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International Journal of Current Trends in Engineering & Research (IJCTER)
e-ISSN 2455–1392 Volume 2 Issue 5, May 2016 pp. 421 – 433
Scientific Journal Impact Factor : 3.468
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@IJCTER-2016, All rights Reserved 421
Analysis of High Rise Building with Outrigger Structural System
Sarfaraz I. Bhati¹, Prof. P. A. Dode², Prof. P. R. Barbude³
¹Department of Civil Engineering, DMCE, Navi Mumbai, [email protected]
²Department of Civil Engineering, DMCE, Navi Mumbai, [email protected]
³Department of Civil Engineering, DMCE, Navi Mumbai, [email protected]
Abstract- This research work is an attempt to study the effect of provision of concrete outriggers in high rise building. Static and dynamic behavior of a 42 storey RCC model was examined for
earthquake and wind loadings using ETABS software. Parameters of earthquake and wind loading
has been defined as per IS 1893 (Part-1):2002 and IS 875 (Part-3):1987 respectively. Linear dynamic
analysis has been carried out by response spectrum analysis. For the various models generated (one
without outrigger and others with outriggers placed at different storey); comparative study has been
carried out to observe the change in parameters such as lateral storey displacements, storey drifts and
base shear. From the results, it was concluded that provision of outrigger is effective in reducing the
displacements and drifts significantly, while base shear of the building showed not much change
with the introduction of outriggers.
Keywords- Outriggers, response spectrum analysis, lateral displacement, storey drift, ETABS
I. INTRODUCTION
Mankind had always been fascinated for height and throughout our history; we have
constantly sought to metaphorically reach for the stars. From the ancient pyramids to today‟s modern
high rise structures, a civilizations power and wealth has been repeatedly expressed through
spectacular and monumental structures. There has been a demonstrated competitiveness that exists in
mankind to proclaim to have the tallest building in the world. Today, high rise tall structures are
considered the symbol of economic power and leadership. As the buildings have gotten taller and
narrower, the structural engineers have been increasingly challenged to meet the imposed drift
requirements while minimizing the architectural impact of the structure. In response to this
challenge, the profession has proposed a multitude of lateral schemes that are now expressed in tall
buildings across the globe.
For buildings taller than a certain height, moment resisting frame structures, shear wall
structures, braced frame structures, tubular structures etc. may not provide adequate stiffness to resist
lateral wind and earthquake loads. In this case the lateral stiffness can be increased by tying the
exterior frames and shear core together by outrigger trusses or girders. In recent decades, outrigger
structural systems have been widely utilized in tall buildings in order to decrease structure‟s
deformation and increase its resistance in lateral loads.
II. OUTRIGGER STRUCTURAL SYSTEM
Outriggers are deep and rigid horizontal beams designed to enhance building overturning
stiffness and strength by connecting the core shear wall or core braced frame to the distant peripheral
column. The basic idea is to make the whole system to act as a single unit in resisting the lateral
load. The core may be centrally located with outriggers extending on both sides or the core may be
located on one side of the building with outriggers extending to the building column on the other
side. Outriggers increase the effective height of the structure. When the outrigger braced structures
are subjected to lateral loads, the exterior column and the outrigger battle the rotation of the central
core and thus considerably reduce the lateral deflection and base moments, which would have arisen
in free core buildings.
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
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Outriggers can be made of steel trusses or concrete beams or composite construction.
Outrigger system can be effectively used for 150 stories height and possibly more. It should be noted
that while the outrigger system is very effective in increasing the structures flexural stiffness, it
doesn‟t increase its resistance to shear, which has to be carried mainly by the core.
Figure 1. Outrigger structural system
III. MODEL SPECIFICATION
A 42 storey RCC model has been considered for analysis. The building dimensions are
such that the building is intentionally kept slender, which is a requirement for the study. The
building plan is symmetrical along both X and Y axis, so as to facilitate the ease in the comparative
study of seismic parameters. ETABS v9.7.4. has been used for analysis purpose.
3.1 Geometry of the model
Details related to geometry and dimensioning of the structure is discussed here.
