Static Analysis and Optimization of Outriggers in A Tall Building
0 likes55 views
This document analyzes the static optimization of outriggers in a 40-story tall building. It summarizes that as buildings get taller, outriggers are often used to increase lateral stiffness but occupy significant floor space. The study models different structural configurations to optimize outrigger design, including varying outrigger location and stiffness, adding composite slab and belt truss systems. Results show placing a single outrigger at the mid-height and increasing its flexural rigidity provides the best stiffness. Including floor slabs and an X-shaped belt truss further reduces displacement and drift, providing equivalent performance to a two-outrigger system while mitigating the space disadvantage of multiple outriggers.
![International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1806
STATIC ANALYSIS AND OPTIMIZATION OF OUTRIGGERS IN A TALL
BUILDING
Pallavi Sao1, B.H.V. Pai2
1Post Graduation Student, Department of Civil Engineering, MIT Manipal, Karnataka, India
2Professor,Department of Civil Engineering, MIT Manipal, Karnataka, India
---------------------------------------------------------------------***---------------------------------------------------------------------
Abstract - As the building goes taller stiffness and stability
becomes important factor in the design. Outriggers areoften
used to give lateral stiffness to tall and slenderbuildings. But
the floor space occupied by these flexurally stiff and deeper
beams is large. In this paper, an attempt has been made to
optimize outriggers providedina buildingandtoincreaseits
lateral stability by including other structural elements.
Different types of floor systems and belt truss are added to
the structure and their effects towards lateral responses are
found out. A 40-storey tall building has been modeled in
Etabs-2015 and analyzed under static wind and earthquake
loads. The key parameters discussed in this paper include
lateral displacement at the top and inter-storey drift ratio.
Key Words: outriggers, flexural rigidity, slab stiffness, belt
truss
1. INTRODUCTION
Due to increase in urbanization and scarcity of
available land, buildings have started moving skywards. The
tall buildings are the solution where more people can be
accommodated on less land space. But as the building
becomes taller, additional lateral forcesstartsactingonthem
and serviceability requirements governs the design. For a
tall building, lateral drift and building accelerationatthetop
should be analyzed and kept within the limits specified on
codes.
1.1 Outrigger
Outriggers are the horizontal members which
resists lateral loads by mobilizing axial stiffnessofperimeter
column by connecting them to the core of the structure.
When lateral loads acts on the structure, column restrained
outriggers resists the rotation by inducing tension force on
windward columns and compression force on leeward
columns thereby generating a restoring tension-
compression couple to resist core overturning moment and
lateral deflection. This system can work efficiently for
buildings upto 150 stories [1].
1.2 Challenges Associated with Outriggers
Outriggers occupying large vertical space upto 2-3
stories deep interferes with floor area in a building
and restricts space utilization on these floors.
Outriggers connecting core and distant columns
undergo additional stressduetodifferential vertical
shortening between them.
The connections betweencoreandoutriggersneeds
to be properly studied and designed especially
when the two are made up of different materials.
Fig -1: “Outrigger system”](https://image.slidesharecdn.com/irjet-v4i6578-180222093024/85/Static-Analysis-and-Optimization-of-Outriggers-in-A-Tall-Building-1-320.jpg)
![International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1807
2. METHODOLOGY
A structure with two outriggers is analyzed and then a
single outrigger system with slabs and belttrussisfound out
which will be equivalent to two-outrigger system in its
lateral stiffness. In this manner, multiple outrigger system
can be reduced to single outrigger and its disadvantage of
occupying large space can be avoided
3. MODEL DETAILS
Plan dimension : 27X24 m
Typical storey height : 3.5m
No. of storey : 40
Beam Details
Breadth – 230mm
Depth – 500mm
Column Details
Breadth – 750mm
Depth -750mm
Concrete Grade : M-40
Steel Grade : Fe-250
Wind load (IS 875(Part-3)-1987)
Design speed – 44m/s
Terrain Category – 2
Structural Class – B
Seismic load (IS 1893(Part-1):2002)
Zone 3 – 0.16
Importance factor – 1.5
Soil type – Medium
Reduction factor – 3
Following Different Models Have Been Prepared
Case1: Two-Outrigger system with
Core and Outriggers -300mm thick
Outriggers placed at ⅓rd and ⅔rd height of the
building i.e at 13th and 26th floor according to
Taranath thumb rule[2] (Fig. 1&2)
Fig -1: Plan
Fig -2: Elevation
Case2: Single outrigger system with
Core -300mm thick
Variation of relative flexural rigidity between
outrigger and core (γ) = (EI)o/(EI)core and
location of the outrigger in the above same plan
and elevation (Fig -1&2).
