Seismic Response of R.C.C Low Rise Frame Structure with Soft Story Effects Using E-Tabs: A Case Study

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Seismic Response of R.C.C Low Rise Frame Structure with Soft Story Effects Using E-Tabs: A Case Study

Preeti1,

1Department of Civil Engineering Department GITAM Kablana, India

Amit Singhal2,

2Department of Civil Engineering Department GITAM Kablana, India

Neeraj Kumar3

3Department of Civil Engineering Department GITAM Kablana, India

The increase in urban population in recent times has raised the requirements of vehicle parking as a major problem, & to resolve this matter first storey of the apartments are generally kept open& hence used for parking purpose .Buildings are classified as soft storey if that level is less than 70% as stiff as the floor immediately above it, or less than 80% as stiff as the average stiffness of the floors above it.A building with soft storey inherently vulnerable to collapse during earthquake due to its reduced stiffness infill walls in frame building provides stiffness & alters the behavior of building under lateral loads.A Soft Storey is a Building in which one or more floors have windows, wide doors, large unobstructed commercial spaces, or other openings in place where shear wall or any other arrangement normally be required for providing stability as a matter of safe structural design.In this paper different iterations are done with programs E-tabs,to study the seismic behavior of soft storey buildings with different arrangements,Parameters discussed here include the comparative study of Stiffness, Design Seismic base shear, fundamental natural time period, lateral displacement.

Index Termswall-strut, E-tabs , Shear walls, Seismic analysis.

  1. INTRODUCTION

    A soft story building is building with one or more floors which are soft due to structural design [1]. These floors can be especially dangerous in earthquakes, because they cannot cope with the lateral forces caused by the swaying of the building during a quake. As a result, the soft story may fail, causing what is known as a soft story collapse.

    A soft story collapse is one of the leading causes of damage to private residences .Soft story buildings are characterized by having a story which has a lot of open space. Parking garages, for example, are often soft stories, as are large retail spaces or floors with a lot of windows. While the unobstructed space of the soft story might be aesthetically or commercially desirable, it also means that there are less opportunities to install shear walls, specialized walls which are designed to distribute lateral forces so that a building can cope with the swaying characteristic of an earthquake [2].

    As the population is increasing day by day in the past few years the trend to utilize in all over world to have one open storey concept in the building but at the same time it is of prime importance to make such buildings earthquake

    resistant. A soft storey is the one in which the rigidity is lesser than other storey above it because of not containing any brick walls of any property which rest of the floors of the same building contains.

    Earthquakes of different intensities produce low-high waves which tend to vibrate & effect the base of the building so due to this some lateral forces generates in the building which bring variations & Complications in structure especially in the open storey. Following is some diagrammatic representation of building showing distribution of displacement in a regular & in a soft storey building [4].

    1. Elevational View Of The Site

      Figure showing Open storey building:- Assumptions made in Current design practice are not consistant with the actual structure.

  2. ANALYSIS OF BUILDING FOR DIFFERENT ARRANGEMENTS

    In this study R.C.C Building is modeled & analyzed in three different arrangements using the same configurations:-

      1. CASE I :- Model with bare frame.

      2. CASE II :- Model with wall-strut above Stilt roof

        Level

      3. CASE III :- Model with shear walls upto Stilt floor Roof Level.

      4. CASE IV :- Model with Wall-Strut at all Storey.

    S.no

    Description

    Information

    1.

    No. of upper storeys above stilt

    4

    2.

    Type of frame

    RCC Frame with Stilt/Ground open

    3.

    Height of Upper floors

    3.0 m

    4.

    Beam sizes

    230×450

    5.

    Soil type/Support

    conditions

    Medium

    6.

    Self weight of Building

    1.0

    7.

    Grade of concrete/Steel

    Slab-M25,Beam- M25,Column-M30 Reinforcement: Fe500/Fe415

    8.

    Minimum cover

    Slab-25mm Beam-25mm Column-40mm Shear wall-25mm

    9.

    Mass Sources

    DL+25%LL

    10.

