Design and construction of artificial infrastructure on the lines of biomimicking principles requires the development of highly advanced structural systems which has the qualities of aesthetic expression, structural efficiency and most importantly geometric versatility. Diagrids, the latest mutation of tubular structures, have an optimum combination of the above qualities.
In this paper, the peculiarities of the Diagrid, its structural behavior under loading and the design and construction of diagrid nodes are described. A case study of some recent diagrid tall buildings, namely the Swiss Re Building in London, the Hearst Tower in New York, and the West Guangzhou Tower in china is also presented.
The Diagrids are perimeter structural configurations characterized by a narrow grid of diagonal members which are involved both in gravity and in lateral load resistance. Diagonalized applications of structural steel members for providing efficient solutions both in terms of strength and stiffness are not new ,however nowadays a renewed interest in and a widespread application of diagrid is registered with reference to large span and high rise buildings, particularly when they are characterized by complex geometries and curved shapes, sometimes by completely free forms.
Among the large-span buildings some examples are represented by the Seatlle Library, the London City Hall, the One Shelley Street in Sydney, and more recently by several outstanding Pavilions realized at the Shanghai 2010 Expo, (e.g. France, UAE) as well as by some dazzling projects like the Astana National library. Among tall buildings, noteworthy examples are the Swiss Re building in London, the Hearst tower in New York, the CCTV headquarters building in Beijing, the Mode Gakuen Spiral Tower in Aichi, the Cyclone Tower in Asan, the West tower in Guangzhou, the Lotte super tower in Seoul, the Capital Gate in Abu Dhabi, the Bow project in Calgary, the Building of Qatar Ministry of Foreign Affairs in Doha.
. The diagrid systems are the evolution of braced tube structures, since the perimeter configuration still holds for preserving the maximum bending resistance and rigidity, while, with respect to the braced tube, the mega-diagonal members are diffusely spread over the façade, giving rise to closely spaced diagonal elements and allowing for the complete elimination of the conventional vertical columns. Therefore the diagonal members in diagrid structures act both as inclined columns and as bracing elements, and carry gravity loads as well as lateral forces; due to their triangulated configuration, mainly internal axial forces arise in the members, thus minimizing shear racking effects. To begin with the behavior of basic Diagrid module is discussed, followed by construction process. Then the merits and demerits of Diagrids are listed.
Diagrid structures, like all the tubular configurations, utilize the overall building plan dimension for counteracting overturning moment and providing flexural rigidity through axial action in the diagonals, which acts as inclined columns; however, this potential bending efficiency of tubular configuration is never fully achievable, due to shear deformations that arise in the building “webs”; with this regard, diagrid systems, which provide shear resistance and rigidity by means of axial action in the diagonal members, rather than bending moment in beams and columns, allows for a nearly full exploitation of the theoretical bending resistance. Being the diagrid a triangulated configuration of structural members, the geometry of the single module plays a major role in the internal axial force distribution, as well as in conferring global shear and bending rigidity to the building structure. While a module angle equal to 35° ensures the maximum shear rigidity to the diagrid system, the maximum engagement of diagonal members for bending stiffness corresponds to an angle value of 90°, i.e. vertical columns. Thus in diagrid systems, where vertical columns are completely eliminated and both shear and bending stiffness must be provided by diagonals, a balance between this two conflicting requirements should be searched for defining the optimal angle of the diagrid module. Usually Isosceles triangular geometry is used.[/justify]