بررسی اثر اتصال سرد بر روی رفتار لرزه‌ای قاب خمشی بتن مسلح

یکی از مسائل رایج در اجرای درجای ساختمان های بتن آرمه، ایجاد اتصال سرد در نقاط قطع بتن ریزی می باشد. طبق آیین نامه aci 224.3r-95 محل قرارگیری اتصال سرد بر روی ستون ها می بایست در زیر تیر و بالای دال سقف باشد. اتصال سرد نوعی ضعف یا نقص در بتن محسوب شده و باعث عدم یکپارچگی بتن سازه می شود. این عدم یکپارچگی می تواند بر روی رفتار بتن و در نهایت بر روی رفتار سازه بتنی تاثیرگذار باشد. اصولاً طراحان، سازه های بتنی را براساس آیین نامه های رایج بدون لحاظ کردن اتصال های سرد و با فرض یکپارچه بودن بتن سازه طراحی می کنند. این در حالی است که در اجرا، بروز اتصال سرد در برخی نواحی خاص از سازه امری اجنتاب ناپذیر است. در این تحقیق ابتدا رفتار اتصال سرد مدل سازی گردیده است. سپس یک قاب بتن آرمه یک طبقه یک دهانه مدل سازی شده است. پس از اطمینان از صحت مدل عددی اتصال سرد و قاب، یک قاب بتن مسلح حاوی اتصال سرد بر روی ستون های آن در محل های زیر تیر و بالای پی مدل سازی شده است. سپس به منظور بررسی رفتار لرزه ای، یک بارگذاری جانبی یکنوا به صورت پوشآور یکبار به قاب با اتصال سرد و بار دیگر به قابی نظیر و یکپارچه بدون اتصال سرد، اعمال گردیده است. با مشاهده نتایج مشخص گردید که تحت بارگذاری یکنوا، وجود اتصال سرد تاثیر چندانی بر روی حداکثر نیروی جانبی قابل تحمل قاب نداشته، اما باعث کاهش حدود 30 درصدی شکل پذیری قاب مورد نظر شده است.

Investigating The Effect of Cold-Joint on Seismic Behavior of Reinforced Concrete Moment Resisting Frame

One of the common issues in the castinsitu reinforced concrete structures is creating a construction joint (cold joint) caused by an interruption or delay in the concreting operations. According to ACI 224.3R95, construction joints in columns are to be provided below the beam for lower story columns and above the floor slab for upper story columns. The cold joint is a weakness or defect in the concrete, which results in the nonintegrity of the concrete. For this reason, the performance of concrete elements with the coldjoint is under the influence of that behavior. The seismic design procedure for insitu construction generally considers that the connection of beam and column that frames into the joint is monolithic in nature. But in actual construction, it is not possible to cast columns of the multistory frame in one go and therefore, a cold joint is inevitable in all the upper story columns immediately above the lower story slab. In this research, firstly, cold joint behavior is modeled. The model of concrete damage plasticity used for the modeling the concrete behavior and the surfacebased cohesive behavior with the tractionseparation response used for the modeling the coldjoint. The threepoint bending beam specimens with the same compressive strengths of concrete on both sides of the coldjoint have been used to verify the opening mode behavior of the cold joint from the experimental results. Three different sizes of the beam were considered to ensure the validation of opening mode behavior for the cold joint. So, the pushoff test specimens have been used to verify the shearfriction behavior of the cold joint from the experimental results. Three same specimens with same compressive strengths of concrete on both sides of the coldjoint and the different number of steel connectors crossing the interface surface of the pushoff specimens were considered to ensure the validation of shearfriction behavior for the cold joint. Then, a singlestory singlebay reinforced concrete frame is modeled. After ensuring the validity of the numerical model of the cold joint and frame, a reinforced concrete frame containing a cold joint is modeled on its columns at the below of the beam and the top of the foundation. Subsequently, in order to investigate seismic behavior, an Inplane monotonic loading, strokecontrolled pushover tests were performed once on a frame containing a cold joint and once again on the same frame but without a cold joint. From the result, prior to the yield point, there was no difference between the loaddisplacement curve of the monolithic frame and frame with cold joints. In the range between the yield point and the failure point in the frame, a relatively small difference was observed between the loaddisplacement curve of the monolithic frame and frame with cold joints. A significant effect on the frame behavior was achieved in monolithic frame and frame with cold joints in their ultimate displacement so that the ultimate displacement in the coldjoint state was reduced by about 30% compared to the monolithic one. In fact, the finding results showed that under monotonic loading, the existence of a cold joint hadn rsquo;t any effect on the maximum lateral force of the frame, but reduced the ductility of it by about 30%.