also Truss element was used for post tensioned bars, gap opening and energy dissipation device. The cyclic loading was applied to this connection. Based on the analysis results, it was found that the introduced connection dissipate energy of earthquake without any residual drift. The effect of post-tensioned bars diameter, level of post tensioning and energy dissipation device size in connection behavior were investigated. The initial stiffness of connection at all component is equal to conventional connection. Buckling restrained plates size affects energy dissipation of connection. When the buckling restrained plate size increases, the amount of energy dissipation is increased. The post yielding stiffness of connection in this condition increased. When the post-tensioning level of bars grows, the compression moment for development of gap opening at grade beam interface and post yielding stiffness of connection increased. The increase of diameter of post-tensioned bars also increases the stiffness of connection.