Shear lag phenomenon in the tubular systems with outriggers and belt trusses

For centuries, mankind have been mesmerized by the notion of building tall structures, which developments in the domain of civil engineering made this dream feasible. By passing the time, different types of lateral resistant systems for high-rise buildings were introduced, as in: tubular systems (tube-in-tube, braced tube and others), braced tall buildings, moment resisting systems, suspended tall buildings with a concrete core and so on. The lateral-resisting system is an important part of the structural system that supports the building against the lateral loads including: wind and earthquake loads. The majority of the lateral-resisting systems can be categorized into three types: 1) shear wall systems, 2) frame systems, and 3) the combination of the previous two systems. Shear wall systems are rather common choices in many earthquake-prone countries. They provide high strength and stiffness simultaneously in the direction of their placement. These structural elements are vertical ones which endure lateral loads in their plane. Core wall systems (shear walls) can reduce the lateral displacement by the core bending resistance and their displacement mode is flexural. However, these systems have a conspicuous problem which is putting the lateral resisting elements close to the neutral axes. This reduces their efficiency because they do not absorb a considerable amount of tensions and their bending rigidity index is low. Then, Fazlur Khan suggested the resisting core had to be put on the perimeter of a structure in which the normal stresses are greater, instead of being close to neutral axes [1]. Besides, their bending rigidity index increases by putting resisting elements far from the central axes. This new system was called a tubular (tube) system. The layout of this system can be rectangular, triangular or square. This system can be recognized as an evolved form of the flexural frames. By passing the time, this system changed gradually into the bundled tube, braced tube and tube-in-tube ones to get rid of the problems of the first generation system of tubular systems. In simple terms, a tube system can be defined as a three-dimensional system that utilizes the entire building perimeter to resist lateral loads. One of the most important deficiencies of the tubular systems is the shear lag phenomenon. The influence of shear lag is to increase axial stresses in the corner columns and reduce the same ones in the inner columns of both the flange and the web panels, as shown in Figure 1. This decreases the moment resistance and Инженерно-строительныйжурнал , No 2(86), 2019Arshadi, H., Kheyroddin, A.bending rigidity of structures. Furthermore, the designers intend to design all the exterior columns as typically as the same as the critical one and this leads to losing materials and money. There are many strategies to overcome this problem: using bundled tube systems, mega bracings, deep spandrel beams, and mega columns at the corner of the structures. However, the most efficient is using the spandrel beams (shear rigidity index), because they distribute forces between columns more uniformly. Then the more shear rigidity index, the less shear lag is observed. It is worth mentioning that the belt trusses (along with the outriggers) can be used to prevent the rotation of the internal tube, decreasing its drift and moment, diminishing shear lag and so on, they also connect exterior columns to each other. The outriggers connect them to the columns of internal tube. These two can distribute forces among columns more uniformly.