Shear Capacity and Shear Reinforcement of Exterior Beam-Column Joint of RC Building

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Shear Capacity and Shear Reinforcement of Exterior Beam-Column Joint of RC Building

Sujan Mishra

M.Sc. Student

(Infrastructure Engineering and, Management), Pashchimanchal Campus,

IOE, Tribhuvan University, Pokhara, Nepal,

Sailesh Adhikari Lecturer Pashchimanchal Campus

IOE, Tribhuvan University, Pokhara, Nepal,

Deepak Thapa, Lecturer Pashchimanchal Campus

IOE, Tribhuvan University, Pokhara, Nepal,

Abstract:- In reinforcement concrete moment resisting frame joints between beams and columns are critical zones which resisted both lateral and vertical load subjected to building. Nepal is seismically active zone due to this earthquake frequently happen and failure beam column joint fail to withstand high shear force developed on joint so this study determined shear reinforcement on joint to resist shear force developed on joint to make the joint ductile. The main objective of this study was to determine shear capacity and shear reinforcement in exterior beam-column joint of RC building. For this study modelling of nine residential building was done in ETABS software. From the obtained results this study indicated that more increase in shear force, column-beam size, longitudinal and shear reinforcement on beam-column by increasing the story height and bay width of building. This study indicated that small change in shear force by changing the concrete grade and number of bays. This study used IS 13920:2016, ACI 352R.02 and NZS 3101.1:2006 codal provision to determine joint shear strength and joint shear reinforcement. NZS 3101.1:2006 codal provision showed that minimum joint shear strength. NZS gave more joint shear reinforcement in external beam-column joint whereas IS 13920:2016 gave minimum joint shear reinforcement.

Keywords RC Building, Exterior Beam-Column Joint, Joint Strength, Joint Shear Reinforcement

  1. INTRODUCTION

    Due to sideways and downward movement of the Indian plate underneath the Eurasian plate causes earthquake frequently in Nepal (Chaulagain et al., 2015). Major Purpose of constructing earthquake resisting building is prevention of failure of building in lateral load. Proper design of beam-column joint is one of the important parameters of earthquake resisting building. Joint could develop sufficient inelastic capacities to disperse seismic energy (Kadarningsih et al., 2014) . In many moment resisting RC frame failure seen beam-column joint. Design check for gravity load and live load is not critical for beam-column joint in normal design but during the earthquake or lateral force heavy distress develops due to shear in joints that lead to failure and studies of seismic effect on joints have undertaken only past three to four decades(Uma & Jain, 2006). Tiwari and Adhikari (2020) studied the behavior of building in the variation in the stiffness and mass subjected to seismic load however the results specified in the joint was not obtained. This study extends the results to the joint results.

    Beam-column joint is the zone of intersection of beams and columns, which is critical zone in moment resisting reinforced concrete frame. During ground shaking large force acting on it which has influence on structure response. For considering effect of shear force developed on joint, it is assumed to be rigid fails. When shear force exceeds the limit shear failure occurs which is always brittle in nature and this failure is not an acceptable structural performance especially in seismic condition so that joints should have adequate strength and stiffness to resist the internal forces developed by the framing members(Uma & Prasad, 2015). Tiwari et. al. (2020) attempted to study different irregular low-rise buildings is considered for the modelling, linear static analysis was performed to check time period, displacement, drift and storey shear of models, but the localized joint output was not reported in the study.

    Columns are constructed earlier than beam slab and less compaction on beam-column joint due to congestion of reinforcement on joint so that joint strength may be differ in same concrete grade of column, beam and slab. Load carrying capacity of joint depends upon the ductility of joint so sufficient ductility should be provided on joint. Visible damage will be seen on joint during inelastic deformation of beams and column and effect of this force is known as plastic hinge. During inelastic rotation, the ductility capacity of all members is transferred to the joint so that the damage at the joint will be substantial and should be avoided. The formation of a plastic hinge is expected, where permitted structural damage occurs. Thus, it is very important in seismic design that the damage of a plastic hinges occurs in the beam, rather than in the column. During horizontal earthquakes,

