Published on Feb 14, 2016
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.
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.
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.
As in the diagrids, diagonals carry both shear and moment. Thus, the optimal angle of diagonals is highly dependent upon the building height. Since the optimal angle of the columns for maximum bendingrigidity is 90 degrees and that of the diagonals for maximum shear rigidity is about 35 degrees, it isexpected that the optimal angle of diagonal members for diagrid structures will fall between these angles and as the building height increases, the optimal angle also increases. Usually adopted range is 60 -70 degree
MATERIALS USED FOR DIAGRIDDS:
Material selection for a Diagrid construction is based on the following factors .
a) Unit weight of the material.
b) Availability of the material.
c) Lead Time.
d) Erection Time.
g) Labor cost.
h) Fire resistance.
The basic materials used in Diagrid construction are Steel, Concrete and Wood. The relative merits and demerits of using them are discussed below.
I. STEEL :
Steel is by far the most popular material for Diagrid constructions. The typical steel sections used are Wide flanges, Rectangular HSS and Round HSS.
Steel Wide Flanges:
Advantages- The weight and Size of wide flanges are optimized to resist the high bending loads many of the members experience. Thus use of wide flanges results in reduced structure weight and flexibility of size. The sections can be prefabricated in multi-panel sections, allowing quick erection by crane, reducing labor costs in the field.
Disadvantages- Pre-fabrication of the Diagrid sections takes a longer lead time.
Rectangular and Round HSS:
Advantages- As with wide flanges, HSS sections can be prefabricated in multi-panel sections, allowing quick erection time, also reducing labor costs in the field.
Disadvantages- Use of HSS sections will need a change in floor layouts as the beams will need to frame into the node points. This reduces the floor flexibility and efficiency.
Concrete is another widespread material for Diagrid constructions. It is used both in Precast and Cast-in-situ forms.
Advantages-The flexibility of precast sections allows them to fit to the complex building geometries. Concrete also offers extreme safety against structural fire damage.
Disadvantages- The use of Concrete increases the dead load on the foundations, deflections of long spans, etc. Creep in concrete is also an issue.
Under an Efficient material management system, cast-in-situ concrete is the best material in terms of material cost. Lead time is virtually nothing as cast-in-situ is available on demand.
Timber is the least popular material for Diagrid constructions.
Advantages- Multi-panel sections can reduce erection time and labor cost.
Disadvantages – Timber cost, both for material and connection, are much higher than the traditional structural materials of steel and concrete. Owing to its lesser material strength, the member sizes would be very large and hence is not preferred for major construction works. Durability and weathering of timber are other major issues.
DIAGRID NODE DESIGN
Figure 8: Load path at Node
The diagrid segments are planned to minimize onsite butt welding and the welding locations illustrated in Figure 9. The load path can be divided into two main scenarios, vertical load and horizontal shear their combination), as shown in Figure 8. The vertical load will be transferred in the form of an axial load from the diagrid members above the node to the gusset plate and stiffeners, then to the diagrid members below the nodes as shown. The horizontal shear will be in the form of axial loads in the diagrid members above the node with one in compression and one in tension to the gusset plate and stiffeners. The force will then be transferred as shear force in the gusset plate and then to the other pair of tensile and compressive forces on the diagrid members below the node. From this load path, the shear force at the location of bolt connections is high under lateral loads. Because this may create weak points at the node particularly during earthquakes, the strength of the bolts should be designed carefully.
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