View of COMPARATIVE STUDY OF EFFECT OF PRESENCE OF FLOATING COLUMNS AND BRACINGS ON RC FRAMED STRUCTURES IN SEISMIC ZONES

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Vol.03, Issue 09, Conference (IC-RASEM) Special Issue 01, September 2018 Available Online: www.ajeee.co.in/index.php/AJEEE

COMPARATIVE STUDY OF EFFECT OF PRESENCE OF FLOATING COLUMNS AND BRACINGS ON RC FRAMED STRUCTURES IN SEISMIC ZONES

Prof. Shreyans Kumar Jain¹, Prof. Supriya Tripathi²

1, 2 Assistant Professor, Department of Civil Engineering, SIRT, SAGE University, Indore, M.P., India

Abstract: Floating columns are now-a-days becoming an unavoidable feature of multistory and high rise buildings. This is due to lack of space available for various accommodations like recreational facility, parking space, etc. This introduces vertical irregularity in the structure and lessens the stiffness of the storey below the floating columns. The stiffness can be improved by providing bracings, shear wall to the structures. In the present paper comparative study of combinations of floating column and bracings are done. For the purpose four models of a 2-D RC framed structure are considered, analysed and designed with the help of STAAD.Pro. The models are assumed to be situated in Seismic Zone IV.

Equivalent Static Method of analysis is applied. Comparisons are made on the basis of various important parameters such as column forces, Beam forces and displacement data.

Models are also compared with economy point of view also. It is concluded that the model with bracings at the storey where floating column is introduced is the best amongst the other model.

Keywords: Floating Column, Bracings, Equivalent static method, Base Shear, Storey displacement, Storey Drift

1. INTRODUCTION

In recent times, multi-storey buildings in urban cities are required to have column free space because of shortage of space, growing population and also for aesthetic and functional requirements. For this, buildings are provided with floating columns at one or more storey. In general, a column is a vertical member starting from foundation level and transferring the load of the structure to the ground.

Floating column is also a kind of column which at its termination level rests on a horizontal member which is generally a beam which in turn, transfers the load to other columns below it.

Floating columns are usually adopted above the ground storey level so that maximum space is made available in the ground floor which is essentially required in apartments, mall or other commercial buildings where parking is a major problem. For example, for hotels, malls, office complex or any commercial building, where the lower floors contain banquet halls, conference rooms, reception lobbies, show rooms or parking areas, large uninterrupted space required for the movement of people or vehicles.

Closely spaced columns based on the layout of upper floors are not desirable in the lower floors. So to avoid that problem concept of floating column has come into existence. Similar is the case for residential buildings as these open spaces

may be required for assembly hall or parking purpose.

Providing floating columns may satisfy some of the functional requirements but structural behaviour changes abruptly. A structure with floating column can be categorized as vertically irregular as it causes irregular distributions of mass, strength and stiffness along the building height. The flexural and shear demand of the beams which supports floating columns are much higher than surrounding beams which results in stiffness irregularities at a particular joint. Columns are main lateral load resisting elements in moment resisting frame and play a vital role in seismic performance of building. The stiffness of the storey below the floating column is usually inferior to the storey above and below it. This stiffness of such storey may be improved by providing shear wall or bracings, etc. Bracings are cross linking between two diagonally opposite joints which minimise the lateral displacement of the frame and also distributes the forces.

Here in the present paper an attempt is made to study the effect of presence of floating columns along with the bracings in a 2-D RC framed structure. Floating columns are introduced to create some space for various purposes, at the same time it also reduces the stiffness of the storey below. Bracings obstruct the

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available free space but at the same time they increase the stiffness of that particular storey. In the present work, the various combinations of floating column and bracings are introduced and the overall effects on the structure is studied with the help of STAAD.Pro on the basis of some important parameters such as axial and shear force, bending moment, base shear, storey displacement and storey drift. Economy is also compared by comparing the quantity of material used.

