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

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.