(γ)
The location of outrigger
(Hs/H) is varied as
0.25,0.5,0.75,1 for each
value of (γ).
0.25
0.5
0.75
1
1.25
Case 3 : Following Floor systems are added
1) Single-outrigger system with composite deck
slab (Fig -3).
Light-weight concrete of 70mm thickness
over 80mm metal deck on all typical floors
and 250mm regular weight concrete of M-
25 grade on outrigger level](https://image.slidesharecdn.com/irjet-v4i6578-180222093024/85/Static-Analysis-and-Optimization-of-Outriggers-in-A-Tall-Building-2-320.jpg)
![International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1811
REFERENCES
[1]. Mir M.Ali, Kyoung Sun Moon (2007) “Structural
Developments in Tall buildings- Current Trend and Future
Scope”.
[2]. Bungale S. Taranath (2004) “ Wind and Earthquake
Resistant Buildings”.
[3]. K.L. Chang , C.C. Chen (2008) “ Outrigger System Study
For Tall Building Structure with Central Core and square
Floor Plate”.
[4]. S.M. Sayeed , Ghulam Ahmed (2013) “Effect of increased
slab stiffness and outrigger system onFlatslabRCbuilding".
[5]. Kiran kamath, Avinash A.R. and S. Upadhyaya(2014) “A
Study on the performance of multi-outrigger structure
subjected to seismic loads”.
[6]. Kiran Kamath,N.Divya andAsha URao(2012) “Optimum
Positioning of Outrigger to Reduce Differential Column
Shortening Due to Long Term Effects in Tall Buildings”.
[7]. Wind loads (IS: 875 Part-3) -1987
[8]. Earthquake loads (IS:1893 Part-1) -2002](https://image.slidesharecdn.com/irjet-v4i6578-180222093024/85/Static-Analysis-and-Optimization-of-Outriggers-in-A-Tall-Building-6-320.jpg)
Static Analysis and Optimization of Outriggers in A Tall Building
- 1. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072 © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1806 STATIC ANALYSIS AND OPTIMIZATION OF OUTRIGGERS IN A TALL BUILDING Pallavi Sao1, B.H.V. Pai2 1Post Graduation Student, Department of Civil Engineering, MIT Manipal, Karnataka, India 2Professor,Department of Civil Engineering, MIT Manipal, Karnataka, India ---------------------------------------------------------------------***--------------------------------------------------------------------- Abstract - As the building goes taller stiffness and stability becomes important factor in the design. Outriggers areoften used to give lateral stiffness to tall and slenderbuildings. But the floor space occupied by these flexurally stiff and deeper beams is large. In this paper, an attempt has been made to optimize outriggers providedina buildingandtoincreaseits lateral stability by including other structural elements. Different types of floor systems and belt truss are added to the structure and their effects towards lateral responses are found out. A 40-storey tall building has been modeled in Etabs-2015 and analyzed under static wind and earthquake loads. The key parameters discussed in this paper include lateral displacement at the top and inter-storey drift ratio. Key Words: outriggers, flexural rigidity, slab stiffness, belt truss 1. INTRODUCTION Due to increase in urbanization and scarcity of available land, buildings have started moving skywards. The tall buildings are the solution where more people can be accommodated on less land space. But as the building becomes taller, additional lateral forcesstartsactingonthem and serviceability requirements governs the design. For a tall building, lateral drift and building accelerationatthetop should be analyzed and kept within the limits specified on codes. 1.1 Outrigger Outriggers are the horizontal members which resists lateral loads by mobilizing axial stiffnessofperimeter column by connecting them to the core of the structure. When lateral loads acts on the structure, column restrained outriggers resists the rotation by inducing tension force on windward columns and compression force on leeward columns thereby generating a restoring tension- compression couple to resist core overturning moment and lateral deflection. This system can work efficiently for buildings upto 150 stories [1]. 1.2 Challenges Associated with Outriggers Outriggers occupying large vertical space upto 2-3 stories deep interferes with floor area in a building and restricts space utilization on these floors. Outriggers connecting core and distant columns undergo additional stressduetodifferential vertical shortening between them. The connections betweencoreandoutriggersneeds to be properly studied and designed especially when the two are made up of different materials. Fig -1: “Outrigger system”
