FIBER REINFORCED CONCRETE IN SHEAR WALL COUPLING BEAMS

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25 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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FIBER REINFORCED CONCRETE IN SHEAR
WALL COUPLING BEAMS

Gustavo J. Parra
-
Montesinos

C.K. Wang Professor of Structural Engineering

University of Wisconsin
-
Madison


James K. Wight

Frank E. Richart Jr. Collegiate Professor

University of Michigan


Cary Kopczynski

Principal, Cary Kopcyznski & Co.



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



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


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√
f
c

(psi)


Little longitudinal reinforcement, terminated at the wall near
the coupling beam end

CURRENT COUPLING BEAM DESIGN
PRACTICE IN USA




(Lequesne, Parra and Wight)

TYPICAL COUPLING BEAM DESIGN


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

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


Constituents

Concrete 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

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


Regular concrete matrix (1/2 in. max. aggregate size)


1.5% volume fraction of high
-
strength hooked steel fibers (
l
f

=1.2
in.;
d
f

= 0.015 in.)


(Naaman et al.)

USE OF SELF
-
CONSOLIDATING HPFRC

(Naaman et al.)

Deflection hardening vs. softening


Strain hardening vs. softening


(Naaman and Reinhardt 2003)


Based on bending and tension behavior

cc

pc

cc

pc

FRC typical;
strain-softening
Matrix
HPFRCC typical
(Strain-hardening and
multiple cracking)
STRESS
STRAIN
(or elongation)
cc
f
r
f
c

u

Deflection-softening
Matrix
(Deflection-hardening
and multiple cracking)
LOAD
DEFLECTION
MOR
cc

pc

cc

pc

FRC typical;
strain-softening
Matrix
HPFRCC typical
(Strain-hardening and
multiple cracking)
STRESS
STRAIN
(or elongation)
cc
f
r
f
c

u

Deflection-softening
Matrix
(Deflection-hardening
and multiple cracking)
LOAD
DEFLECTION
MOR
CLASSIFICATION OF FRCs

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
Tensile Stress (MPa)
Tensile Strain
Damage Localization
0
10
20
30
40
50
0
0.005
0.01
0.015
0.02
Compressive Stress (MPa)
Compressive Strain

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

SLENDER COUPLING BEAMS (
l
n
/
h

≥ 2.2)


#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 (
l
n
/
h

= 2.75)


-8
-6
-4
-2
0
2
4
6
8
0
2
4
6
8
10
12
Drift (%)
Shear Contribution, (psi)
CB1


Diagonal bars
Stirrups
HPFRC
'
c
f
-8
-6
-4
-2
0
2
4
6
8
0
2
4
6
8
10
12
Drift (%)
Shear Contribution, (psi)
CB2


Stirrups
Diagonal bars
HPFRC
'
c
f
-8
-6
-4
-2
0
2
4
6
8
0
2
4
6
8
10
12
Drift (%)
Shear Contribution, (psi)
CB3


Diagonal bars
Stirrups
HPFRC
'
c
f
CB
-
1

CB
-
2

CB
-
3

SHEAR CONTRIBUTION FROM DIAGONAL BARS




(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 BEHAVIOR

ELIMINATION OF DIAGONAL BARS (
l
n
/
h

≥ 2.2)

(Sektik, Parra and Wight)

COUPLING BEAM BEHAVIOR

SLENDER COUPLING BEAM DESIGN (
l
n
/
h

≥ 2.2)

BEHAVIOR of COUPLING BEAM with NO
DIAGONAL BARS (
l
n
/
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
Average shear stress [(
f
c
'
)
1/2
, psi]
Drift (%)
Average shear stress [(
f
c
'
)
1/2
, MPa]
SLENDER COUPLING BEAM with NO
DIAGONAL BARS AT 6% DRIFT




(Sektik, Parra and Wight)

BEHAVIOR of COUPLING BEAM with NO
DIAGONAL BARS (
l
n
/
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 (%)
Shear Stress (psi)

Diagonal bars can be eliminated in HPFRC coupling
beams with
l
n
/
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