Vo Manh Tung RESEARCH THE IMPACT OF THE BEAM-COLUMN JOINT DEFORMATION ON SEISMIC BEHAVIOR OF REINFORCED CONCRETE FRAME Major: Civil Engineering Code: 9580201 SUMMARY OF DOCTORAL DISS
Trang 1Vo Manh Tung
RESEARCH THE IMPACT OF THE BEAM-COLUMN JOINT DEFORMATION ON SEISMIC BEHAVIOR OF
REINFORCED CONCRETE FRAME
Major: Civil Engineering Code: 9580201
SUMMARY OF DOCTORAL DISSERTATION
Ha Noi –2018
Trang 2The work was completed at:
NATIONAL UNIVERSITY OF CIVIL ENGINEERING
Academic supervisor: Assoc Prof PhD Nguyen Le Ninh
Reading Committee:
Prof PhD Nguyen Tien Chuong
Assoc Prof PhD Nguyen Ngoc Phuong
PhD Nguyen Dai Minh
The doctoral dissertation will be defended at the level of the State Council of Dissertation Assessment's meeting at the National University of Civil Engineering
at hour ', day month year 2018
The dissertation is available for reference at the libraries as follows:
- National Library of Vietnam;
- Library of National University of Civil Engineering
Trang 3INTRODUCTION
1 The necessity of the topic
The beam-column joint (BC joint) is the intersection of beams and columns Underneath the earthquake effects, reinforced concrete BC joint have complex behavior and their destructive behavior often leads to the collapse of the frame Determination
of shear strength and simulated behavior of BC joint has been proposed However, these models are not general and have a broad consensus In design, the BC joint is still considered to be the rigid zone
At present, in the modern design of earthquake resistance, the
BC joint must have sufficient strength to ensure that the beams and columns around it develop the desired plastic deformation
In order to solve this problem, the design codes of earthquakeresistance have very strict requirements for the calculation and details, but avoid the problem of distortion, a very important factor affecting behavior of the frame when earthquake
In Vietnam, there are currently no studies on the behavior of reinforced concrete BC joint The BC joint is considered an rigid zone Therefore, the "Research the impact of the BC joint deformation on seismic behavior of reinforced concrete frame" is very necessary
2 Aims of the research
a An overview of the models of durability and behavioral simulations of earthquake resistant reinforced concrete BC joint;
b Experimental study of the types of reinforced concrete BC joint in Vietnam in order to clarify the problems: the possibility
of deformation and force bearing, criteria for shear strength, suitable to the plastic responseof frame
c Study of nonlinear calculation frames taking into consideration
BC joint deformation designed in accordance with TCVN 9386:
2012
3 Object and methodology of the research
Object: the reinforced concrete BC joint are available in actual
Trang 4construction in Vietnam
Methodology: theory combined with experiments
4 Significant contributions of the dissertation
a The experiments showed that the types of reinforced concrete BC joint available in Vietnam are deformed when earthquake; The joint is designed in accordance with TCVN 9386:2012, which is inelastic failure, and the TCVN 5574:2012 and SP 14.13330.2011 (Russian Federation) are brittle failure, not suitable to create ductile mechanism for the frame structure Identification of the main factors affecting the behavior of the
BC joint and the conditions to ensure the shear strength of the joint designed in Vietnam
b The proposed three models simulate the shear deformation and bond slip of the BC joint The use of these models in static and dynamic analysis of reinforced concrete frames designed in accordance with TCVN 9386: 2012 shows that the deformation
of the BC joint significantly changes the overall response of the structural frame
5 Dissertation structure
The dissertation includes Introduction, 5 main chapters,
Conclusion, list of published works by author, references and appendices
CHAPTER 1 BACKGROUND OF THE REINFORCED
