IRJET- Diagrid Structural System for Tall Buildings: State of the Art Review

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 06 Issue: 06 | June 2019 www.irjet.net p-ISSN: 2395-0072
© 2019, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 3406
Diagrid Structural System for Tall Buildings: State of the Art Review
Premdas S1, M. Sirajuddin2
1M. Tech Student, Department of Civil Engineering, TKM College of Engineering, Kollam, Kerala, India,
2Professor, Department of Civil Engineering, TKM College of Engineering, Kollam, Kerala, India,
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Abstract - Over the past decade several research works
have been carried out about diagrid buildings. Having
established the importance of diagrid structures, researchers
and practitioners have developed advanced design strategies
to make diagrid structural systems efficient and economical.
The structural efficiency and flexibility in architectural
planning makes diagrid an innovative structural system for
tall buildings. Several studies have pointed out the design
methodology to be followed and researches have been carried
out about the joint connections as well. Hence the purpose of
this paper is to provide a systematic summary of the existing
research achievements of the diagrid structuralsystemfortall
buildings.
Key Words: Diagrid structural system, Framed structures,
Lateral loads, Optimum diagonal angle, Concrete filled steel
tube
1. INTRODUCTION
The increase in the number of high-rise buildings is
contributed mainly by the evolution of efficient structural
system, advances in construction technology, scarcity and
high cost of urban land and also the advancement in
computational techniques which allows the analysis and
design of complex structures less time consuming and easy.
Along with gravitational loading lateral loads such as wind
loads and seismic loads proves to be major governing factor
in the design of high-rise buildings. In order to resist lateral
loads in tall structures mainly interior structural systems
and exterior structural systems are provided. The widely
used internal lateral load resisting systems are: rigid frame,
braced frame, shear wall and outrigger structure. The
exterior systemsare:tubularstructureanddiagridstructure.
Out of many framed structures the uniqueness that
differentiate diagrid is the ability to form wide range of non-
rectilinear geometric structures which includes angles and
curves. This makes diagrids as an excellent architectural
choice in the creation of contemporary buildings. Diagrid is
particular form of space truss, which does not have any
conventional column on the exterior periphery of the
structure. The efficiency of diagrids in sustaining lateral
loads makes it an innovative and reliable technology. A
series of triangulated truss system is formed by thediagonal
columns and horizontal beams which facilitate efficientload
transfer and increases stability. Deshpande et al. [22]
compared diagrid structures to conventional structures.The
authors came to the conclusion that diagrid has better
overall performance in terms of efficiency as well as
sustainability. Diagrid was found to have less deflection,
used less steel and was eco-friendlier. More recently,
diagrids are getting more and more attention owing to their
unique form and properties. The unique compositional
characteristics of diagrids providegreatstructural efficiency
and aesthetic potential [15]. Swiss Re Tower in London is
one of the latest examples of diagrid structures.
Fig -1: Swiss Re Tower. [14]
Boake [25] explained the fact that even though the first
building to use a diagrid was constructed around 1965 in
Pittsburgh the method was not really used again until
around the year 2000 when several high-profile projects
were in their design phase. The first building to use diagrid
was the IBM Building constructed in 1965 at Pittsburgh. Its
aesthetic appearance was enhanced by providing glazing
system and resulted in oddly shaped windows.
2. DIAGRID CONFIGURATION AND DESIGN
2.1 Diagonal angle
Moon [16] explained the practical design guidelines. The
methodology followed by Moon [16] was stiffness-based
design methodology. This methodology was used to
determine the preliminary member sizes for the diagonals.
Moon [15] studied the optimal configuration of structural
systems for tall buildings and in his study, he investigates
optimal configurations of today’s prevalent structural
systems for tall buildings. Among various structural systems
developed for tall buildings, the systems with diagonals are
generally more efficient because they carry lateral loads by
their primary structural members’ axial actions [24]. From
the studies it is evident that by placing the lateral load
system throughout the perimeter of the building the overall
efficiency of the system can be maximised. Varyingangles as
well as uniform angles are studied in order to determine the

optimum angle so as to form an efficient configuration. By
varying the diagrid angles, the system’s efficiency can be
further increased. Seismic performance of diagrid systems
highly depends upon the diagonal angles. Moreover,
increasing the diagonal angles will increase the related
ductility ratios [23]. Moon [16] explained the practical
design guidelines and optimal configuration for diagrid
system. The optimal angle is usually unique for any
particular building. The performance of the structure is
influenced by brace angle and aspect ratio [13]. Lee et al. [4]
used a Solid Isotropic Material with Penalization
optimization approach to find the optimal topology and
diagonal angle of a two‐dimensional triangulated diagrid
mesh under static and dynamic loadings. Hence authors
recommend a formal optimization approach for finding the
optimal diagonal angle for any particular diagrid structure.
