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.

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