بررسي اثر اتصال سرد بر روي رفتار لرزه‌اي قاب خمشي بتن مسلح

يكي از مسائل رايج در اجراي درجاي ساختمان­هاي بتن­آرمه، ايجاد اتصال سرد در نقاط قطع بتن­ريزي مي­باشد. طبق آيين­نامه ACI 224.3R-95 محل قرارگيري اتصال سرد بر روي ستون­ها مي بايست در زير تير و بالاي دال سقف باشد. اتصال سرد نوعي ضعف يا نقص در بتن محسوب شده و باعث عدم يكپارچگي بتن سازه مي­شود. اين عدم يكپارچگي مي­تواند بر روي رفتار بتن و در نهايت بر روي رفتار سازه بتني تاثيرگذار باشد. اصولاً طراحان، سازه­هاي بتني را براساس آيين­نامه­هاي رايج بدون لحاظ كردن اتصال­هاي سرد و با فرض يكپارچه بودن بتن سازه طراحي مي­كنند. اين در حالي است كه در اجرا، بروز اتصال سرد در برخي نواحي خاص از سازه امري اجنتاب­ناپذير است. در اين تحقيق ابتدا رفتار اتصال سرد مدل­سازي گرديده است. سپس يك قاب بتن­آرمه يك طبقه يك دهانه مدل­سازي شده است. پس از اطمينان از صحت مدل عددي اتصال سرد و قاب، يك قاب بتن مسلح حاوي اتصال سرد بر روي ستون­هاي آن در محل­هاي زير تير و بالاي پي مدل­سازي شده است. سپس به منظور بررسي رفتار لرزه­اي، يك بارگذاري جانبي يكنوا به صورت پوش-آور يكبار به قاب با اتصال سرد و بار ديگر به قابي نظير و يكپارچه بدون اتصال سرد، اعمال گرديده است. با مشاهده نتايج مشخص گرديد كه تحت بارگذاري يكنوا، وجود اتصال سرد تاثير چنداني بر روي حداكثر نيروي جانبي قابل تحمل قاب نداشته، اما باعث كاهش حدود 30 درصدي شكل­پذيري قاب مورد نظر شده است.

One of the common issues in the cast-in-situ reinforced concrete structures is creating a construction joint (cold joint) caused by an interruption or delay in the concreting operations. According to ACI 224.3R-95, 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 non-integrity of the concrete. For this reason, the performance of concrete elements with the cold-joint is under the influence of that behavior. The seismic design procedure for in-situ 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 multi-story 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 surface-based cohesive behavior with the traction-separation response used for the modeling the cold-joint. The three-point bending beam specimens with the same compressive strengths of concrete on both sides of the cold-joint 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 push-off test specimens have been used to verify the shear-friction behavior of the cold joint from the experimental results. Three same specimens with same compressive strengths of concrete on both sides of the cold-joint and the different number of steel connectors crossing the interface surface of the push-off specimens were considered to ensure the validation of shear-friction behavior for the cold joint. Then, a single-story single-bay 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 In-plane monotonic loading, stroke-controlled 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 load-displacement 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 load-displacement 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 cold-joint 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’t any effect on the maximum lateral force of the frame, but reduced the ductility of it by about 30%.