CONCRETE BEAM-COLUMN JOINT
AND RESULTS ACHIEVED 1.1 THE FAILURE OF BC JOINT UNDER SEISMIC ACTIONS
There are two types of failure that are commonly observed after earthquakes: (a) shear failure of the joint and (b) failure of the reinforcing anchor Cause due to lack of stirrups and anchor reinforcement is not enough in the joint area
1.2 CLASSIFICATION OF BC JOINTS
The joints are classified according to: (a) the geometry and way of anchoring the reinforcement of beam (outer, inner), (b) the behavior of the joint (elastic, inelastic), (c) the detail of the joint(brittle, ductile)
1.3 THE FORCE IMPACT ON BC JOINT
Trang 5Consider an interior joint, which is subjected to forces acting from the beams and columns (Figure 1.9a), resulting in internal forces as shown in Figure 1.9b Balancing these internal forces will be the shear force horizontally Vjh:
V = C +C +T -Vjh b sb sb c(1.1) orV = T +Tjh sb1 sb2-Vc(1.2)
SoV = (A +A ) f -Vjh s1 s2 0 y c(1.3) in which Cb1- compression forced in concrete, Tsb1, Tsb2 or Csb1-tensile and compression in beam reinforcements , Vc- shear force of columns above and below the joint, As1 or As2– the areas of beam reinforcements,
λ0and fy – overstrength factor and tensile strength of reinforcement
Figure 1.9
the interior joint
Horizontal shear stress τ jh and vertical shear stress τ jv:
For exterior joint:V = Ajh s1 0f - Vy c(1.7)
1.4 METHODS FOR DETERMINING SHEAR STRENGTH OF JOINT
1.4.1.The model determines the shear resistance of joint
There are many computational models that have been proposed and classified in four ways The following are the two most commonly used computing models
1.4.2.Model of Paulay and Priestley
Trang 6Figure 1.12 Transmission shear
mechanisms: a) strut; b) truss;
According to Paulay and Priestley, the shear strength of the jointcombination of mechanism and
mechanism The first
one contributes to horizontal shear
With this model, Paulay and Priestley set up equations for
calculating the horizontal strength V jh and vertical shear strength
V jv
1.4.3 Model of A G Tsonos (1999, 2001)
Tsonos' shear strengthmodel is also comprised of two mechanisms as proposed by Paulay and Priestley, but Tsonos argues that these mechanisms produce an evenly distributed stress field Tsonos then establishes the relationship between the vertical compression stress σ and the shear stress τ in the and consequently horizontal shear of joint: V = jh h b c j(1.45)
1.5 THE SHEAR STRENGTH OF THE JOINTS IN ACCORDANCE WITH THE DESIGN STANDARDS
This section deals with the determination of shear strengththe design standards ACI 318M-2011, NZS 3101 (2016), TCVN 9386: 2012, EN 1998-1-1: 2004 and AIJ 1999
1.6 COMMENTS ON METHODS FOR DETERMINING THE
STRENGTH OF THE JOINTS
The theoretical basis used to determine the shear strengthare very different in the design of the earthquake resistance US standards focus on the dimensions of joint and the concrete
strength f c The Vietnamese and European standards focus on the
amount of stirrups in the joint and the axial force N c of column
According to Paulay and Priestley, the shear
joint is a combination of strut
and truss The first one contributes to
shear force
al shear (Fig 1.12a),
force V sh
out through the
of the reinforcement of the column and beams in the joint With this model, Paulay and Priestley set up equations for
strength
is also comprised of two
as proposed by Paulay and Priestley, but Tsonos
distributed stress field Tsonos then establishes the relationship between the
stress τ in the joint
column
Trang 71.7 BC JOINT MODELS FOR NONLINEAR ANALYSIS
There are many models for simulating the deformation of the
BC joint that have been proposed for more than 60 years The following are the most prominent models of computation
• The model is based on the experimental studies of Townsed and Hanson (1973), Anderson and Towsend (1977)
