Comparative Study and Analysis of Unbraced RCC Framed Structure with Steel Braced RCC Framed Structure using Response Spectrum Method

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Comparative Study and Analysis of Unbraced RCC Framed Structure with Steel Braced RCC Framed Structure using Response Spectrum Method

Comparative Study and Analysis of Unbraced RCC Framed Structure with Steel Braced RCC Framed Structure using Response Spectrum Method

Jonty Choudhary1,

1Student,

  1. Tech (Structural Engineering) GEC Jagdalpur (C.G.)

    Dr. G. P. Khare2

    2 Principal & Professor, GEC Jagdalpur (C.G.)

    Abstract- Lateral forces induced on the structure due to seismic waves have given rise to the study of lateral load resisting elements such as braces, shear wall, dampers etc. These lateral load resisting elements should be analysed before arriving on a best orientation and configuration. This work is an analysis of braces and there orientation, configuration and arriving on a best bracing system. In this project there is a comparison of unbraced RCC framed structure and steel Braced RCC framed structure the steel braces are of different configurations such as X, V, inverted V and their different arrangements have been studied, analysed using ETABS 2015 and results are compared. For the analysis a Special Moment Resisting Frame of G+15 Storey is modelled in ETABS 2015 which is 33m×15m. The objective is to have a comparative perspective of unbraced and steel braced RCC frame, all the frames are subjected to same DL, IL and Earthquake loads all the structure are in the same earthquake zone i.e.; Zone V. It is found that when X-braced RCC frame structure performed best when compared with the parameters of Maximum Storey Displacement, Maximum Storey Drift, Base Shear, Overturning moments and Time period of unbraced RCC frame structure. With the use of X-braced steel bracings the structure can be designed using lighter section and thus economic efficiency can be achieved. High construction quality should be maintained while embedding steel bracing at beam-column joints. IS 13920 (1993): Ductile detailing of reinforced concrete structures subjected to seismic forces Code of practice should be preferred and the codal provisions should be achieved as far as possible.

    Key words- Maximum storey displacement, Maximum storey drift, Base shear, Resisting moments, Time period, ETABS, RCC.

    1. INTRODUCTION

      Earthquake can be understood as the shaking of earths crust which can be due to relative movement in tectonic plates, volcano eruptions and even in case of explosions. There can be huge loss of life as experienced in past earthquakes in India and the world. Apart from loss of life there is economic loss also. Industrialization has given rise to many things one of which is high rise buildings. High rise building requires in depth analysis of response of the building for a given set of load conditions. In general structure is designed for gravity loads, imposed loads, wind loads, snow loads and earthquake loads. Earthquake loads have a different tendency as compared to other loads, these loads induces lateral forces on to the structure. Lateral force induces forces parallel to the plane of the structure this has given rise to study of lateral force resisting elements such as shear wall, dampers, bracings etc. Steel which have good compressive and tensile load carrying property are used as braces. Depending upon the configuration steel braces can

      be classified as concentric and eccentric. Concentric braces such as X, V, inverted V are used.

      In the previous works many such bracing configurations have been studied. This work aims at studying the response of G+15 storied special moment resisting RCC framed structure subjected to earthquake excitation in seismic Zone V( as per IS 1893: (Part 1) 2002). X, V and inverted V braces are used in different arrangements (shown in modelling). All the models are in same seismic zone with same structural properties.

    2. OBJECTIVES OF PRESENT STUDY

      The objective of this work is to know the response of a G+15 storey unbraced RCC framed structure in comparison to concentrically steel braced G+15 storey RCC framed structure with different arrangements of steel bracing. To have an insight in the performance of these steel braced RCC framed structure linear dynamic analysis is performed i.e.; Response Spectrum Analysis. Following are the aspects which are attempted to study:

      1. Comparison of storey displacement for G+15 storied unbraced RCC framed structure and concentrically steel braced G+15 RCC framed structure.

      2. Comparison of storey drift for G+15 storied unbraced RCC framed structure and concentrically steel braced G+15 RCC framed structure.

      3. Comparison of base shear for G+15 storied unbraced RCC framed structure and concentrically steel braced G+15 RCC framed structure.

      4. Comparison of base reaction for G+15 storied unbraced RCC framed structure and concentrically steel braced G+15 RCC framed structure.

      5. Comparison of time period for G+15 storied unbraced RCC framed structure and concentrically steel braced G+15 RCC framed structure.

