A Review of Research on Shear Strength Decay in Members under Load Reversals

In the design of reinforced concrete earthquake-resisting frame members, it is critical that shear distress be limited in order to ensure acceptable deformation capacity and reduce damage. Accordingly, several ACI Building Code1 provisions for beams and columns of frames categorized as “special moment frames” are aimed at minimizing shear distress. Pertinent requirements include using a capacity design approach to calculate demand, neglecting any contribution of the concrete to nominal shear strength in beams, and limiting hoop spacing to one-fourth of the effective (beams) or overall (columns) member depth. These provisions are largely based on findings from early research aimed at understanding the behavior of frame members subjected to cycles of load reversals. The aim of this paper is to review relevant research on the behavior of frame members under earthquake-type demands, beginning with the first tests of flexural members subjected to fully reversed loads and ending with the 1983 ACI Building Code2, as it was the first ACI Code edition to incorporate several provisions aimed at minimizing shear strength decay. This paper describes the basis for pertinent ACI Building Code provisions (other code or design documents were not included in this review), emphasizes the importance of low shear stress demands, and highlights reinforcement detailing options that have been shown to improve member behavior. This review should therefore be of interest to students and structural engineers, particularly those learning or involved in earthquake-resistant design of reinforced concrete structures

Field Tests of Two Prestressed Concrete Girder Bridges for Live Load Distribution and Moment Continuity

Two similar bridges constructed with a live load continuous connection were tested for live load distribution and joint continuity. Girder distribution factors (GDFs) were compared to AASHTO equivalent values. For positive moments on all girders as for negative moments on interior girders, results using AASHTO equivalent GDFs were found to be generally conservative. However, for negative moments on exterior girders, test results exceeded AASHTO values, with 2-lane results significantly so. GDF results were verified with a FEA model, which produced similar behavior to the field tests. With respect to joint continuity, it is estimated that a simple span would produce a maximum positive moment about 7% higher than the actual structure, while a completely continuous structure would produce a maximum positive moment about 16% lower than the actual structure. For negative moments, the actual structure experienced about 28% of that of a continuous structure. Shear reactions were also investigated. Recommendations are proposed for evaluating transverse distribution of live load as well as the longitudinal distribution of live load, accounting for the effect of joint continuity

Seismic Behavior and Detailing of High-Performance Fiber-Reinforced Concrete Coupling Beams and Coupled Wall Systems

The seismic behavior of coupling beams and walls constructed with tensile strain-hardening, high-performance fiber-reinforced concrete (HPFRC) was studied through tests of large-scale precast coupling beams and coupled walls. A precast coupling beam design was developed to speed up construction and minimize interference with wall reinforcement. Three isolated precast coupling beam specimens with a span-to-depth ratio of 1.75 were tested under large displacement reversals. Test results indicate the use of HPFRC allows a reduction of the reinforcement required to achieve a stable coupling beam response by providing confinement and contributing to beam shear strength. A concrete design shear stress capacity of 0.41√fc’, [MPa] (5√fc’, [psi]), where fc’ is the compressive strength of the concrete, was found to be appropriate. In addition to the coupling beam tests, two four-story coupled wall specimens with precast HPFRC and regular concrete coupling beams were tested under lateral displacement reversals. Besides allowing the evaluation of seismic behavior of coupled walls with HPFRC coupling beams, the use of HPFRC in the plastic hinge regions of the walls as a means of relaxing transverse wall reinforcement was evaluated. The two coupled wall specimens exhibited drift capacities of at least 2.5%. The HPFRC coupling beams were more ductile and damage tolerant than the regular concrete beams. The incorporation of an HPFRC material in the wall allowed the use of a concrete design shear stress capacity of 0.33√fc’, [MPa] (4√fc’, [psi]) and a wider spacing of transverse reinforcement confining the wall boundary regions

Seismic Response of Fiber-Reinforced Concrete Coupled Walls

The behavior of coupled T-shaped structural walls was studied through tests of two large-scale four-story specimens under reversed cyclic lateral displacements. The use of tensile strainhardening, high-performance fiber-reinforced concrete (HPFRC) in coupling beams and walls was evaluated as a means to reduce diagonal and confinement reinforcement. The Specimen CW-1 walls were constructed with reinforced concrete (RC) designed to satisfy ACI Building Code (ACI 318-08) seismic provisions. The walls in Specimen CW-2 were constructed with HPFRC and reduced shear and confinement reinforcement. Each specimen included one RC and three HPFRC precast coupling beams with span-depth ratios of 1.75. Both specimens sustained 80% of the peak lateral strength through loading cycles to at least 2.5% drift. Inelastic flexural deformations were more concentrated near the foundation in the HPFRC walls than in the RC walls, which led to a higher curvature demand at the base of the HPFRC walls. Although the walls in both specimens exhibited a flexuraldominated behavior, shear distortions in the first story of the walls reached 0.01 rad. Detailed data are presented regarding specimen behavior, including wall and coupling beam deformations

Test of a Coupled Wall with High Performance Fiber Reinforced Concrete Coupling Beams

Results from the test of a large-scale coupled-wall specimen consisting of two T-shaped reinforced concrete structural walls joined at four levels by precast coupling beams are presented. Each coupling beam had a span length-depth ratio (ln/h) of 1.7, and was designed to carry a shear stress of 7vfc' [psi], (0.59vfc' [MPa]). One reinforced concrete coupling beam was included along with three strain-hardening, high-performance fiber-reinforced concrete (HPFRC) coupling beams to allow a comparison of their behavior. When subjected to reversing lateral displacements, the system behaved in a highly ductile manner characterized by excellent strength retention to drifts of 3% without appreciable pinching of the lateral load versus drift hysteresis loops. The reinforced concrete structural walls showed an excellent damage tolerance in response to peak average base shear stresses of 4.4vfc' [psi], (0.34vfc' [MPa]). This paper presents the observed damage patterns in the coupling beams and the structural walls. The restraining effect provided by the structural walls to damage-induced lengthening of the coupling beams is discussed and compared with that observed in component tests. Finally, the end rotations measured in the coupling beams relative to the drift of the coupled-wall system are also presented

