The inspiration for this study arises from the impact that the construction industry has in the environment, in terms of use of resources and production of waste, and the social need that calls for investigating sustainable solutions. Tall building structural design developed rapidly in the last decades. One of the evocative structural design solutions is embraced by the diagrid (diagonal grid) structural scheme [1].The diagrid structural scheme (with a perimeter structural configurations characterized by a narrow grid of diagonal members involved both in gravity and in lateral load resistance) has emerged as a new design trend for tall-shaped complex structures due to aesthetics and because it requires less structural steel than a conventional steel frame. This study focuses on the structural assessment of diagrid structures, using FEM nonlinear analyses. In particular, numerical comparisons are performed between an ordinary high-rise building (outrigger) and three comparable in size diagrid structures.The aim is to identify the configuration that minimizes the structural weight guaranteeing a suitable structural behaviour in terms of safety and serviceability. In the conceptual design, comparisons are performed to select the most efficient structural system and to reduce the material used, between two proposed different structural design solutions: outrigger and diagrid. The considered structure is a 40-story building, for a total height of 160 m, and a footprint of about 36 m × 36 m. Its function is for not-public offices. The outrigger structure has been realized in order to make a diagonal bracing system resisting horizontal wind actions. To reduce the building deformability, a rigid plane is introduced. This plane (reinforcement) is called outrigger and located at the 29th floor. It is realized by introducing braces expanded vertically for all facades. These outriggers are located on two facades at Y={4, 31} and on the other two facades at X={4, 31} m. The diagrid structures are all subdivided longitudinally into 4-story modules according to the repeated diagrid pattern. The optimal angle of diagonals is highly dependent on the building height. Since the optimal angle of the columns for maximum bending rigidity is 90 degrees and that of the diagonals for maximum shear rigidity is about 35 degrees, it's expected that the optimal angle of diagonal members for diagrid structures will fall between these angles. This study introduces three intermediate angles: 42, 60 and 75 degrees (figure 1 ).The diagrid structures have been modeled with two structural systems working in parallel: the first is internal and consists of a rigid frame system that only reacts to gravity loads, while the second is perimetral and it is made of a diagonal grid system which reacts both to vertical and horizontal loads. For additional modelling details, sustainability aspects and robustness analyses see [2] and [3]. In this study the weight reduction is considered as the most significant aspect from a sustainability point of view. For all diagrid buildings an important weight saving occurs, therefore, in all cases the diagrid system results better than the ordinary outrigger for what regards sustainability.
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