Research Report 2000-1 January 2000 ISSN 0110 - 3326 44'' '' *UN 3 Seismic Behaviour and Design of Reinforced Concrete Interior Beam Column Joints Cheng-Ming Lin Supervised by J. 1. Restrepo and R. Park %*44 -1 - ./.::: .1 - ....%K 4*: ' *OF 64 :Atft *« P JIVERSITY OF ANTERBURY QISTCHURCH • NEW ZEALAND 4 . . ./ 4 ·31-....% r 0 Cheng-Ming Beam Colum Seismic Bell: 14 1|||U|il :30»99*... fl 49% '4 >443.
Seismic Behaviour and Design of Reinforced Concrete Interior Beam Column Joints
Apr 05, 2023
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276-Seismic behaviour and design of reinforced concrete interior beam column jointsBeam Column Joints
Cheng-Ming Lin Supervised by J. 1. Restrepo and R. Park
%*44 -1
14 1|||U|il
CONCRETE INTERIOR BEAM COLUMN JOINTS
by
A report to the Sponsor
Research Foundation of EQC New Zealand
Eg#*e6tmotyAU94
The physical model of shear transfer mechanism in reinforced concrete beam-column
joints in which the New Zealand Concrete Structures Standard NZS 3101:1995 is based
provides a good insight into the seismic behaviour of joints. However there are still some
issues observed in the laboratory work that can not be fully explained with such model. This
research project is aimed at improving the understanding of the seismic behaviour of joints.
The research work seeks the endorsement of the design recommendations for interior joints
given by the Concrete Structures Standard.
The lower bound theorem of plasticity was applied to find the internal force trajectories
within the joint panel. The diagonal compressive stress field of the joint was modeled with
variable angle struts-and-ties. Relative importance of parameters influencing the shear
strength of the joint panel was identified through the series of parametric analysis.
The database consisting of 60 tests were processed to be used in conjunction with the
analytical work. It was found that a clear trend exists between the ductility of the frame
subassemblies and the joint shear stress ratios equivalent to a reference joint. This relationship
was used to derive the design recommendations for the requirements of horizontal joint shear
reinforcement ofjoints of ductile frames and limited ductility frames.
An experimental programme was conducted to validate the analytical results with
particular emphasis given to parameters that were found to be in disagreement between the
analysis and the current design recommendations. Eight cruciform subassemblies were tested
under simulated earthquake loading. Precast concrete was incorporated in the fabrication of
the test units to simulate the design practice. There are five units in which beam bars are
lumped at the top and bottom beam chords, while three units incorporate distributed
longitudinal beam reinforcement. Grade 500 reinforcing bars were used as beam and column
longitudinal reinforcement in all units. Test results showed good agreement with the analytical
model within reasonable accuracy. Some of the important findings are summarized below.
First, column compressive loads are not always beneficial to the joint strength. When the
column axial load level exceeds 0.3fcAg, it becomes detrimental to the joint. Second,
according to the results obtained in this study, the design recommendations given by NZS
I
.
.
.
3101:1995 are conservative in general and could be relaxed, except for some rare cases. Third,
the horizontal joint reinforcement is strongly influenced by the ratio Vm / fc rather than by the
bond force of the longitudinal beam bars. Forth, the requirement of horizontal joint
reinforcement given by NZS 3101:1995 for joints in which the amount of top and bottom
beam bars is unequal was found to be unduly stringent. Fifth, the shear strength ofthe joints in
which beam bars are distributed along the web is very similar to that of the conventionally
reinforced joints. Therefore, no relaxation of amount of horizontal joint reinforcement can be
expected when using this design alternative.
Test results showed that the theoretical model established in this study is able to predict
the joint strength in correlation with the ductility. Joint design procedures based on the
traditional forced based and displacement based design are discussed in this work. The effect
of using high-grade reinforcement on the bond strength within the joint is also studied.
Test results and theoretical predictions conclusively showed that the yield drift of the
frame subassembly becomes large when Grade 500 longitudinal reinforcement is incorporated.
As a result, full ductility can seldom be achieved before reaching the interstorey drift
limitation of 2.5% given for the ultimate limit state by the loadings code, NZS 4203:1992.
Drift limitations are expected to control the design of reinforced concrete moment resisting
frames when Grade 500 reinforcement is used as longitudinal bars in columns and beams. It is
suggested that, except for low-rise structures in which the drift limit can be easily met,
moment resisting frames designed using Grade 500 bars be designed only for limited ductility
response.
II
ACKNOWLEDGMENTS
The research work reported in this thesis was carried out in the Department of Civil
Engineering, University of Canterbury, New Zealand under Dr.s N. Cooke and K. McManus
as heads of Department.
I express my deepest gratitude to Dr. J. Restrepo and Professor R. Park, supervisors of
this project, for their invaluable advice and encouragement.
The financial support and the scholarship provided by the Research Foundation of the
Earthquake Commission of New Zealand ( project 97/294) is greatly acknowledged. Also
acknowledged is the donation ofthe Grade 500 reinforcing steel by Pacific Steel Ltd.
The excellent assistance of the technical staff of the Department of Civil Engineering
under the direction of Mr. G. Clarke is acknowledged. Thanks are given to Mr. J. Maley, S.
Toase, R. McConchie and G. Harvey for their contribution to the construction of the test
specimens, reaction frames and the testing. Thanks are also given to Mr. N.H. Hickey for his
advice and friendship.
I also wish to thank Emeritus Professor T. Paulay for his interest and fruitful discussions
on some of the topics in this project and Dr. N. Cooke for his time on correcting English
writing on some of chapters. Warm gratitude is given to my fellow postgraduate students for
their friendship and discussions.
