A Study on Tall RC Structure with Outrigger System Subjected to Seismic and Wind Loading

A Study on Tall RC Structure with Outrigger System Subjected to Seismic and Wind Loading

Roy Shyam Sundar

PG Student Department of Civil Engineering MGMS College of Engineering Kamothe, Navi Mumbai, India

Gore. N. G

Assistant Professor Department of Civil Engineering MGMS College of Engineering Kamothe, Navi Mumbai, India

Abstract – Finite Element Analysis of the tall slender RC structure with and without outrigger system has been carried out to study the behaviour of tall RC structure in terms of time period, base shear, base moment, story displacement & story drift. Due to reduction of lateral stiffness with the increase in height of structures, outrigger system has been proposed in the present study to minimize the effect due to loss of stiffness. The three dimensional model has been considered and analyzed for the gravity loading, seismic and wind loading, for seismic loading both Equivalent Static Method (Static) and Response Spectrum Method (Dynamic) has been carried out, and in the same manner for wind loading both IS Code coefficient method (Static) and Gust Factor method (Dynamic) has been carried out, IS 875 part III has been used for wind loading calculation and analysis, and IS 1893:2002 has been used for seismic loading calculation and analysis.

Keywords – Outrigger, Wind Load, Seismic Load, Gust Factor, Response Spectrum, Tall

1. INTRODUCTION

   Now a day it is the most common trend in the world of structures to go for the tall buildings, this trend has raised many issues with it to be taken into consideration; the major issue that affects the design of tall structures is its sensitivity to the lateral load. One of the important criteria for the design of tall buildings is lateral sway (deflection) and storey drift together along with the strength criteria.

   Now the question is how to ensure that the considered structure is safe and stable from the deflection and drift point of view, to ensure this IS code has laid some guidelines, according to the IS456:2000, Clause 20.5 Under transient wind load the lateral sway at the top of should not exceed H/500, where H is the total height of the building and according to IS1893:2002, Clause 7.11.1 The storey drift in any storey due to the minimum design lateral force, with partial load factor of 1.0 shall not exceed

   1. times the storey height, and as per Clause 4.23, Storey drift may be defined as the displacement of one level relative to the other level above or below.

      OUTRIGGER

      outriggers are deep, stiff beam which connects the central core to the exterior columns which help in keeping the columns in their position in turn reducing the sway, Outriggers act as stiff arms engaging outer columns/shear

      walls, when the lateral wind or earthquake force attacks the central core, central core tries to tilt, its rotation at the outrigger level induces a tension-compression couple in the outer columns acting in opposition to that movement acting on the core at that level.

      Outriggers have been used in tall buildings for nearly half a century, but the design principles has been used for millennia, the oldest outriggers were horizontal beams connecting the main canoe shaped hulls of Polynesian oceangoing boats to outer stabilizing floats or amas, the outrigger concept is in widespread use today in the design of tall buildings. In this concept, outrigger trusses extend from a lateral load-resisting core to columns at the exterior of the building.

      The key idea in conceptualizing the structural system for a narrow tall building is to think of it as a beam cantilevering from the earth, The laterally directed force generated, either due to wind blowing against the building or due to the inertia forces induced by ground shaking, tends both to snap it (shear), and push it over (bending), therefore, the building must have a system to resist shear as well as bending. In resisting shear forces, the building must not break by shearing off and must not strain beyond the limit of elastic recovery, when an outrigger-braced building deflects under wind or seismic load, the outrigger which connects to the core wall and the exterior columns/shear walls, makes the whole system to act as a unit in resisting the lateral force, the primary result of the outrigger trusses is the development of axial forces in the exterior columns due to lateral load action.

      Outrigger serve to reduce the overturning moment in the core that would otherwise act as a pure cantilever, and to transfer the reduced moment to columns/shear walls, outside the core by the way of tension-compression coupled, which take advantage of the increase moment arm between these columns, in high-rise building this same benefit can be realized by a reduction of the base core over- turning moments and the associated reduction in the potential core uplift forces.

      There are two types of outrigger system

      1. Conventional outrigger system

      2. Virtual outrigger system

   Conventional outrigger system

   In the conventional outrigger system, the outrigger trusses or girders are connected directly to shear walls or braced frames at the core and to columns located outboard of the core. Typically (but not necessarily), the columns are at the outer edges of the building. Figure 1.9 is an idealized section through a tall building.

