مطالعه اثر رانش معکوس در سازه‌های بلند دارای هسته مقاوم بتن‌آرمه با در نظر گرفتن اثر تغییر شکل‌های برشی

نوع مقاله : Articles

نویسندگان

1 دانشکده مهندسی عمران، دانشگاه سمنان

2 دانشکده مهندسی عمران، دانشگاه فردوسی مشهد

چکیده

سیستم هسته مقاوم بتن‌آرمه به دلیل داشتن سختی و مقاومت بالا، یکی از سیستم‌های جانبی مقاوم سازه‌ای مناسب و کارآمد برای مقابله با نیروهای جانبی باد و زلزله در سازه‌های با ارتفاع بلند محسوب می‌شود. از سوی دیگر در طراحی سازه‌های بلند وجود یک یا چند طبقه زیرزمین به دلیل ملاحظات معماری و یا سازه‌ای امری است که در اکثر ساختمان‌ها مشاهده می‌گردد. وجود طبقات تحتانی زیر تراز زمین که توأم با به‌کارگیری دیوارهای بتن‌آرمه پیرامونی به‌عنوان دیوارهای حائل، برشی و یا هر دو عملکرد می‌باشد در کنار وجود هسته مقاوم بتن‌آرمه باعث بروز پدیده‌ای به نام اثر رانش معکوس در این‌گونه سازه‌ها می‌شود. این پدیده که بر اثر وجود سختی زیاد دیوارهای بتن‌آرمه پیرامونی همراه با دیافراگم تراز زمین حادث می‌گردد باعث می‌شود که در هنگام اعمال نیروهای جانبی و انتقال آن به هسته یک نیروی معکوس از طرف دیافراگم ذکر شده به هسته وارد شده و عملکرد آن را در این ناحیه تحت تأثیر شدید قرار دهد. مرور تحقیقات پیشین بیانگر آن است که نسبت‌های سختی و نسبت‌های ابعادی ریشه ساختمان نقش مستقیم در میزان نیروی به وجود آمده دارند. در این تحقیق به بررسی نقش تغییر شکل‌های برشی هسته در پدیده ذکر شده پرداخته و با بررسی این عامل روابطی را برای برآورد این نیرو پیشنهاد نموده است. روابط تحلیلی به‌دست‌آمده با نتایج حاصل از یک تحلیل عددی مورد مقایسه قرار گرفته و حاکی از آن است که در موارد کاربردی صرف‌نظر نمودن از تغییر شکل‌های برشی خطای بزرگی در نتایج ایجاد نموده و لذا لحاظ نمودن اثر تغییر شکل‌های برشی با استفاده از روابط به‌دست‌آمده قابل توصیه می‌باشد.

کلیدواژه‌ها


عنوان مقاله [English]

Study of Backstay Effect in Tall Buildings with Core-Wall System by Involving of Shear Deformation

نویسندگان [English]

  • Mahdi Karimi 1
  • Ali Kheyroddin 1
  • Hashem Shariatmadar 2
1 Faculty of Civil Engineering, Semnan University, Iran
2 Structural Engineering Department, Faculty of Civil Engineering, Ferdowsi University of Mashhad, Iran
چکیده [English]

