reinforcing effects of stirrups on concrete …niwa-lab/hp2015/research/research...reinforcing...
TRANSCRIPT
Reinforcing Effects of Stirrups on Concrete Contribution in Shear
コンクリートが負担するせん断抵抗に及ぼすせん断補強鉄筋の補強効果
JCI Annual Convention in KOBE (2018.7.4~2018.7.6)
Devin GUNAWAN Tokyo Institute of TechnologyDr. Takuro NAKAMURA Tokyo Institute of Technology
(Present: Civil Engineering Research Institute for Cold Region)
Prof. Junichiro NIWA Tokyo Institute of Technology
Research Background 1Shear capacity of RC beams =
Concrete contribution + Stirrups contribution
arch action
CL Compressionstrut
Tie member
CL
Beam action
Different stirrup ratio in a same shear span :Calculation :No effect on concrete contribution
Observed : Different crack patterns⇒ Effect on concrete contribution
(arch and beam actions)
It is required to investigate the effects of stirrups on concrete contribution, focusing on the arch and beam actions.
Interlockingaction
Dowel action
Research Objectives & Experimental OutlineHow to evaluate the arch action?
2
Research Objectives :1.To investigate the applicability of the method above2.To investigate the effects of stirrups on concrete contribution
(M. Nakamura et al.,2017)
strain at support ≈ arch action CL• Simple• Requires further investigation
about the applicability
a = 750, 1050Test shear span b =150
d=
300
(mm)• Stirrup ratio rw = Aw /(bs)
rw = 0.00% ~ 0.65%• Stirrups :D6 (mostly) & D10• Shear span ratio a/d
a/d = 2.5 (4 specimens)a/d = 3.5 (8 specimens)
Loading test specimens
s
CL
Evaluation Method (1) 3
120 kN
240 kN
338 kN
0
1000
2000
3000
Stra
in (μ
)
Strain gauges
0 kN(Load)
CL
Varch = (AsEsεs) z/a
Varch = D’sinθstrut
= (T/cosθstrut)sinθstrut
= T tanθstrut
strain at support
θstrutεs
CL
θstruta
z VarchT
D’
M. Nakamura et al. (2017)Varch : contribution of arch action
Shear force V = Varch + Vcbeam + VstirrupsConcrete in beam action StirrupsArch action
CL
Closest strain gauges
Evaluation Method (2) 4
Strain of stirrups that intersected the critical diagonal crack
Vstirrups : contribution of stirrups
Varch = (AsEsεs) z/a
θstrut
Varch : contribution of arch action
Vcbeam : contribution of concrete in beam action
Vcbeam = V - Vstirrups - Varch
εs
CL
a
z
Shear force V = Varch + Vcbeam + VstirrupsM. Nakamura et al. (2017)
bdda
..dp'f.cal_Vc wc
417501000200 413131 (Niwa et al.)
fc‘ : concrete compressive strength pw : tensile steel bar ratio
d : effective depth b : beam width
a : shear span
Calculated value of concrete contribution :
Applicability of the Method 5
Vconcrete (=Vcbeam + Varch) ≈ Vc_calVarch compensated the drop of Vcbeam
Transition of shear resisting mechanism :1. No diagonal crack Vcbeam2. Diagonal crack~ Vstirrups3. stirrups yield~ Varch
• The results have good agreements with previous researches.
• Similar results were also seen in other specimens.
• The method is applicable to evaluate arch and beam actions.
0
50
100
150
200
0 5 10 15
V(k
N)
Δ(mm)
Vcbeam
Varch
Vstirrups
V
0
50
100
150
200
0 5 10 15
Vstirrups
V
Vconcrete
Vc_calV(k
N)
Δ(mm)
1
23
CL
(a/d = 2.5 rw = 0.38%)
CL
1 2
Effects of Stirrups a/d = 2.5 6
0
25
50
75
100
0 0.17 0.28 0.38rw(%)
Increasing
0
25
50
75
100
0 0.17 0.28 0.38rw(%)
Decreasing
Stirrups ratio increases⇒ contribution of arch action tends to increase⇒ contribution of concrete in the beam action decreases
Vcbeam_peak (kN)Varch_peak (kN)
Effect of stirrups :⇒ difference of each mechanism during peak shear force
0
50
100
0
50
100 V…fc'Varch_peakfc‘
Varch_peak (kN) fc‘ (N/mm2)
Effects of Stirrups on Varch a/d = 3.5 7
similar
rw (%)High stirrup ratio⇒ maximum contribution of arch action
⇒ concrete crush near loading point
Varch_peak
Stirrup ratio rw(%)max
0
rw = 0.65%
rw = 0.38%D10rw(%)
Hig
h f c’
rw = 0.00%
0
50
100
Effects of Stirrups on Vcbeam a/d = 3.5 8
similar
Higher stirrup ratio⇒ maintained at the minimum value
⇒ similar crack pattern, crack opening⇒ There might be exceptions.
Vcbeam_peak
Stirrup ratio rw(%)min
0
Vcbeam_peak (kN)rw = 0.38%
rw = 0.65%
exce
ptio
nrw(%)
rw = 0.38%D10Gently
sloped
CL
Wide spacing
Conclusions1.Evaluation method using the strain of tensile steel bar at the
support is applicable to evaluate arch and beam actions.
2.As stirrup ratio increase, the contribution of arch action increases, and the contribution of concrete in the beam action decreases.
3.With high stirrup ratio, the contribution of arch action reaches its maximum value,and the contribution of concrete in the beam action is maintained at its minimum value.
9