    Thickness of slab/shear wall

    Slab-125/150mm,shear wall- 230mm

    11.

    Thickness of partions& brick /block used

    230mm/115mm

    12.

    Dead Loads/Density of different material

    RCC-25Kn/m3

    Brickwork-18.0 Kn/m3

    13.

    Imposed load

    2/3 Kn/m3

    14.

    Floor finish

    1.35 Kn/m3

    15.

    Importance Factor

    1.0

    16.

    Seismic Zone

    Z-4

    17.

    Response reduction factor

    5

    1. STRUCTURAL CONFIGURATION OF BUILDING

  3. DESCRIPTIONS OF STRUCTURAL MODELS WITH ARCHITECTURAL AND STRUCTURAL

DRAWING [5]

  1. ARCHITECTURAL TYPICAL FLOOR PLAN

    STRUCTURALTYPICAL FLOOR PLAN

    1. CASE I :- MODEL WITH BARE FRAME [5]

      TYPICAL FLOOR PLAN IN E-TABS

      ISOMETRIC VIEW OF CASE III

      ISOMETRIC VIEW OF CASE I

    2. CASE II:- MODEL WITH WALL-STRUT BOVE STILT ROOF LEVEL [5]

      TYPICAL FLOOR PLAN IN E-TABS

      ISOMETRIC VIEW OF CASE II

    3. CASE III:-MODEL WITH SHEAR WALLS UPTO STILT FLOOR ROOF LEVEL [5]

      TYPICAL FLOOR PLAN IN E-TABS

    4. CASE IV:- MODEL WITH WALL-STRUT AT ALL STOREY [5]

    TYPICAL FLOOR PLAN IN E-TABS

    ISOMETRIC VIEW OF CASE IV

    1. ANALYSIS RESULT AND DISCUSSION

      1. Storey Stiffness

        .It is the rigidity of an object the extent to which it resists deformation in response to the applied force.

        K = P /

        Hence after analyzing the Building the results obtained maximum of both longitudinal and transverse direction comparisons are presented in tabular form.

        CASEI -Calculation of stiffness for bare frame model:-

        Storey No.

        Load(KN)

        Stiffness(KN/M)

        Roof

        861

        443368

        4th

        1547

        496057

        3rd

        1958

        505674

        2nd

        2165

        523705

        1st

        2236

        684561

        Stilt

        2238

        3349887

        CASE-II: Calcuation of stiffness with wall strut above stilt roof level :-

        Storey No.

        Load(KN)

        Stiffness(KN/M)

        Roof

        962

        2332499

        4th

        1783

        7014699

        3rd

        2275

        9081545

        2nd

        2523

        11067384

        1st

        2601

        940321

        Stilt

        2603

        3666969

        1.50

        1.00

        0.50

        0.00

        CASE-III :Calculation of stiffness with shear wall upto stilt roof level:-

        CASE-IV: Calculation of stiffness with wall strut at all storey:-

        Storey No.

        Load(KN)

        Stiffness(KN/M)

        Roof

        992

        4128167

        4th

        1840

        7287135

        3rd

        2345

        9621057

        2nd

        2600

        11622938

        1st

        2686

        8573513

        Stilt

        2690

        8737476

        Fig1:-Showing stiffness comparison of all four cases

      2. Fundamental Natural Period (Ti) [8]

        The Analysis as per code (IS :1893-1984) and analysis as per (E-TABS) natural periods of the building models is seen that the analytical natural periods do not match with the natural periods extracted from the empirical formula in the code. It is the first modal time period of vibration The Lower the time period estimated imposes larger base shear in the building .As we have introduced infill in models the time period get reduced .Hence after analyzing the Building the results obtained for three models in both longitudinal and transverse direction and there comparisons are presented in tabular form

        Table 1. Showing Time Period Comparison of all four cases

        Storey No.

