Analysis of High Rise Building with Outrigger Structural ... · PDF fileAnalysis of High Rise Building with Outrigger Structural System ... high rise structures, a ... the structural

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.


Volume 02, Issue 05; May – 2016 [Online ISSN 2455–1392]


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




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)


Floor

500 mm X 500 mm M40 11th

Floor to 20th

Floor


th Floor


Mega-Columns

(Modeled as

Shear Walls)


Floor

375 mm X 1850 mm M45 11th

Floor to 20th

Floor


th Floor


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




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




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




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




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


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%




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


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


Without Outrigger

11 Outriggers




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




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




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




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




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.

REFERENCES

[1] P. N. Biradar and M. S. Bhandiwad, “A Performance based study on Static and Dynamic behavior of Outrigger

Structural System for Tall Buildings”, International Research Journal of Engineering and Technology, Volume 2,

Issue 5, 2015.

[2] Karthik and N. Jayaramappa, “Optimum Position of Outrigger System for High Raised RC Buildings using Etabs

2013.1.5”, International Journal of Advanced Technology in Engineering and Sciences, Vol. 2, Issue No.12, 2014.

[3] Dr. K. S. Sathyanarayanan, A. Vijay, S. Balachandar, “Feasibility Studies on the Use of Outrigger System for RC

Core Frames”, International Journal of Advance Innovations, Thoughts & Ideas, 2012.

[4] N. Herath, N, Haritos, T. Ngo and P. Mendis, “Behavior of Outrigger Beams in High Rise Buildings under

Earthquake Loads”, 2009, Proceedings of Australian Earthquake Engineering Society Conference.

[5] R. K. Nanduri, B. Suresh and I. Hussain, “Optimum Position of Outrigger System for High-Rise Reinforced

Concrete Buildings under Wind And Earthquake Loadings”, 2013, American Journal of Engineering and Research.

[6] K. Shivcharan, S. Chandrakala and N. M. Karthik, 2015, “Optimum Position of Outrigger System for Tall Vertical

Irregularity Structures”, IOSR Journal of Mechanical and Civil Engineering, 2015.

[7] K. Kamath, N. Divya and A. U. Rao, “Static and Dynamic Behavior of Outrigger Structural System”, Bonfring

International Journal of Industrial Engineering and Management Science, Volume 2, Issue 4, 2012.

[8] J. Ahmed and Y. Sreevalli, “Application of Outrigger in Slender High Rise Buildings to Reduce Fundamental Time

Period”, Proceedings of 6th IRF International Conference, 2014.

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Analysis of High Rise Building with Outrigger Structural ... · PDF fileAnalysis of High Rise Building with Outrigger Structural System ... high rise structures, a ... the structural