Thanks are also given to the Department and the University for the opportunity they gave
to me to study here. Being the first Ph.D. student came from Taiwan in this Department
conducting seismic reinforced concrete research, what I have learned here and the life in New
Zealand will be always in my mind.
Finally, I would like to thank my wife, Sherry Huang for her continuous support, care
and encouragement during the past years. Also thanks our parents for their understanding and
encouragement of my studying here. And, most importantly, thanks to the Lord, Jesus Christ.
III
FRAMES IN NEW ZEALAND 1
1.2 DESIGN CRITERIA FOR BEAM-COLUMN JOINTS 7
1.3 REVIEW OF THE SEISMIC DESIGN METHOD OF BEAM-COLUMN JOINTS IN
NZS 3101:1995 6
1.3.1 Background 6
1.3.2 The Design Provisions of Interior Beam-Column Joints in NZS 3101:1995 ............................... ,
1.4 INCORPORATION OF GRADE 500 LONGITUDINAL REINFORCEMENT 10
1.5 AIMS OF THE RESEARCH PROJECT 11
1.6 SCOPE OF THE THESIS................ ........................................................................................................12
CHAPTER 2 ASSESSMENT OF THE STRENGTH OF INTERIOR BEAM-COLUMN JOINTS
2.1 INTRODUCTION ....................................................14
REINFORCEMENT 16
2.2.4.2 Effects ofColumn Axial Loads and Horizontal Joint Reinforcement Ratio, VshVjh ···26
2.2.4.3 Horizontal Joint Shear Stress Ratio, VjU C 77
2.2.4.4 Effect ofUnequal Top and Bottom Beam Longitudinal Reinforcement......................28
2.2.5 Data Reduction 31 2.3 PROCESSING OF EXISTING TEST RESULTS 31
2.3.1 Evaluation ofthe Joint Shear Stress at Overstrength, v», of Tests 3?
2.3.2 Evaluation of (Vib' 38
W
2.3.4 Ductility Relationships 39
2.3.5 Calculation of Column Deformation as a Component of System Yield Displacement............42
2.3.6 Observed Trenaq 43 2.4 ANALYTICAL RESULTS OF INTERIOR JOINTS INCORPORATING LUMPED BEAM BARS....45
2.4.1 Design ofthe Horizontal Joint Reinforcement 45
2.4.2 Evaluation of Cracking ofthe Joint Panel 47
2.4.3 Comparison ofAnalytical Results With Requirements ofNZS 3101:1995.................................49
2.4.4 Proposed Design Recommendations 49
2.5 VERTICAL JOINT SHEAR REINFORCEMENT 57
2.6 ANALYTICAL WORK OF JOINTS INCORPORATING BEAMS WITH DISTRIBUTED
REINFORCEMENT 57
2.6.1 Moment-Curvature Analysis on Beam Sections Reinforced with Lumped and Distributed
Bars..... 57
2.7 CONCLUSIONS 59-1
3.2.1 Design Considerationg 60
3.2.2.1 Units 1 and 7 63
3.2.2.2 Units 3 and 4 66
3.2.2.3 Unit 8 68
3.2.3.1 General Considerations.................................................................................................68
3.2.3.2 Units 5 and 6 69
3.2.3.3 Unit 7. 71 3.3 LOADING FRAME AND TEST SET-UP 71
3.4 CONSTRUCTION OF THE TEST SPECIMENS....................................................................................76
3.4.1 Formwork 76
3.4.4 Preparation of Connection Surface...............................................................................................80
3.4.5 Assembling ofthe Beam-Column Sub-assemblages....................................................................80
3.6 INSTRUMENTATION 88
3.6.2.1 Measurement of Displacements....................................................................................89
3.6.2.3 Beam Bar Slippage 91
3.6.2.4 Measurement of Strains in Reinforcing Bars................................................................93
3.7 TEST PROCEDURE AND LOADING SEQUENCE... 97
3.8 DECOMPOSITION OF INTERSTOREY DISPLACEMENTS 98
3.8.1 General ... - -.. 98
3.8.4 Beam Deformations....................................................................................................................102
3.9 CONCLUSIONS 107
4.1 INTRODUCTION 108
4.2.4 Decomposition ofLateral Displacements........... ........................................................................116
4.2.5 Joint Behaviolir 117
4.2.5.2 Joint Shear Distortion 117
4.2.5.3 Beam Bar Slip 170
4.2.5.4 Beam Bar Strains and Bond Stresses in Joint Region 1?1
4.2.5.5 Bar Stresses ofthe Column Vertical Reinforcement Within Joint Region.................126
4.2.6 Beam Behaviour 128
4.2.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................129
4.2.6.3 Beam Elongation 179
4.2.7 Column Behaviniir 132
4.3.5.2 Joint Shear Distortion 143
4.3.5.3 Beam Bar Slip 144
4.3.5.4 Bar Strain And Bond Stress ofthe Beam Longitudinal Reinforcement Passing Through
Joint Region 144
4.3.5.5 Bar and Bond Stresses ofthe Column Vertical Reinforcement within Joint Region..149
4.3.6 Beam Behaviour 149
4.3.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................151
4.3.6.3 Beam Elongation 154
4.3.7 Column Behaviolir 155
5.1 INTRODUCTION 157
5.2.5 Joint Behaviour 164
5.2.5.2 Joint Shear Distortion Versus Lateral Loads 168
5.2.5.3 Beam Bar Slip Within Joint Region. 168
5.2.5.4 Bar Strain And Bond Stress ofthe Beam Longitudinal Reinforcement Passing Through
Joint Region 170
5.2.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Rpginn 174
5.2.6.2 Beam Strain Profiles at the Level ofthe Longitudinal Reinforcement.......................176
5.2.6.3 Beam Elongation......... ................................................................................................179
5.3.5.1 Strains in the Horizontal Joint Shear Reinforcement ..................................................185
5.3.5.2 Joint Shear Distortion Versus Lateral Loads 189
5.3.5.3 Beam Bar Slip Within Joint Region............................................................................190
5.3.5.4 Bar Strain And Bond Stress of the Beam Longitudinal Reinforcement Passing Through
Joint Reginn 190
5.3.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Region ........................................................................................................................195
5.3.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................198
5.3.6.3 Beam Elongation .........................................................................................................198
6.1 INTRODUCTION 703
