Diagrid structural system

Contents
• Introduction
• Diagrid Components
• Triangular Diagrid Module
• Structural action of Diagrid Module
• Node Design
• Node construction
• Varied Angle Diagrid
• Merits and demerits of Diagrid Structures
• Examples
• Conclusion
• References

Introduction
• Tall commercial buildings are primarily a response to the intense
pressure on the available land. Advances in materials, construction
technology, analytical methods and structural systems for analysis
and design accelerated the development of tall structures.
• The lateral loading due to wind and earthquake is the major factor
that causes the design of high-rise buildings. These lateral loads are
resisted by exterior structural system or interior structural system.
The lateral load resisting systems that are widely used are mainly
rigid frame, shear wall, wall-frame, braced tube system, outrigger
system, diagrid system and tubular system.
• Recent trend shows that the Diagrid structural system is becoming
popular in the design of tall buildings due to its inherent structural
and architectural advantages.
• Diagrid is an exterior structural system in which all perimeter vertical
columns are eliminated and consists of only inclined columns on the
façade of the building.
• Shear and over-turning moment developed are resisted by axial
action of these diagonals compared to bending of vertical columns in
framed tube structure.

• Vertical columns in the core are designed
for carrying gravity loads only and the
diagrid is useful for both gravity and lateral
loading.
• 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 it and a wide spread 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.

• The diagrid systems are the evolution of
braced tube structures.
• The major difference between a braced
tube building and a diagrid building is
that, there are no vertical columns
present in the perimeter of diagrid
building.
• The diagonal members in diagrid
structures act both as inclined columns
and as bracing elements and due to
their triangulated configuration, mainly
internal axial forces arise in the members.
• Diagrid structures do not need high shear
rigidity cores because shear can be
carried by the diagrids located on the
perimeter.
Braced Tube Structure( John
Hancock Center, Chicago)
Diagrid Structure ( Hearst
Tower, New York)

• Diagrid has good appearance and it is easily recognized.
• The configuration and efficiency of a diagrid system reduce
the number of structural element required on the façade of
the buildings, therefore less obstruction to the outside
view.
• The structural efficiency of diagrid system also helps in
avoiding interior and corner columns, therefore allowing
significant flexibility with the floor plan.
• Perimeter “diagrid” system saves approximately 20 percent
structural steel weight when compared to a conventional
moment-frame structure.
• An early example of the diagrid structure is the IBM
Building in Pittsburgh built in the early 1960s, with its 13-
storey building height.
IBM Building , Pittsburgh, Pennsylvania

• The Swiss Re tower in
London, Hearst tower
in New York, CCTV
headquarters building
in Beijing, Mode
Gakuen Spiral Tower in
Aichi, West tower in
Guangzhou, Lotte
super tower in Seoul,
Capital Gate in Abu
Dhabi etc. are some
other popular diagrid
buildings.
Mode Gakuen
Spiral Tower
Capital Gate Super Lotte Tower CCTV Headquarters

History
• The Shukhov tower in Polibino is the world's
first diagrid hyperboloid structure designed in
1896 by Russian engineer and architect
Vladimir Shukhov.
• Built in the period 1920–1922.
• 160 m high.
• Its steel shell experiences minimum wind load.

Diagrid Components
• Nodes
• Diagonal Members
• Ring Beams
• Tie Beams
• Core
• Floor Slab

Nodes
 Joints that connects all the members.
 Typically formed by bolting or welding the ends of the members to a gusset plate.
 When diagrid is to be exposed internally or externally then we have to weld to achieve seamless connections
otherwise bolting can be done easily.
 Nodes can be designed as hinged or fixed depending upon condition.
 In case of concentric load and high diagrid angles hinged support is enough but when loads is eccentric and
diagrid angle is small then we have to provide fixed nodes as it becomes difficult to erect diagonal beams at
nodes.

Diagonal Members
• Members that transfer both lateral and gravity loads through axial action.
• Can be made of steel , concrete , timber and composite materials.
• Usually steel diagonal members are used.