Moon [16] suggest that the optimal angle may vary from
structure to structure and also depends on the height to
width ratio. Jani and Patel [14] in their study carried out
analysis of 40, 50, 60, 70 and 80 storey diagrid structureand
determined that the optimal angle structurerangesbetween
65 degrees to 75 degrees. Panchal et al [21] conducted a
similar study to determinetheoptimumangleandconcluded
that Diagrid angle in the region of 65° to 75° provides more
stiffness to the diagrid structural system. Milana et al [9]
quantified the considerable reduction of steel compared to
traditional forms of construction like outrigger structures.,
three geometric configurations with inclination of diagonal
members of 42°, 60°and 75° were used in the analysis and
concluded that the one with the best overall behaviour
results to be the one with 60°diagonal element inclination.
2.2 Preliminary analysis and design
To propose a practical form for the building, engineers need
to have an approximate method in order to determine the
preliminary sizes of the structural members quickly. Hence
number of approximatepreliminarydesign procedureswere
mentioned in number of studied one of them is ofMoon etal.
[16]. The authorsintroduced a simpledesign methodologyto
find the required cross-sectional areas of diagonals
approximately considering a lateral stiffness‐based design
criterion. The allowable maximum displacement was given
as H/500 where H is the height of structure [10].Thediagrid
structure is divided into a number of diagrid modules (a
group of adjacent stories) along the height. This diagrid
modules were interconnected using diagonals. Diagrid
structures do not need high shear rigidity cores because
shear can be carried by the diagridslocatedontheperimeter
[21]. Approximate cross sections of diagonals are calculated
for the web and flange diagrids (diagrids parallel and
perpendicular to the lateral force, respectively)basedonthe
stiffness criterion. The authors report reasonable accuracy
compared with using a linear finite element(FE)packagefor
tall diagrid structures with an aspect ratio equal to or
greater than five and diagonal angle in the range 60° to 70°.
Moon [16] revised the method slightly by introducing a
factor to include the contribution of web diagonals to
bending rigidity. Mele et al [7] propose a simplified analysis
method by dividing the diagrids into a number of triangular
elements. Hasnet, A [1]. Analysed buildingswithvertical and
stiffness irregularity subjected to lateral loading and
concluded that complex shaped buildings are getting
popular, but they carry a risk of sustaining damages during
earthquakes. Hence for their safe construction proper care
should be given for the dynamic behaviour of the buildings.
Moehle, J.P [11] studied seismic response of vertically
irregular structure and concluded that certain parameters
like storey drift changed abruptly on reaching the
irregularity. Stiffness and strength approaches are both
necessary and unavoidable;theyarenotseparatelysufficient
for an exhaustive sizing processofthediagonal members [8].
Lee and shin [5] performed nonlinear static push over
analysis using ABAQUS software to verify the structural and
economic superiority of convex shaped diagrids over flat-
shaped and concave diagrids.
2.3 Joints in diagrids
The most complex part in the designofdiagridstructuresare
the Joints. Joints connection may vary depending upon the
type of material used and the amount of loading acting on
the structure. Diagrid columns can be constructed with
different type of materials like Steel,reinforcedconcreteand
CFST tubes. Each of the material have different type of joint
connection. The joints have to be stiff and also capable of
transferring the axial loading effectively to the adjacent
members [27]. Ling et al [18] investigated the behaviour of
the intersecting connection of CFST columns in diagrid
structure, a typical reduced scale planner connection
specimen is tested under monotonic compressive loading.
Experimental results as well as numerical simulation
concluded that the intersecting CFST connection was able to
develop fully plastic deformation with sufficient capacities.
Huang et al [3] conducted a number of experiments on eight
circular CFST diagrid node specimens under monotonic
static axial compressive loading. Two types of CFST
connections are tested In CFST connections, elliptical steel
bearing plate is welded to the four intersecting columns to
resist axial compressive loads. Twodifferentdetailsareused
to strengthen the connections (i) a lining plate with a ring
reinforcing plate, it consists of four semi-tubular lining
plates which are welded at the intersecting ends at each
column, and one ring reinforcing plate at the centre of the
connection.(ii) flange plates, it consists of four semi-ring
flange plates which are welded to the columns andthenthey
are connecting each other with bolts. All these components
are strengthening the confinement effect of the structural
members. So as to improve the load bearing capacity of the
connections. Compared to flange plates lining plates offers
more confinement effect. Huang etal [3]suggeststhatfailure
occurs in these connections are mainly depends on the
interacting angle between the columns. For the specimen
with an interacting angle of 20°then steel tube bulging
occurs within the connection zone. But specimen with an
interacting angle of 35° the failure occurs in the column

zone. Therefore, the connection with smaller interacting
angle requires appropriate reinforcementsuchasincreasing
the thickness of the steel tube or the elliptical plate to avoid
failure.
Zhou et al [26] conducted study in RC diagrid structures and
x-shaped reinforced concrete joints were provided in the
experimental study. These joints consist of four diagonal
columns and horizontal girders. volumetric ratio of the
stirrups loading patterns are the main design factors for x-
shaped reinforced concrete joints. Expanding deformation
may occurs at the joint concrete region under loads. The
stirrups confine the lateral expandingdeformation.So,joints
with normal volumetric ratio of stirrups are designed for
large stiffness and high load bearing capacity.