    3. MODELLING

The structure which is considered in this project work have been analysed in ETABS 2015. A G+15 storied structure is considered for the analysis using Response Spectrum Method. The structure is 33×15 m in plan. Columns are placed in 3m interval in both X and Y direction. The structure lies in earthquake zone V and has a importance

factor 1. Seismic zone factor is 0.36 for zone V as per IS 1893 (Part-1): 2002. The factors which affect the calculation

of seismic forces induced on the structure are:

Structure

SMRF

Number of storey

G+15

Storey height

3m

Seismic zone

V

Seismic zone factor (Z)

0.36

Soil type

medium

Damping ratio

5%

Importance factor

1

Table 3.1- Structural Parameters

Materials

Concrete grade

M30

Steel

Fe 415

Masonry

As per ETABS 2015

Table 3.2- Material Property

Frame Property Dimension

Outer column C1

400*400 (mm)

Inner column C2

450*450 (mm)

Outer beam B1

450*300 (mm)

Inner bream B2

500*300 (mm)

Slab thickness

125mm Membrane type

Bracings

ISA 100*100*8 (mm)

Table 3.3- Frame property and Slab property

Loads

Dead load (DL)

As per the unit weight of material and their volume

Dead load on slab (DL)

2 KN/mm2 (floor finish + partition wall)

Imposed load (IL)

5 KN/mm2

As per IS 875 (Part 2):1987

Masonry load outer wall

6 KN/m

Masonry load inner wall

13.8 KN/m

Earthquake load

As per IS 1893 part I 2002

Table 3.4- Dead Load and Live Load considered

Support/Diaphragm

Support

Fixed

Membrane rigidity

Rigid

Table 3.5- Support and Diaphragm Property

Load combination Load combinations which are used in this work are based on codal provisions which are given in IS 857 part5. Following are load combinations are used to analyse the structure. When earthquake loads are applied to a structure then the loads are resolved in mutually perpendicular directions (as IS 857 part5).

1. 0.9*(DL+Masonry)-1.5*EY

2. 0.9*(DL+Masonry)+1.5*EY

3. 0.9*(DL+Masonry)-1.5*EX

4. 0.9*(DL+Masonry)+1.5*EX

  1. 1.2*(DL+Masonry+IL+EX)

  2. 1.2*(DL+Masonry+IL-EX)

  3. 1.2*(DL+Masonry+IL+EY)

  4. 1.2*(DL+Masonry+IL-EY)

  5. 1.5*(DL+Masonry+EX)

  6. 1.5*(DL+Masonry+EY)

  7. 1.5*(DL+Masonry+IL)

  8. 1.5*(DL+Masonry-EX)

  9. 1.5*(DL+Masonry-EY Models and their bracings

  1. Model 1 -Bare frame

    Fig 1- Side view and Front view of Model 1

    Fig 2- Plan View

  2. Model 2 – X bracing in pairs

  3. Model 3- X bracing alternate arrangement

    Fig 3- Side view and Front view of Model 2

    Fig 4- Side view and Front view of Model 3

  4. Model 4- V bracing in pair

  5. Model 5- V bracing alternate arrangement

    Fig 5- Side view and Front view of Model 4

    Fig 6- Side view and Front view of Model 5

  6. Model 6- Inverted V bracing in pair

    Fig 7- Side view and Front view of Model 6

  7. Model 7- Inverted V bracing alternate arrangement

Fig 8- Side view and Front view of Model 7

Fig 9- 3D model of Unbraced RCC frame

4. RESULTS AND DISCUSSION

Bracings are used to reduce the seismic forces which are developed under the earthquake excitation. As the objectives of the work were to have comparative analyses of unbraced RCC framed structure and steel braced RCC structure. The seismic responses of the models are shows in graphical as well as tabular fashion:

Maximum storey displacement

Displacement is the parameter of maximum importance as it governs the failure pattern of the structure. From the present study, the displacement of the model with and without braces is tabulated and the results are plotted.

Maximum storey Displacement(mm)

S. NO.

Name

X-Direction

Y-Direction

1

Bare RCC frame

34.544

38.182

2

X bracing

18.858

23.225

3

X bracing alternate

31.013

42.278

4

V bracing

31.224

42.338

5

V bracing alternate

31.268

36.377

6

Inverted V bracing

29.864

35.637

7

Inverted V bracing alternate

31.086

36.251

Table 4.1- Maximum Storey Displacement

Comparison of Maximum Storey Displacment

45mm

42.338

40mm

36.377

35.637

Bare RCC X bracing X bracing V bracing V bracing Inverted V Inverted V

frame alternate alternate bracing bracing alternate

38.182 42.278

36.251

35mm

30mm

34.544

31.086

23.225 31.224 31.268

25mm

31.013

29.864

20mm

15mm

18.858

10mm

X Direction

Y Direction

05mm

0mm

Maximum Storey Displacement

Graph 4.1- Comparison of Maximum Storey Displacement

Storey drift

Storey drift is the drift of one level of a multi storey building relative to the level below.

