International Concrete Abstracts Portal

The present study demonstrates the feasibility of using longitudinal hybrid reinforcement in concrete columns in seismic zones. In this research, four concrete columns were constructed and subjected to quasi-static cyclic loading, featuring a combination of steel and glass fiber-reinforced polymer (GFRP) longitudinal reinforcement. Two reference columns were fabricated and reinforced in the longitudinal direction with steel bars. These columns had a 400 × 400 mm (15.8 × 15.8 in.) cross-section and 1850 mm (72.8 in.) overall height. All the columns were reinforced with GFRP crossties and spirals in the horizontal direction. The variable parameters were the transverse reinforcement spacing, axial load ratio, and column configuration. The outcomes of this research clearly showed that reinforced concrete (RC) columns that are properly designed and detailed longitudinally with hybrid reinforcement (GFRP/steel) could achieve the drift limitation in building codes with no strength degradation. Further, these hybrid-RC columns displayed enhanced energy dissipation capacity, superior ductility, and improved post-earthquake recoverability compared to columns reinforced longitudinally with steel. The promising results of this study represent a step towards the use of longitudinal hybrid reinforcement in lateral-resisting systems.

Eleven large-scale reinforced concrete coupling beam specimens were tested under reversed cyclic displacements of increasing magnitude. The main variables included: yield stress (f_y ) of the primary longitudinal reinforcement, span-to-depth (aspect) ratio, and layout of the primary longitudinal reinforcement (diagonal [D] or parallel [P]). Specimens had the same nominal concrete strength and cross-section and were designed for nominal shear stresses for D-type beams and P-type beams. Transverse reinforcement was Grade 80 (550) in all but one beam with Grade 120 (830) reinforcement.

Test results show that, on average, D-type beams had chord rotation capacities in excess of 5, 6, and 7% for beams with aspect ratios of 1.5, 2.5, and 3.5, respectively. P-type beams with Grade 80 or 100 (550 or 690) longitudinal bars, tested only for an aspect ratio of 2.5, had chord rotation capacities of approximately 4%. Based on these results, the authors recommend permitting the use of high-strength steel, Grade 80 (550) and higher, in D-type and P-type coupling beams for earthquake-resistant design. The spacing of confining reinforcement should be limited to 5d_b for f_y = 80 ksi (550 MPa) and 4d_b for f_y = 100 or 120 ksi (690 or 830 MPa). Consistent with prior findings, the results show that deformation capacity correlates with the span-to-depth ratio and is more sensitive to the spacing of the confining reinforcement than to the uniform elongation of the reinforcement. Finally, the test results illustrate the effects of reinforcement grade on stiffness and energy dissipation of pseudo-statically loaded coupling beams.

Under earthquake excitations, reinforced concrete (RC) columns could be subjected to lateral drift reversals and a combination of axial forces, bending moments, and torsional effects. This paper investigates the behavior of glass fiber-reinforced polymer (GFRP)-RC columns under seismic-simulated loading, including torsion, which has not been studied previously. Seven large-scale circular GFRP-RC column-footing connections were cast and tested under various combined reversed cyclic loading configurations to examine the effects of torsion-bending moment ratio (tm), transverse reinforcement ratio, and concrete compressive strength. The test results revealed that increasing the tm reduced the lateral load capacity and deformability of the GFRP-RC column, but resulted in a more symmetric torque-twist relationship. Increasing the transverse reinforcement ratio mitigated core damage and provided additional support (for example, spiral turns) for torsion-induced tensile stresses. Moreover, increased concrete compressive strength bolstered torque capacity and torsional stiffness, while, under a tm of 0.4, it resulted in decreased twist capacity. When torsion was present, increasing the concrete compressive strength had an insignificant impact on the bending-shear response, differing from findings for GFRP-RC columns subjected to seismic loading without torsion.

The adequate seismic behavior of slender reinforced concrete (RC) structural walls relies heavily on the effectiveness of the boundary element (BE) in providing stable resistance against combined axial and flexural-shear compression demands resulting from gravity loading and lateral earthquake deformations. The geometric properties of the BE, including thickness and confined length, as well as the arrangement, detailing, and quantity of transverse reinforcement, play crucial roles in achieving a stable compressive response. Laboratory tests on isolated BE specimens subjected to uniform axial compression or cyclic axial tension and compression have been instrumental in understanding the influence of these variables on the compressive behavior of wall BEs. This study uses a database of experimental results from 45 rectangular BE specimens to establish empirical relationships between compressive force and strain, accounting for geometric and transverse reinforcement design parameters. A novel auto-regularizing model is proposed to estimate the compressive behavior within the damaged zone of a BE, based on its geometry and transverse reinforcement.

Seven one-half-scale reinforced concrete coupling beams, designed using ACI 318-19, were tested with constant stiffness axial restraint. Test variables were span-to-depth ratio, reinforcement configuration (conventional or diagonal), primary reinforcement ratio and bar diameter, and level of axial restraint. Six beams consisted of three nominally identical pairs, with the two beams in each pair tested at a different level of axial restraint. The two conventionally reinforced beams reached peak strength at 2.0% and 3.0% chord rotation and experienced rapid post-peak strength degradation with the opening of diagonal cracks and the formation of splitting cracks along longitudinal reinforcement. Strength degradation in diagonally reinforced beams initiated with buckling of diagonal reinforcement, and variation in axial restraint on identical pairs of beams did not lead to a significant difference in deformation capacity. Deformation capacity was larger for beams with larger diagonal bar diameters, which corresponded to a larger reinforcement ratio and a larger ratio of transverse reinforcement spacing to diagonal bar diameter. For the diagonally reinforced test beams, the maximum measured shear strength reached as high as 2.4 times the nominal shear strength computing using ACI 318-19 and exceeded the limit on nominal shear strength by more than a factor of 2.0 in the test with the smallest span-to-depth ratio. Based on strut-and-tie behavior, modifications to the ACI 318-19 equation to include axial load were examined. When the location of the compressive strut and tension tie at the beam ends was consistent with nominal moment calculations, the resulting ratio of the average maximum measured shear strength in the positive and negative loading direction to shear strength calculated using the modified equation ranged from 1.16 to 1.33. For the diagonally reinforced beams, a larger span-to-depth ratio, bar size, and reinforcement ratio were associated with larger rotation at yielding and larger effective stiffness.