Optimization of the structural systems using the concept of outrigger braced frame system for high-rise structure using three dimensional

analyses

Abstract – Control over the economy and use of the efficient structural systems for high rise structure are the two main factors which appreciate the use and construction of tall buildings. This paper presents the use of an efficient structural system which is more economical, more resistive to earthquake forces, wind forces, controlling the overall displacement and the relative drifts of the high rise structure named “Outrigger braced frame system”. Economy and optimization are the main concern of this system. The system has been compared with basic structural system, i-e moment resisting frame and building frame system and the efficiency of Outrigger Braced frame system over the basic structural systems has been concluded, Beside this, trail method is used to locate the optimum location of outrigger system in a high rise structure, different cases were made according to the location using extended three dimensional analysis of building system “ETABS”. Index term-----Economy, Optimization and Efficient Structural system for high rise structure OUTRIGGER SYSTEM AND ITS USE IN OPTIMIZATION OF THE STRUCTURAL SYSTEMS

Outrigger braced frame system is a type of structural system in which the internal core wall is connected to the exterior periphery columns by the flexurally stiff horizontal cantilever known as Outrigger Arms [1]. Outrigger arms are generally the deep beam having depth equal to the full story height in which the outrigger system has been provided. In order to involve the periphery columns a belt truss is provided in the form of reinforced wall or steel girder [2]. The main purpose of providing outrigger system is providing lateral stiffness to the high rise structure against lateral forces which mainly include Seismic and wind forces. The system is generally used for slender high rise building. Outrigger arms can be provided at only one side of the high rise structure or at the both side of the structure. See Fig 1.1, 1.2

OUTRIGGER SYSTEM

Outrigger braced frame is a system which consist the concept of providing outrigger arms in the form of deep beams which will extend from the core wall to the exterior columns for the bracing of core wall which is provided in the center of the building as without bracing the internal core wall is just like a free standing cantilever and due to its free standing behavior the moments produced due to both seismic and wind forces will also be greater as a result the lateral displacement of the high rise structure increased an ultimately the relative drift too, while in an outrigger system contain bracing for the core wall in the form of outrigger arms which turns to reduce the effect of lateral loads( seismic and wind) and provide more stiffness to the core wall which has been designed for resisting the effect of the lateral forces Fig 1.3 [3].Beside this the use of outrigger belts truss engages the overall parameter column in the phenomena of reducing the lateral displacement by the use of tension-compression phenomenon which include the concept that when the structure is under the action of lateral forces the building will deflect inducing tension in one side of the building and compression on the other side[4] Fig 1.4

1.1 1.2

Providing outrigger system means is providing stiffness to the tall structure on the particular level which causes a resistance to break the bending moments which are induced in the structure. Outrigger arms are provided according to the height of the building and location of outriggers. The story where the outrigger system is installed is known as outrigger story, and it is generally used for the mechanical purpose like heavy machinery [5]. each installment of outrigger system reduces the drift by 30 % [6].

COMPARISON OF DIFFERENT FRAMMING SYSTEMS WITH

OUTRIGGER

ANALYZATION

To check the optimization of the structural system for a high rise structure a thirty story building having 360 feet height is analyzed and designed through ETABS using the four structural system i-e Moment resisting frame system, Ordinary Building frame system, Dual frame system and Outrigger braced frame system for the same loading condition and for the same seismic zone.

All the input data was the same according to the behavior of the structural system and four different models were prepared according to the preliminary design of the structural elements the same loading was applied to all the structural members and the models were analyzed in order to check the parameters like Displacement & drifts under the action of lateral loads.

The first model of moment resisting frame was found to have higher drifts value not following

the allowable limits provided by the ACI code and with greater lateral displacement was recorded Fig 2.1. The second model of building frame was found to have less displacement and drifts as compared to the moment resisting frame, the building frame was found in following the ACI provision for drifts and Displacement Fig 2.2 the last model of Outrigger braced frame system was analyzed and the output received was more appreciable and efficient than all the three system

OPIMIZATION IN DIPSLACMENTAGAINST EARTH QUAKE AND WIND

FORCESThe analyzed models were checked for the maximum displacement under the action of seismic forces and wind forces, the

(2.1)Exceed the allowable drift limit

(2.2)Drift has been reduced from 0.004 to 0.0013

( 2.3)Drift has been reduced from 0.0025 to 0.0013 and 0.003 to 0.0013

1.3 1.4

comparison which shows the efficiency of the outrigger braced frame system over the others system are given in the charts and Fig 3.1 ,3.2 forBoth the seismic and wind forces

(3.1)LATERAL DISPLACMENT DUE TO EARTH QUAKE

(3.2)LATERAL DISPLACMENT DUE TO WIND FORCES

DESIGNING

The preliminary design for gravity loads was used and form that minimum sizes of the structural members were chosen and the models were designed through “ETABS”.