    moments and shear forces acting on the beams and columns of the frame building are resulting in internal-vertical and horizontal forces on the face of the joint core. The internal forces produce a resultant acting in the joint core, either a diagonal tensile or compression stress. Diagonal tensile stresses and compressive forces result in cracking and crushing of the concrete core. If the shear resistance at the joint core is insufficient, there will be failures along the diagonal of the joint core. The design of the shear beam-column joint of steel reinforced concrete (SRC) contributed much to the design of joints under seismic loads (Kadarningsih et al., 2014). According to report ACI Committee 352 by (Bonacci & Leon, 1988) beam-column joints are interior, exterior, corner, roof interior, roof external and roof corner which is shown in figure 1. In figure slabs are not shown for clarity and wide beam cases are not shown.

    Figure 1: Typical Beam-Column Joint

    Tran et al. (2018) developed formula to predict the shear strength of exterior RC beam-column joints where beam longitudinal reinforcements are anchored into the joint by headed bars and those formula based on a regression analysis which used the database collected from 30 experiments. It was shown that, concrete compressive strength, beam rebars details and joint shear reinforcement also play vital role in the joint shear strength. Pimanmas & Chaimahawan (2011) focused on joint strengthening method by expanding the joint area by experimentally with under control specimen to investigate the shear strength of specimens with expanded joint with non-ductile reinforcement details. They found that sufficiently joint enlargement is effective to reduce shear stress transmitted in the joint panel, failure mode is changed from brittle joint shear failure to moderately ductile flexural failure in beams, plastic hinge is moved from column face to the edge of enlargement and if the size of enlarged area is small, the failure mode is concrete crushing in the joint panel and expanded areas.

  2. MODELLING BY ETABS

    In the present study 9 different model of reinforced concrete building is modelling and analyzing in ETABS software. This study has been chosen RC building of 3 story building and variation in parameters such as building story height, bay width, number of bays, size of beam column and grade of concrete. Following parameters of building has been taken for modelling and analyzing of this study.

    1. Width of the bays: 3m, 3.5m and 4m

    2. Floor heights: 3m 3.5m and 4m

    3. Number of the bays: 2, 3 and 4

    4. Grade of the concrete: 20MPa, 25MPa and 30 MPa

    5. Size of column: 300mmx300mm, 350mmx350mm

    6. Size of beam: 230mmx300mm, 230mmx350mm 300mmx350mm

    7. Earthquake load: Zne V, importance factor 1, response reduction factor 5, soil type II, mass source took from (IS 1893 part 1, 2002)

    8. Wall load: 230mm thick brick wall of unit weight 18.85 KN/m3 from IS 875(Part 1) reported by (IS 875 part 1, 1987)

    9. Live load: 2 KN/m2 for all room and 1.6 KN/m2 for roof had taken from (IS 875 part 2, 1987)

    From above parameter this study categorized 9 models of RC building and modelling is done by using ETABS software. In all model Fe415 reinforcement is assigned in all frame members. This study assigned square column as frame element, rectangular beam as frame element and slab as membrane element and only one parameter is changed in each model.

    Model 1: M20 concrete, bay width 3m, story height 3m, 3 story, 3×3 bay building Model 2: M25 concrete, bay width 3m, story height 3m, 3 story, 3×3 bay building Model 3: M30 concrete, bay width 3m, story height 3m, 3 story, 3×3 bay building Model 4: M20 concrete, bay width 3m, story height 3.5m, 3 story, 3×3 bay building Model 5: M20 concrete, bay width 3m, story height 4m, 3 story, 3×3 bay building Model 6: M20 concrete, bay width 3.5m, story height 3m, 3 story, 3×3 bay building Model 7: M20 concrete, bay width 4m, story height 3m, 3 story, 3×3 bay building Model 8: M20 concrete, bay width 3m, story height 3m, 3 story, 4×4 bay building Model 9: M20 concrete, bay width 3m, story height 3m, 3 story, 2×2 bay building

    Figure 2: 3D Model of 2×2 Bay Building with Plan

    Figure 3: 3D Model of 3×3 Bay Building with Plan

    Figure 4: 3D Model of 4×4 Bay Building with Plan

  3. RESULTS AND DISCUSSIONS

    The nine RC building models are analyzed. This study finds exterior beam-column joint with maximum shear force on beam and column of all nine models, longitudinal and shear reinforcement of beam and column exterior joint, joint shear strength, joint shear reinforcement of exterior joint and their spacing according to IS 13920:2016, ACI 352R.02 and NZS 3101.1:2006 code provision. The results of each parameters are discussed below.

