Rémy D. Lequesne

Seismic Detailing and Behavior of Coupling Beams with High-Peformance Fiber- Reinforced Concrete

This paper provides a summary of a series of tests that were performed on strain hardening High-Performance Fiber-Reinforced Concrete (HPFRC) coupling beams with span length-to-depth ratios (ln/h) of 1.75 and 2.75. These tests show that incorporating HPFRC simplifies the detailing required to ensure a stable response of coupling beams that are subjected to earthquake induced displacement reversals. Results from five tests of precast coupling beams, three with ln/h = 1.75 and two with ln/h = 2.75, are reported in the paper. Strategies for embedding the precast HPFRC coupling beams into the structural walls without interfering with boundary element reinforcement were explored. Test results confirm that HPFRC can reliably confine diagonal reinforcement and ensure stable hysteresis behavior. HPFRC was also found to significantly increase shear strength, thereby forcing a flexurally dominated failure mode with modest stiffness degradation and excellent energy dissipation. A revised coupling beam design philosophy is outlined in order to ensure ductile flexural behavior.

BEHAVIOR OF BIAXIALLY LOADED SLAB-COLUMN CONNECTIONS WITH SHEAR STUDS

Results are presented from four non-prestressed concrete slabcolumn connection subassemblies tested under simulated gravity and earthquake-type loading. Each specimen consisted of a largescale first-story interior slab-column connection reinforced with headed shear studs, loaded to a gravity-shear ratio of 50%, and subjected to biaxial lateral displacements. The slabs, which were nominally identical aside from the shear stud reinforcement design, had a flexural reinforcement ratio in the column strip, based on the effective depth, of 0.7%. Shear stud reinforcement in the test specimens varied in terms of amount and spacing, both between and within stud peripheral lines. All four specimens exhibited drift capacities significantly lower than shown by previous studies. Although the lateral strength of the specimens was governed by the flexural capacity of the slab, severe concrete degradation ultimately limited the drift capacity of the connections. Signs of punching-related damage were first observed during the cycle to 1.85% drift in each loading direction. Test results suggest that the minimum amount of shear reinforcement required in Section 21.13.6 of ACI 318-11 when neither a drift nor a combined shear-stress check is performed (vs ≥ 3.5√fc′, psi [0.29√fc′, MPa]) is adequate for connections subjected to a gravity shear ratio of up to 50% and resultant drifts from biaxial displacements of up to 2.0% if studs are spaced at less than 2d within the first two peripheral lines. For larger drift demands, a maximum stud spacing within the first three peripheral lines of 1.5d is recommended.

A Summary of Ten Years of Research on HPFRC Coupling Beams

The design of coupling or “link” beams connecting structural walls in earthquake-resistant construction requires the use of intricate diagonal and transverse reinforcement detailing to ensure adequate strength, stiffness and energy dissipation during an earthquake event. The tensile strain-hardening behavior exhibited by high-performance fiber reinforced concretes (HPFRCs), along with their compression behavior that resembles that of well-confined concrete, led the senior writers to consider their use as a means to simplify the required reinforcement detailing in coupling beams, while leading to comparable or even enhanced seismic performance. Short coupling beams with a span-to-depth ratio (l n /h) of 1.0 were first investigated. Test results showed that HPFRC provides confinement to the diagonal reinforcement and increases coupling beam shear strength and drift capacity. This allows for a substantial reduction in both diagonal and confinement reinforcement without compromising deformation capacity. A follow-up study on coupling beams with l n /h = 1.75 showed that a ductile flexural mechanism with high damage tolerance can be achieved through the use of HPFRC. A precast scheme with a short embedment length was shown to effectively anchor the beam into the wall without interfering with the wall reinforcement. Also, an HPFRC mixture with high-strength (2300 MPa) hooked steel fibers in a 1.5% volume fraction was found to be the most promising in terms of structural performance, economy and ease of construction. In order to cover the range of l n /h ratios common in practice, additional studies were conducted on more slender coupling beams, with l n /h = 2.75 and 3.3. It was shown that slender precast HPFRC coupling beams can develop a high drift capacity and damage tolerance, even when diagonal reinforcement is eliminated. The results from this work thus provide structural engineers and contractors a viable design alternative for use in earthquake-resistant coupled wall construction.