All the models were designed for the same loading condition. The design results of moment resisting frame shows the maximum number of columns and beams failure, the ordinary building frame was found having failure in columns beams and shear wall the same results was found in the dual system, the outrigger braced frame system was designed and it was found that all the structural member were passed to the most safer and economical design.

OPTIMIZATION IN SIZES For optimization, all the

structural models should have no failure in their members for that we have increase the member sizes in the model for bearing the gravity loads and as well as lateral loads, the outrigger braced frame system was found that do not need any change in their structural member sizes, the member sizes and increase

in their sizes are given in the table 3.3.

STORY LEVEL

MOMENT RESISTING

FRAME

BUILDINGFRAME

OUTRIGGER BRACED FRAME

30th 20.35” 13.64” 8.941”20th 14.96” 9.69” 5.815”

10th 6.94” 4.10” 2.54”

SYSTEM

MRF BFS OUTRIGGER

BEAM

18” X 40” (OVERALL)

12”X 32”(OVERALL)

10” X 30” (OVER

ALL)

COLUMN

42” X 42” LEVEL26-30

39” X 39” LEVEL 26-30

30” X 30” LEVEL 26-

30

COLUMN

45” X 45” LEVEL 17- 25

42” X 42” LEVEL 17- 25

33” X 33” LEVEL 17-

25

COLUMN

48” X 48” LEVEL 10- 16

45” X 45” LEVEL 10- 16

36” X 36” LEVEL 10-

16

COLUMN

51” X 51” LEVEL 1 - 9

48” X 48” LEVEL 1 - 9

39” X 39” LEVEL 1 – 9

SHEAR

WALL

_ _24” THICKLEVEL 4 –

30

SHEAR

WALL

_30” THICK

LEVEL 1 – 3034” THICK

STORY 1 – 3

STORY LEVEL

MOMENT RESISTING

FRAME

BUILDINGFRAME

OUTRIGGER BRACED

FRAME

30th 8.96” 6.19” 4.43”20th 7.22” 4.77” 3.32”

10th 3.75” 2.2” 1.62”

OPTIMIZATION FOR THE BEST LOCATION OF OUTRIGGER SYSTEMTrail method was used in order to find the optimum location for outrigger system in a high rise structure in order to bear the lateral forces efficiently for these five different cases were made which are as underCase1: Providing outrigger arm at the 30th

level (Top) of the structure Case 2: Providing Outrigger arm at both 15th

and 30th level of the structure Case 3: Providing Outrigger arm at the 15th

level (mid height) of the structure Case 4: Providing Outrigger arm at both 10 th

and 20th level of the structure Case 5: With no Outrigger arm (Ordinary Building Frame system) The cases were checked for criteria of minimum drift and displacement, the given graph show the use of best location for outrigger system which can bear both the wind and seismic forces efficiently.

OPTIMIZATION IN DRIFT CONTROLThe model were analyzed and the following results were obtained using the same loading condition Fig 3.1, 3.2, 3.3, 3.4, 3.5

(3.1) Drift reduced from 0.004 to 0.0013

(3.2) Drift reduced from 0.0040 to 0.0013 & 0.0030 to 0.0009

(3.3)Drift reduced from 0.0041 to 0.0023

(3.4) Drift reduced from 0.0025 to 0.0013 &

from 0.003 to 0.0013(3.5) Drift reduced from 0.004 to 0.0023

OPTIMIZATION IN DISPLACMENT DUE EARTH QUAKE FORCES

The cases were checked according to the condition and the following results were obtained given in the following figure and chart Fig 3.6

(3.6)Lateral Displacement under Earth Quake Forces

OPTIMIZATION IN DISPLACMENT DUE TO WIND FORCES

The models were checked for the wind speed of 100mph and the optimized outrigger results

are given in the following figure and chart.

