Analyzing Utility of Component Elements of Outrigger System

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 01 | Jan-2018 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1141
Analyzing Utility of Component Elements of Outrigger System
Vardhan Dongre1, Dr. Vivek Garg2
1 UG Scholar, Department of Civil Engineering, Maulana Azad National Institute of Technology, Bhopal, India
2 Assistant Professor, Dept. of Civil Engineering, Maulana Azad National Institute of Technology, Bhopal, India
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Abstract - Outriggers are rigid horizontal structures
designed to improve the building overturning stiffness and
strength by connecting the building core or spine to distant
columns in the perimeter. In any tall building its structural
efficiency depend upon its lateral stiffness and lateral load
resistance capacity. There are various structural systems
available to resist lateral loads in tall buildings. Outrigger
system is one of the lateral load resisting systems which is
widely used for tall buildings. Present work aims at
understanding the fundamental functionality ofthisoutrigger
system, the utility of each component element involved viz.
Belt Truss, Core and Outrigger Trusses and understandingthe
economic implications of using outrigger system. A
comparative study between Virtual and Conventional
outrigger is performed to understand their effect in resisting
lateral load in a 30 m high building.
Key Words: Outrigger System, Structural Analysis,Static
Analysis, Optimization, Utility, Economy, Structural
Performance
1. INTRODUCTION
Economic prosperity and population increase in the urban
areas increases the necessity of judicial use of the land
available, thus, points towards a future with increased
number of high-rise structuresofcommercialandresidential
use. As a result of this tall building developments have been
rapidly increasing worldwide. A number of structural
systems have been evolved over the years depending upon
various complex factors such as economics, aesthetics,
technology, municipal regulations.Variousstudiesshowthat
the structural efficiency of tall buildings mainly depends on
the lateral stiffness and resistance capacity of the structure.
Out of all the systems Outrigger is one of the most efficient
systems especially for buildings with regular floor plan. The
use of outrigger in building structure can be traced back
from the concept of deep beams. As the building height
increases, deep beams become concrete walls or large steel
truss type outrigger. This paper focuses on analysing the
structural components of outrigger system viz. Outrigger
truss, Belt truss, stiff core and the load transferring
mechanism and in the pursuit of the same aims to
understand the utility and contribution of each component
elements by carrying out a comparative study on models
simulated in STAAD. The paper also discussesthedifferences
in virtual and conventional outriggers after performing
comparative studies on models in STAAD.
2. LITERATURE REVIEW
Case Study: Taipei 101
In order to understand the practical application of outrigger
system in a high-rise a case study was performed to identify
the key areas influenced by the use of outriggers. At 101
stories and 508 m above ground, the Taipei 101 is one of the
world’stallest buildingsthat adopted the outriggersystem.It
consists of a structural framing system of braced core and
multiple outriggers along with a system of perimeter frames
and connections to the core that resists lateralloadsspecially
the seismic forces. The Structure hasadoptedauniquewayof
controlling the drift by using large box type columns of steel
filled with high-strength concrete that were termed as Mega
columns. A central braced core stiffened by providing
connections (outrigger trusses) to the perimeter columns
adds to the lateral load resisting system. The primary
structural skeleton of a tall building can be visualized as a
vertical cantilever beam with its base fixed in the ground.
The structure has to carry the vertical gravity loads and the
lateral wind and earthquake loads. Gravity loads are caused
by dead and live loads. Lateral loads tend to snap the
building or topple it. The building must therefore have
adequate shear and bending resistance and must not lose its
vertical load-carrying capability.
Takeaways of Study
Provision of outrigger system provided the required lateral
stiffness to the building via transferring the forces from the
core to the coupled mega columns. The component systems
viz. the outrigger truss, stiff core and the mega columns
when clubbed together behaved more efficiently. The
outrigger system engaged the perimeter columns that
otherwise would have meant only as gravity-only elements.