Maximum Storey Drift

S. NO.

Name

X-Direction

Y-Direction

1

Bare RCC frame

0.001065

0.001075

2

X bracing

0.000517

0.000603

3

X bracing alternate

0.00087

0.001126

4

V bracing

0.000904

0.001147

5

V bracing alternate

0.000922

0.000993

6

Inverted V bracing

0.000866

0.00096

7

Inverted V bracing alternate

0.000912

0.000986

Table 4.2- Maximum Storey Drift

0.0014

Comparision of Maximum Storey Drift

0.0012

0.001

0.001075

0.001065

0.001126

0.001147

0.000993

0.00096

0.000986

Storey Drift

0.0008

0.0006

0.0004

0.000603

0.000517

0.00087

0.000904 0.000922

0.000866

0.000912

X Direction Y Direction

0.0002

0

Bare RCC frame X bracing X bracing

alternate

V bracing V bracing alternate

Inverted V bracing

Inverted V bracing alternate

Graph 5.2- Comparison of Maximum Storey Drift

Base shear

Base shear is an estimate of the maximum expected lateral force that will occur due to seismic ground motion at the base of a structure. Calculation of base shear depends on:

    • Soil condition at site.

    • The level of ductility and over strength associated with various structural configurations and total weight of the structure

    • The fundamental (natural) period of vibration of the structure when subjected to dynamic loading

Comparison of Base Shear (in KN)

S. NO.

Name

X Direction

Y Direction

1

Bare RCC frame

3640.4808

3434.6744

2

X bracing

3098.344

2651.5037

3

X bracing alternate

4062.8458

3159.2638

4

V bracing

4011.3911

3126.5164

5

V bracing alternate

4071.3135

3631.4122

6

Inverted V bracing

4296.8971

3771.0553

7

Inverted V bracing alternate

4098.3585

3649.8014

Table 4.3- Comparison of Base Shear

Comparision Of Base Shear

Inverted V bracing alternate

3649.8014

4098.3585

Inverted V bracing

3771.0553

4296.8971

V bracing alternate

3631.4122

4071.3135

V bracing

3126.5164

4011.3911

X bracing alternate

3159.2638

4062.8458

X bracing

2651.5037

3098.344

Y Direction

Bare RCC frame

3434.6744

3640.4808

X Direction

0KN 1,000KN 2,000KN 3,000KN 4,000KN 5,000KN

Resisting Moments

Graph 4.3- Comparison of Base Shear

Comparison of Resisting Moments (in KN-m)

S. NO.

Name

Mx

My

Mz

1

Bare RCC frame

99043.58

106972.24

62906.41

2

X bracing

75129.73

91786.4

49538.18

3

X bracing alternate

89597.52

118577.05

60380.6

4

V bracing

89362.6

117526.48

59719.39

5

V bracing alternate

104004.44

119435.68

67250.13

6

Inverted V bracing

107877.74

126468.51

70072.75

7

Inverted V bracing alternate

104496.57

120197.71

67612.57

Graph 4.4- Comparison of Resisting Moments

Comparision of Resisting Moments

Inverted V bracing alternate

Inverted V bracing

V bracing alternate

V bracing

67612.57

70072.75

67250.13

59719.39

120197.71

126468.51

119435.68

117526.48

104496.57

107877.74

104004.44

Mz

X bracing alternate

X bracing

Bare RCC frame

60380.6

49538.18

62906.41

89362.6

My

118577.05 Mx

89597.52

91786.4

75129.73

106972.24

99043.58

0KN 50,000KN 1,00,000KN 1,50,000KN

Graph 4.4- Comparison of Resisting Momen

Time Period

2.4sec 2.35sec 2.3sec 2.25sec 2.2sec 2.15sec 2.1sec 2.05sec

Time Period (sec)

2.sec 1.95sec 1.9sec 1.85sec 1.8sec 1.75sec 1.7sec 1.65sec 1.6sec 1.55sec 1.5sec 1.45sec 1.4sec 1.35sec 1.3sec 1.25sec 1.2sec 1.15sec 1.1sec 1.05sec

1.sec 0.95sec 0.9sec 0.85sec 0.8sec 0.75sec 0.7sec 0.65sec 0.6sec 0.55sec 0.5sec 0.45sec 0.4sec 0.35sec 0.3sec 0.25sec 0.2sec 0.15sec 0.1sec 0.05sec

0.sec

Comparison of Time Period Model 1 Model 2

Model 3

Model 4

Model 5

Model 6

Model 7

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Mode number

Graph 4.5- Comparison of Time Period

DISPLACEMENT

Displacement is the parameter of maximum importance as it governs the failure pattern of the structure. From this present study, the displacement of the model with and without bracings is observed. By providing the bracings to the structure we observe that the displacement of the structure is reduced for X-bracing used in pairs shown in model 2.