   Virtual outrigger system

   1. To prepare a three dimensional model in ETABS and to analyze the structure using Finite element analysis approach

      1. METHODOLOGY

         The said structure is modeled as three dimensional structure and all the loads are applied, gravity loading such as dead load and live load in the direction of gravity, lateral loads such as seismic and wind, and the behaviour of the structure has been studied and it has been insured the drift and displacements are within the limits specified by Indian standards.

         Fig. 1Types of Outrigger

         Virtual outrigger system

         Conventional Virtual

         Conventional Virtual

         Model Data

         Material Properties

         • Shear wall, core wall & column M70

         • Slab & beam M35

         • Reinforcement FE415

         • Fe490 for structural steel FE490

           Section Properties

         • Beam – 300X450/700mmm modeled as line

         • Shear wall – 750mm thk. modeled as shell

         • Slab 150 mm thk. Modeled as shell

         • Outrigger SHS 200X200X30mm

           Gravity loading

         • Floor Finish 1.5 kN/m2

         • Water proofing – 1.5 kN/m2

         • Live Load 2 kN/m2, 3 kN/m2

         • Terrace Live load 1.5 kN/m2

         • Wall load 230 mm thk. 9.16 kN/m,

   In the conventional outrigger system, outrigger trusses connected directly to the core and to outboard columns convert the virtual outrigger concept, the same transfer of overturning moment from the core to elements outboard of the core is achieved, but without a direct connection between the outrigger trusses and the core. The elimination of a direct connection between the trusses and the core avoids many of the problems associated with the use of outriggers.

2. OBJECTIVES OF STUDY

1. To study various parameters such as Base shear, Base moment, Time period, Story displacement & Story drift.

2. To compare the behaviour of the structure with & without outrigger system.

3. To carryout static and dynamic analysis along with gust factor analysis.

4. To compare the effect of outriggers y both Equivalent static method and Dynamic Analysis method (Response spectrum method).

5. To insure that the deflection and displacement are within the permissible limits as specified by IS standards.

   • Wall load 150 mm thk. 6.09 kN/m

   • parapet wall load 3.8 kN/m

Seismic loading

Zone factor 0.36, for zone V, Table 2 of IS1893:2002 Building Frame Systems Ductile shear walls Response reduction factor 4, Table 7 of IS1893:2002 Soil Type I, Hard soil, Table 1 of IS1893:2002 Importance factor 1, Table 6 of IS1893:2002

Time Period 3.3Sec, user defined No. of modes to be considered 12 Modal Analysis Ritz

Modal combination CQC, cl 7.8.4.4 of IS1893:2002 Directional combination SRSS

Initial scale factor Ig/2R Minimum eccentricity 0.05

Damping 5 percent, cl 7.8.2.1 of IS1893:2002 Mass source 1DL + 0.25 LL

Diaphragm type Semi rigid

Wind loading

Wind speed 44m/s Terrain category 2 Structure class B Risk coefficient, K1 1

Topography factor 1 Parapet height 1.2m

Windward coefficient, Cp 0.85 Leeward coefficient, Cp 0.75

Fig. 2 Typical Plan View

Method of analysis

Following analysis has been carried in addition to analysis for gravity loading

Seismic analysis

Static method Equivalent static method Dynamic method Response spectrum method

Wind analysis

Static method IS 875 coefficient method Dynamic method Gust factor method

Fig. 3 Typical Elevation view

Load case

DL Selfweight of structure SDL floor finish, waterproofing LIVE live load on floors

TERRACE LIVE Live load on terrace LLNR Non reducible live load, MEP load EQX, EQY Seismic load

WX, WY wind load

SPECX, SPECY Response spectrum case GUSTX, GUST Y Gust factor load

Load Combinations

Limit state of strength 1. 1.5DL

2. 1.5DL+1.5LL

3. 1.5DL+1.5WL/EQ

4. 0.9DL+1.5WL/EQ

5. 1.2DL+1.2LL+1.2WL/EQ

Limit state of Serviceability

1. 1DL

2. 1DL+1LL

3. 1DL+1WL/EQ

4. 1DL+0.8LL+0.8WL/EQ

P-Delta load combination 1.2DL+1.2LL

1. RESULTS AND DISCUSSION Case 1 Conventional, without outrigger, G+50-C Case 2 With outrigger system, G+50-OUT

   1. Time Period

      Fundamental Natural Time Period of Structure
      Type of	G+50-C	G+50-OUT
      ESA	3.302	3.302
      RSA	7.639	6.821
      ESA-Equivalent Static Analysis, MA-Modal Analysis