2. METHODOLOGY

To study the effect of various combinations of floating column and bracings, four 2-D RC framed structures are modelled, analysed and designed in STAAD.Pro. A frame of 18m width (6 bays of 3m each) and 36m height (12 floors of 3m each) is considered to be situated in seismic zone IV. The beams are supposed to support a hypothetical slab which is 125mm thick and 1.5m in width either side of beam running along the full length. A dead load of magnitude 12.375kN/m and live load of 9kN/m is assumed to be transferred from slab to beam. The cross section of columns (400x400mm) and beams & bracings (200x400mm) are kept same for all the models.

2.1 Modelling

Following four models are prepared:

i) Model 0: It is taken as basic model with neither floating column nor bracings. (Fig. 1)

ii) Model A: A floating column is created by removing the central column at ground floor (at +3m from foundation level) and thus forming Model A. (Fig.

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iii) Model B: In this model, to increase the stiffness, cross bracings in the bays either side of removed column is provided which are occupying the space in two bays. (Fig. 3)

iv) Model C: In this model, the central column at first floor is also removed and bracings are provided instead.

(Fig. 4)

Fig.1: Model 0

Fig.2: Model A

Fig.3: Model B

Fig.4: Model C

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2.2 Method of Analysis

The Equivalent Static Method in accordance with code IS:1893 – 2002 is being adopted to analyse the structures. It is a simple method of analysis and needs less computational efforts as the forces depend on the fundamental natural period of structures based on the code with some empirical modification. In this method, initially the design base shear or lateral forces are computed for the structure as a whole. Then this design lateral force is distributed to the various floor levels along the height of structure based on simple formulae with regular distribution stiffness and mass. Therefore the overall design lateral or seismic forces are obtained at each floor level and then distributed to the individual lateral load resisting elements.

The Base Shear is calculated by the following expression:

VB = Ah W Where

Ah = Design horizontal seismic coefficient by using fundamental natural period (Ta) = ^𝑍𝐼𝑆_2𝑅𝑔^𝑎

W = Seismic weight of the whole building as per clause 7.4.2

Z = Zone factor.

I = Importance factor

R = Response reduction factor

Sa /g = Average response acceleration coefficient for rock and soil sites.

Ta = Approximate fundamental natural period of vibration for moment resisting frame building in seconds = ^0.09ℎ_√𝑑 h = Height of the building, in m.

d = Base dimension of the building, in m, along the considered principal direction of the lateral force.

This computed design base shear (VB) shall be distributed along the building height by following expression:

Qi = VB 𝑊_𝑖ℎ_𝑖² 𝑊_𝑗ℎ_𝑗² 𝑛𝑗 =1

Where,

Qi = Design lateral force at floor i.

Wi = Seismic weight of floor i.

hi = Height of floor i measured from base.

n = Number of storey in the building (number of levels at which the masses are located)

STAAD.Pro V8i software calculates and applies the static seismic forces to analyse the structure in accordance with the procedures as recommended by the relevant IS Codes.

3. RESULTS & DISCUSSION

Analysis results for all models are obtained and summarised by taking maximum absolute values of each parameter with their location of appearance and compared.

Table 1: Results and Comparison of Column Forces, Beam Forces, Base Shear, Storey displacements and Storey Drift and Quantities of Concrete& Reinforcement Steel

Models Model

0 Model

A Variation

* Model

B Variation

* Model

C Variation

*

COLUMN FORCES Axial

Force (Fx) kN 1454.