- 2. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072 © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1807 2. METHODOLOGY A structure with two outriggers is analyzed and then a single outrigger system with slabs and belttrussisfound out which will be equivalent to two-outrigger system in its lateral stiffness. In this manner, multiple outrigger system can be reduced to single outrigger and its disadvantage of occupying large space can be avoided 3. MODEL DETAILS Plan dimension : 27X24 m Typical storey height : 3.5m No. of storey : 40 Beam Details Breadth – 230mm Depth – 500mm Column Details Breadth – 750mm Depth -750mm Concrete Grade : M-40 Steel Grade : Fe-250 Wind load (IS 875(Part-3)-1987) Design speed – 44m/s Terrain Category – 2 Structural Class – B Seismic load (IS 1893(Part-1):2002) Zone 3 – 0.16 Importance factor – 1.5 Soil type – Medium Reduction factor – 3 Following Different Models Have Been Prepared Case1: Two-Outrigger system with Core and Outriggers -300mm thick Outriggers placed at ⅓rd and ⅔rd height of the building i.e at 13th and 26th floor according to Taranath thumb rule[2] (Fig. 1&2) Fig -1: Plan Fig -2: Elevation Case2: Single outrigger system with Core -300mm thick Variation of relative flexural rigidity between outrigger and core (γ) = (EI)o/(EI)core and location of the outrigger in the above same plan and elevation (Fig -1&2). (γ) The location of outrigger (Hs/H) is varied as 0.25,0.5,0.75,1 for each value of (γ). 0.25 0.5 0.75 1 1.25 Case 3 : Following Floor systems are added 1) Single-outrigger system with composite deck slab (Fig -3). Light-weight concrete of 70mm thickness over 80mm metal deck on all typical floors and 250mm regular weight concrete of M- 25 grade on outrigger level
- 3. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072 © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1808 Fig -3 : Composite deck slab 2) Single–outrigger system with horizontal steel bracing (Fig -4). X-shaped horizontal steel bracing made up of hollow pipe sectionsofdiameter500mm and 30mm thickness is modeled on all typical floors Fig -4: Horizontal steel bracing Case4: Following belts truss are added (Fig 5&6) Single outrigger system with composite deck slab and X-shape belt truss Single outrigger system with composite deck slab and inverted V-shape belt truss Fig -5 :Elevation showing X-shape belt truss Fig -6: Elevation showing inverted V-shape belt truss 4. RESULTS AND DISCUSSION For case 3, the effect on lateral deflection due to variation in flexural rigidity and location of the outrigger is obtainedand shown in (Fig 7 &8). 0 50 100 150 200 250 0.25H 0.5H 0.75H H β = β = β = β = β = Storey Level D e f l e c t i o n Fig -7: Variation of lateral deflection for wind load
- 4. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072 © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1809 Fig -8: Variation of lateral deflection for seismic load Fig-7 & Fig-8 indicates that deflection decreases by increasing outrigger rigidity. It is observed that the maximum reduction of 29% and 25% is obtained when outrigger is placed at mid-heightofthe buildingfor windand seismic case respectively. Top displacement gets reduced to 6.2% and 3.31% by varying ‘γ’ from 0.25 to 0.5 and 0.5 to 0.75 respectively. It is observed that any further increase in flexural rigidity has relatively less effect in reducing deflection. Hence an increase in relative flexural rigidity to 0.75 and locating single outrigger at 0.5H is taken as the most optimum condition. For Case 3, effects of adding composite deck slabs and horizontal steel bracing to the lateral deflection is shown in Chart -1 and Chart -2 0 20 40 60 80 100 120 140 160 180 WIND LOAD SEISMIC LOAD Single- outrigger with β= 0.75 and Hs/H =0.5 Single- outrigger with composite deck slab Single- outrigger with horizontal steel bracing Chart -1: Lateral deflection due to wind and seismic load It is noticed that including floor diaphragms reduces lateral deflection and makes the structure stiffer. A reduction upto 6.85% and 2.67% is gained by adding composite slabs for wind and seismic load respectively. Horizontal steel bracing reduces deflection due to winds to 15% but it increases to 10% for seismic case due to an increase in buildings overall weight. 