• Models based on theoretical research combined with experiments:Plastic hinge models of Otani (1974), Banon et all (1981), Fillipou et al (1983, 1988), El-Metwally (1988), Alath
và Kunnath (1995), Pampanin (2002); Nonlinear springs models
of Biddah và Ghobarah (1999), Elmorsi et al (2000), Lowes et
al (2003), Altoontash (2004), Shin và LaFave (2004), Unal and Burak (2010)
Model reviews: the models based on experiments are
non-typical and objective, nonlinear springs models that reflect the actual behavior of the joints rather than the plastic hinge models, but need for specific software and large computing volume
1.8 COMMENT FROM THE STUDY OF OVERVIEW
1 Under the impact of earthquakes, in the region of the BC joint appears large vertical and horizontal forces
2 There is not yet a rational model of the shear bearing mechanism of BC joints that is unanimously accepted The criteria for evaluating shear strength in the standards differ considerably Paulay's and Priestley's models allow for the most rational interpretation of the shear bearing of joint and are included in many standards including Vietnam
3 The recent nonlinear springs models are considered to be the most practical simulation of BC joint, but its application is limited due to the need for specific software and large computing volumes At present, there are no studies on the behavior of RC reinforced concrete under earthquake in Viet Nam
CHAPTER 2 DEFORMATIONS OF BC JOINTS
Trang 8Figure 2.2 Joint deformation a)
Shear deformation, b) fixed rotation
The deformation of the BC jointconsists of two components: the shear deformation of the
panel γ j and the θ sl rotation of fixed-end (Figure 2.2)
2.3.THE FIXED-END ROTATIONS
The longitudinal bars of the beam usually passes through or anchors to the joints They tend to be pulled out of the anchorzone when subjected to the force induced by the rotating θ
end of the beam (fixed-end rotation) Let s be the slip of the
(Fig 2.4):
= (1 )
to the effective depth, d
The fixed-end rotation at yielding is: , =
(2.5) in which:
ϕ y , d b , f y và f c corresponding tocurvature of beam, the diameter of bar, the tensile strength of the reinforcement and the compressive strength of the concrete yielding.With the increase in the loads, the deformation of the bar extends into the joint zone The length of yield deepcause the additional slip ( l y,p) as well as additional fixed end
rotation at ultimate: Δθ u,sl= (2.6) with - ultimate curvature
at end of beam According to
Fardis Δθ u,sl = 5,5d b ϕ u (2.8cyclic load
The bond of bars in the region is the decisive factor for the magnitude of the rotation
BC joint the joint
the beam usually passes through or They tend to be pulled out of the anchorage
θsl at the
of the bars
: ξ – is neutral axis depth, normalised end rotation at yielding
in which: corresponding to the , the tensile strength of the reinforcement and the compressive strength of the concrete at
, the deformation of the
deeper l y,p
as well as additional fixed end
= ϕ u l y,p
ultimate curvature According to 8) with
in the joint region is the decisive factor for
the rotation θ sl
Trang 9The strength of bond in the joint depends on many factors:
confined concrete, diameter of bar d b, compressive strength of
the concrete f c, surface of bar …bond slip models have been proposed by many authors, that's remarkable is the model of Biddah (Hình 2.8) Based on the results of the Morita and Kaku
experiments, Biddah determined the slips of the bars as well as
the slopes K1 and K2 of the model
2.4 THE SHEAR DEFORMATION OF JOINT
The shear deformation γ of joint caused by shear stress τ
determined by (1.5) mainly due to the bond along the longitudinal bars of the beams and the columns passing through the joints The shear stress is important factor affecting the durability and stiffness of the joint The standards ACI 318M-11, NZS 3101 (2006), TCVN 9386:2012 and EN 1998-1-1:2004 has evaluated shear resistance of joint as function of compression
