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FIBER REINFORCED CONCRETE IN SHEAR WALL COUPLING BEAMS

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.

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FIBER REINFORCED CONCRETE IN SHEAR WALL COUPLING BEAMS

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  1. FIBER REINFORCED CONCRETE IN SHEAR WALL COUPLING BEAMS Gustavo J. Parra-MontesinosC.K. Wang Professor of Structural EngineeringUniversity of Wisconsin-MadisonJames K. WightFrank E. Richart Jr. Collegiate ProfessorUniversity of MichiganCary KopczynskiPrincipal, Cary Kopcyznski & Co.

  2. 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

  3. COUPLED WALLS • 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

  4. CURRENT COUPLING BEAM DESIGN PRACTICE IN USA • 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

  5. TYPICAL COUPLING BEAM DESIGN (Lequesne, Parra and Wight)

  6. MOTIVATION • 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

  7. 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

  8. MATERIAL-RELATED ASPECTS 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)

  9. 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

  10. USE OF SELF-CONSOLIDATING HPFRC • 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.)

  11. (Naaman et al.)

  12. CLASSIFICATION OF FRCs • Based on bending and tension behavior • Strain hardening vs. softening • Deflection hardening vs. softening (Naaman and Reinhardt 2003)

  13. 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

  14. FIBER REINFORCED CONCRETE IN EARTHQUAKE-RESISTANT COUPLING BEAMS 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)

  15. SLENDER COUPLING BEAMS (ln/h ≥ 2.2)

  16. SLENDER COUPLING BEAM (ln/h = 2.75) • 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%

  17. SHEAR CONTRIBUTION FROM DIAGONAL BARS CB-1 CB-2 CB-3

  18. ELIMINATION OF DIAGONAL BARS (ln/h ≥ 2.2) COUPLING BEAM BEHAVIOR • 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) (Sektik, Parra and Wight)

  19. SLENDER COUPLING BEAM DESIGN (ln/h ≥ 2.2) COUPLING BEAM BEHAVIOR (Sektik, Parra and Wight)

  20. BEHAVIOR of COUPLING BEAM with NO DIAGONAL BARS (ln/h = 3.3) (Sektik, Parra and Wight)

  21. SLENDER COUPLING BEAM with NO DIAGONAL BARS AT 6% DRIFT (Sektik, Parra and Wight)

  22. BEHAVIOR of COUPLING BEAM with NO DIAGONAL BARS (ln/h = 2.2) (Comforti, Parra and Wight)

  23. CONCLUSIONS – SLENDER COUPLING BEAMS • 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 • 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

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