Kim et al [28] studied the steel joints through number of
experiments. The node core is designed with a box type
section such as H-section. It consists of two diagonal brace
members, side stiffeners, centre stiffener. The flanges are
attached at the node section. The web braces are not
continuous therefore the axial forces from the webs are
transferred to side stiffeners at the node.Failuremechanism
of a diagrid node is governed by combined mechanism of
axial force and additional moment from the two axial forces.
For the design of joints consider stiffener configuration,
welding method, overlapping length of stiffeners as
fundamental parameters. Roundedstiffenersofferreduction
of the strain concentration near the side stiffener ends. FP
welding method is recommended for the flange to flange
welds in order to avoid detrimental behaviour. PP welding
method is recommended for remaining welds for load
transfer mechanism. Overlapping length of stiffeners also
plays an important role in structural performance of diagrid
structures. Overlappedlengththatbeyondstandardlengthis
recommended for adequate safety.
3. TYPES OF DIAGRID STRUCTURES
3.1 Concrete diagrids
Due to the ease in construction and fabricationsteel sections
commonly W‐shaped and circular or square hollow
structural sections are used for diagonals of a diagrid
structure. The added advantage of steel since it can
withstand large tension as well as moderate compressive
strength also attribute to the fact of preferring steel over
concrete for diagrid structures. However, a number of
diagrid structures have been built with concrete. Some
common examples are the 20‐story 170 AmsterdamAvenue
building in New York City completed in 2014 is an example
of exposed concrete diagrids where the large concrete
diagonals form the exterior of building, the so‐called
exoskeleton. Yet little research has been reported on
concrete diagrid frames [17]. Hence the scope of research is
more for concrete diagrids.
3.2 Steel diagrids
The most common form of diagrid frames are steel frames.
The advantage of ease in fabrication and construction paved
the way for steel frames over concrete frames. The sections
commonly used are rectangular HSS, rounded HSS and wide
flanges [20]. The weight and size of the sections are made so
as to resist the high bending loads. They can be quickly
erected and the cost of labour for the installation is low. One
of the major disadvantages that can be attributed against
steel is that since steel sections are factory made the
flexibility of construction complex shaped structures is
limited. Majority of diagrid structures built today is of steel.
3.3 CFST diagrids
The new type of composite construction of Concrete Filled
Steel Tubular elements used as diagrid frames is gaining
importance in recent years. The combined structural
performance of steel as well as concrete justifies the use of
CFST in diagrid frames [29]. They can be further
strengthened using Fibre reinforced polymer [19].
Completed in 2010, the 103‐story Guangzhou West Tower
aka the Guangzhou International Finance Centre in China is
known as the tallest diagrid structure built. The structure
consists of an RC internal core and an exterior
curved‐shaped concrete‐filled steel tubular (CFST) diagrid
frame. Huang et al [13] conducted Experimental and
Numerical InvestigationoftheAxial BehaviourofConnection
in CFST Diagrid Structures. But however very little research
study has been conducted in using CFST as diagrid frames.
4. SUSTAINABILITY OF DIAGRID STRUCTURES
The Leadership in Energy andEnvironmental Design(LEED)
uses a rating system to evaluate the sustainability of a new
or constructed building based on material,energyandwater
efficiency, environmental quality, and innovative design
criteria. The inclined members of the diagrid carry most of
the lateral loads and hence the interior columns may be
designed for gravity loading only which leads to less
consumption of steel [6]. Also, the diagrid offers flexibilityin
designing complex geometrical forms which can create
beautiful architectural buildings. The other sustainability
feature of Diagrid has been the large column less spaces
which may be used as rental spaces or can be allocated to
sky gardens, atriums, and so forth. Also, the structural
efficiency lead to reduced use of structural material. One of
the examples of a LEED gold rated diagrid building is The
Bow Tower completed in 2012. It is a 47‐m and 57‐story
steel diagrid office building with unique curved shape and
distinctive sustainable features. It is the tallest building in
Calgary, Canada. Charnish and McDonnell [2] claimed that a
great deal of thought and design effort was placed on
creating a progressive sustainable building within Canada.
Multi-story sky gardens were incorporated into thebuilding
at three separate levels to provide added green space. And
finally, in order to maximize sunlight several studies were
also done to position the building accordingly.

5. CONCLUSION
The mechanism of load transfer, structural style and
aesthetic appearance of diagrid structures areverydifferent
form orthogonal structures. The design of safe joint
connection is an important step in the design of diagrid
structures. Better seismic resistance of the structure makes
it ideal in seismic vulnerable areas as well. Many studies
prove that diagrid not only enhances strength and stiffness
but also improves the ductile capacityofthestructure.Hence
the potential of diagrids to become more popular in the
construction of high rise and mid-rise buildingsishighasthe
new technology in the field of construction will aid the
complex construction to become easier. The flexibility of
using diagrids to form architecturally complex shaped
structures has been recognised all over the world. A
limitation to this is the lack of specific design procedures for
the safe and conservative design.
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