The Diagrid System

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Introduction
The term Diagrid is a portmanteau of ‘diagonal grid’ and is a technique typically used for constructing large steel buildings by creating external triangular structures with horizontal support rings. The diagonal members intersect each other and the horizontal rings at a single node. The members combine to create an element similar to a moment frame. In a Diagrid structure, gravity and lateral loads are resisted by all of the members, creating efficiency and large amounts of structural redundancy. A Diagrid structure effectively creates a rigid shell which can be thought of as a very thin, deep beam.

Some noteworthy examples of buildings that have the Diagrid incorporated are The Swiss Re building in London known as The Gherkin, The Hearst Tower in New York, The CCTV Headquarters in Beijing, The West tower in Guangzhou, The Capital Gate in Abu Dhabi.

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 facade, 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 minimising the shear effects. To begin with the behaviour of basic Diagrid module is discussed, followed by construction process. Then the advantages and disadvantages of Diagrids are listed.

The History of the Diagrid
Although the Diagrid is now seen as a modern new technology in the construction industry, the Diagrid is not necessarily a new concept. The Diagrid concept may not have been primarily related to buildings, the ideas and structural inherent found with Diagrids can be seen in the early forms of aviation. In terms of buildings it is still not a recent idea however many buildings from many years ago that have incorporated the diagrid system ended in colossal failures these where often parking buildings.

Figure 1: Diagrid technology used in other formats

The Triangular Module Diagrid
Diagrid structure is modelled as a beam, and subdivided longitudinally into modules according to this repetitive diagonal pattern. Each Diagrid module is defined by a single level of diagonals that extend over stories.

Module Geometry
Diagrid structures, like all the tubular configurations, utilise 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”. 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 bending resistance. The Diagrid being 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 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....
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