The Hearst Magazine Tower, completed in 2006, in New York City.
Attributes and terms of the diagrid. The “module” refers to the number of floors that the diamond shape of the grid spans from tip to tip. The “node” is the point of intersection of the diagonal members. A “horizontal bracing ring” is created by the connection of the diagrid nodes to the floor edge beam. The steepness of the angle of the diagrid is measured as the angle formed between the diagonal and the floor. This diagram illustrates the application of the system to a rectangular plan. This can result in large cantilevers of the corner floor areas, as the corners are without vertical columns to assume their support. (Diagram after K.S. Moon, see note 1.)
CONCEPT AND DEFINITION The term “diagrid” is somewhat misleading. Diagrid is commonly used to describe a diagonal structural grid. The system is comprised of diagonal members, normally fabricated from structural steel, that are joined at nodal points. The diagonal grid, although often presented as the dominant visual feature in the design of diagrid buildings, is by itself unstable. The diamond-shaped system requires triangulation in order to create sufficiency in the structure. Diagrids or diagonal grids are a structural design strategy for constructing buildings that combine the resistance to gravity and lateral loads into a triangulated system of members that eliminates the need for vertical columns. This system is usually placed on the perimeter of the building. Triangulation is normally achieved where the floor edge beams tie into the grid. The primary idea behind the development of the diagrid system was the recognition of the savings possible in the removal of (most of) the vertical columns. Vertical columns, engineered to carry gravity loads, are incapable of providing lateral stability. The diagonal grid, if properly spaced, is capable of assuming all of the gravity loads as well as providing lateral stability due to its triangular configuration. A pure diagrid structure does not require the traditional reinforced concrete or steel core to provide lateral stability. This will be further addressed in Chapter 7: Core Design. A diagrid tower is modeled as a vertical cantilever. The size of the modules of the diagonal grid is normally determined by evenly dividing the height of the tower. Numerous studies have been conducted toward the optimization of the module size as a function of the building height and angles of the inclined members.1 The diamond-shaped modules typically span six to eight floors tip to tip, although shorter modules are used for buildings with irregular geometries or tighter curves. Normally, the height of the base module of the diamond grid will extend over several storeys. In this way the beams that define the edge of the floors can also frame into the diagonal members, providing the required stiffness to the unsupported length of the diagonal members. Floor framing will usually connect these edge beams back to the core. As a significant portion of the expense of the structure lies in the fabrication of the nodes, efforts are toward minimizing their variation and frequency and simplifying the connection between the node and the diagonal to speed up erection. The major structural intersections occur at the nodes. The diagonal members are typically continuous from node to node-conversely inferring that they are erected as discrete members as there is a physical connection between the member and the node. This will vary from project to project as a function of the module height.
module height
horizontal bracing rings node
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