strength f c of concrete, not to take into account the stirrup ratios
2.5 COMMENTS ON DEFORMATIONS OF THE JOINTS
1 The joint deformation consists of: rotation of the θ sl at the
end of the beams and shear deformation γ j of the joint core
2 The magnitude of θ sl is the consequence of the loss of bond and the yield of the longitudinal bar of beam The bondstrength depends on many factors, in which the confinement of concrete core effect is the most important
3 The magnitude of γ j is determined indirectly through the
shear stress τ jh from the experiment In order to minimize the deformation joint, the design standards introduce different limit
Trang 10Figure 3.3 Model NK1 Figure 3.4 Model NK2
3.2.DESIGN THE SPECIMENS
Experimental specimens were interior BC joints in 1:1 scale, extracted from a 3-storey spaceframe constructed in Thanh Xuan district, Hanoi, designed in three scenarios Detailed of the specimens is given in Fig 3.3 (NK1 according to TCVN 9386: 2012), Figure 3.4 (NK2 according to TCVN 9386: 2012 and TCVN 5574: 2012) and Figure 3.5 (NK3 according to SP 14.13330.2011)
3.3.THE PROPERTIES OF MATERIAL
The mechanical properties of concrete and reinforcement are given respectively in Tables 3.2 and 3.3 The specimens and mechanical properties of the materials were determined at the Laboratory and Building Inspection of the University of Civil Engineering
Trang 11Figure 3.9 Schematic of test setup
Table 3.2 Properties of concrete
Specimen NK1 NK2 NK3 Age at test (days) 83 90 80
f c at test (MPa) 31.5 32 31.7
εc 0.002 0.002 0.002
Ec (MPa) 30000 30000 30000
Table 3.3 Properties of reinforcing steel
Bar Ф18 – AII Ф16 - AII Ф6 - AI
fy (MPa) 310 320 235
fu (MPa) 480 510 400
Es (MPa) 210000 210000 210000
3.4 SCHEMATIC OF TEST SETUP AND LOAD ROUTINE
The installation and loading of the test specimens is shown in Figure 3.9 with a pin at the bottom of the column
and free to move at the two ends of the beam and the top of the free column subjected to the vertical action P = 300
kN and horizontal reversal effect
The loading process consisted of two stages: force control and displacement control (Figure 3.12)
At the displacement control stage, the yield deflection Δy of the samples was approximated as shown in Figure 3.10b
a) Ideal relationship b) Actual relationship
Figure 3.10 Definition of displacement
ductility factor
Trang 12Figure 3.17 Failure of NK1 Figure 3.20 Failure of NK2
The devices used were LVDT, strain gauges, TDS 530 and data recorders, hydraulic jack control devices
3.6 BEHAVIOR OF SPECIMENS
In Cycle 19 (last), NK1 is inelastic failure by plastic hinges that appear at the ends of the beams near the joint The joint deformation is evenly distributed (Figure 3.17) NK2 is brittle failure in the 17th cycle, the ends of the column and the beam is not deformed The perimeter of the core is crushed locally and the center is intact (Figure 3.20)
Figure 3.12 Load routine
Trang 13Figure 3.26: NK1
NK3 was crushed in cycle 14th similar to NK2 (Figure 3.23)
Partial damage to the joint of the focus at the corners The column and beam ends are deformed by bending and shear
3.7.2 Story shear
Experiment results show that the maximum story shear
of NK1 appears at higher ductile levels, at later cycles and more ductile than the NK2 and NK3 (Table 3.7)
Table 3.7 Parameters related to maximum story shear
Figure 3.41 Relation
from column (brach +)
NK3 was crushed in cycle 14th similar to NK2 (Figure 3.23)
nt of the focus at the corners The column and beam ends are deformed by bending and shear
The hysteresis show the
story shear - ement Δ of all three samples are symmetric and
to different degrees
of the beam , especially in NK3
story shear V tb,max
levels, at later cycles and more
NK3
8 và 9 -68,0
The behavior of beams
of NK1 does , does not slip, can develop full plastic deformation, as opposed to NK2 and NK3 This is due to the NK1 with the
column and the content 3.7 times greater than that of NK2 and NK3 The