Reliability Analysis for Buckling Restrained Braced Frame (BRBF)

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Reliability Analysis for Buckling Restrained Braced Frame (BRBF)

Malavika G Babu

M. tech Student [Structural Engineering] Dept of Civil Engineering

Younus College of Engineering and Technology Kollam, India

AbstractStructural demands in high seismic zones require the use of strong lateral framing systems. The structure must have adequate strength and stiffness to resist smaller, frequent earthquakes with limited damage, but must also be able to sustain large inelastic cyclic deformations to economically assure safety and stability during large, infrequent earthquakes. To overcome these problems, a new type of braces called buckling restrained braces (BRBs) was introduced. They are structural steel frame that provide lateral resistance to buckling during seismic activity. This study aims to assess the seismic performance of buckling restrained braced frames (BRBFs) by Eigen value modal analysis, equivalent static analysis and time history plots should be created by spectral matching to find out peak response of the building. The reliability index can be calculated by varying building height and plotting the safety margin curves. The modeling and analysis of building is carried out using ETABS software.

Keywords Buckling restrained braced frames (BRBFs), Eigen value modal analysis, ETABS software, Reliability index.

  1. INTRODUCTION

    Earthquake causes economic losses as well as losses of lives due to the collapse of structures. During strong seismic waves structural elements like beams and columns are seriously affected. Under moderate to severe earthquakes conventional lateral load resisting systems are not effective to overcome these problems, a new type of braces called buckling restrained braces (BRBs) were introduced. They are structural steel frame that provide lateral resistance to buckling during seismic activity. The main components of braces are steel core, bond-preventing layer and outer casing. The steel core is able to resist axial force acting on bracings. Core is divided into three parts, central yielding part and rigid non yielding parts at both ends. The bond preventing layer separates core and casing. It allows free expansion and contraction of core during tension and compression. The casing envelops the inner parts and restraining the steel core from buckling

    In this work the seismic performance of a10 storey I shaped BRBF building is evaluated by equivalent static analysis, Eigen value model analysis. Time history data collected for spectral matching and peak response of building is obtained. Reliability index of structure can be calculated by plotting safety margin curves for different building height.

  2. METHODOLOGY DATA COLLECTION

    LITERATURE REVIEW

    SOFTWARE STUDY MODELLING

    CONCLUSIONS

    CONCLUSIONS

    ANALYSIS RESULTS & DISCUSSION

  3. MODELLING

    A 10 storey I shaped building is modelled using etabs software. The materials used are M30 grade concrete and Fe415 grade steel. Height of each storey is 3 m. Number of bays in X and Y direction are 7 and 4 respectively. The span of each bay is 3m.The beam section of size 250 mm x 250 mm and Column of size 500 mm x 500 mm is used. Here the provided Slab thickness is 120 mm.

    Fig 1.Plan and elevation of building

    TABLE I. .DEAD LOAD (IS 875: 1987 PART 1)

    Parapet

    2kN/m

    Wall Load

    12 kN/m

    Floor finish

    1KN/m2

    TABLE II. LIVE LOAD (IS 875: 1987 PART 2)

    Residential building

    2 KN/m2

    Roof

    1.5 KN/m2

    TABLE III. WIND LOAD (IS 875: PART 3)

    Risk factor,(k1)

    1.0

    Topography factor,(k2)

    1.0

    Terrain factor,(k3)

    1.0

    Terrain category

    2

    Building class

    B

    TABLE IV. SEISMIC LOAD (IS1893:2002)

    Seismic zone factor, Z

    0.16

    Importance factor, I

    1

    Response reduction factor ,R

    5

    TABLE V. LOAD COMBINATIONS (IS 1893: 2002 PART 1)

    1.5 DL

    0.9 DL + 1.5 WL-Y

    1.5 ( DL + LL)

    1.2 ( DL + LL + EQX )

    1.2 (DL + LL + WLX )

    1.2 ( DL + LL + EQ-X )

    1.2 ( DL + LL + WL-X )

    1.2 ( DL + LL + EQY )

    1.2 ( DL + LL + WLY )

    1..2 ( DL + LL + EQ-Y )

    1.2 ( DL + LL + WL-Y )

    1.5 ( DL + EQX )

    1.5 ( DL + WLX )

    1.5 ( DL + EQ-X )

    1.5 ( DL + WL-X )

    1.5 ( DL + EQY )

    1.5 ( DL + WL-Y)

    1.5 ( DL + EQ-Y )

    0.9 ( DL + 1.5 WLX )

    0.9 ( DL + 1.5 EQX )

    0.9 ( DL + 1.5 WL-X )

    0.9 ( DL + 1.5 EQ-X )

    0.9 ( DL + 1.5 WLY )

    0.9 ( DL + 1.5 EQY )

    1.5 ( DL + WLY )

    0.9 ( DL + 1.5 EQ-Y )

  4. ANALYSIS

    1. Buckling Analysis

      Buckling analysis is carried out to predict the maximum load a structure can support prior to instability or Collapse The colored portions indicates regions with buckling load is maximum. The buckling load factors can be obtained from the software.

