بررسي آزمايشگاهي و تحليلي تقويت اتصالات تير به ستون بتني آسيب ديده توسط ورقه هاي كامپوزيتي CFRP

Experimental and Analytical Investigation of Damaged Concrete Beam-Column Joints Strengthened by CFRP Composites

در اين مقاله اتصالات خارجي تير به ستون بتني تحت اثر بارهاي چرخه اي مورد آزمايش قرار گرفتند. پس از آسيب اوليه در نمونه ها، تقويت توسط ورقه هاي FRP انجام شده و اتصالات مجددا آزمايش شدند. سپس اتصال مبنا و اتصالات تقويت شده با FRP توسط نرم افزار Opensees مدلسازي گرديده و نتايج با رفتار آزمايشگاهي آن ها مقايسه گرديد. در ادامه تاثير عواملي همچون افزايش بار محوري ستون و وجود دال عرضي متصل به تير در حالت هاي با و بدون بار ثقلي بر روي كف بصورت تحليلي با استفاده از نتايج مدلسازي مورد بررسي قرار گرفت. با توجه به نتايج بدست آمده، با افزايش خسارت در اتصالات، ظرفيت باربري، سختي و استهلاك انرژي آنها كاهش مي يابد. همچنين خسارت در اتصالات تا سطح آسيب اوليه به ميزان 1/5% دريفت طبقه با استفاده از ورقه هاي FRP قابل جبران مي باشد. مقايسه ي پارامترهاي لرزه اي اتصالات نشان مي دهد افزايش بار محوري ستون از 5% به 10% ظرفيت فشاري اسمي مقطع ستون باعث افزايش 5 تا 12 درصدي ظرفيت باربري و سختي اتصال مي گردد. بعلاوه، افزايش بار محوري اثرات چنداني بر افزايش انرژي تلف شده ندارد. همچنين وجود دال عرضي افزايش ظرفيت و استهلاك انرژي را در پي دارد. اثرات دال عرضي در حالت بدون بار ثقلي بيش از حالت وجود بار مي باشد.

The scope of this study is to investigate the rehabilitation of concrete beam-column joints retrofitted by carbon-fiber-reinforced plastics (CFRPs), to achieve a safe and economic level of seismic damage. This paper, efficiency investigates the mentioned strengthening technique in improving the seismic behavior of damaged structures, analytically and experimentally. Four beam-column connections are tested under reversed cyclic loading. No specific seismic detail is used for connections, i.e. no transverse rebar and seismic stirrups are used in critical end zones of joint core, beam and column. Joints are damaged in different levels. Thereafter, they are retrofitted by carbon fiber reinforced materials (CFRP sheets). The strengthened joints were tested again to reach the ultimate drift capacity. The experimental results show that the beam column joints could be retrofitted by external wrapping of FRP sheets until a limited level. This level is approximately equal to 1.5% story drift for tested joints. Specimens which were initially damaged with reference to 1% and 1.5% drifts showed an increase in their capacity up to 5% and 3%, respectively. This is called the repair-ability level and for the cases with higher damage levels, other rehabilitation methods may be useful. In order to simulate the behavior of joints, a numerical model was developed in the OpenSees framework version 2.4.0. The tested joints including reference joint and retrofitted joints are analyzed by nonlinear tools of the software. The software was selected regarding the available models for concrete and reinforcement rebar materials, which are enhanced with the consideration of reloading/unloading stiffness deterioration and hysteretic energy dissipation during reversed cyclic loads. Nonlinear beam-column elements with spread or concentrated plasticity can be evaluated in this software with accurate simulation. The analytical models are used to assess the efficiency of the CFRP rehabilitation to predict an optimum level of damage that the seismic behavior parameters could be compensated, safely and economically. The results of joint analysis are compared with experimental behavior of specimens. The hysteresis curves of the modeled beam column joints had a high level of accuracy in terms of stiffness degradation, moment carrying capacity, capacity degradation and energy dissipation. Thus, the model is calibrated for each level of damage intensities. Results showed that the model had a good accuracy in terms of load carrying capacity, secant stiffness, energy dissipation and joint ductility, and the error was reported less than 10% comparing analytical and experimental results. Effect of other variables such as column axial load and the existence of transverse slab connected to the beam was analytically investigated. Results showed that increasing the axial load on the column results in increase in the load carrying capacity and stiffness from 5% to 12% (depending on the initial damage intensity of the joint). However, it had negligible effect on dissipated energy. On the other side, transverse slab modeling revealed an increase in the capacity, stiffness and energy. The positive effect was higher in the absence of gravity loads on the slab. Thus, the existence of transverse slab with gravity load had negative effect on secant stiffness in specimens with initial damage higher than 1.5% of story drift.