Perimeter Girders or Ring Beams
• They comprise of ring structure at the periphery
of the building connected at the nodes which
are further connected with the diagrid columns.
• Extremely important in maintaining the stability
of the system.
• Constrain the shape and act in a very similar
manner to the hoops in the Shukhov towers.
• In smaller modules, those with heights tip to tip
of the diamond in the range of 2 to 4 floors, the
beams are designed in conjunction with the
edges of the floors.
• For taller modules there may need to be some
interaction between the floor edge beams and
the long diagonals, in addition to the dedicated
function of the horizontal members that frame
directly into the node.
• The longer the diagonals the more likely
they are to require additional lateral
bracing if the desire is for slenderness.
• Alternately, if unbraced, the member size
must be substantially increased to provide
self-support. This is common if the diagrid
is used in conjunction with a large atrium
space where there are no floors to assist
with this function.

Tie Beams
 Transfer load from RC core to Diagrid structures.
 Unbalanced forces can be balanced by ring beams and
tie beams.

Core
• To carry gravity loads
• To break up the span of the floor beams.
• Can be made of RCC (Canton Tower in
Guangzhou, China)
or Steel Columns ( Swiss Re Tower,
London)
• Capital Gate in Abu Dhabi, UAE
designed by RMJM Architects has also
used a reinforced concrete core, in
this instance prestressed to
counterbalance the 18 degree lean of
the tower.
Fig: . BIM Model of Al Dar Headquarters and Capital
Gate Building

Triangular Diagrid Module
• A diagrid structure is modelled as a vertical cantilever beam on the ground, and
subdivided longitudinally into modules according to the repetitive diagrid
pattern.
• Each Diagrid module is defined by a single level of diagonals that extend over ‘n’
stories.
• The geometry of the single module plays a major role in the internal axial force
distribution, as well as in conferring 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.
• Vertical columns are almost 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.

Optimal Angle:
Usually adopted range is 60 to 70 degrees
• The diagonal member of the diagrid carries both shear and moment. So the optimal angle of placing of the
diagonals is dependent of building height. The optimal angle of the columns for maximum bending rigidity in
the normal building is 90 degree and for the diagonals for shear rigidity is 35 degree. It is assumed that the
optimal angle of the diagrid falls in between the both.
• A sample study done of a 60 storey tall building, measuring 36m x 36m with an 18m x 18m gravity core at the
center with floor to floor heights of 3.9m confirmed 69 degree as the most effective angle for a uniform
diagrid as it results in the least amount of steel by weight. (Moon, 2007, 2008, 2009, 2010,2011) This angle
changes as a function of the building height as well as its width to height ratio.

Module Dimensions:
• Height of the module: It depends on the number of stories stacked per module. Usually 2 – 6 stories are
stacked per diagrid with average floor height varying from 3.5 - 4.15 m on an average.
• The relationship between the overall height and number of floors of a tower is more critical in the case of a
diagrid tower as the façade expression needs to terminate at a full module. Therefore the number of storeys
will directly influence the decision on the primary module height.
• Larger modules allow more flexibility in the choice of curtain wall. They are easier to “fit” on the façade and
can more easily accommodate standard rectilinear based curtain wall systems as infill.
• Smaller modules are more restrictive and invariably lead to more complex glazing systems that tend to be
triangulated. This impacts labour costs as these tend to be more time consuming to install and maintain as
they cannot be fitted with standard window washing equipment.
• Base of the module: It depends on the height and optimal angle (apex angle) of the diagrid.

Structural action of a Diagrid module
• Effects of gravity loading,
lateral loading and shear
loading
• Effect of non-apex loading :
Bending moment and shear
force are expected due to
this load condition. However
the introduction of a
horizontal member at each
floor girder to diagonal
intersection allows for the
absorption of the force
component orthogonal to
the diagonal direction, thus
preserving the prevailing
axial force condition.
Courtesy: Kyoung Sun Moon

Node Design
• The load path can be divided into two main scenarios, vertical load and horizontal shear .
• The vertical load will be transferred in the form of an axial load from the diagrid members above the node to
the gusset plate, then to the diagrid members below the nodes.
• 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. 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 nodes.

• 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.

Node construction for diagrid structure
• Both the engineering and fabrication of the joints are more complex than
for an orthogonal structure, and this incurs additional cost.
• Due to the triangular configuration of the diagrid structural system, rigid connections are not necessary at
the nodes, and pin connections using bolts can be made more conveniently at the jobsite.
• In order to reduce jobsite work, prefabrication of nodal elements is essential.
The prefabricated nodes are connected to the large built-up diagonal members by bolts at the jobsite.
• The precision of the geometry of the connection nodes is critical, so it is advantageous to maximize shop
fabrication to reduce difficulties associated with erection and site work. Some nodes are many tonnes and
it is desirable to be able to lift and turn the elements with a crane to provide access for fabrication, welding
and finishing.