      Fig 2.Column axial load

      In conventional method, to avoid buckling we should multiply these buckling load factors as factor of safety to the loads acting on the building and design as per the resultant loads. Instead of this, the provision of using

      buckling restrained braced frame on the building should be analyzed.

      BUCKLING RESTRAINED BRACED FRAME

      Star Seismic is an international manufacturer of Buckling restrained braces. The details of braces can be collected from star seismic manual. Star Seismic sections of size 1 incp to 52 incp are available. By selecting auto selection option the software itself select the suitable section for the structure. Here Star BRB of cross section 26.5 incp is selected. This process is called optimization of BRB.

      TABLE VI. BRBF MATERIAL PROPERTIES

      Core Material Density

      7850 KN/m3

      Modulus of Elasticity

      2×105 MPa

      Poissons Ratio

      0.3

      Shear Modulus

      76903.07 Mpa

      Minimum Yield stress

      262 Mpa

      Minimum Tensile Stress

      399.9 Mpa

      Effective Yield Stress

      327.5 Mpa

      Effective Tensile Strength

      499.87 Mpa

      TABLEVII. BRBF SECTION DETAIL

      Weight

      25.54 kN

      Depth

      406.4 mm

      Width

      304.8 mm

      Area of yielding core

      171 cm2

      Stiffness of elastic segment

      4334353.557 kN/m

      Length of yielding core

      4.2672

      Length of elastic segent

      2.2713

      Linear Effective Axial Stiffness

      676133.98 kN/m

    2. Equivalent Static Analysis

      Here parameters like storey displacement, storey drift and storey shear values of Conventional Buckling resistance building and BRBF building are compared.

      Fig 3. Storey displacement Without BRB and With BRB Maximum value of storey displacement for conventional building is 29.57 Mm and for building with BRB the maximum storey displacement is 12.02 mm.

      Fig 4. Storey drift Without BRB and with BRB

      Maximum value of storey drift for building without is 29.5mm and maximum value of storey drift for building with BRB is 4.92 mm.

      Fig 5. Storey shear Without BRB and with BRB Maximum storey shear for building Without BRB is 726.78 KN and Maximum storey shear for building with BRB 1990 kN. Maximum Displacement and Story Drift get reduced when BRB sections wereprovided.The Base shear increases with increase in weight.

    3. Eigen Value Modal Analysis

      Fig 6 .Building With BRBF

      Fig 7 .Building Without BRBF

      For building without BRBF, the modal participating mass ratio becomes 99% atMode 20 andfor building with BRBF the modal participating mass ratio becomes99% atMode

      1. Less the number of modes, less will be the distortion of building.Circular frequency zero means applied load is close to critical buckling load.

        TARGET RESPONSE SPECTRUM

        According to IS 1893 there are four seismic zones in India.Code provide acceleration time graph for each seismic zone based on previousearthquake datas.

        TIME HISTORY DATA FROM STRONG MOTION VIRTUAL DATA CENTRE

        Collect datas of past 12earthquake occurredin India from strong motion virtual data centre.

        SPECTRAL MATCHING

        In spectral matching the response spectrum for different seismic zones provided by the code combines with acceleration time graph of various earthquakes collected from strong motion virtual data centre

        Fig 8. Matched Response Spectrum

        RELIABILITY INDEX

        As per IS1893, total drift of the building should not exceed

        0.004 times the buildingheight.The reliability index is typically used to measure the reliability of the building ,by using the maximum roof displacement of the building. MARGIN OF SAFETY

        • The margin of safety of a building is given by the following equation:

      M = – (1)

      where,

      = allowable drift

      = maximum roof displacement

      maximum roof displacement for building at different heights are Plotted

      30m 24m 18m 12m 6m

      Figure 11.Safety margin curve for (6m)

      Safety margin curve for 6 m height building is plotted.The standard deviation value is .004 and mean value is .021 and reliability index value is 5.27 along X direction.For Y direction the standard deviation value is .003 mean value is

      1. and reliability index is 8.05.

        ROORKEE UTTARKASHI

        TEHRI SHILLONG PANIMUR JELLALPUR GHANSIALI

        DAUKI CHERRAPUNJI

        CHAMOLI CHAMBA

        BHUJ

        0 200 400 600 800

        Figure 12. Safety margin curve for 12 m building

        Safety margin curve for 12 m height building is plotted.The standard deviation value is .015 and mean value is .037 and reliability index value is 2.41 along X direction.For Y direction the standard deviation value is .035 mean value is

        .026 and reliability index is .76.

        Fig 9. Roof displacement in x direction(mm)

        For building with more height ,displacement is less because lateral load get diminished along the height. The Cherrapunji earthquake induce more roof displacement and Shillong earthquake induce least roof displacement.

        30m 24m 18m 12m 6m

        30m 24m 18m 12m 6m

        UTTARKASHI SHILLONG JELLALPUR

        DAUKI CHAMOLI

        BHUJ

        UTTARKASHI SHILLONG JELLALPUR

        DAUKI CHAMOLI

        BHUJ

        0

        200

        400

        600

        800

        0

        200

        400

        600

        800

        Fig 10.Roof displacement in y-direction (mm)

        The Cherrapunji earthquake induce more roof displacement and Shillong earthquake induce least roof displacement.