Thus when the core tries to tilt, a tension-compression
couple is induced in the outrigger in opposition that actsasa
restoring moment. Outrigger system provesto be a solution,
adopted for structuresto be built in areas of highturbulence
and seismicity. For a high-rise structure multiple outriggers
can be provided at suitable locations by identifying their
optimum locations. This system also provides a dampening
effect in case of seismic force.
3. METHODOLOGY
3.1 Problem Formulation
A space frame model of a typical commercial / office type G +
9 building having each story of 3 meters and having 3 bays

along X-axis and having 5 bays along Z-axis is considered.
After the dimensions of the frame have been finalized 4
different frames having combinations of various component
systems - Bracing of core, Belt Truss in periphery and
Outrigger Truss connecting core to exterior of structure or a
rigid diaphragm are modeled in the STAAD.PROsoftware.The
structure considered is a steel frame providedwithcladdingof
glass as curtain wall / façade.
Fig. 1 Plan of Structure
Fig. 2 Elevation
3.2 Parameters of Models
Length along X- axis = 15 m
Length along Z- axis = 17 m
Height along y- axis = 3m per floor
Total Height = 30m
3.3 Load Calculation
DEAD LOAD
 Self-weight
 Floor Load – Thickness of Slab = 15 cm
Unit Weight of Concrete = 25 KN/m3
Intensity of Floor Load = 3.75 KN/m2
 Member load – Annealed Glass of Density 2520
kg/m3. Intensity of member load = 1 KN/m
(Applicable on peripheral beams)
LIVE LOADS
Imposed Loads - The imposed loads to be assumed in the
design of buildings shall be the greatest loads that probably
will be produced by the intended use or occupancy, but shall
not be less than the equivalent minimum loads specified in
Table 1 and clause 3.1 of IS 875 part II. Therefore, a floor load
of 4 kN/m2 is considered by considering the buildingasOffice
/ Business type building.
SEISMIC LOAD
 Importance Factor, I = 1.5
 Zone Factor, Z = 0.24
 Response Reduction, R = 4
 Damping Ratio, D = 0.05
Accordingto IS 1893-2002 Imposed Load to beconsideredin
seismic weight calculation is 50%. The seismic loadcalculator
of STAAD is used for load calculations.
3.4 Load Combinations
When earthquake forcesare considered on a structure, these
shall be combined as per guidelines mentioned in Clause
6.3.1.1 and 6.3.1.2 of IS 1893:2002 where the terms DL, IL
and EL stand for the response quantities due to dead load,
imposed load and designated earthquake load respectively.In
the limit state design of the structures, the following load
combinations are considered:
1) 1.5(DL+ IL) 3) 1.5(DL+EL)
2) 1.2(DL+IL+EL) 4) 0.9DL+ 1.5EL
3.5 Modeling Structures
Sections used for modeling were:
 Beams: ISMB 500
 Columns: User Defined Tube of Width = 600 mm,
Depth = 600 mm and Thickness = 8 mm.
 Bracing: ISMC 300
 Belt Truss: ISMC 300
 Outrigger Truss: ISMC 300
Using these STAAD modelsare obtained shown in Fig 3 to Fig
6.
Fig. 3 Simple Steel Frame (Model 1)

Fig.4 Steel Frame with Belt Truss and Outrigger Trusses
(Model 2)
Fig. 5 Steel Frame with Braced core, Belt Truss and
Outrigger trusses (Model 3)
Fig.6 Steel Frame with Virtual Outrigger and Floor
diaphragm (Model 4)
3.6 Parameter of Analysis
In order to compare the structures in this study, lateral
displacement is considered as a parameter. This lateral
displacement is indicative of the stiffness of the structure as for
tall buildings, the higher we go the more influence lateral drift
has on the analysis and design of the structure. Analysis of the
structure that is particularly elastic analysis is carried out in
STAAD. From Post- Processing results of analysis, Maximum
Lateral Displacement values are observed for all the models for
various load combinations and a comparative study is
performed.