Table 1. Geometry of the model
Model Geometry
01. Number of bays in X-direction :7 05. Typical storey height :4 m
02. Number of bays in Y-direction :5 06. Bottom storey height :5 m
03. Largest dimension of building :26 m 07. Total height of bldg. :169 m
04. Least dimension of building :17 m 08. Aspect ratio „H/B‟ :9.94
International Journal of Current Trends in Engineering & Research (IJCTER)
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Figure 2. Grid plan of the building
Table 2. Element Details
Element Dimensioning Concrete
Grade Remarks
Slabs 125 mm thick M25 Two-way Slab
Central shear
wall core 350 mm thick M45 Two C-shaped lift core
(4 m X 3 m – each)
Beams a) 230 mm X 600 mm M25 Replicated on all floors
b) 300 mm X 650 mm M25 Replicated on all floors
Columns
a)
425 mm X 1200 mm M40 Base to 10th
Floor
400 mm X 1100 mm M40 11th
Floor to 20th
Floor
350 mm X 1000 mm M40 21st Floor to 30
th Floor
300 mm X 900 mm M40 31st Floor to Terrace
b)
550 mm X 550 mm M40 Base to 10th
Floor
500 mm X 500 mm M40 11th
Floor to 20th
Floor
450 mm X 450 mm M40 21st Floor to 30
th Floor
400 mm X 400 mm M40 31st Floor to Terrace
Mega-Columns
(Modeled as
Shear Walls)
375 mm X 1900 mm M45 Base to 10th
Floor
375 mm X 1850 mm M45 11th
Floor to 20th
Floor
350 mm X 1800 mm M45 21st Floor to 30
th Floor
350 mm X 1750 mm M45 31st Floor to Terrace
It should be noted that mega-columns on which outriggers are to be connected from central
core have been modeled as shear walls; as the size of mega-columns was larger hence they have
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
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been dimensioned in a way that their width does not bring in obstruction in the occupiable space in
adjacent rooms. The depth of mega-columns is greater than 4 times its width, hence modeled as
shear walls.
3.2 Static and dynamic loading
Details related to static and dynamic loading are given below. Various parameters related
to seismic and wind load cases are mentioned below, as they have been given as input in ETABS
v9.7.4.
Table 3. Static load cases
Static Load Cases
01. Dead Load : 2 kN/m²
02. Live Load : 3 kN/m²
03. Earthquake in X – direction : Auto generated as per IS 1893 (Part 1) - 2002
04. Earthquake in Y – direction : Auto generated as per IS 1893 (Part 1) - 2002
05. Wind load in X – direction : Auto generated as per IS 875 (Part 3) - 1987
06. Wind load in Y – direction : Auto generated as per IS 875 (Part 3) - 1987
As per the provision of IS 1893 (Part-1):2002, while defining „mass source‟, mass
multiplier for live load has been kept as „0.25‟; as only 25% of live load is to be considered for
calculation of seismic weight for live load class upto 3 kN/m².
Table 4. Dynamic load cases
Dynamic Load Cases
01. Response Spectra in X- dir. : Auto generated as per IS 1893 (Part 1) - 2002
02. Response Spectra in Y - dir. : Auto generated as per IS 1893 (Part 1) - 2002
Table 5. Wind loading parameters
Parameters of wind loading as per IS 875 (Part-3) : 1987
01. Structure class : C
02. Terrain Category : 2
03. Basic wind speed, Vb : 44 m/s
04. Risk coefficient (k1 factor) : 1
05. Topography coefficient (k3 factor) : 1
06. Design wind speed (Vz = Vbk1k2k3) : 53.24 m/s
International Journal of Current Trends in Engineering & Research (IJCTER)
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Table 6. Earthquake loading parameters
Parameters of seismic loading as per IS 1893 (Part-1) : 2002
01. Seismic zone : III (Mumbai)
02. Seismic zone factor, Z : 0.16
03. Importance factor, I : 1
04. Response reduction factor, R : 5
05. Tx = 0.09*(H/√D) : 2.983 secs
06. Ty = 0.09*(H/√D) : 3.689 secs
07. Soil Type : II (Medium)
IV. RESULTS AND DISCUSSIONS
The results studied for the 42 storey structure are discussed below. Response spectrum
analysis has been carried out. The significant parameters monitored throughout the study were
lateral storey displacement, inter-storey drift of the building and base shear.
4.1 Results of the bare frame without any outriggers
Table 7. Maximum lateral displacements for bare frame without outriggers
Load Case Maximum top lateral
displacement
Direction & position of
displacement
Earthquake in X-direction 90.3 mm. Along X-direction at top
Earthquake in Y-direction 137.86 mm. Along Y-direction at top
Wind in X-direction 189.02 mm. Along X-direction at top
Wind in Y-direction 542.31 mm. Along Y-direction at top
Table 8. Maximum inter-storey drift for bare frame without outriggers
Load Case Maximum inter-
storey drift Direction & position of drift
Earthquake in X-direction 0.694 mm. Along X-direction at 21st floor
Earthquake in Y-direction 1.073 mm. Along Y-direction at 21st floor
Wind in X-direction 1.36 mm. Along X-direction at 15th
floor
Wind in Y-direction 3.863 mm. Along Y-direction at 16th
floor
It should be noted that the building is more slender in Y-direction and hence
displacements and drifts are considerably more for the load cases in Y-direction. Inter-storey drift is
in control for all the cases as the actual drifts are much below the maximum allowable criteria of
„0.004 times the storey height‟ as given in IS 1893(Part-1):2002. The top lateral displacement is in
control in all the cases except „Wind in Y-direction case‟; as the displacement for the WY case is
542.31 mm and maximum allowed is „H/500‟, which comes out to be 338 mm. Since the governing
load case is wind in Y-direction for the particular structure under consideration. Hence for the study
International Journal of Current Trends in Engineering & Research (IJCTER)
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the arrangement of outrigger system was decided in such a way that 8 number of outriggers were
given in Y-direction and only 4 in X-direction.
