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 = 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

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)

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)

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)

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)

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