4. RESULTS AND DISCUSSION
The results of linear elastic analysis are made available by the
software and areshown in table 1. To analyze the utility of each
component element a comparative study is carried out. To
establish the utility of stiffening of core a comparison is done
between Model 2, Model 3 as the only distinguishing
parameter between the two structures is the bracing that
stiffens the core. The comparison is illustrated graphically in
fig 7. Similarly, to establish the utility of outrigger a similar
comparison between Model 1 and Model 3 is done. To
determine the effect of using a complete outrigger on the
lateral load resisting capacity of the structure a comparison
between Model 1 and Model 3 is shown graphically in fig 6.
4.1 Utility of Component Elements
 Utility of Stiff Core (Bracings): Models 2 and 3 differ
with one another in terms of stiffness of core. It is
understood that the stiffness of the structure is directly
proportional to its lateral load resisting capacity. The
bracings in the coreprovides additionalstiffnesstoresist
the lateral force thereby reducing the lateral drift in
Model 3 by 24.56% in the direction in which the bracing
is provided (X axis).
 Utility of Belt Truss: On comparing results of Model 1
and 2 it can be seen that belt truss and outrigger trusses
together reduce the displacement values by 20.33mm
which is approximately 39.47% that of Model 1. From
these results a comment can be made on the interaction
between the Belt truss and the outrigger trusses. The
Belt truss when used withtheoutriggertrussesprovides
a suitable load transferring mechanism from the core to
the perimeter columns. It distributes the tensile and
compressive forces to a large number of exterior
columns and also helps in minimizing their differential
elongation and shortening.
 Utility of Outrigger System: Reduction in lateral
displacements can be seen in results of Model 3 from
those of Model 1. The difference betweenthetwomodels
is the outrigger trusses spanning between core and
perimeter columns together with the belt truss and
braced core. On comparing it is seen that the system
reduced the lateral displacements by a significant
47.67%. From this we can infer that Outriggers along
with the belt truss act as a backbone of the braced core
system which help in force relaxation. If a link of
appropriate stiffness that connectsthebracedcoretothe
exterior column is absent then the braced core deforms
without transferring the forces to the perimeter column
and thus results in larger displacements.
Fig. 7 Comparisonof lateral displacementsofModel1&3

4.2 Comparing Conventional and Virtual Outrigger
In virtual outrigger the transfer of overturning moment
from the core to elements is achieved by a stiff floor
diaphragm. In the study the virtual outrigger system is
modelled by providing a rigid floor diaphragm at the top
story, the diaphragm provides the necessary stiffness that is
otherwise offered by the trusses.
Fig. 8 Comparison of lateral displacements of Model 2 &
3
On comparing the displacements of the two structures from
Table 2 it can be seen that virtual outrigger shows an
improved efficiency of36.42%in the Xdirectionand52.37%
in the Z direction than the conventional outrigger system.
Table -2: Comparison of Optimized model 1 & model 4
5. CONCLUSION
The presentwork compares the difference in thebehavior of
the building in presence and absence of an outrigger system.
The following conclusions were drawn based on the study:
 Use of outrigger system inthe buildingimprovesthe
efficiency of the building in comparison to the one
without outrigger system.
 Outriggers increase the flexuralstiffnessbyreducing
base shear. Provision of stiff core along with the
outriggersin the building decreasesthe forcesinthe
core.
 The outrigger trusses improve the building
overturning stiffnessand strengthbyconnectingthe
building core to the columns.
 The Belt truss engages multiple columns and
improvesthe efficiency ofthe system byproviding a
load distribution mechanism between the outrigger
truss and the columns.
 Belt truss also provides more gravity load to the
columns that minimizes the net uplift, thereby
minimizing the reinforcement needed to resist
tension.
 Distribution of forces between the component
elements of the outrigger system depends on the
relative stiffness of each element. Stiffness of the
component elements can significantly affect the
outcome asvisible fromthecomparisonbetweenthe
concrete outrigger and the steel truss outrigger.
REFERENCES
[1] Wu J.R., Li Q.S. (2002), “Structural Performance of
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[3] Mulla A.K., Srinivas B.N. (2007), “A Study on
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Bracing,” International Journal of Engineering
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CTBUH 2012 9th World Congress, Shanghai pp 271-
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