      MA

      CONVENTIONAL MA OUTRIGGER

      G+50

      ESA

      9

      8

      7

      6

      5

      4

      3

      2

      1

      0

      TIME PERIOD(SEC)

      Table 1- Fundamental Natural Time Period

      TIME PERIOD CHART FOR

      EQUIVALENT STATIC ANALYSIS & MODAL ANALYSIS

      *EFFECT OF OUTRIGGER

      Fig. 4 Time Period Comparison

      Modal Time Period, Modal Analysis
      Mode No.	G+50-C	G+50-OUT
      1	7.639	6.821
      2	7.256	5.652
      3	5.867	4.483
      4	2.049	1.662
      5	1.465	1.239
      6	0.981	0.98
      7	0.644	0.784
      8	0.593	0.576
      9	0.377	0.414
      10	0.375	0.355
      11	0.196	0.229
      12	0.188	0.124

      Table 2 Time Period from Modal analysis

   2. Base Shear

      TABLE: Base Reactions
      	G+50-C-1		G+50-OUT-1
      Load Case	Shear	Moment	Shear	Moment
      	kN	kN-m	kN	kN-m
      SPECX Max	30361.17	2581028	43840.13	2558829
      GX	31153.56	3077529	31153.56	2917261
      EQXD 1	34203.53	4684011	34512.26	4477789
      SPECY Max	30906.61	2393452	33229.19	2382900
      GY	34268.92	3198649	34268.92	3081384
      EQYD 1	34203.53	4447141	34512.27	4321560

      CONV

      SPEC X OUT GUST X

      CONV

      GUST X OUT EQ X

      CONV

      EQ X OUT

      BASE MOMENT CHART OF G+50

      SPEC X

      5000000

      4000000

      3000000

      2000000

      1000000

      0

      BASE MOMENT, My(kN-M)

      Table 3 Base Shear of conventional & outrigger system

      CONV

      SPEC Y OUT GUST Y

      CONV

      GUST Y OUT EQ Y

      CONV

      EQ Y OUT

      BASE MOMENT CHART OF G+50

      SPEC Y

      5000000

      4000000

      3000000

      2000000

      1000000

      0

      *EFFECT OF

      OUTRIGGER

      BASE MOMENT, Mx(kN-M)

      Fig. 5 Base Moment Comparison, My

      *EFFECT OF

      OUTRIGGER

      Fig. 6 Base Moment Comparison, Mx

      GUST X

      CONV GUST X OUT SPEC X CONV SPEC X OUT EQ X CONV EQ X OUT

      STORY V/S DISPLACEMENT

      G+50, SOIL TYPE I, X-DIRECTION

      55

      50

      45

      40

      35

      30

      25

      20

      15

      10

      5

      0

      STORY

   3. Story Displacement.

      100 200 300 400

      DiISPLACEMENT, mm

      0

      STORY

      Fig. 7 Story Displacement Comparison

      STORY V/S DISPLACEMENT

      G+50, SOIL TYPE I, Y-DIRECTION

      55

      50

      45

      40

      35

      30

      25

      20

      15

      10

      5

      0

      0 100 200 300

      DiISPLACEMENT, mm

      GUST Y

      CONV GUST Y OUT SPEC Y CONV SPEC Y OUT EQ X CONV EQ X OUT

      Fig. 8 Story Displacement Comparison

   4. Top Story Displacement

      Top Story Displacement
      		G+50-C	G+50-OUT
      Load Case	Direction	Maximum	Maximum
      		mm	mm
      SPECX	X	199.3	116.7
      GX		230.9	132.437
      EQX	X	368.6	205.264
      SPECY	Y	140.1	78.3
      GY	Y	179.8	98.0
      EQY	Y	267.9	143.3