6.2.3 Decomposition ofthe Lateral Displacement 708
6.2.4 Joint Behavini,r 710
6.2.4.4 Bar Strain and Bond Stresses in Joint Region 715
6.2.4.5 Bar Stresses in the Column Vertical Reinforcement Passing Through the Joint
Region 718
6.2.5.2 Beam Strain Profiles at the Level ofthe Longitudinal Reinforcement.......................221
6.2.5.3 Beam Elongation............ .............................................................................................226
6.3.5.4 Bar Strain and Bond Stress in Joint Region................................................................238
6.3.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Reginn 738
6.3.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................243
6.3.6.3 Beam Elongation.........................................................................................................245
6.4.4 Decomposition of Lateral Displacement 753
6.4.5 Joint Behavin,ir ?55
6.4.5.2 Joint Shear Distortion ?59
6.4.5.3 Beam Bar Slip 759
6.4.5.4 Bar Strain And Bond Stress in Joint Region...............................................................261
6.4.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Reginn 767
6.4.6.2 Beam Strain Profiles at the Level of Longitudinal Reinforcement.............................268
6.4.6.3 Beam Elongation...... ...................................................................................................268
IX
7.2.3 Decomposition ofLateral Displacements........... ........................................................................277
7.2.4 Joint Behavinilr 781
7.2.4.2 Joint Shear Distortion 081
7.2.4.3 Beam Bar Slip 284
7.2.4.4 Beam Bar Strains And Bond Stresses in Joint Region................................................286
7.2.4.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Rpginn 790
7.2.5.2 Beam Strain Profiles at the Level ofthe Longitudinal Reinforcement.......................293
7.2.5.3 Beam Elongatinn 793
7.2.6 Column Behaviolir 796
8.1 DISCUSSION OF THE TEST RESULTS 798
8.1.1 Significance of Yield Drift Level on the Seismic Design of Reinforced Concrete Moment
Resisting Frampq 298
8.1.1.3 Comparison ofMeasured and Predicted Yield Drift...................................................305
8.1.1.4 Available Displacement Ductility And the Significance of Using Grade 500
Longitudinal Reinforcement... ....................................................................................307
8.1.2 Bond Stress Distribution of Beam and Column Bars in the Joint and Its Influence on the Joint
Strength......................................................................................................309
8.1.3 Influence of Column Axial Loads Level on the Joint Shear Strength 313
8.1.4 Influence ofAs /AsRatio on the Joint Shear Strength 313
8.1.5 The Role ofv / fc 316
8.1.6 Study ofFrame Sway Mechanisms With Joint Failurpq 316
8.2 DESIGN RECOMMENDATIONS FOR JOINT INCORPORATING BEAMS WITH TOP AND
BOTTOM REINFORCEMENT 370
8.2.1 Comparison ofTest Results........................................................................................................320
8.2.5 Refined Design Recommendations....... ......................................................................................330
8.2.5.2 Ductility Base Horizontal Joint Reinforcement Design ..............................................331
8.2.5.3 Performance-Based Horizontal Joint Reinforcement Design .....................................332
8.2.5.4 Prediction ofthe Available Displacement Ductility in Joints ofExisting Buildings...334
8.2.6 Joint Incorporating Shear Reinforcement Without an Apparent Yield Plateau..........................336
8.3 BEHAVIOUR OF JOINTS INCORPORATING BEAMS WITH DISTRIBUTED
REINFORCEMENT... .. 338
8.3.1.1 Bond Strength ofBeam Bars Passing Through Joint Region 119
8.3.1.2 Shear Strength ofthe Joint..........................................................................................339
8.4 SEISMIC STRENGTH ASSESSMENT OF INTERIOR BEAM-COLUMN JOINTS IN EXISTING
BUILDINGS 340
8.4.3 Degradation Model of Shear Strength of Reinforced Interior Joints..........................................342
8.5 BOND STRENGTH OF INTERIOR BEAM-COLUMN JOINTS 344
8.5.1 Introduction 344
8.5.2 Evaluation of Average Bond Stress of Beam Bars Passing Through Interior Beam-Column Joint
345
8.5.3 Enhancement Effects on the Bond Strength Caused by Column Axial Loads...........................350
8.5.4 Discussion 350
8.6.1 Introduction 355
8.6.2 Prediction ofBeam Elongation...................................................................................................356
9.1 GENERAL 366
9.2.1 Analytical Investigation..............................................................................................................366
9.2.2 Experimental Evidence...............................................................................................................367
XI
Longitudinal Bars Distributed Through the Web 369
9.2.3 Recommendations for the Sesimic Design of Interior Beam-Column Joints.............................369
9.2.4 Bond Strength of Beam Bars Passing through Joint Region 370
9.2.5 Beam Elongation 370
9.3 SEISMIC DESIGN OF REINFORCED CONCRETE MOMENT RESISTING FRAMES US[NG
GRADE 500 LONGITUDINAL STEEL 371
9.4 SUGGESTED FUTURE RESEARCH 371
REFERENCE............ .374
area of top beam bars
area of bottom beam bars
effective joint shear area
diameter of top beam bar passing through joint region
diameter of bottom beam bar passing through joint region
effective beam depth measured from the centroid of the top beam bars
to the bottom beam chord
effective beam depth measured from the centroid of the bottom beam
bars to the top beam chord
Young's Modulus of concrete
Young's Modulus of steel
ultimate tensile yield strength of reinforcing steel
concrete compressive strength
average uniaxial compressive stress in the central strut of the joint
joint diagonal tension cracking stress
diagonal tensile strength of the concrete
grout compressive strength
dependable lateral load capacity
measured properties
XIII
.