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.Network for Earthquake Engineering Simulation (NEES) Program (Grant No. 0936519

Experimental Evaluation of Design Procedures for Shear Strength of Deep Reinfoced Concrete Beams

In this paper, results from the monotonic testing of four reinforced concrete deep beams are presented. The behavior of the deep beams is described in terms of cracking pattern, load-versus-deflection response, failure mode, and strains in steel reinforcement and concrete. Despite different failure modes, the failure loads and corresponding ultimate deflections were similar in all four specimens. Yielding of both longitudinal and transverse reinforcement occurred prior to failure. Based on the test results, the shear design procedures contained in the ACI 318-99 Code and Appendix A of the ACI 318-02 Code were evaluated. Both design procedures yielded conservative predictions of the shear strength of the single-span deep beams

High-performance fiber reinforced concrete coupling beams: from research to practice

Results from experimental research that led to the development of a new design of coupling beams constructed with High-Performance Fiber Reinforced Concrete (HPFRC) and simplified reinforcement detailing are presented, along with information related to its implementation in a high-rise building in the city of Seattle, WA. The experimental program consisted of the testing, under large displacement reversals, of a series of large-scale HPFRC coupling beams with span-toheight ratios ranging between 1.75 and 3.3. The main goal of the experimental program was to evaluate the possibility of simplifying diagonal and confinement reinforcement detailing without compromising seismic performance. Experimental results indicate that the use of HPFRC allows the complete elimination of diagonal reinforcement in beams with span-to-height ratios greater than or equal to approximately 2.2. Also, special confinement reinforcement, as used in regular reinforced concrete coupling beams, was found to only be required over a distance of half the beam height from each beam end. For beams with span-to-height ratios smaller than approximately 2.2, a 2/3 reduction in diagonal reinforcement was found to be possible, with the same relaxation in confinement reinforcement as for the more slender coupling beams. Drift capacities of the HPFRC coupling beam specimens, when subjected to shear reversals with amplitudes comparable to the upper shear limit allowed in the ACI Building Code, ranged between approximately 5% and 7% for span-to-height ratios of 1.75 and 3.3, respectively

Cultural validity of cognitive markers for Alzheimer's disease (AD) : evidence for global strategies

Background: Availability of culture-free cognitive tests with marker properties for AD has proved a barrier to global harmonization strategies. The European Neurodegenerative Diseases Working Group suggested that the Short-Term Memory Binding Test (STMBT) and the Free and Cued Selective Reminding Test (FCSRT) are useful tests for the early detection of AD (Costa et al., 2017). Yassuda et al. (2019) showed that STMB is insensitive to age and education among healthy Brazilian adults. Parra et al. (2019) suggested that these tests should enter global strategies to aid the early detection of AD. Evidence is still needed to ascertain that such a validity translates to the assessment of affected individuals from underrepresented populations. The current study aimed to shed new light on such an outstanding question. Methods: We recruited 64 healthy controls (HC), 60 patients with Mild Cognitive Impairment (MCI), and 63 patients with mild AD from [Lima at the regional area health clinics of the “Dirección Regional de Salud (DIRESA)” of the “Gobierno Regional del Callao” between June 2018 and May 2019]. They were all illiterate. We considered Illiterate individuals who (1) attended no school or were enrolled for less than one year and (2) could not read or write (a booklet was given which showed a simple sentence). We assessed them with the STMBT, the visual FCSRT, and a brief clinical-neuropsychological protocol. Results: The assessment confirmed the healthy (CDR=0.0, pFAQ=2.2, BDI=5.9), MCI (CDR=0.5, pFAQ=3.7, BDI=6.2), and dementia (CDR=1.3, pFAQ=16.5, BDI=7.2) status of our groups. Significant between-group differences were found with both the STMBT (F(2,184)=590.1, p>>MCI>>>AD. ROC analysis with STMB revealed AUC=0.98 for HC vs. MCI, AUC=1.00 for HC vs. AD, and AUC=0.97 for MCI vs. AD. For the visual FCSRT, an AUC=1.00 was found for HC vs (MCI & AD), and AUC=0.99 for MCI vs AD. Conclusion: The two cognitive markers recently recommended for harmonisation of neuropsychological assessment in neurodegenerative dementias in Europe seem suitable to support such practices in illiterate populations. Parra et al. (2019) recently suggested that only global strategies will help meet global challenges. Here we provide evidence of cognitive markers for AD that can reliably enter such strategies

Earthquake-Resistant Fiber Reinforced Concrete Coupling Beams Without Diagonal Bars

Results from large-scale tests on fibre-reinforced concrete coupling beams subjected to large displacement reversals are reported. The main goal of using fibre reinforcement was to eliminate the need for diagonal bars and reduce the amount of confinement reinforcement required for adequate seismic performance. Experimental results indicate that the use of 30 mm long, 0.38 mm diameter hooked steel fibres with a 2300 MPa minimum tensile strength and in a volume fraction of 1.5% allows elimination of diagonal bars in coupling beams with span-todepth ratios greater than or equal to 2.2. Further, no special confinement reinforcement is required except at the ends of the coupling beams. The fibre-reinforced concrete coupling beam design was implemented in a high-rise building in the city of Seattle, WA, USA. A brief description of the coupling beam design used for this building, and construction process followed in the field, is provided