      Table 4 Top Story Displacement Comparison

   5. Story Drift.

   STORY V/S STORY DRIFT

   G+50, SOIL TYPE I, X-DIRECTION

   55

   50

   45

   40

   35

   30

   25

   20

   15

   10

   5

   0

   GUST X

   CONV

   GUST Y

   OUT

   SPEC Y CONV

   SPEC Y OUT

   EQ Y CONV

   EQ Y OUT

   0

   0.004

   0.002

   STORY DRIFT

   GUST X

   OUT

   SPEC X CONV

   SPEC X OUT

   EQ X CONV

   EQ X OUT

   0

   STORY

   STORY

   Fig. 9 Story Drift Comparison, X-Direction

   STORY V/S STORY DRIFT

   G+50, SOIL TYPE I, Y-DIRECTION

   55

   50

   45

   40

   35

   30

   25

   20

   15

   10

   5

   0

   GUST Y

   CONV

   0.004

   0.002

   STORY DRIFT

   Fig. 10 Story Drift Comparison, Y-Direction

2. CONCLUSION & FUTURE COPE OF STUDY The most effective and deciding basic parameter studied during this whole analysis was drift and deflection of the structure. Fig 7 to fig. 10 shows the variation of drift and deflection of the structure with & without outrigger system. It is observed that the deflection is reduced by 40% after adding outriggers to the conventional structure. Base moment is also reduced by adding outriggers to the conventional structure is controlled by providing the outrigger at 0.3H, 0.7H & 1H, where H is the height of structure.

The following conclusions are made from the present study.

1. The use of outrigger system in high rise structure increases the stiffness and makes the structure more efficient under seismic and wind loading.

2. The maximum displacement at the top of structure Without outrigger system was 368.6 mm, which was reduced to 202.5 mm after providing outrigger at 0.4H 0.7H & 1H.

3. The reduction in drift is more than 50 % at the story where outrigger is provided.

4. It can be concluded from this study that the outrigger system provides reduction in displacement, drift and base moment, which will further the size and depth of foundation.

5. It can be concluded after reviewing the values in table no.03 that Equivalent static analysis load case governs the analysis as well as design, as the base shear generated by the Gust Factor load case among all load case.

Scope of future study

1. More detailed analysis such as Pushover analysis and time history analysis will carried out of the said structure and their results will be compared

2. Structural design of the members namely beam, column slab, shear wall will be done

3. Ductile detailing of the structure will be done

4. Further optimization of the sizes of the members will be done

5. Analysis will be done on one unsymmetrical plan and the results will be compared with the symmetrical layout plan.

REFERENCES

1. Dr. Vinod Hosur Earthquake Resistant Design of Building Structures willey publications, first edition 2013.

2. Krunal z. Mistry, proff. Dhruti j. Dhyani: Optimum outrigger location in outrigger structural system for high rise building, International Journal of Advance Engineering and Research Development Volume 2,Issue 5, May -2015

3. Kiran Kamath, N. Divya, Asha U Rao: A Study on Static and Dynamic Behaviour of Outrigger Structural System for Tall Buildings, Confirming International Journal of Industrial Engineering and Management Science, Vol2, No 4, December 2012

4. Krunal z. Mistry, proff. Dhruti j. Dhyani: Optimum outrigger location in outrigger structural system for high rise building, Intern

5. National Journal of Advance Engineering and Research Development Volume 2,Issue 5, May -2015

6. Alpana L. Gawate J.P. Bhusari: Behaviour of outrigger structural system for high rise building, International Journal of Modern Trends in Engineering & Research, e-

   ISSN No.:2349-9745, Date: 2-4 July, 2015

7. Vijaya Kumari Gowda M R and Manohar B C: A Study on Dynamic Analysis of Tall Structure with Belt Truss Systems for Different Seismic Zones, International Journal of Engineering Research & Technology (IJERT) Vol. 4 Issue 8,

   August 2015

8. Kiran Kamath, Shashi kumar Rao and Shruthi : Optimum Positioning of Outriggers to Reduce Differential Column Shortening Due to Long Term Effects in Tall Buildings, International Journal of Advanced Research in Science and Technology, Volume 4, Issue 3, 2015, pp.353-357.

9. Prateek N. Biradar, Mallikarjun S. Bhandiwad:A performance based study on static and dynamic behaviour of outrigger structural system for tall buildings, International Research Journal of Engineering and Technology (IRJET),

   Volume: 02 Issue: 05 Aug-2015

10. S. Fawzia and T. Fatima: Deflection Control in Composite Building by Using Belt Truss and Outriggers Systems, World Academy of Science, Engineering and Technology Vol:4 2010-12-22

11. Shivacharan K, Chandrakala S , Karthik N M: Optimum Position of Outrigger System for Tall Vertical Irregularity Structures, IOSR Journal of Mechanical and Civil Engineering, Volume 12, Issue 2 Ver. II (Mar – Apr. 2015), PP 54-63