50 2 :F: 2: Er © Gr, 1,) r p r'- 6-' 'ri Fl Fl C+ 01 P. f. 2 2 07- .Ir cr .2, P - P. 2, 2,01,>
cr
n " " " " " " n " " " " " n
P- delta effect
moment of inertia of gross section area of structure members
internal level arm of beams, measured from the centroid of the bottom
beam bars to the centroid of concrete stress block
internal level arm of beams, measured from the centroid of the top
beam bars to the centroid of concrete stress block
distance between the centroids of top and bottom beam bars, = ( d-d' )
joint stiffness
beam span length between two pin ends
half beam span length measured from one pin end to the column face
half beam span length measured from one pin end to the controid of
column exterior bars
half column height measured from one pin end to the column face
ideal beam moment capacity, calculated from T+(id+)
ideal beam moment capacity, calculated from T-(id-)
column axial load
compressive force of the central strut in the joint
circumference of beam bar
tensile force of bottom beam bars based on measured properties
without counting over-strength
tensile force of top beam bars based on measured properties without
counting over-strength
beam shear force associated to Mit calculated from Mi / 1'
beam shear force associated to Mi-, calculated from Mi- / 1'
beam shear force
11 11 11 11 "
H H " H " I
I n n li ll il il il li ll " H n li 11
11 11 11 11 11
0
1
4 2 2 2 19 19 -2 -9 9 *Z c. -0. 0: 1 2 2 2 E.2 E.5 67 2 0 23 > >7 +
,jd
joint shear force taken by the provided joint hoops
ideal joint shear force
horizontal joint shear resistance due to joint concrete
vertical joint shear force taken by the vertical joint reinforcement
vertical joint shear force
width of the diagonal concrete central strut in the joint
reference yield displacement
storey displacement due to beam flexural deformation
storey displacement due to beam fixed-end rotation
storey displacement due to beam shear
storey displacement due to joint shear distortion
displacement duetility factor
rotational ductility factor
curvature ductility factor
plastic curvature developing in beam plastic hinge
beam ultimate curvature
: left beam end vertical displacement
: right beam end vertical displacement
: top column end lateral displacement
: bottom column end lateral displacement
: elongation of beams taking place in the first load cycle toward a new
displacement ductility
: column displacement associated to 0.75 Ha due to fixed-end rotation
: column flexure displacement associated to 0.75 Ha
: column fixed-end rotation
yj = joint shear distortion
Esh = tensile strain at onset of work hardening
su = ultimate tensile strain of reinforcing bars
= As'/As
Ajo = over-strength factor for evaluating joint shear stress at over-strength
CHAPTER 1
RESISTING FRAMES IN NEW ZEALAND
Moment resisting frames are broadly recognized as an efficient structural system for
providing the lateral load resistance in reinforced concrete building structures. Frames are
generally designed and detailed for ductility to survive a major seismic event.
In New Zealand, with the incorporation of precast structural concrete, designers often
separate the structural system into a lateral force resistance system and a gravity load carrying
system. Normally the lateral load resistance system is allocated to perimeter frames, which
have squat beams and columns, while the interior frames are more flexible and predominately
carry gravity loads. The use of perimeter moment resisting frames has the advantage of
simplifying the structural analysis and design as well as the detailing of the structural
members. This is particularly the case when precast structural concrete is used. Another
advantage is that as beams of perimeter frames carry little gravity loads, consequently the
gravity beam shear is relatively less than that induced by the lateral force. Hence, under the
action of reversed lateral loads, positive and negative plastic hinges can form in beams at the
same location adjacent to column faces; thus, uni-directional plastic hinges can easily be
avoided. The displacement capacity of moment resisting frames with reversing plastic hinges
in beams is generally larger than that of frames with uni-directional plastic hinges. This is
because the curvature ductility demands of uni-directional plastic hinges are much larger than
those of reversal plastic hinges [I)4].
Some concepts of the seismic design of moment resisting frames of multistorey buildings
are broadly recognized in the New Zealand Concrete Structures Standard, NZS 3101:1995 [Sl]
[A6]. First, it is preferable to ensure "strong column-weak beam response so that the adverse
soft-storey mechanism" can be avoided. Second, the interstorey drift under the design lateral
action needs to be restricted to below a certain limit. This is to prevent excessive non-
structural damage and P-delta effects.
In New Zealand, a further step has been taken with the use of a deterministic capacity
design procedure [P2]. According to this procedure, a suitable sway mechanism is chosen for
the structure. Plastic hinge regions are detailed to be ductile and other undesirable failure
modes are precluded to ensure that the preferable sway mechanism can develop and be
sustained. The input actions for structural members have to be quantified with some accuracy.
In order to provide adequate protection against the formation of plastic hinges in columns, a
dynamic magnification factor is introduced to magnify the column design moment and shear
resulting from input actions of adjacent members to account for the higher mode effects [P2.1
[Sl].
This research project concentrates on the seismic design and behaviour of interior joints.