Figure 3. Plan showing outrigger layout
Table 9. Base shear sharing between columns and shear walls
Load
Case
Total base
shear
Base shear shared by
columns
Base shear shared by
shear walls
EQX 1757 kN 113 kN → 6.43 % 1644 kN → 93.57 %
EQY 1421 kN 173 kN → 12.17 % 1248 kN → 87.83 %
4.2 Result for models with single outrigger system
Displacement result for the governing load case is given below. Result is given for a single
outrigger system located at different heights along the structure namely 0.25H, 0.33H, 0.5H, 0.67H,
0.75H and at top. Each outrigger is 350 mm thick and 1 storey deep (4 m.) and of M45 grade
concrete.
Figure 4. Displacement for wind y-direction load case for ‘single outrigger system’ models
As it can be seen that the displacement has come nowhere close to the limit (338 mm), we
increase the number of outrigger systems for the structure.
0
10
20
30
40
50
0 200 400 600
Nu
mb
er
of
sto
reys
Displacement in mm. for WY
Without Outrigger
1 - 25% Height
1 - 50% Height
1 - 75% Height
1 - 100% Height
1 - 33% Height
1 - 66% Height
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4.3 Result for models with double outrigger system
Displacement result for the governing load case is given below. Result is given for double
outrigger system located at different heights along the structure namely 0.25H & 1H, 0.5H & 1H,
0.75H & 1H, 0.33H & 1H, 0.66H & 1H, 0.25H & 0.5H, 0.25H & 0.75H, 0.5H & 0.75H, 0.33H &
0.66H. Each outrigger is 350 mm thick and 1 storey deep (4 m.) and of M45 grade concrete.
Figure 5. Displacement for wind y-direction load case for ‘double outrigger system’ models
As again it can be seen that the displacement has come nowhere close to the limit (338
mm), hence we further increase the number of outrigger systems for the structure.
4.3 Result for models with multiple outrigger system
Displacement result for the governing load case is given below. Result is given for multiple
outrigger system located at different heights along the structure. But as we increase the number of
outriggers used we decrease the size of the outrigger. For this case each outrigger is 350 mm thick
and 2 m. deep and of M45 grade concrete.
Table 10. Details of multiple outrigger system
Number of
outrigger storeys Positions of outrigger systems
3 H/3, 2H/3 & top ( 1/3rd
height interval)
4 H/4, H/2, 3H/4 & top (1/4th
height interval)
6 Floors: 7th
, 14th
, 21st, 28
th, 35
th & top
8 Floors: 5th
, 10th
, 15th
, 20th
, 25th
, 30th
, 35th
& top
11 Floors: 5th
, 9th
, 13th
, 16th
, 19th
, 22nd
, 25th
, 28th
, 32nd
, 36th
and top
0
5
10
15
20
25
30
35
40
45
0 200 400 600
Nu
mb
er
of
sto
reys
Displacement in mm. for WY
Without Outrigger
2 - 25%+100%
2 - 50%+100%
2 - 75%+100%
2 - 33%+100%
2 - 66%+100%
2 - 25%+50%
2 - 25%+75%
2 - 50%+75%
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Figure 6. Displacement for wind y-direction load case for ‘multiple outrigger system’ models
As again it can be seen that the displacement has come in control to the limit (338 mm), for
the model where 11 number of outriggers have been used. The maximum displacement for wind y-
direction load case for the model with 11 number of outrigger is 334.9 mm (< 338 mm).
4.3 Result for model with 11 number of outrigger floors across the structure height
Given below are the results of the model with eleven number of outrigger system layout in
comparison with the bare frame model without any outrigger system. The results include
comparisons between top lateral displacement, inter-storey drift and base shear.