      1. Shear force on Beam Column of exterior Joint

        Maximum shear force develops in 2×2 bay building at column 4 at story 1 and beams connected to column 4 are beam 3, beam 7, beam 8 which formed an exterior beam-column joint. Maximum shear force develops in 3×3 bay building at column 9 at story 1 and beams connected to column 9 are beam 7, beam 14, beam 15 which formed an exterior beam-column joint. Maximum shear force develops in 4×4 bay building at column 16 at story 1 and beams connected to column 16 are beam 13, beam 23, beam 24 which formed an exterior beam-column joint. Maximum shear force develops in model 7 that means shear force also increases with increases in story height of building. As increases in bay width of building shear force is also increasing. There is small variation in shear force as increases in grade of concrete and number of bays.

        Table 1: Shear Force on Beam and Column of Exterior Joint

        Column 9 top shear (KN)

        Column 9 bottom shear (KN)

        Beam 7 shear (KN)

        Beam 14 shear (KN)

        Beam 15 Shear (KN)

        Model 1

        35.46

        21.90

        75.07

        68.35

        85.67

        Model 2

        37.58

        23.03

        77.23

        70.76

        88.10

        Model 3

        38.72

        24.01

        79.08

        72.38

        88.67

        Model 4

        32.92

        20.31

        83.25

        76.04

        94.63

        Model 5

        33.15

        21.78

        95.65

        88.48

        102.78

        Model 6

        45.65

        30.13

        87.04

        80.42

        99.84

        Model 7

        61.54

        40.00

        104.82

        99.00

        114.35

        Model 8

        35.84

        21.64

        75.11

        69.53

        85.63

        Model 9

        36.59

        22.23

        75.31

        70.23

        86.02

      2. Longitudinal and Shear Reinforcement of Exterior Beam-Column Joint

        As increases in grade of concrete longitudinal reinforcement of column decreases. Similarly, as increases in bay width of building and story height of building longitudinal reinforcement in both beam and column increases more. Longitudinal reinforcement of external beam column of each model is tabulated below.

        Table 2; Longitudinal Reinforcement of Exterior Beam-Column Joint

        Column 9

        Beam 7

        Beam 14

        Beam 15

        Top

        Bottom

        Top

        Bottom

        Top

        Bottom

        Top

        Bottom

        (mm2)

        (mm2)

        (mm2)

        (mm2)

        Model 1

        1041

        1678

        524

        349

        474

        254

        510

        255

        Model 2

        1006

        1575

        531

        364

        478

        268

        520

        260

        Model 3

        970

        1548

        538

        378

        486

        281

        518

        259

        Model 4

        1254

        4168

        595

        397

        531

        275

        564

        282

        Model 5

        2484

        5920

        707

        470

        640

        360

        625

        313

        Model 6

        1246

        2258

        552

        347

        503

        251

        565

        283

        Model 7

        1267

        2119

        731

        409

        682

        344

        729

        364

        Model 8

        1039

        1729

        536

        359

        478

        257

        509

        255

        Model 9

        1086

        1745

        529

        349

        502

        277

        512

        256

        In model 5 shear reinforcement is maximum in both column and beam. As increases in bay width and story height shear force increases so shear reinforcement is also increased. Shear reinforcement of beam and column exterior joint tabulated below.