0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016 0.0018 WIND LOAD SEISMIC LOAD Single- outrigger with β= 0.75 and Hs/H =0.5 Single- outrigger with composite slab Single- outrigger with horizontal steel bracing Chart -2 Storey drift due to wind and seismic load A similar trend in the reduction of storey drift is also observed. A maximum reduction of 22.26% storeydriftdue to wind load and 12.15% due to seismic load is obtained. For Case 4, results are found by adding belt truss and its effect on different parameters is shown in Chart -3 and Chart-4 112 114 116 118 120 122 124 126 128 130 WIND LOAD SEISMIC LOAD Single outrigger with composite slab and X- shape belt truss Single outrigger with composite slab and inverted V- shape belt truss Chart -3: Lateral deflection due to wind and seismic load
- 5. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072 © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1810 0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 WIND LOAD SEISMIC LOAD Single outrigger with composite slab and X- shape belt truss Single outrigger with composite slab and inverted V- shape belt truss Chart -4: Storey drift due to wind and seismic load A minor difference of 0.45% between performance of X and inverted V-shape towards lateral stiffnessisobserved.Butit is clearly seen that X shape gives more stability to the structure compared to inverted V-shape. Type of structure Lateral Deflection (mm) due to wind load Lateral Deflection (mm) due to seismic load Two-outrigger system 126 131 Single-outrigger system 146 150 Single-outrigger system with composite deck slab 136 146 Single-outrigger system with composite deck slab and X-shape belt truss 118.47 128.9 Table-1: Comparison of lateral deflection for different structures Type of structure Storey Drift due to wind load Storey Drift due to seismic load Multi-outrigger system 0.001248 0.001321 Single-outrigger system 0.001298 0.001613 Single-outrigger system with composite deck slab 0.001194 0.001417 Single-outrigger system with composite deck slab and X-shape belt truss 0.001057 0.001273 Table -2: Comparison of storey drift for different structures From Table-1 and Table 2, it can be seen that single outrigger system with slabs and belt truss is found to have equal lateral stiffness compared to the two outriggers system. Hence the optimization process adopted is found to be feasible and effective. 5. CONCLUSIONS 1) When lateral deflection is considered providing a single outrigger at mid height of a building is found to be the optimum location for both wind and seismic loads. 2) Floor diaphragms provided in the form of compositedeck slab and horizontal steel bracing enhances building lateral behaviour. 3) The maximum reduction of 15% due to wind loads by horizontal steel bracing and 2.67% due to seismic loads by composite slabs is achieved. 4) Further increase in lateral stiffness is obtained by adding belt truss on outrigger floors . X-shaped is found to have performed better than inverted V-shape. 5) It is concluded that a two-outrigger structure can be replaced by single outrigger structure by adding slabs and belt truss and is found to be equivalent stiffer in resisting lateral loads. Hence this proves to be a good alternative solution to mitigate outrigger disadvantage of occupying more space in a building.
- 6. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072 © 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1811 REFERENCES [1]. Mir M.Ali, Kyoung Sun Moon (2007) “Structural Developments in Tall buildings- Current Trend and Future Scope”. [2]. Bungale S. Taranath (2004) “ Wind and Earthquake Resistant Buildings”. [3]. K.L. Chang , C.C. Chen (2008) “ Outrigger System Study For Tall Building Structure with Central Core and square Floor Plate”. [4]. S.M. Sayeed , Ghulam Ahmed (2013) “Effect of increased slab stiffness and outrigger system onFlatslabRCbuilding". [5]. Kiran kamath, Avinash A.R. and S. Upadhyaya(2014) “A Study on the performance of multi-outrigger structure subjected to seismic loads”. [6]. Kiran Kamath,N.Divya andAsha URao(2012) “Optimum Positioning of Outrigger to Reduce Differential Column Shortening Due to Long Term Effects in Tall Buildings”. [7]. Wind loads (IS: 875 Part-3) -1987 [8]. Earthquake loads (IS:1893 Part-1) -2002