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 http://www.ijcter.com @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.
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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
http://www.ijcter.com
@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
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%
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
@IJCTER-2016, All rights Reserved 428
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
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
@IJCTER-2016, All rights Reserved 429
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
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
@IJCTER-2016, All rights Reserved 430
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
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
@IJCTER-2016, All rights Reserved 431
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
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
@IJCTER-2016, All rights Reserved 432
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
International Journal of Current Trends in Engineering & Research (IJCTER)
Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]
@IJCTER-2016, All rights Reserved 433
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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