        Table 3: Shear Reinforcement of Exterior Beam-Column Joint

        Column (mm2)/m

        Beam 7 (mm2)/m

        Beam 14 (mm2)/m

        Beam 15 (mm2)/m

        Model 1

        332

        887

        943

        681

        Model 2

        332

        915

        974

        700

        Model 3

        332

        936

        998

        705

        Model 4

        332

        988

        1055

        772

        Model 5

        388

        1178

        1241

        839

        Model 6

        332

        847

        898

        667

        Model 7

        387

        1049

        1094

        720

        Model 8

        332

        896

        956

        679

        Model 9

        332

        923

        991

        687

      3. Joint Shear Strength and Joint Shear Reinforcement of Exterior Beam-Column Joint

        According to IS code shear strength of joint is proportional to the effective area of joint and grade of concrete so shear strength of exterior beam column joint of model 5 and model 7 had maximum shear strength. This study had taken 2 leg of 8mm stirrups (100mm2) confining reinforcement for all nine models and spacing of these confining reinforcements calculated from clause 8.1 b and clause 8.1.c.2 and provide over length lo which specified in clause 8.1.a of IS 13920:2016.

        Table 4:Joint Shear Strength, confining reinforcement According to IS 13920:2016

        bj (mm)

        wj (mm)

        jc (KN)

        Ash (mm2)

        S

        (mm)

        Ash mm2/m

        Lo (mm)

        Model 1

        240

        240

        309.11

        100

        75

        1333

        675

        Model 2

        240

        240

        345.6

        100

        65

        1538

        715

        Model 3

        240

        240

        378.58

        100

        55

        1818

        715

        Model 4

        240

        240

        309.11

        100

        75

        1333

        675

        Model 5

        290

        290

        451.33

        100

        85

        1176

        680

        Model 6

        240

        240

        309.11

        100

        75

        1333

        825

        Model 7

        290

        290

        451.33

        100

        85

        1176

        935

        Model 8

        240

        240

        309.11

        100

        75

        1333

        675

        Model 9

        240

        240

        309.11

        100

        75

        1333

        675

        According to ACI code shear strength of joint is proportional to the effective area of joint and square root of grade of concrete so shear strength of exterior beam column joint of model 7 had maximum shear strength. This study had taken 2 leg of 10mm stirrups (157mm2) confining reinforcement for all nine models and spacing of these confining reinforcements calculated from clause 4.2.2.2 and clause 4.2.2.3 and provide over length lo which specified in ACI 352R.02.

        Table 5:Joint Shear Strength, Confining Reinforcement According to ACI 352R.02

        bj (mm)

        hc (mm)

        n (KN)

        Ash (mm2)

        S

        (mm)

        Ash mm2/m

        Lo (mm)

        Model 1

        265

        240

        354.11

        157

        75

        2093

        675

        Model 2

        265

        240

        395.91

        157

        60

        2617

        720

        Model 3

        265

        240

        433.69

        157

        50

        3140

        700

        Model 4

        265

        240

        354.11

        157

        75

        2093

        675

        Model 5

        290

        290

        468.25

        157

        80

        1963

        720

        Model 6

        265

        240

        354.11

        157

        75

        2093

        825

        Model 7

        325

        290

        524.76

        157

        80

        1963

        880

        Model 8

        265

        240

        354.11

        157

        75

        2093

        675

        Model 9

        265

        240

        354.11

        157

        75

        2093

        675

        According to NZS 3101.1:2006 V*jh calculated by clause 15.3.4. is 0.75 for not based on overstrength. Ajh and Ajv calculated by using NZS code. NZS code is provided vertical and horizontal shear stirrups over a length of effective depth of column and beam. As increasing the grade of concrete joint shear reinforcement is also increased.