Lateral Displacement Due To Wind Forces

OPTIMIZATION IN CONTROLLING BASE SHEAR

An increase in the structural member sizes increase the dead load of the structure and ultimately this dead load increases the base shear of the Structure. The increase in structural member sizes in moment resisting frame and building frame system results in increase in the base shear while outrigger braced frame system needs no change and so the base shear is constant and minimum as compared to both the structural system. Difference in base shear, dead load and Ft are given in the chart

EONOMICAL BENEFITS AND COST COMPARISON

The main part which include the optimization in economy by using outrigger braced frame system all the structure were designed in the most economical range and their respective sizes are selected for the cost comparison of all the three structure, mainly concrete and steel has been calculated for all the structure and their quantities are listed down in the following figures.

The difference in the quantity of steel and concrete are due to the varying sizes of the structural member for the different system which greatly affect the economy of the structural system. Difference in the quantities and their respective cost and their difference are given in the following graphs

Story level

No out-rigger

@ 15th

level

@ 30th

level@

10th

& 20th

level

@ 15th & 30th level

10th

level13.64” 12.17” 16.08” 8.941” 10.93”

20th level

9.69” 7.82” 12.23” 5.185” 7.75”

30th level

4.10” 4.19” 4.98” 2.54” 4.21”

Story level

NO Out-rigger

@ 15th

level@30th

level@10th & 20th level

@ 15th & 30th

level30th

level6.19” 6.1” 8.18” 4.43” 5.7”

20th level

4.84” 4.462” 6.65” 3.35” 4.44”

10th level

2.25” 2.66” 2.65” 1.628” 2.62”

SYSTEM MRF BFS OUT-RIGGER

DEAD LOAD

104264 Kips

97442.86 Kips

86337.87 Kips

Ft USED 502.02 416.63 369.22R USED 5.5 5.5 5.5TOTAL BASE SHEAR

2918.56 Kips

2571.96 Kips

2279.32 Kips

The economic benefit of outrigger braced frame system is that it is 25% economical than ordinary building frame system and 34% than moment resisting frame system.Conclusion:COMPARISON WITH OTHER SYSTEMSThe use of outrigger braced frame system for a high rise structure is quite efficient comparatively with the three basic structural system i-e Moment resisting frame system, ordinary building frame system, and dual or hybrid system in controlling the following parameters.Controlling the Drift, Controlling the overall Displacement, Controlling the Base Shear, Controlling the core wall moments, Providing Stiffness to structure, Controlling the Weight of the Structure, Control over the economy

COMPARISON FOR OPTIMIZATIONChanging the location of the outrigger system greatly affect the economy and as well as the behavior of the structure in controlling the effects of the wind and seismic forces, providing outrigger at 10th and 20th level was found to be more efficient in controlling the overall effects and outrigger at 15th level was found to be the most economical one

REFERENCES[1]Choi,H,Ho.,G.,Joseph,L.&Matthias(2012)Outrigger Design for High-RiseBuildings:And output of the CTBUH Outrigger Working Group, Council on Tall Building and Urban Habitat:Chicago

[2] Fawzia, S., Fatima, T (2010) Deflection control in composite building by using Belt truss and Outrigger System: World Academy of Science, Engineering and Technology

[3]Kian,Seng,Po.,Siahaan,Torang,First.,(2001)THE USE OF OUTRIGGER AND BELTS TRUSS SYSTEM FOR HIGH RISE CONCRETE BUILDINGS: Department of Civil Engineering Petra Christian University & Department of building Science, National University Singapore

[4]Nanduri,Kiran,Raj.,Suresh,B.,Hussain,Ihtesham(2013)Optimum Position of Outrigger system for High-Rise Reinforced concrete Buildings Under Wind and Earthquake forces: And an output from American Journal of Engineering Research(AJER)2013

[5]Hoenderkamp, Bakker. Snijder, H., Preliminary design of high-rise outrigger braced shear wall structure on flexible foundation: Eindhoven University of Technology, Eindhoven, and The Netherland

[6]Bayati, Z., Mahdikhani, M., Rahaei, A., (2008) Optimized use of Multi-Outriggers System to stiffen Tall Buildings: World Conference on Earthquake Engineering, Amirkabir University of Technology