Figure 1.1 shows a displaced frame structure under the action of lateral load with the
classification of exterior and interior beam-column joints. Reinforced concrete beam-column
joints are very important because of their role in the behaviour of moment resisting frames
designed for earthquake resistance. Joints are subjected to reversing bending moments on
opposite adjacent members, and also the input shear force in the joint region is typically of the
order of 5 times the column shear force. This can be seen in Fig. 1.2 which depicts the
bending moments and shear force along the column of a frame…
Cheng-Ming Lin Supervised by J. 1. Restrepo and R. Park
%*44 -1
14 1|||U|il
CONCRETE INTERIOR BEAM COLUMN JOINTS
by
A report to the Sponsor
Research Foundation of EQC New Zealand
Eg#*e6tmotyAU94
The physical model of shear transfer mechanism in reinforced concrete beam-column
joints in which the New Zealand Concrete Structures Standard NZS 3101:1995 is based
provides a good insight into the seismic behaviour of joints. However there are still some
issues observed in the laboratory work that can not be fully explained with such model. This
research project is aimed at improving the understanding of the seismic behaviour of joints.
The research work seeks the endorsement of the design recommendations for interior joints
given by the Concrete Structures Standard.
The lower bound theorem of plasticity was applied to find the internal force trajectories
within the joint panel. The diagonal compressive stress field of the joint was modeled with
variable angle struts-and-ties. Relative importance of parameters influencing the shear
strength of the joint panel was identified through the series of parametric analysis.
The database consisting of 60 tests were processed to be used in conjunction with the
analytical work. It was found that a clear trend exists between the ductility of the frame
subassemblies and the joint shear stress ratios equivalent to a reference joint. This relationship
was used to derive the design recommendations for the requirements of horizontal joint shear
reinforcement ofjoints of ductile frames and limited ductility frames.
An experimental programme was conducted to validate the analytical results with
particular emphasis given to parameters that were found to be in disagreement between the
analysis and the current design recommendations. Eight cruciform subassemblies were tested
under simulated earthquake loading. Precast concrete was incorporated in the fabrication of
the test units to simulate the design practice. There are five units in which beam bars are
lumped at the top and bottom beam chords, while three units incorporate distributed
longitudinal beam reinforcement. Grade 500 reinforcing bars were used as beam and column
longitudinal reinforcement in all units. Test results showed good agreement with the analytical
model within reasonable accuracy. Some of the important findings are summarized below.
First, column compressive loads are not always beneficial to the joint strength. When the
column axial load level exceeds 0.3fcAg, it becomes detrimental to the joint. Second,
according to the results obtained in this study, the design recommendations given by NZS
I
.
.
.
3101:1995 are conservative in general and could be relaxed, except for some rare cases. Third,
the horizontal joint reinforcement is strongly influenced by the ratio Vm / fc rather than by the
bond force of the longitudinal beam bars. Forth, the requirement of horizontal joint
reinforcement given by NZS 3101:1995 for joints in which the amount of top and bottom
beam bars is unequal was found to be unduly stringent. Fifth, the shear strength ofthe joints in
which beam bars are distributed along the web is very similar to that of the conventionally
reinforced joints. Therefore, no relaxation of amount of horizontal joint reinforcement can be
expected when using this design alternative.
Test results showed that the theoretical model established in this study is able to predict
the joint strength in correlation with the ductility. Joint design procedures based on the
traditional forced based and displacement based design are discussed in this work. The effect
of using high-grade reinforcement on the bond strength within the joint is also studied.
Test results and theoretical predictions conclusively showed that the yield drift of the
frame subassembly becomes large when Grade 500 longitudinal reinforcement is incorporated.
As a result, full ductility can seldom be achieved before reaching the interstorey drift
limitation of 2.5% given for the ultimate limit state by the loadings code, NZS 4203:1992.
Drift limitations are expected to control the design of reinforced concrete moment resisting
frames when Grade 500 reinforcement is used as longitudinal bars in columns and beams. It is
suggested that, except for low-rise structures in which the drift limit can be easily met,
moment resisting frames designed using Grade 500 bars be designed only for limited ductility
response.
II
ACKNOWLEDGMENTS
The research work reported in this thesis was carried out in the Department of Civil
Engineering, University of Canterbury, New Zealand under Dr.s N. Cooke and K. McManus
as heads of Department.
I express my deepest gratitude to Dr. J. Restrepo and Professor R. Park, supervisors of
this project, for their invaluable advice and encouragement.
The financial support and the scholarship provided by the Research Foundation of the
Earthquake Commission of New Zealand ( project 97/294) is greatly acknowledged. Also
acknowledged is the donation ofthe Grade 500 reinforcing steel by Pacific Steel Ltd.
The excellent assistance of the technical staff of the Department of Civil Engineering
under the direction of Mr. G. Clarke is acknowledged. Thanks are given to Mr. J. Maley, S.
Toase, R. McConchie and G. Harvey for their contribution to the construction of the test
specimens, reaction frames and the testing. Thanks are also given to Mr. N.H. Hickey for his
advice and friendship.
I also wish to thank Emeritus Professor T. Paulay for his interest and fruitful discussions
on some of the topics in this project and Dr. N. Cooke for his time on correcting English
writing on some of chapters. Warm gratitude is given to my fellow postgraduate students for
their friendship and discussions.
Thanks are also given to the Department and the University for the opportunity they gave
to me to study here. Being the first Ph.D. student came from Taiwan in this Department
conducting seismic reinforced concrete research, what I have learned here and the life in New
Zealand will be always in my mind.
Finally, I would like to thank my wife, Sherry Huang for her continuous support, care
and encouragement during the past years. Also thanks our parents for their understanding and
encouragement of my studying here. And, most importantly, thanks to the Lord, Jesus Christ.
