fiber reinforced concrete in shear wall coupling beams gustavo j. parra-montesinos c.k. wang...
TRANSCRIPT
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FIBER REINFORCED CONCRETE IN SHEAR WALL COUPLING BEAMS
Gustavo J. Parra-MontesinosC.K. Wang Professor of Structural Engineering
University of Wisconsin-Madison
James K. WightFrank E. Richart Jr. Collegiate Professor
University of Michigan
Cary KopczynskiPrincipal, Cary Kopcyznski & Co.
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OUTLINE
• Current design practice for coupling beams
• Research motivation
• Classification of Fiber Reinforced Concretes (FRCs)
• Experimental program
• Coupling beams
• Coupled walls
• Implementation of fiber reinforced concrete coupling beams into practice
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• Two or more walls connected by short beams referred to as coupling beams
• Commonly used in medium- and high-rise structures in combination with RC or steel moment frames
COUPLED WALLS
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• Typical span-to-depth ratios between 1.5 and 3.5
• Diagonal reinforcement, designed to carry the entire shear demand, is required in most cases
• Column-type transverse reinforcement must be provided to confine either diagonal reinforcement or entire member
• Maximum shear stress of 10√fc’ (psi)
• Little longitudinal reinforcement, terminated at the wall near the coupling beam end
CURRENT COUPLING BEAM DESIGN PRACTICE IN USA
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(Lequesne, Parra and Wight)
TYPICAL COUPLING BEAM DESIGN
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• Reinforced concrete coupling beams require intricate reinforcement detailing to ensure stable seismic behavior, leading to severe congestion and increased construction cost
• Use of a material with tension ductility and confined concrete-like behavior should allow for substantial simplification in confinement and shear reinforcement without compromising seismic behavior
MOTIVATION
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FIBER REINFORCED CONCRETE
• Concrete reinforced with discontinuous fibers
• Commonly used steel fibers have deformations to improve bond with surrounding concrete. However, fibers are ultimately expected to pullout
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ConstituentsConcrete matrix in fiber reinforced concrete is made of same constituents used in plain concrete
• Aggregates (fine and course)• Cement• Water• Mineral admixtures• Water reducing agents (high-range water-reducing agents)
MATERIAL-RELATED ASPECTS
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Aggregates• Sufficient fine aggregates to ensure adequate volume of
paste
• Control volume and size of course aggregate– Increase in course aggregate size has been associated with
poor fiber distribution and a reduction in tensile performance– Maximum aggregate size in fiber reinforced concrete used in
coupling beams has been limited to ½ in.
Workability• For large fiber dosages as used in coupling beams, use self-
consolidating mixture or a mixture with high slump (at least 8 in.) prior to addition of fibers
MATERIAL-RELATED ASPECTS
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• Regular concrete matrix (1/2 in. max. aggregate size)
• 1.5% volume fraction of high-strength hooked steel fibers (lf =1.2 in.; df = 0.015 in.)
(Naaman et al.)
USE OF SELF-CONSOLIDATING HPFRC
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(Naaman et al.)
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Deflection hardening vs. softening Strain hardening vs. softening
(Naaman and Reinhardt 2003)
• Based on bending and tension behavior
CLASSIFICATION OF FRCs
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FIBER REINFORCED CONCRETE IN EARTHQUAKE-RESISTANT COUPLING BEAMS
Fiber reinforced concrete with tensile strain-hardening behavior (HPFRC) and compression behavior similar to well-confined concrete
RC
HPFRC
13
0
0.5
1
1.5
2
2.5
3
0 0.005 0.01 0.015 0.02 0.025 0.03
Ten
sile
Str
ess
(MP
a)
Tensile Strain
Damage Localization
0
10
20
30
40
50
0 0.005 0.01 0.015 0.02
Co
mp
ress
ive
Str
ess
(MP
a)
Compressive Strain
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• High-strength hooked steel fibers have been the most investigated fiber type for use in coupling beams
• Volume fraction = 1.5% (200 lbs/cubic yard)
FIBER REINFORCED CONCRETE IN EARTHQUAKE-RESISTANT COUPLING BEAMS
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SLENDER COUPLING BEAMS (ln/h ≥ 2.2)
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#3
#3
#4
6 in.
24 in.
6.5 in.3.25 in.
#4
66 in.
#6#5
7 in.
• Target shear stress 8-10√f’c , psi
• Approximately 25% of shear resisted by diagonal bars , 45% of shear carried by stirrups, and 30% of shear resisted by HPFRC
• Transverse reinforcement ratio = 0.56%
SLENDER COUPLING BEAM (ln/h = 2.75)
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-8 -6 -4 -2 0 2 4 6 80
2
4
6
8
10
12
Drift (%)
Sh
ea
r C
on
trib
utio
n,
(p
si)
CB1
Diagonal bars
Stirrups
HPFRC
' cf
-8 -6 -4 -2 0 2 4 6 80
2
4
6
8
10
12
Drift (%)
Sh
ea
r C
on
trib
utio
n,
(p
si)
CB2
Stirrups
Diagonal bars
HPFRC
' cf
-8 -6 -4 -2 0 2 4 6 80
2
4
6
8
10
12
Drift (%)
Sh
ea
r C
on
trib
utio
n,
(p
si)
CB3
Diagonal bars
Stirrups
HPFRC
' cf
CB-1 CB-2
CB-3
SHEAR CONTRIBUTION FROM DIAGONAL BARS
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(Sektik, Parra and Wight)
• Complete elimination of diagonal reinforcement in coupling beams with length-to-depth ratios ≥ 2.2
• No special confinement, except for beam ends
• Shear strength up to 10√f’c (psi)
COUPLING BEAM BEHAVIORELIMINATION OF DIAGONAL BARS (ln/h ≥ 2.2)
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(Sektik, Parra and Wight)
COUPLING BEAM BEHAVIORSLENDER COUPLING BEAM DESIGN (ln/h ≥ 2.2)
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BEHAVIOR of COUPLING BEAM with NO DIAGONAL BARS (ln/h = 3.3)
(Sektik, Parra and Wight)
-10
-5
0
5
10
-10 -8 -6 -4 -2 0 2 4 6 8 10
-0.8
-0.4
0
0.4
0.8
Ave
rag
e s
hea
r s
tres
s [
(f c')
1/2, p
si]
Drift (%)
Ave
rag
e s
hea
r s
tres
s [
(f c')
1/2, M
Pa
]
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SLENDER COUPLING BEAM with NO DIAGONAL BARS AT 6% DRIFT
(Sektik, Parra and Wight)
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BEHAVIOR of COUPLING BEAM with NO DIAGONAL BARS (ln/h = 2.2)
(Comforti, Parra and Wight)
-10 -8 -6 -4 -2 0 2 4 6 8 10-1500
-1000
-500
0
500
1000
1500
Drift (%)
Sh
ea
r S
tre
ss (
psi
)
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• Diagonal bars can be eliminated in HPFRC coupling beams with ln/h ≥ 2.2 when reinforced with a 1.5% volume fraction of high-strength hooked steel fibers and subjected to shear stress demands up to the upper limit in ACI Building Code
• When diagonal reinforcement was used in slender HPFRC coupling beams, shear resistance provided by that reinforcement was estimated at or below 15% of the total shear, which suggested elimination of diagonal bars in such beams
CONCLUSIONS – SLENDER COUPLING BEAMS