        Table 6:Joint Shear Strength, Confining Reinforcement According to NZS 3101.1:2006

        Vjh (N)

        V*jh N

        Ajh mm2

        Sh (mm)

        Ash mm2/m

        Ajv (mm2)

        Sv (mm)

        Model 1

        307200

        230400

        157

        60

        2617

        157

        80

        Model 2

        384000

        288000

        157

        45

        3489

        157

        60

        Model 3

        460800

        345600

        157

        40

        3925

        157

        60

        Model 4

        307200

        230400

        157

        60

        2617

        157

        80

        Model 5

        448533

        336400

        157

        50

        3140

        157

        70

        Model 6

        307200

        230400

        157

        60

        2617

        157

        80

        Model 7

        448533

        336400

        157

        50

        3140

        157

        60

        Model 8

        307200

        230400

        157

        60

        2617

        157

        80

        Model 9

        307200

        230400

        157

        60

        2617

        157

        80

      4. Comparison of Shear Capacity of Exterior Beam-Column Joint

        According to ACI code 352R.02 gave maximum shear strength of joint of external beam-column joint. In all these code shear capacities of joint depend upon dimension of beam-column joint and grade of concrete. Bar chart of shear capacity obtained from IS code, ACI code and NZS code shown in figure below.

        Figure 5:Shear Capacity of Exterior Beam-Column Joint

      5. Comparison of Shear Force on Beam and Column of Exterior Joint

        Shear force obtained from ETABS software presented on bar chart. Figure 6 shows that the as increase in grade of concrete there is slightly change in shear force in both beam and column. As increase in story height, there is more increase in the shear force in beam and but slightly change in column. As increase in bay width of building there is more increase of sear force in both beam and column. As increase in number of bays of building there is no such variation in of shear force in both beam and column.

        Figure 6:Shear Force due to Change of Concrete Grade Figure 7: Shear Force due to Change in Story Height

        Figure 8:Shear Force due to Change in Bay Width Figure 9:Shear Force due to Change in Number of Bays

        Figure 10: Shear Force on Beam and Column of Exterior Joint

      6. Comparison of Joint Shear Reinforcement of Exterior Beam-Column Joint

    As increase in grade of concrete shear reinforcement on joint increases according to IS, ACI and NZS code provision. As increase in story height and bay width column and beam size increased. As increases in beam and column size joint shear reinforcement decreased according to IS and ACI code but increased according to NZS code. As increases in number of bay joint shear reinforcement remains same for IS, ACI and NZS code which is represented in figure below.

    Figure 11: Joint Shear Reinforcement of Exterior Beam-Column Joint

  4. CONCLUSIONS

In the present study, modelling and analysis of 9 building of 3 story was done in ETABS software. This study examined the exterior beam-column joint of highest shear force, tabulated beam-column shear force in each model building and represented in bar chart. Longitudinal reinforcement and shear reinforcement of exterior beam-column joint obtained from ETABS software. Shear capacity, shear reinforcement and spacing of shear reinforcement of exterior beam-column joint was calculated form IS 13920:2016 code, ACI 352R.02 code and NZS 3101.1:2006 code.

  1. Maximum shear force in both beam and column was developed in Model 7 of bay width 4m and maximum longitudinal and shear reinforcement of beam and column in Model 5 of story height 4m.

  2. There is small change in shear force on both beam and column of exterior beam-column joint by changing the grade of concrete. There is more increase in shear force on beam but less increases in column shear by increasing the story height of building, there was more increase in shear force on both beam column by increasing the bay width of building. There is very less increment in shear force on both beam and column by increasing the number of bays of building.

  3. By using the beam-column dimension of exterior joint ACI 352R.02 code gave maximum joint shear strength of exterior beam-column joint whereas NZS 3101.1:2006 code gave minimum joint shear strength with the same dimension of exterior beam-column joint. By increasing the grade of concrete shear capacity of joint increases. Similarly, shear capacity of joint increases with increase in dimension of exterior beam-column joint.

  4. By using dimension of exterior beam-column joint NZS 3101.1:2006 code gave maximum joint shear reinforcement whereas IS 13920:2016 code gave minimum joint shear reinforcement with same dimension of exterior of beam-column joint.

  5. With increase in grade of concrete shear reinforcement on joint increases more. As increases in number of bay joint shear reinforcement remains same. As increase in story height and bay width column and beam size increased so shear capacity of joint increased due to this joint shear reinforcement decreased according to IS 13920:2016 and ACI 352R.02 code.

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