III
FRAMES IN NEW ZEALAND 1
1.2 DESIGN CRITERIA FOR BEAM-COLUMN JOINTS 7
1.3 REVIEW OF THE SEISMIC DESIGN METHOD OF BEAM-COLUMN JOINTS IN
NZS 3101:1995 6
1.3.1 Background 6
1.3.2 The Design Provisions of Interior Beam-Column Joints in NZS 3101:1995 ............................... ,
1.4 INCORPORATION OF GRADE 500 LONGITUDINAL REINFORCEMENT 10
1.5 AIMS OF THE RESEARCH PROJECT 11
1.6 SCOPE OF THE THESIS................ ........................................................................................................12
CHAPTER 2 ASSESSMENT OF THE STRENGTH OF INTERIOR BEAM-COLUMN JOINTS
2.1 INTRODUCTION ....................................................14
REINFORCEMENT 16
2.2.4.2 Effects ofColumn Axial Loads and Horizontal Joint Reinforcement Ratio, VshVjh ···26
2.2.4.3 Horizontal Joint Shear Stress Ratio, VjU C 77
2.2.4.4 Effect ofUnequal Top and Bottom Beam Longitudinal Reinforcement......................28
2.2.5 Data Reduction 31 2.3 PROCESSING OF EXISTING TEST RESULTS 31
2.3.1 Evaluation ofthe Joint Shear Stress at Overstrength, v», of Tests 3?
2.3.2 Evaluation of (Vib' 38
W
2.3.4 Ductility Relationships 39
2.3.5 Calculation of Column Deformation as a Component of System Yield Displacement............42
2.3.6 Observed Trenaq 43 2.4 ANALYTICAL RESULTS OF INTERIOR JOINTS INCORPORATING LUMPED BEAM BARS....45
2.4.1 Design ofthe Horizontal Joint Reinforcement 45
2.4.2 Evaluation of Cracking ofthe Joint Panel 47
2.4.3 Comparison ofAnalytical Results With Requirements ofNZS 3101:1995.................................49
2.4.4 Proposed Design Recommendations 49
2.5 VERTICAL JOINT SHEAR REINFORCEMENT 57
2.6 ANALYTICAL WORK OF JOINTS INCORPORATING BEAMS WITH DISTRIBUTED
REINFORCEMENT 57
2.6.1 Moment-Curvature Analysis on Beam Sections Reinforced with Lumped and Distributed
Bars..... 57
2.7 CONCLUSIONS 59-1
3.2.1 Design Considerationg 60
3.2.2.1 Units 1 and 7 63
3.2.2.2 Units 3 and 4 66
3.2.2.3 Unit 8 68
3.2.3.1 General Considerations.................................................................................................68
3.2.3.2 Units 5 and 6 69
3.2.3.3 Unit 7. 71 3.3 LOADING FRAME AND TEST SET-UP 71
3.4 CONSTRUCTION OF THE TEST SPECIMENS....................................................................................76
3.4.1 Formwork 76
3.4.4 Preparation of Connection Surface...............................................................................................80
3.4.5 Assembling ofthe Beam-Column Sub-assemblages....................................................................80
3.6 INSTRUMENTATION 88
3.6.2.1 Measurement of Displacements....................................................................................89
3.6.2.3 Beam Bar Slippage 91
3.6.2.4 Measurement of Strains in Reinforcing Bars................................................................93
3.7 TEST PROCEDURE AND LOADING SEQUENCE... 97
3.8 DECOMPOSITION OF INTERSTOREY DISPLACEMENTS 98
3.8.1 General ... - -.. 98
3.8.4 Beam Deformations....................................................................................................................102
3.9 CONCLUSIONS 107
4.1 INTRODUCTION 108
4.2.4 Decomposition ofLateral Displacements........... ........................................................................116
4.2.5 Joint Behaviolir 117
4.2.5.2 Joint Shear Distortion 117
4.2.5.3 Beam Bar Slip 170
4.2.5.4 Beam Bar Strains and Bond Stresses in Joint Region 1?1
4.2.5.5 Bar Stresses ofthe Column Vertical Reinforcement Within Joint Region.................126
4.2.6 Beam Behaviour 128
4.2.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................129
4.2.6.3 Beam Elongation 179
4.2.7 Column Behaviniir 132
4.3.5.2 Joint Shear Distortion 143
4.3.5.3 Beam Bar Slip 144
4.3.5.4 Bar Strain And Bond Stress ofthe Beam Longitudinal Reinforcement Passing Through
Joint Region 144
4.3.5.5 Bar and Bond Stresses ofthe Column Vertical Reinforcement within Joint Region..149
4.3.6 Beam Behaviour 149
4.3.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................151
4.3.6.3 Beam Elongation 154
4.3.7 Column Behaviolir 155
5.1 INTRODUCTION 157
5.2.5 Joint Behaviour 164
5.2.5.2 Joint Shear Distortion Versus Lateral Loads 168
5.2.5.3 Beam Bar Slip Within Joint Region. 168
5.2.5.4 Bar Strain And Bond Stress ofthe Beam Longitudinal Reinforcement Passing Through
Joint Region 170
5.2.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Rpginn 174
5.2.6.2 Beam Strain Profiles at the Level ofthe Longitudinal Reinforcement.......................176
5.2.6.3 Beam Elongation......... ................................................................................................179
5.3.5.1 Strains in the Horizontal Joint Shear Reinforcement ..................................................185
5.3.5.2 Joint Shear Distortion Versus Lateral Loads 189
5.3.5.3 Beam Bar Slip Within Joint Region............................................................................190
5.3.5.4 Bar Strain And Bond Stress of the Beam Longitudinal Reinforcement Passing Through
Joint Reginn 190
5.3.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Region ........................................................................................................................195
5.3.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................198
5.3.6.3 Beam Elongation .........................................................................................................198
6.1 INTRODUCTION 703
6.2.3 Decomposition ofthe Lateral Displacement 708
6.2.4 Joint Behavini,r 710
6.2.4.4 Bar Strain and Bond Stresses in Joint Region 715
6.2.4.5 Bar Stresses in the Column Vertical Reinforcement Passing Through the Joint
Region 718
6.2.5.2 Beam Strain Profiles at the Level ofthe Longitudinal Reinforcement.......................221
