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Diagrid Structures

 

A Diagrid system is a complex frame system, which has been implemented in construction of buildings for aesthetical and structural reasons. The most common system uses are with skyscrapers, bridges and sculptures. As such, diagrids have a limited number of materials that can be used in their construction. They are mostly constructed form structural steel, wood, or reinforced concrete.  Structural steel is used for tall buildings (40 stories or more), reinforced concrete can be also used for tall buildings, but it does not have strength of steel. Application of wooden diagrid systems can be found in interior design, ceilings and canopies. However, using wood and timber is not advisable when we are talking about high buildings, such as skyscrapers. Wood does not have enough strength to support dead load.

In current standard high rise buildings, diagrid systems use an exterior frame comprised exclusively of diagonal members as the primary means of support. When we are comparing diagrids with other structural building systems we can see that the amount of steel used for a diagrid is much smaller, which makes them more efficient. However, that does not mean the price of the construction of a diagrid system is cheaper. Using a diagrid frame system, engineers and architects can produce column free interiors, which would eliminate columns between the exterior structure and the core (same structural stability, more interior space).

Limitations

  • The complex design of the diagrid system can present challenges in the computation and construction process.

  • Because of the design variables like the diagonal angle and the bending to shear flexibility ratio, it is not possible to predict in advance, which approach will govern, either global stiffness demand or member strength demand.

  • There is a height limitation of 100 stories for diagrid constructed of steel and 60 stories for diagrid constructed of concrete.

  • Concrete diagrid system is very complex. Thus, it requires a large amount of form work, which eventually leads to higher construction cost.

  • Similarly, steel members are pre-fabricated due to their complexity. This also increases the construction cost.

The most widely used material in order to construct a diagrid is system is steel due to it’s ability to withstand tensile and compressive forces. Advantages of using a diagrid system includes the reduction in steel needed in comparison to traditional steel framing as it is lighter, stiffer and more efficient. Diagrid also reduces the amount of welding required.

 

Other materials that can be used for the diagrid system are wood and concrete however, these are less common due to their lower tensile strengths. Diagrid has so far been used mainly in high rise structures where high tensile stresses occur. Using steel for diagrid proves to be most cost effective as timber is a considerably expensive building material despite the fact that timber structures can be erected quicker.

 

Advantages

  • Behaves like a membrane, structurally and aesthetically effective

  • Eliminates vertical columns allowing diagonal members to carry gravity loads as well as lateral forces

  • Minimize shear deformation since structure carries shear by axial action of the diagonal members

  • Provides bending & shear rigidity

  • Spans greater distances compared to traditional steel structures

  • Amount of steel used is reduced compared to ordinary steel structures.

  • Inherent intelligent load distribution attributes thus accounting for structural instability.

 

Disadvantages

  • Designing the diagrid can be very complicated due to the complex computer models and construction methods

  • It is difficult to predict the which approach will regulate the global stiffness demand or member strength demand due to the design variables such as diagonal angle and bending shear flexibility ratio

  • The height limits for concrete diagrid and steel diagrid are 60 and 100 respectively

  • In case of a concrete diagrid, the vast form work required to construct such a structure raises the construction costs.

  • In the case of steel diagrid, the prefabrication of the complex design increases the costs.

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