• As this type of structure is more expensive to fabricate, cost savings are to be realized if there is a high
degree of repetition in the design and fabrication of the nodes.
• An architecturally exposed diagrid is more likely to require fully welded connections – both within the node
and for the connections between the nodes and the diagonals. This puts added pressure on the fabrication
tolerances in order to ensure that the butt welds are closely aligned to ensure a seamless transition. Site
welding may also require pre-heating of the material, so scaffolding and weather protection might also be
required.

Storey Drift for different grid systems under
earthquake load case
The drift ratio is defined as the ratio of maximum
lateral drift to total height of the specimen.
 Normal conventional building without any load
resisting system [R1]
 Diagrid building with diagonal angle of 45 [M1]
 Hexagrid building [M7]
Courtesy : Deepika R1, Shivanand C.G2, Dr.Amarnath K3
P G Student1, Assistant Professor2, Professor and HOD3
Department of Civil Engineering
The Oxford College of Engineering, Bangalore, Karnataka India

Storey Drift for different grid systems under wind
load case
PARAMETERS
Seismic parameters Wind parameters
Location: Amritsar Wind speed, Vb: 47m/s
Zone: IV Terrain category: 3
Importance factor: 1.0 Class C
Response reduction factor: 5  < 3
Soil type: medium soil k1,k3 = 1
Table II: wind and seismic parameters
Courtesy : Deepika R1, Shivanand C.G2, Dr.Amarnath K3
P G Student1, Assistant Professor2, Professor and HOD3
Department of Civil Engineering
The Oxford College of Engineering, Bangalore, Karnataka

Varied Angle Diagrid
• As a tall building functions as a vertical cantilever, the taller and more slender the building, the
greater will be the differentiation between the function of the diagrid elements at the base to
those towards the top. This has suggested that a variation in the inclination angle of the diagonals
can be used to reduce the amount of steel and provide needed resistance in the structure. The
members and connections at the base of the building must be designed to resist moment while
those at the top to resist shear.
• The diagonal members towards the top of the tower should be more inclined and the module
smaller as these members have less gravity loading associated with their function and must resist
more wind loading.

Comparison between uniform and varied angle
diagrids
Fig. Different configurations of diagrids for
an 80 storey building
A comparative study was conducted on 20
models ( 4 different configurations of 36, 50,
60, 70 and 80 storey buildings)
in terms of top storey displacement and
maximum forces developed on the diagrid
member.

Maximum storey drift for different models
Courtesy : Nijil George Philip, Dr.Shashidharan

Maximum force on diagrid for different models
Courtesy : Nijil George Philip, Dr.Shashidharan

Results
• Building of 36 and 50 storeys yield lowest displacement when configured with uniform diagrid angle.
• In 60, 70 and 80 storey buildings, non-uniform diagrid configuration provided lower displacement and
diagrid forces when the diagrid angle changed from 75° to 50°.
The forces on diagrids reduced around 2.5%, 3% and 5.5% in 60, 70 and 80 storey buildings respectively.
• So designing the diagrid building with changing diagrid angle could reduce the section size required and
thereby reduce the material required for the building.

Twisted Diagrids vs Braced Tube
Courtesy: Kyoung Sun Moon

Examples
Swiss Re Tower , London
• First modern application of diagrid structure.
• Designed by Sir Norman Foster, with 40 stories and an inter-
story height of 4.15 m, the tower is 180 meters tall.
• Consists of a central core and a perimeter “diagrid’.
• Core is required to act only as a load-bearing element and is
free from the diagonal bracing, producing more flexible floor
plates.
• Tower’s circular plan widens as it rises from the ground and
then tapers toward its apex.
• The aerodynamic form of the tower encourages wind to flow
around its face, minimizing wind loads on the structure and
cladding, and enables the use of a more efficient structure.

