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.

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√fc’ (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

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

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 (lf =1.2 in.; df = 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

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

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

• 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 (ln/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 (ln/h = 2.75)

-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

(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)

(Sektik, Parra and Wight)

COUPLING BEAM BEHAVIORSLENDER COUPLING BEAM DESIGN (ln/h ≥ 2.2)

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

]

SLENDER COUPLING BEAM with NO DIAGONAL BARS AT 6% DRIFT

(Sektik, Parra and Wight)

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

)

• 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