6.2.5.3 Beam Elongation............ .............................................................................................226
6.3.5.4 Bar Strain and Bond Stress in Joint Region................................................................238
6.3.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Reginn 738
6.3.6.2 Beam Strain Profiles at the Level of the Longitudinal Reinforcement.......................243
6.3.6.3 Beam Elongation.........................................................................................................245
6.4.4 Decomposition of Lateral Displacement 753
6.4.5 Joint Behavin,ir ?55
6.4.5.2 Joint Shear Distortion ?59
6.4.5.3 Beam Bar Slip 759
6.4.5.4 Bar Strain And Bond Stress in Joint Region...............................................................261
6.4.5.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Reginn 767
6.4.6.2 Beam Strain Profiles at the Level of Longitudinal Reinforcement.............................268
6.4.6.3 Beam Elongation...... ...................................................................................................268
IX
7.2.3 Decomposition ofLateral Displacements........... ........................................................................277
7.2.4 Joint Behavinilr 781
7.2.4.2 Joint Shear Distortion 081
7.2.4.3 Beam Bar Slip 284
7.2.4.4 Beam Bar Strains And Bond Stresses in Joint Region................................................286
7.2.4.5 Bar Stresses ofthe Column Vertical Reinforcement Passing Through the Joint
Rpginn 790
7.2.5.2 Beam Strain Profiles at the Level ofthe Longitudinal Reinforcement.......................293
7.2.5.3 Beam Elongatinn 793
7.2.6 Column Behaviolir 796
8.1 DISCUSSION OF THE TEST RESULTS 798
8.1.1 Significance of Yield Drift Level on the Seismic Design of Reinforced Concrete Moment
Resisting Frampq 298
8.1.1.3 Comparison ofMeasured and Predicted Yield Drift...................................................305
8.1.1.4 Available Displacement Ductility And the Significance of Using Grade 500
Longitudinal Reinforcement... ....................................................................................307
8.1.2 Bond Stress Distribution of Beam and Column Bars in the Joint and Its Influence on the Joint
Strength......................................................................................................309
8.1.3 Influence of Column Axial Loads Level on the Joint Shear Strength 313
8.1.4 Influence ofAs /AsRatio on the Joint Shear Strength 313
8.1.5 The Role ofv / fc 316
8.1.6 Study ofFrame Sway Mechanisms With Joint Failurpq 316
8.2 DESIGN RECOMMENDATIONS FOR JOINT INCORPORATING BEAMS WITH TOP AND
BOTTOM REINFORCEMENT 370
8.2.1 Comparison ofTest Results........................................................................................................320
8.2.5 Refined Design Recommendations....... ......................................................................................330
8.2.5.2 Ductility Base Horizontal Joint Reinforcement Design ..............................................331
8.2.5.3 Performance-Based Horizontal Joint Reinforcement Design .....................................332
8.2.5.4 Prediction ofthe Available Displacement Ductility in Joints ofExisting Buildings...334
8.2.6 Joint Incorporating Shear Reinforcement Without an Apparent Yield Plateau..........................336
8.3 BEHAVIOUR OF JOINTS INCORPORATING BEAMS WITH DISTRIBUTED
REINFORCEMENT... .. 338
8.3.1.1 Bond Strength ofBeam Bars Passing Through Joint Region 119
8.3.1.2 Shear Strength ofthe Joint..........................................................................................339
8.4 SEISMIC STRENGTH ASSESSMENT OF INTERIOR BEAM-COLUMN JOINTS IN EXISTING
BUILDINGS 340
8.4.3 Degradation Model of Shear Strength of Reinforced Interior Joints..........................................342
8.5 BOND STRENGTH OF INTERIOR BEAM-COLUMN JOINTS 344
8.5.1 Introduction 344
8.5.2 Evaluation of Average Bond Stress of Beam Bars Passing Through Interior Beam-Column Joint
345
8.5.3 Enhancement Effects on the Bond Strength Caused by Column Axial Loads...........................350
8.5.4 Discussion 350
8.6.1 Introduction 355
8.6.2 Prediction ofBeam Elongation...................................................................................................356
9.1 GENERAL 366
9.2.1 Analytical Investigation..............................................................................................................366
9.2.2 Experimental Evidence...............................................................................................................367
XI
Longitudinal Bars Distributed Through the Web 369
9.2.3 Recommendations for the Sesimic Design of Interior Beam-Column Joints.............................369
9.2.4 Bond Strength of Beam Bars Passing through Joint Region 370
9.2.5 Beam Elongation 370
9.3 SEISMIC DESIGN OF REINFORCED CONCRETE MOMENT RESISTING FRAMES US[NG
GRADE 500 LONGITUDINAL STEEL 371
9.4 SUGGESTED FUTURE RESEARCH 371
REFERENCE............ .374
area of top beam bars
area of bottom beam bars
effective joint shear area
diameter of top beam bar passing through joint region
diameter of bottom beam bar passing through joint region
effective beam depth measured from the centroid of the top beam bars
to the bottom beam chord
effective beam depth measured from the centroid of the bottom beam
bars to the top beam chord
Young's Modulus of concrete
Young's Modulus of steel
ultimate tensile yield strength of reinforcing steel
concrete compressive strength
average uniaxial compressive stress in the central strut of the joint
joint diagonal tension cracking stress
diagonal tensile strength of the concrete
grout compressive strength
dependable lateral load capacity
measured properties
XIII
.