        Case I (Sec)

        Case II (Sec)

        Case III (Sec)

        Case IV (Sec)

        IN X

        Direction

        1.13

        0.69

        0.26

        0.38

        IN Y

        Direction

        0.88

        0.53

        0.21

        0.27

        Fig1:-Showing stiffness comparison of all four cases

        Storey No.

        Load(KN)

        Stiffness(KN/M)

        Roof

        1013

        5245846

        4th

        1877

        9216545

        3rd

        2395

        12477596

        2nd

        2655

        18490000

        1st

        2748

        51476334

        Stilt

        2750

        185096979

        COMPARISON OF FUNDAMENTAL TIME PERIOD

        IN BOTH DIRECTIONS

        CASE I

        CASE II

        CASE III

        CASE IV

        IN"X "DIRECTION IN"Y "DIRECTION

      3. Lateral Displacements [6]

        Lateral displacement is caused due to the Lateral Force on the each storey level of building. Lateral displacement will be higher on top storey. Hence after analyzing the Building the results obtained for four models are below in the table.

        Kindly Note:-Maximum Displacement of both longitudinal and transverse direction are observed for comparison

        Table 1. Showing Lateral Displacement comparisons of all four cases

        STOREY NO

        Case I (MM)

        Case II (MM)

        Case III (MM)

        Case IV (MM)

        roof

        28

        7.6

        1.6

        3.0

        4th

        25

        7.2

        1.3

        2.5

        3rd

        20

        6.7

        0.9

        2.1

        2nd

        13.5

        6.3

        0.5

        1.6

        1st

        6.6

        5.8

        0.2

        1.2

        Stilt

        0

        0

        0

        0

    2. DESIGN SEISMIC BASE SHEAR (VB)

      It is the total design lateral force at the base of a structure. Hence after analyzing the Building the results obtained for four models in both longitudinal and transverse direction. The comparisons are presented in tabular for

      Case I

      (KN)

      Case II

      (KN)

      Case III

      (KN)

      Case IV

      (KN)

      IN X

      Direction

      2238

      2603

      2751

      2690

      IN Y

      Direction

      2238

      2603

      2751

      2690

    3. RESULT AND CONCLUSSION

RCC Frame building with open at stilt will to perform poorly during in strong earthquake shaking .In this paper ,the seismic vulnerability of buildings with soft storey is shown through an example building .The strength demands in first storey building is very large for buildings with soft stilt storey .It is not feasible to provide such large column sizes at stilt storey level.

The lateral displacement of the bare frame model is on the higher side as no infill are modelled in it ,hence gives less resistance to lateral forces and less stiffness of storey.

In case III by using diagonal wall strut at specific locations it significantly increases the stiffness of the building and also reduces the lateral displacement,it also reduces the Fundamental time period of the building.

The analysis result shows some observations:-

    • The Fundamental time period of the model with bare frame gives 2.97 times more time period than the model with infill effect.

    • The base shear lateral forces of the model with wall strut modelled is 20% higher then the bare frame modal

    • The lateral displacement of the bare frame modal are almost 8 times then the modal with infill effect

REFERENCES

  1. DevendraDohare, Dr. SavitaMaru. SiesmicBehaviour of Soft Storey Building:-Acritical View, International Journal of Engineering Research & General Science Volume 2,Issue 6, October-November-2014.

  2. Mr. Raghavendra S. Deshpande, Dr. Mrs. Surekha A. Bhalchandra Siesmic analysis of rcc building with soft storey,International Journal of Scientific & Engineering Research,Volume 5, Issue 5, May-2014 ISSN 2229-5518.

  3. Nikhil Aggarwal, Prof. PB Kulkarni, PoojaRaut Analysis of MasonaryinfilledRc Frame with and without opening including soft storey by usingEqaution Diagonal Strut Method.International Journal of Scientific and research Publication ,volume 3,issue 9,September 2013 ISSN 2250-3153

  4. D.K Bell and B.J.Devidson Evaluation of earthquake resistant building with masonary infill panels NZSEE 2001 Conference

  5. ETABS Non linearvesion 13.1.4 Extended 3D Analysis of USA.

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