استفاده از ميلگردهاي كلاهك دار نزديك به هم در اتصال هاي تير- ستون تحت بارگذاري چرخه ‌يي

Use of Closely Spaced Headed Bars in Beam-Column Joints Subjected to Cyclic Loading

در دو دهه‌ي اخير، ميلگردهاي كلاهك‌دار به عنوان يكي از مناسب‌ترين انتخاب‌ها جهت كاهش ازدحام ميلگردها در اتصال‌هاي تير- ستون بتن مسلح مطرح شده‌اند. از طرفي، آيين‌نامه‌هاي موجود از جمله آيين‌نامه‌ي 11-318ACI در مورد فاصله‌ي آزاد ميلگردهاي كلاهك‌دار و استفاده از آن‌ها در چند رديف، كاستي‌هايي دارند. به علاوه، نياز به داده‌هاي آزمايشگاهي بيشتري در اين زمينه احساس مي‌شود. از طرف ديگر، اثر توام پارامترهاي طراحي ميلگردهاي كلاهك‌دار ممكن است پاسخ اتصال را تحت تاثير قرار دهد. در اين پژوهش، 4 نمونه‌ي اتصال خارجي تير- ستون با تركيب‌هاي متفاوت طول مهار، فاصله‌ي آزاد، و چيدمان متفاوت ميلگردهاي كلاهك‌دار براساس ضوابط لرزه‌‌يي ويژه با مقياس 2:3 اندازه‌ي واقعي ساخته شده و مورد آزمايش چرخه‌‌يي قرار گرفته‌ و رفتار آن‌ها براساس معيار‌هاي پذيرش براي قاب‌هاي خمشي تحت شرايط ويژه‌ي لرزه‌يي ارزيابي شده است. نتيجه‌ي بررسي‌ها نشان داده است كه اتصال‌هاي آزمايش‌شده با ميلگردهاي با فاصله‌ي آزاد كمتر از مقدار تعيين‌شده در آيين نامه ها، رفتار مناسب لرزه‌يي نشان داده‌اند.

Generally, a conventional 90-degree standard hook is used for the anchorage of reinforcing bars terminating within a beam-column joint. It is well accepted that hooked bars should be bent into the joint with the hook embedded as far as possible from the critical section. This requirement and the specified dimensions of standard hooks in the ACI 318 Code, however, often cause steel congestion in an exterior beam-column joint, thereby, making fabrication and construction difficult. Using headed bars offers a potential solution to this problem and can also ease fabrication, construction and concrete placement. Relevant provisions and limitations of headed deformed bars have been provided in the ACI 318-11 Code. Over the last two decades, many experimental studies have been carried out to evaluate the provisions of the ACI 318 Code for anchored headed bars. There is still a need for additional data for various design parameters, such as headed-bar clear spacing and the number of layers of beam reinforcing bars. In this regard, the current study investigates the applicability of closely spaced headed bars in the exterior beam-column joints of earthquake resistant structures. Four 2/3-scale exterior beam-column joint specimens were manufactured and tested under cyclic lateral loading. The variables in different specimens include clear spacing and the number of layers of the headed bars of the beam, and also the development length of the headed bars. The seismic performance of the tested beam-column joints is qualitatively evaluated based on the acceptance criteria in ACI374.1–05 entitled: “Acceptance Criteria for Moment Frames Based on Structural Testing and Commentary”. Evaluation of the specimens’ load-displacement hysteretic behaviour and comparison of the test results with ACI 374.1-05 criteria, show that beam-column joints with closely spaced headed bars can exhibit satisfactory performance and adequate anchorage capacity. Test results also indicate that headed bars with a development length shorter than that provided by the ACI 318-11 Code can be effectively anchored in exterior beam-column joints under inelastic deformation reversals.