Figure 7. Storey displacement in y-direction for wind load in y-direction
0
5
10
15
20
25
30
35
40
45
0 200 400 600
Nu
mb
er
of
sto
reys
Displacement in mm. for WY
Without Outrigger
3 - 33% Interval
4 - 25% Interval
6 Outriggers
8 Outriggers
11 Outriggers
0
5
10
15
20
25
30
35
40
45
0 200 400 600
Nu
mb
er
of
sto
reys
Displacement in mm. for WY
Without Outrigger
11 Outriggers
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Figure 8. Storey displacement in x-direction for wind load in x-direction
Figure 9. Storey displacement in y-direction for earthquake load in y-direction
Figure 10. Storey displacement in x-direction for earthquake load in x-direction
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200
Nu
mb
er
of
sto
reys
Displacement in mm. for WX
Without Outrigger
11 Outriggers
0
5
10
15
20
25
30
35
40
45
0 50 100 150
Nu
mb
er
of
sto
reys
Displacement in mm. for EQY
Without Outrigger
11 Outriggers
0
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100
Nu
mb
er
of
sto
reys
Displacement in mm. for EQX
Without Outrigger
11 Outriggers
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Figure 11. Inter-storey drift in y-direction for wind load in y-direction
Figure 12. Inter-storey drift in x-direction for wind load in x-direction
Figure 13. Inter-storey drift in y-direction for earthquake load in y-direction
0
5
10
15
20
25
30
35
40
45
0 1 2 3 4 5
Nu
mb
er
of
sto
reys
Drift in mm. for WY
Without Outrigger
11 Outriggers
0
5
10
15
20
25
30
35
40
45
0 0.5 1 1.5
Nu
mb
er
of
sto
reys
Drift in mm. for WX
Without Outrigger
11 Outriggers
0
5
10
15
20
25
30
35
40
45
0 0.5 1 1.5
Nu
mb
er
of
sto
reys
Drift in mm. for EQY
Without Outrigger
11 Outriggers
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Figure 14. Inter-storey drift in x-direction for earthquake load in x-direction
Table 11. Reduction in maximum top lateral displacement
Load
Case
Direction of
displacement
Maximum top lateral storey displacement Percentage
reduction in
displacement For model without
outriggers
For model with
outriggers laid on
11 storeys
WY Y-direction 542.31 mm 334.9 mm 38.35 %
WX X-direction 189.02 mm 120.72 mm 36.13 %
EQY Y-direction 137.86 mm 89.48 mm 35.09 %
EQX X-direction 90.3 mm 60.09 mm 33.45 %
Table 12. Reduction in average inter storey drift
Load
Case
Direction of
drift
Average inter storey displacement Percentage
reduction in
displacement For model without
outriggers
For model with
outriggers laid on
11 storeys
WY Y-direction 3.24 mm 2.00 mm 38.27 %
WX X-direction 1.13 mm 0.73 mm 40.00 %
EQY Y-direction 0.88 mm 0.59 mm 32.95 %
EQX X-direction 0.57 mm 0.39 mm 31.57 %
0
5
10
15
20
25
30
35
40
45
0 0.2 0.4 0.6 0.8
Nu
mb
er
of
sto
reys
Drift in mm. for EQX
Without Outrigger
11 Outriggers
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Figure 15. Base shear for EQX load case
Figure 16. Base shear for EQY load case
Table 13. Base shear comparison
Load
Case Model
Total Base
shear
Base shear
shared by
columns
Base shear shared
by shear walls
EQX
Without
outriggers 1757 kN 113 kN→6.43 % 1644 kN→93.57 %
With 11 number
of outrigger
storeys
1820 kN 105 kN→5.76 % 1715kN→94.24%
EQY
Without
outriggers 1421 kN 173 kN→12.17 % 1248 kN→87.83 %
With 11 number
of outrigger
storeys
1472 kN 155 kN→10.53% 1317 kN→89.47%
1757
1820
1500
1550
1600
1650
1700
1750
1800
1850
Without outriggers With 11 number of outriggerstoreys
Bas
e s
hea
r in
kN
.
Base shear
1421
1472
1150
1200
1250
1300
1350
1400
1450
1500
Without outriggers With 11 number of outriggerstoreys
Bas
e s
hea
r in
Kn
.
Base shear
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V. CONCLUSION
The study assessed the behavior of outrigger braced structure under the influence of earthquake and
wind loading from which the following conclusions can be drawn based upon the results shown
above:
The use of outriggers increases the stiffness of the building and makes it more efficient in resisting the lateral loads.
The most critical lateral displacement for wind in y-direction loading was reduced by
38.35% and brought under the limit to satisfy the criteria of „Displacement < H/500‟.
Inter-storey drifts were also considerably reduced.
Use of outriggers did not show any significant change in base shear, as the total force acting on the structure does not change with addition of outriggers. Small increment which is seen
in base shear is due to the effect of increment in total seismic weight due to the addition of
self weight of outriggers.
Hence it can be concluded that outriggers are efficient in controlling the displacements,
while they do not have noticeable effect on the lateral force acting on the structure.
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