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Farhangian H, Shakib H. Reduction of shear lag and improvement of performance in tubular structures. MCEJ 2019; 19 (5) :181-193
URL: http://mcej.modares.ac.ir/article-16-31731-en.html

The tubular structures are having the capability of resisting wind and earthquake loads with the exterior tube system. Tube systems consist of closely spaced exterior columns and deep beams around the corner of the plan that provides sufficient rigidity and stability for tall/high-rise buildings. Another advantage of the tubular system is the significant reduction in the building materials and increasing the architectural space in the internal plan. The mentioned cases have increased the popularity of this kind of structural system. But the most important problem in the tube system is the shear lag. Shear lag is the non-uniform distribution of stress on the face columns when the tube system is subjected to lateral loads. Shear lag can occur in any box-shaped structural system that is loaded laterally.  Shear lag increases structural displacement, limiting the use of maximum structural capacity and causing warping of the floors.
The purpose of this research is to study the effects of shear lag on the tube system and find how to reduce shear lag. In order to do this, a tubular structure is analyzed and designed based on the Capacity Design Approach and Performance Base Design with the LATBSDC, ASCE 7-16, and AISC 360-10. To evaluate the seismic performance of the tubular structure and braced tube structure, the Nonlinear Dynamic Procedure (NDP) for two ground motion intensity levels based on LATBSDC is used. Nonlinear dynamic response analyses for two earthquake ground motion intensities done and acceptability criteria demonstrated. In the next step shear lag in the designed structures is investigated. Also, the relationship between shear lag and the stiffness of the peripheral beams is studied. The result shows that increasing peripheral beam stiffness is not a good way to reduce the effects of shear lag because of economic issues. Also, other proposed methods, such as the addition of core or internal tubes, are not a suitable solution to reduce the effects of shear lag, given the high cost they impose on the project.
Due to the development process and the effect of shear lag on tubular structures, adding braces to one-fifth of the height of the structure in lower floors have been proposed to reduce the effects of shear lag. In order to calculate the shear lag, the distribution of axial stress in the tubular structure columns is considered as the basis and compared with the axial stresses of the columns in the proposed system. The proposed tube system and tube system have been analyzed by different levels of earthquakes and compared for performance purposes. In all the analysis and evaluations carried out in this study, the performance and behavior of the proposed systems were better than the tubular structure. With the addition of braces to one-fifth of the height of the structure in lower floors, the stress in the corner columns has been significantly reduced. The proposed system has been able to significantly reduce the shear lag. Therefore, in order to reduce shear lag and achieve proper behavior in tube systems, it is recommended to use the proposed systems in this study.