        Fig 13.Safety margin curve for 18 m

        Safety margin curve for 18 m height building is plotted.The standard deviation value is .011 and mean value is .059 and reliability index value is 5.34 along X direction.For Y direction the standard deviation value is 0.086 mean value is .016 and reliability index is .18.

        Fig 14.Safety margin curve for 24 m height building

        Safety margin curve for 18 m height building is plotted.The standard deviation value is .142 and mean value is .029 and reliability index value is .20 along X direction.For Y direction the standard deviation value is 0.088 mean value is .005 and reliability index is .05.

        Fig 15.Safety margin curve for 30 m height building

        Safety margin curve for 30 m height building is plotted.The standard deviation value is .043 and mean value is .063 and reliability index value is 1.47 along X direction.For Y direction the standard deviation value is 0.091 mean value is .010 and reliability index is .11.

        TABLE VIII. DEVELOPED RELIABILTY INDEX FOR BRBF

        Storey Height(m)

        Minimum Range

        Maximum Range

        30

        0.11

        1.70

        24

        0.05

        0.20

        18

        0.18

        5.34

        12

        0.76

        2.41

        6

        5.27

        8.05

        time period, higher circular frequency and higher eigenvalue, so BRBF is more buckling resistant.The reliability index of a structure increases when the probability of failure reduces.Here for 30 m building reliability index ranges from .11 to 1.7 and for 24 m building reliability index ranges from .50 to .2. For 18 m building reliability index varies from .18 to 5.34 and for 12 m building reliability index is from .76 to 2.41.In the case of 6 m heigh buiding the value varies from 5.27 to 8.05.

        REFERENCES

        1. Kushagra Ashara1, Keval Patel, Comparative Study On Performance Of Rc Building With Outrigger System Incorporating Buckling Restrained Bracings ,International Research Journal of Engineering and Technology ,Vol: 07 Issue: 06,June 2020

        2. Y. D. Kumbhar , Study of Buckling Restrained Braces in Steel Frame Building ,Int. Journal of Engineering Research and Applications , Vol. 4, Issue 8, August 2014, pp.71-74

        3. Arunraj E, Vincent Sam Jebadurai S, Samuel Abraham D, Daniel C, Hemalatha G. Analytical Investigation of Buckling Restrained Braced Frame Subjected To Non-Linear Static Analysis ,

          Volume-8 Issue-5, June 2019

        4. Ferdinand, Niyonyungu & Jianchang, Zhao & Qiangqiang, Yang & Wang, Guobing & Junjie, Research on Application of Buckling Restrained Braces in Strengthening of Concrete Frame Structures , Civil Engineering Journal. 6. 344-362. 10.28991/cej-2020- 03091475.

        5. Kurdi Mohammed Suhaib, Sanjay Raj A and Dr. Sunil Kumar Tengli (2018). Analysis of Flat Slab Structures with Outriggers , International Journal of Applied Engineering Research ,Vol 13,

          Number 7 (2018) pp. 72-77

        6. Lin, Pao-Chun & Takeuchi, Toru & Matsui, Ryota, Seismic performance evaluation of single dampedoutrigger systemincorporatingbucklingrestrained braces ,Earthquake Engineering & Structural Dynamics. 47. 2343- 2365.10.1002/eqe.3072.,201

        7. Najia, Syeda ,Dynamic Response of Rcc and Composite Structure with Brb Frame Subjected To Seismic and Temperature Load 2248-962279 .IJERA 2016.

        8. Smith, Rob & Willford, Michael. (2007), The damped outrigger concept for tall buildings ,The Structural Design of Tall and Special Buildings. 16. 501 517

        9. Viise, J., P. Ragan, and J. Swanson. "BRB and FVD alternatives to conventionalsteel brace outriggers." In Proceedings of the CTBUH Shanghai conference, pp. 691-9. 2014.

        10. Watanabe, A, Design and applications of buckling-restrained braces ,International Journal of High-Rise Buildings. 7. 215-221.

          July 2018

        11. Ryan A. Kersting, Larry A. Fahnestock, Walterio A. López , Seismic Design of Steel Buckling-Restrained Braced Frames NEHRP Seismic Design Technical Brief No. 11.

        12. IS: 1893(Part-I)-2016, Criteria for Earthquake Resistant Design of Structures. Bureau of Indian Standard, New Delhi.

        13. IS: 875(Part 1)-1987 Indian Standard Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures (Part 1: Dead Loads).

        14. IS: 875(Part 2)-1987 Indian Standard Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures (Part 2: Live Loads).

  5. CONCLUSION

By using Buckling restrained braced frames 40% reduction in storey displacement and storey drift value became negligible when compared with conventional building.The storey shear of structure increases due to increase in weight. So BRBF is better than conventional buckling resisting buildings . FEMA356 recommends 99% mass participation is required to obtain requirednumber of modes in modal analysis.Less the number of modes, less will be the distortion of building. BRBF have less fundamental

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