Introduction Regarding to the efficiency of core-wall resisting system, it has become one of the most widely used structural resisting system. On the other hand, existence surrounding walls at subterranean levels together with diaphragm of grade level cause to comprise a stiff concrete box. This stiff box can create a large force at diaphragm of grade level whenever a lateral load is imposed to the structure. Due to a largeness of mentioned generated force, it may reverse the internal shear force of below grade in the core-wall. This phenomenon is often recognized as “Backstay Effect”. Some literatures such as PEER/ATC 72-1 have been prescribed various certain values for upper and lower bound of stiffness of effecting components. By utilization of these specified values, the aforesaid structural components must be designed for all the critical conditions. A study was performed by Karimi and Kheyroddin determined various limit states of backstay effect. These limit states were investigated by various boundary condition assumptions for core-wall support and ground level diaphragm. Another research was performed by these authors, presented a relationship for prediction of backstay effect; however, this relationship was not considering of shear deformation that may be important for mentioned investigation. Involving Shear Deformation This paper is focused on involving the shear deformation of the core-wall in backstay formulation. Large depth of section to the length ratio of a frame element causes to increase the contribution of the shear deformation in total deformation. Therefore, due to the largeness of the core-wall section dimensions relative to embedment length of structure in the ground, accounting of this aforementioned impact must be considered. In this research, the effect of shear deformation is comprised as a parameter named β. The β parameter is related to a shape of the element section and the length of that element, and can be calculated from the Elasticity of Material science. This parameter was obtained for a core with a shape of square thin walled section and then the aspect ratio of core width to the subterranean height is related to the generated force at the diaphragm of grade level. All the parameters exist in the presented formulas are dimensionless, that make convenient for the usage of them. Results and Discussion Obtained formula is depicted in the form of some curves as a function of an aspect ratio of the subterranean height to the core width at different individual stiffness ratios (the stiffness of a concrete box relative to the stiffness of a core-wall). Investigation of these curves (or main formula) shows when the value of stiffness ratios is very high; the result of obtained formula is closed to a limit state of the obtained results of the previous research. Results show that considering of shear deformation cause to decrease the core-wall stiffness and also decrease generated force at diaphragm of grade level. Furthermore, concerning shear deformation is quite important for a low ratio of the embedment length to the dimension of core-wall section. If this ratio is bigger than about 10, the effect of shear deformation is not considerable.
Besides, for verification of the achieved formula, a numerical case study is performed. For this purpose, a building of 21 stories with a core-wall resisting system is investigated. This building has one story of basement and 20 stories of the superstructure with a quadrilateral typical plan (five bays of 6 m in each side). The core-wall section with a square shape of 6 by 6 m is placed at the centre of the plan. A notable point that must be considered at modelling time is not using the rigid diaphragm constraint at the diaphragm of grade level. Analysing of the explained mentioned building by ETABS program and comparing its result to the obtained result from the proposed formula showed a good acceptable match.

کلیدواژه‌ها [English]

  • Core-Wall System
  • Stiff Concrete Box
  • Backstay Effect
  • Shear deformation
  • Grade Level
  1. Kheyroddin, A. and Aramesh, S. (2012) Lateral Resisting Systems in Tall Buildings. Semnan University Press, Semnan, Iran (in Persian).
  2. Bryan, S.S. and Coull, A. (1991) Tall Building Structures: Analysis And Design. John Wiley & Sons, New York.
  3. Tocci, N. and Levi, S. (2012) Basement modeling in tall buildings: backstay effect. Structure Magezine, June, 23-24.
  4. Karimi, M. and Kheyroddin, A. (2016) Study of backstay effect in tall buildings and presentation of governed relationships of structural behavior from this viewpoint. Proceedings of the 2nd National Conference on Iranian structural Engineering, Iran, Tehran, Amirkabir University (in Persian).
  5. Moehle, J. (2015) Seismic Design of Reinforced Concrete Buildings. McGraw-Hill Education, New York.
  6. Karimi, M. and Kheyroddin, A. (2015) Intrudoction and study of backstay effect in high-rise core-wall buildings. Iranian Concrete Institute, 59, 34-42 (in Persian).
  7. Adebar, P. (2008) Design of high-rise core-wall buildings: a canadian perspective. Proceedings of the 14th World Conference on Earthquake Engineering, China, Beijing.
  8. Rad, B.R. and Adebar, P. (2009) Seismic design of high-rise concrete walls: reverse shear due to diaphragms below flexural hinge. ASCE Journal of Structural Engineering, 135(8).
  9. LATBSDC (2014) An Alternative Procedure for Seismic Analysis and Design of Tall Buildings Located in the Los Angeles Region. Los Angeles Tall Buildings Structural Design Council, Los Angeles, USA.
  10. PEER/ATC-72-1 (2010) Modeling and Acceptance Criteria for Seismic Design and Analysis of Tall Buildings. Applied Technology Council, Redwood City, California.
  11. Kheyroddin, A. and Emami, E. (2016) Shear Walls. Semnan University Press, Semnan, Iran (in Persian).
  12. Gere, J.M. and Timoshenko, S.P. (Ed.) (1991) Mechanics of Materials. Springer-Science+ Business Media, UK.
  13. Office of Iranian National Building Regulations (2014) Iranian National Building Code, Devision 9: Design and Construction of Concrete Structures. Tehran, Iran (in Persian).
  14. PEER/TBI (2017) Guidelines for Performance-Based Seismic Design of Tall Buildings. Pacific Earthquake Engineering Research Center as part of the Tall Buildings Initiative, Headquarters at the University of California, Berkeley.