Implementation of High-Performance Fiber Reinforced Concrete Coupling Beams in High-Rise Core-Wall Structures

Synopsis: Experimental and analytical studies that led to the incorporation of strain-hardening, high-performance fiber reinforced concrete (HPFRC) coupling beams in the design of a high-rise core-wall structure in Seattle, WA, are described. A total of eight HPFRC coupling beams with span-to-depth ratios ranging between 1.75 and 3.3 were tested under large displacement reversals. The tension and compression ductility of HPFRC materials allowed an approximately 70% reduction in diagonal reinforcement, relative to an ACI Building Code (318-08) compliant coupling beam design, in beams with a 1.75 span-to-depth aspect ratio and a total elimination of diagonal bars in beams with a 2.75 and 3.3 aspect ratio. Further, special column-type confinement reinforcement was not required except at the ends of the beams. When subjected to shear stress demands close to the upper limit in the 2008 ACI

Conventional and high-strength hooked bars-Part 2: Data analysis

Empirical equations are developed to characterize the anchorage strength of hooked bars. The equations are based on tests of 245 simulated beam-column joint specimens with two hooked bars: 146 with confining reinforcement and 99 without. Bar stresses at failure for specimens used in the analysis ranged from 30,800 to 144,100 psi (212 to 994 MPa), and concrete compressive strengths ranged from 2570 to 16,200 psi (17.7 to 112 MPa). For the specimens analyzed, hooked bar anchorage strength was proportional to concrete compressive strength raised to the 0.29 power. For confining reinforcement parallel to and located within eight or 10 bar diameters of the straight portion of the hooked bar, the contribution to anchorage strength was proportional to the area of confining reinforcement; for confining reinforcement perpendicular to the straight portion of the bar, more legs of the confining reinforcement contributed to anchor strength, but each leg made a smaller contribution.

Conventional and High-Strength Hooked Bars—Part 1: Anchorage Tests

This paper presents the results of an experimental study on the anchorage strength of conventional and high-strength steel hooked bars. Three hundred and thirty-seven exterior beam-column joint specimens were tested with compressive strengths ranging from 4300 to 16,500 psi (30 to 114 MPa). Parameters investigated included the number of hooked bars per specimen, bar diameter, side cover, amount of confining reinforcement, hooked bar spacing, hook bend angle, hook placement, and embedment length. Bar stresses at failure ranged from 22,800 to 144,100 psi (157 and 994 MPa). The majority of the hooked bars failed by a combination of front and side failure, with front failure being the dominant failure mode. Test results show that development lengths of hooked bars calculated based on ACI 318-14 are very conservative for No. 5 (No. 16) bars and become progressively less conservative with increasing bar size and concrete compressive strength.

Earthquake-Resistant Squat Walls Reinforced with High- Strength Steel

Results are reported from reversed cyclic tests of five large-scale squat wall specimens reinforced with steel bars having a specified yield strength of either 60 or 115 ksi (413 or 792 MPa). Two specimens were designed for a shear stress of 5√fc′ psi (0.42√fc′ MPa) and the other three 9√fc′ psi (0.75√fc′ MPa). Boundary element confining reinforcement complied with the requirements of Chapter 18 of ACI 318-14 in all but one specimen, which had 50% of the required transverse boundary element reinforcement. Specimens constructed with Grade 115 steel had similar strength and exhibited 20% greater drift capacity than those with Grade 60 steel. Use of Grade 115 steel tended to control the softening effect of sliding at the base of the wall and to increase the component of drift due to reinforcement strain penetration into the foundation.

Conventional and high-strength steel hooked bars: Detailing effects

Findings from a study on the effect of hook bend angle, concrete clear cover, and orientation of confining reinforcement on hook anchorage strength are presented. The range of test parameters was much broader than in previous studies. Bar stress at anchorage failure ranged from 33,000 to 137,400 psi (228 to 947 MPa) and concrete compressive strengths ranged from 4300 to 16,500 psi (30 to 114 MPa). Anchorage strength of hooked bars was insensitive to bend angle (90 or 180 degrees) and side cover (between 2.5 and 3.5 in. [65 and 90 mm]). Confining reinforcement was found to increase anchorage strength for 180-degree hooked bars regardless of orientation (parallel or perpendicular to the embedment length). For 90-degree hooked bars, reinforcement oriented parallel to the embedment length had a greater effect on anchorage strength than reinforcement oriented perpendicular to the embedment length.

Seismic Response of Fiber-Reinforced Concrete Coupled Walls

to high shear and deformation reversals. These effects are attributed to the post-cracking toughness of HPFRC in tension and its response in compression, which resembles that of well-confined concrete. However, there have been no tests of coupled HPFRC structural walls, for which the distribution of base shear stresses and wall deformations differ significantly from those in isolated slender walls. This paper presents a detailed comparison of the behavior of four-story RC and HPFRC coupled wall specimens, with an emphasis on deformations in the first story of the walls and in the coupling beams. RESEARCH SIGNIFICANCE Data from large-scale tests of coupled walls linked by RC and HPFRC coupling beams are presented. The testing program included the first HPFRC coupled wall test and one of few tests of large-scale T-shaped RC coupled walls. The results presented should be useful to researchers, designers, and code officials interested in the seismic performance and modeling of RC structures constructed with HPFRC. EXPERIMENTAL PROGRAM