Guangzhou West Tower
• The Guangzhou West Tower, designed by Wilkinson Eyre architects,
London with 103 stories
• Height of 440m, is the tallest building in China and one of the tallest in
the world.
• Curvilinear shape along elevation and the floor plate is an equilateral
triangle with round-corners.
• Central concrete core and perimeter diagrid structure.
• Diagrid module expanding on six stories, 12.4 m wide and 24.8 m high.
• The diagonals are steel tubular members filled by concrete (CFST), with
size ranging between 1080 x 55 mm at the first floor and 700 x 20mm
at the top.
• The concrete core has a triangle shape and fully participates to the
lateral resistance up to the seventh floor, where it is eliminated, and
leaving place to a central giant atrium for the hotel which occupies the
upper floors.

Merits
 The Diagrid structures have mostly column free exterior
and interior, hence free and clear, unique floor plans are
Possible.
 The Glass facades and dearth of interior columns allow
generous amounts of day lighting into the structure.
 The use of Diagrids results in roughly 1/5th reduction in
steel as compared to Braced frame structures.
 Sustainability: - energy efficiency
- low environmental cost
- efficient use of available resources
The Diagrids makes maximum exploitation of the structural
Material.
 The diagrid Structures are aesthetically dominant and
expressive.
Redundancy in the Diagrid design is obvious. It is this
redundancy then that can transfer load from a failed portion of
the structure to another.
Skyscraper structural failure, as it is such an important/
prominent topic, can be minimized in a Diagrid design. A Diagrid
has better ability to redistribute load than a Moment Frame
skyscraper. Thus creating a deserved appeal for the Diagrid in
today’s landscape of building.

Demerits
 The number of storeys directly depend upon the
primary module height.
 It is hard to design windows that create a regular
language from floor to floor.
 Both the engineering and fabrication of the joints
are more complex than for an orthogonal
structure, and this incurs additional cost.
 Erection of nodes is a difficult process.
 As of yet, the Diagrid Construction techniques
are not thoroughly explored.
 Lack of availability of skilled workers having
experience in Diagrid Construction.

Conclusion
• One of the best structural system for high rise buildings especially irregular shaped (tilted, twisted,
freeform etc.)
• The majority of the published research has been conducted within the University setting and has
focused primarily on idealized optimization, leaving the applied realities of the practising professionals,
hidden or internalized.
• Using Diagrid we can built skyscrapers even without inner core providing vast floor area. The Leaden
hall Building in London is the first skyscraper without a bearing inner core thanks to diagrid structural
system.
• Diagrids help in sustainable development as amount of construction material required is less and
energy is saved due to less obstruction to incoming light at the periphery of building.
• The diagrid Structures are aesthetically dominant.

References
• Analysis of Circular Steel Diagrid Buildings with non-Uniform Angle Configurations Nijil George Philip1, Dr.Shashidharan.2, October-2016
• Performance Study of High Rise Buildings with Diagrid and Hexagrid Systems under Dynamic Loading Deepika R1, Shivanand C.G2, Dr.Amarnath K3 P G
Student1, Assistant Professor2, Professor and HOD3 Department of Civil Engineering The Oxford College of Engineering, Bangalore, Karnataka, India
DOI 10.4010/2016.1084 ISSN 2321 3361 © 2016 IJESC
• Optimized Topology Extraction of Steel-Framed Dia Grid Structure for Tall Buildings Dong-Kyu Lee1, Uwe Starossek2, and Soo-Mi Shin International
Journal of Steel Structures June 2010, Vol 10, No 2, 157-164
• Advantage of Steel Diagrid Building Over Conventional Building Saket Yadav1, Dr. Vivek Garg2 1 Post Graduate Student, 2 Assistant Professor,
Department of Civil Engineering, M.A.N.I.T., Madhya Pradesh, India
• Diagrid Structures: Innovation and Detailing, T. M. Boake ,School of Architecture, University of Waterloo, Canada
• Design and Construction of Steel Diagrid Structures by K. Moon , School of Architecture, Yale University, New Haven, USA
• Terri Meyer Boake “ DIAGRID STRUCTURES SYSTEM,CONNECTION,DETAILS”
• Mir M. Ali and Kyoung Sun Moon “Structural Developments in Tall Buildings: Current Trends and Future Prospects” Architectural Science Review
Volume 50.3, 205-223
• Terri Meyer Boake “Elegant Structures: Diagrids take to the Sky”

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