45mm

42.278

42.338

Comparison of Displacment

40mm 38.182

36.377

35.637

36.251

35mm

30mm 25mm

23.225

Displacment

0mm

Model 1

Model 2

Model 3

Model 4

Model 5

Model 6

Model 7

20mm

15mm 10mm 5mm

Displacment (mm)

Graph 4.6- Comparison of Displacement

5. CONCLUSIONS

Steel braced RCC frame was analysed with different arrangements of steel braces and with various types of concentric braces such as X, V, inverted V. After the analysis the results were represented in tabular as well as graphical manner. On the basis of the graphs following are the conclusion inferred:

Maximum Storey Displacement:

Comparative to unbraced RCC frame there is 64.40% reduction in maximum storey displacement in X- direction and 83 18% reduction in Y-direction when X- braced frame (Model 2) arrangement is used.

Maximum Storey Drift:

Comparative to unbraced RCC frame there is 78.27% reduction in maximum storey drift in X-direction and 105.99% reduction in Y-direction when X-braced frame (Model 2) arrangement is used.

Base Shear:

Comparative to unbraced RCC frame there is 17.5% reduction in base shear in X-direction and 29.54% reduction in Y-direction when X-braced frame (Model 2) arrangement is used.

Resisting Moments:

Comparative to unbraced RCC frame there is 31.83% reduction in overturning moments in X-direction, 16.54% reduction in Y-direction and 26.99% reduction in Z- direction when X-braced frame (Model 2) arrangement is used.

Time Period:

When braces are used the time period decreases this makes the structure stiffer. More stiff structure will have increased frequency and there is more chance of failure.

Resonance condition can be seen when the frequency induced by the seismic waves will be equal to the natural frequency of the structure but for this condition to happen the seismic waves should not end before the structure reaching is resonating frequency.

REFERENCE and BOOKS

[1] A. Massumi and A. A. Tasnimi Strengthening of low ductile reinforced concrete frames using steel X-bracing with different details The 14th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China.

[2] Agrawal Pankaj, Shrikhande Manish, Earthquake Resistant Design of Structures.

[3] Chadhar Shachindra Kumar and Dr. Sharma Abhay Sesmic behavior of RC Buliding frame with steel bracing system using various arrangements International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056 Volume: 02 Issue: 05 Aug-

2015 p-ISSN: 2395-0072.

[4] Desai J. P., Jain A. K. and Arya A. S., Seismic response of R. C. braced frames, Computers and Structures Volume 29 No.4, pp 557568, 1988.

[5] Duggal S.K, Earthquake-Resistant Design of Structures

[6] Mahmoud R. Maher & R. Akbari Seismic behaviour factor, R, for steel X-braced and knee-braced RC buildings Elsevier Science direct Engineering Structure Volume 25, Issue 12, October 2003, Pages

1505-1513.

[7] Mishra Rishi, Dr. Sharma Abhay, Dr. Garg Vivek Analysis of RC Building Frames for Seismic Forces Using Different Types of Bracing Systems International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2278 -0181 Volume: 03 Issue: 07 July-

2014.

[8] Prof. Patil S.S, Miss. Ghadge S.A, Prof. Konapure C.G, Prof. Mrs.

Ghadge C.A

[9] Seismic Analysis of High-Rise Building by Response Spectrum Method International Journal Of Computational Engineering Research (Ijceronline.Com) Vol. 3 Issue. 3 March 2013.

[10] R Sabelli, S Mahin, C Chang Seismic demands on steel braced frame buildings with buckling-restrained braces Elsevier Science direct Engineering Structure Volume 25, Issue 05, April 2003, Pages 655-

666.

[11] Viswanath K.G, Prakash K.B, Anant Desai Seismic Analysis of Steel Braced Reinforced Concrete Frames International Journal of Civil and Structural Engineering Volume: 01 Issn 0976-4399.

[12] IS 1893 (Part I) 2002: Criteria for earthquake resistant design of structures.

[13] IS 13920 (993): Ductile detailing of reinforced concrete structures subjected to seismic forces

[14] IS 875 (Part 2):1987 Code of practice for design loads (other than earthquake) for buildings and structures.

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