50 2 :F: 2: Er © Gr, 1,) r p r'- 6-' 'ri Fl Fl C+ 01 P. f. 2 2 07- .Ir cr .2, P - P. 2, 2,01,>
cr
n " " " " " " n " " " " " n
P- delta effect
moment of inertia of gross section area of structure members
internal level arm of beams, measured from the centroid of the bottom
beam bars to the centroid of concrete stress block
internal level arm of beams, measured from the centroid of the top
beam bars to the centroid of concrete stress block
distance between the centroids of top and bottom beam bars, = ( d-d' )
joint stiffness
beam span length between two pin ends
half beam span length measured from one pin end to the column face
half beam span length measured from one pin end to the controid of
column exterior bars
half column height measured from one pin end to the column face
ideal beam moment capacity, calculated from T+(id+)
ideal beam moment capacity, calculated from T-(id-)
column axial load
compressive force of the central strut in the joint
circumference of beam bar
tensile force of bottom beam bars based on measured properties
without counting over-strength
tensile force of top beam bars based on measured properties without
counting over-strength
beam shear force associated to Mit calculated from Mi / 1'
beam shear force associated to Mi-, calculated from Mi- / 1'
beam shear force
11 11 11 11 "
H H " H " I
I n n li ll il il il li ll " H n li 11
11 11 11 11 11
0
1
4 2 2 2 19 19 -2 -9 9 *Z c. -0. 0: 1 2 2 2 E.2 E.5 67 2 0 23 > >7 +
,jd
joint shear force taken by the provided joint hoops
ideal joint shear force
horizontal joint shear resistance due to joint concrete
vertical joint shear force taken by the vertical joint reinforcement
vertical joint shear force
width of the diagonal concrete central strut in the joint
reference yield displacement
storey displacement due to beam flexural deformation
storey displacement due to beam fixed-end rotation
storey displacement due to beam shear
storey displacement due to joint shear distortion
displacement duetility factor
rotational ductility factor
curvature ductility factor
plastic curvature developing in beam plastic hinge
beam ultimate curvature
: left beam end vertical displacement
: right beam end vertical displacement
: top column end lateral displacement
: bottom column end lateral displacement
: elongation of beams taking place in the first load cycle toward a new
displacement ductility
: column displacement associated to 0.75 Ha due to fixed-end rotation
: column flexure displacement associated to 0.75 Ha
: column fixed-end rotation
yj = joint shear distortion
Esh = tensile strain at onset of work hardening
su = ultimate tensile strain of reinforcing bars
= As'/As
Ajo = over-strength factor for evaluating joint shear stress at over-strength
CHAPTER 1
RESISTING FRAMES IN NEW ZEALAND
Moment resisting frames are broadly recognized as an efficient structural system for
providing the lateral load resistance in reinforced concrete building structures. Frames are
generally designed and detailed for ductility to survive a major seismic event.
In New Zealand, with the incorporation of precast structural concrete, designers often
separate the structural system into a lateral force resistance system and a gravity load carrying
system. Normally the lateral load resistance system is allocated to perimeter frames, which
have squat beams and columns, while the interior frames are more flexible and predominately
carry gravity loads. The use of perimeter moment resisting frames has the advantage of
simplifying the structural analysis and design as well as the detailing of the structural
members. This is particularly the case when precast structural concrete is used. Another
advantage is that as beams of perimeter frames carry little gravity loads, consequently the
gravity beam shear is relatively less than that induced by the lateral force. Hence, under the
action of reversed lateral loads, positive and negative plastic hinges can form in beams at the
same location adjacent to column faces; thus, uni-directional plastic hinges can easily be
avoided. The displacement capacity of moment resisting frames with reversing plastic hinges
in beams is generally larger than that of frames with uni-directional plastic hinges. This is
because the curvature ductility demands of uni-directional plastic hinges are much larger than
those of reversal plastic hinges [I)4].
Some concepts of the seismic design of moment resisting frames of multistorey buildings
are broadly recognized in the New Zealand Concrete Structures Standard, NZS 3101:1995 [Sl]
[A6]. First, it is preferable to ensure "strong column-weak beam response so that the adverse
soft-storey mechanism" can be avoided. Second, the interstorey drift under the design lateral
action needs to be restricted to below a certain limit. This is to prevent excessive non-
structural damage and P-delta effects.
In New Zealand, a further step has been taken with the use of a deterministic capacity
design procedure [P2]. According to this procedure, a suitable sway mechanism is chosen for
the structure. Plastic hinge regions are detailed to be ductile and other undesirable failure
modes are precluded to ensure that the preferable sway mechanism can develop and be
sustained. The input actions for structural members have to be quantified with some accuracy.
In order to provide adequate protection against the formation of plastic hinges in columns, a
dynamic magnification factor is introduced to magnify the column design moment and shear
resulting from input actions of adjacent members to account for the higher mode effects [P2.1
[Sl].
This research project concentrates on the seismic design and behaviour of interior joints.
Figure 1.1 shows a displaced frame structure under the action of lateral load with the
classification of exterior and interior beam-column joints. Reinforced concrete beam-column
joints are very important because of their role in the behaviour of moment resisting frames
designed for earthquake resistance. Joints are subjected to reversing bending moments on
opposite adjacent members, and also the input shear force in the joint region is typically of the
order of 5 times the column shear force. This can be seen in Fig. 1.2 which depicts the
bending moments and shear force along the column of a frame…
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