Diagonal Compression Failure in RC Beams RC

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Diagonal Compression Failure in RC
BeamsRC?????????

• M2 KOBAYASHI Chikaharu

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Background
Recently, high-strength and high-performance
concrete and steel have been developed.

• High-strength concrete (fc?60MPa)
• High-strength steel (fy?700MPa)

RC member
?Very thin web ?Dense shear reinforcement
Ultimate capacity for diagonal compression
failure decrease
Accurate calculation of load carrying capacity
for diagonal compression failure is needed.
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Background
Not yet fully understood because of the lack of
sufficient research in this failure.
Proposed equation by Placas et al.
VPlacas0.157(6.631.32rw)fc1/2bwd
Standard Specification for Concrete Structures of
JSCE
Vwc1.25fc1/2bwd

• Quite conservative for safety reasons

• This equation is covered only for concretes not
  exceeding fc 50 N/mm2

Objective
To propose calculation formula of diagonal
compression failure in order to estimate the
ultimate capacity of RC beams
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Specimen
Three points bending tests were conducted on RC
beams with very thin I shaped web using large
amount of shear reinforcement.
Compression bar
A-A
B-B
B
A
240
240
a660
a660
250
250
50
h260
d220
40
70
A
B
146
146
Tensile bar (fy?930MPa)
Unit mm
Stirrup (fwy?954MPa)
High-fluidity concrete
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Parameter
Proposed equation by Placas et al.
VPlacas0.157(6.631.32rw)fc1/2bwd
Experiments were conducted in RC beams using
different fc and rw.
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Load-Displacement Curves
Ratio of stirrup rw3
fc100MPa
Sharp drop
rw4
fc100MPa
rw3
rw2
Load (kN)
Load (kN)
fc65MPa
fc35MPa
Displacement (mm)
Displacement (mm)
In case of high-strength concrete, web
compression failure was brittle.
Under using high-strength concrete (fc100MPa),
peak load increased as rw increased.
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Dependence of fc
Proposed equation by Placas et al.
VPlacas0.157(6.631.32rw)fc1/2bwd
The dependence of fc was discussed by vu/fc1/2.
vu shear strength (Vexp/(bwd))

• The values of vu/fc1/2 are not horizontally
  distributed.

increasing
The equation cannot evaluate the dependence of
fc by fc1/2.
rw3
rw2

• As fc increased, vu/fc1/2 increased.

The dependence of fc was larger than evaluated
by VPlacas.
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Discussion of vu/fc
Equilibrium conditions of a web
C

• The capacity can be calculated by compressive
  strength concrete crossed by shear cracks and ?.

steel
D
z
sd
?

• The angles of diagonal cracks were nearly the
  same in all specimens.

T
V
Cutting plane
Assuming the capacity is proportional to fc
The value decreased as fc increased.
rw3
rw4
rw1
The value increased as rw increased.
rw2
vu/fc got close to 0.20 by fully shear
reinforcement.
There is a possibility that localized compressive
failure affected the decrease of vu/fc.
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Localized Compressive Failure and Capacity
rw4
rw3
rw1
?
?
?
rw2
?
?
The compressive failure of ? using high strength
concrete was more localized than that of ? and
vu/fc decreased.
?
The compressive failure of ? with dense shear
reinforcement was not localized and vu/fc got
close to 0.20.
There is a possibility that fully shear
reinforcement can prevent localization of
compressive failure and decreasing of diagonal
compressive capacity.
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Spacing of Stirrup
It is considered that the area of compressive
failure is affected by the position of stirrups.

• Heterogeneity of stress by steel

• Capacity of load distribution

All stirrups did not reach yield points.

• There is a possibility that the area of
  compressive failure is affected by spacing of
  stirrup.

The effect by different spacing of stirrup was
examined by setting the same condition except for
nominal diameter of stirrups and spacing of
stirrup.
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fc102.4MPa, rw2.0, s90mm
fc109.5MPa, rw2.0, s50mm
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Spacing of Stirrup
rw2, s50mm
rw2, s90mm
increasing
Under the same condition except for nominal
diameter of stirrups and spacing of stirrup, the
specimen which have narrow spacing of stirrup
crushed more widely in shear span and got larger
capacity.
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Summary

• Diagonal compression failure was more brittle and
  localized as compressive strength of concrete
  increased.
• It is considered that fully shear reinforcement
  can prevent localization of compressive failure
  and decreasing of diagonal compressive capacity.
• There is a possibility that the localization of
  compressive failure is affected by spacing of
  stirrup, not ratio of stirrup.

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Thank you for kind attention!