87 1999.2

2 1.37

1843.6

6 1.27

2027.6

3 1.39

Location of Col. BGF BGF BGF BGF

Shear-Y (Fy) kN 30.88 59.07 1.91

62.81 2.03

49.73 1.61 Location of Col. 2nd

Flr 1st Flr 1st Flr GF

Moment-

Z (Mz)

kNm 54.81 90.78 1.66

104.26 1.90

90.54 1.65

Location of Col. BGF 1st Flr 1st Flr GF

BEAM FORCES Shear-Y (Fy) kN 70.32 148.72

2.12

149.47

2.13

104.67

1.49 Location of Beam 4th

Moment-

Z (Mz)

kNm 65.82 163.44 2.48

164.93 2.51

99.41 1.51 Location of Beam 3rd

Base Shear 119.3

6 119.02 1.00 119.50 1.00 119.16 1.00

Avg. Disp. of

Top Storey (mm) 27.92 28.21 1.01 24.34 0.87 24.20 0.87 Storey Drift

(mm)

At GF 2.83 3.16 1.12 0.62 0.22 2.46 0.87

Max. 3.01 3.16 1.05 2.94 0.98 2.84 0.94

Location 0.33h 0.17h 0.42h 0.42h

Concrete (M35) Cum. 57.60 57.10 0.99 57.80 1.00 57.30 0.99 Reif. Steel (Fe415) MT 3.95 4.26 1.08 4.22 1.07 3.94 1.00

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* Ratio of Parameters of respective models w.r.t. Model 0

Graph 3.1: Column Axial Force Graph 3.2: Column Shear Force

Graph 3.3: Column Moments Graph 3.4: Base Shear

Graph 3.5: Beam Shear Force Graph 3.6: Beam Moments

1454.87

1999.22

1843.662027.63

0.00 500.00 1000.00 1500.00 2000.00 2500.00

Model 0 Model A Model B Model C Column Axial Force (Fx) kN

30.88

59.07 62.81

49.73

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00

Model 0 Model A Model B Model C Column Shear-Y (Fy) kN

54.81

90.78

104.26 90.54

0.00 20.00 40.00 60.00 80.00 100.00 120.00

Model 0 Model A Model B Model C Column Moment-Z (Mz) kNm

119.36

119.02

119.50

119.16

115.00 116.00 117.00 118.00 119.00 120.00

Model 0 Model A Model B Model C Base Shear

70.32

148.72 149.47

104.67

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00

Model 0 Model A Model B Model C Beam Shear-Y (Fy) kN

65.82

163.44 164.93

99.41

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 180.00

Model 0 Model A Model B Model C Beam Moment-Z (Mz) kNm

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Graph 4.12: Average Displacement Graph 4.13: Storey Drift

It can be seen from above table and graphs that while comparing with Model 0, all parameters are increased when floating column is introduced (Models A, B, C) except displacement parameters which are reduced when bracings are also provided i.e. in Model B & C. Comparing Model B with A where bracings are provided at ground floor, the parameters further increased except column axial force, displacements and drift. It can be noted that the displacement parameters are lesser than the Model 0. But these bracings also blocks free space within the bays. When bracings are provided at first floor as in Model C, all the parameters are reduced w.r.t. Model B except column axial force and drift at ground floor. As the bracings are provided at first floor storey drift is reduced thereat instead of ground floor. Bracings are blocking bay- space at first floor but at the same time extra space is also created by removing the central column thereat and also column free space are not interrupted by bracings. In Model C, the displacement parameters are lesser than that of Model 0. Requirement of concrete and steel is also reduced as compared with Model 0.

4. CONCLUSION

From above study it can be concluded that:

i) Floating column reduces stiffness while bracing provides extra stiffness to the structure.

ii) When floating columns are unavoidable, bracings may be provided at the floor where floating column terminates in the pattern as in Model C.

iii) Model C gives the most economical design.

5. FUTURE SCOPE

Research can be further extended to 3-D model, with different height, different configurations of floating columns and bracings. Study can be done by using other software like SAP, ETABS, etc. and by adopting other methods of analysis.

0 3 6 9 12 15 18 21 24 27 30 33 36 39

0 10 20 30

Height (m)

Avg. Disp. (mm)

Model 0 Model A Model B Model C

0 3 6 9 12 15 18 21 24 27 30 33 36 39

0 1 2 3 4 5

Height (m)

Drift (mm)

Model 0 Model A Model B Model C