International Concrete Abstracts Portal

The calculation of the nominal flexural strength of concrete members prestressed with hybrid (that is, a combination of bonded and unbonded [steel and/or carbon fiber-reinforced polymer (CFRP)]) tendons is dependent on determining the stress in the unbonded prestressed reinforcement. Current provisions in ACI CODE-318-25 are only applicable to members with either unbonded or bonded steel tendons. Additionally, while ACI 440.4R-04 is adopted for unbonded CFRP tendons, neither ACI provision addresses the use of hybrid tendons. This paper presents a closed-form analytical solution for the stress at ultimate derived based on the modified deformation-based approach (MDBA) that is applicable to beams prestressed with unbonded, hybrid (steel or FRP), external with deviators or internal tendons, with and without nonprestressed reinforcement. An assessment of its accuracy and applicability in calculating the nominal flexural strength is examined using a large database of 330 beams and slabs (prestressed with steel and/or CFRP tendons) compiled from test results by the authors as well as those available in the literature. Results predicted by the proposed approach exhibited excellent accuracy when compared to those predicted using ACI CODE-318 or ACI 440.4R stress equations. They also show that the approach is universally applicable to any combination of bonded and/or unbonded (steel and/or CFRP) tendons, span-depth ratio, and loading applications.

Progressive collapse behavior of reinforced concrete frame buildings has been studied extensively, but most research has focused on frames with seismic details. This paper presents results from analysis of the progressive collapse behavior of reinforced concrete frame buildings containing details used in regions of low seismicity following ACI 318-19. The analytical simulations presented in this paper include the effect of moment redistribution that occurs after plastic moments are reached at sections of maximum moment. Ten-story three-dimensional (3-D) frame models were designed in accordance with ACI 318-19 and analyzed under progressive collapse scenarios involving the non-simultaneous removal of an interior and a corner perimeter column following ASCE 76-23. Nonlinear material behavior in these analytical models was captured using a lumped plasticity approach using hinge properties calibrated using results from laboratory experiments of full-scale sub-assemblages representing a portion of the perimeter frame containing details corresponding to non-seismic zones. The effect of catenary action in beams after column removal was included in the analysis, and the potential for premature shear failure of beams was assessed. Furthermore, models were also constructed to investigate the beneficial effects of increased rotational capacity of perimeter beams that result from using closer stirrup spacing at beam ends. This study demonstrates the importance of incorporating properly detailed continuous longitudinal bars enclosed within closely spaced closed stirrups at ends of beams of reinforced concrete frames in non-seismic zones to provide progressive collapse resistance. The study also highlights the importance of accounting for 3-D effects in frame models to capture out-of-plane moment redistribution after the loss of supporting elements.

For the assessment of existing reinforced concrete slab bridges, the shear capacity under concentrated loads and transition to flexural failure are under discussion. Previous research showed an increased shear capacity for slabs under concentrated loads close to the support, so that for assessment, positions farther from the support became governing. This experimental research studies the flexural and shear capacity of reinforced concrete slabs under concentrated loads. For this purpose, six slabs representing 1:2-scale continuous slab bridges were tested at various positions from the support and along the width. The results show two main failure modes: flexural failure (onset of yielding of the reinforcement), and shear failure. Secondary punching was observed as well. The comparison between the test results and calculation methods shows that all considered methods perform reasonably well when both shear and flexure are considered, and the effective width in shear is included, with average tested-to-predicted capacities between 0.92 (Regan’s method) and 1.39 (Extended Strip Model [ESM]) and coefficients of variation between 15% (Regan’s method) and 25% (ACI 318-19 and Eurocode 2). These insights can be used for the assessment of existing reinforced concrete slab bridges.

This study evaluates the seismic performance of circular columns reinforced with fiber-reinforced polymer (FRP) bars, focusing on the efficacy of existing code provisions (ACI 318-19, CSA A23.3-24, CSA S806-12, and CSA S6-25) in predicting drift and moment capacities. A database of 38 full-scale columns tested under lateral cyclic loading with varying axial load levels, spiral pitches, and reinforcement types (glass fiber-reinforced polymer [GFRP]/steel longitudinal bars) was analyzed to assess code provisions, confinement effectiveness, and strength enhancements. Results demonstrate that CSA S6-25, which incorporates updated FRP compressive strain limits (0.008Ef for spirals), outperformed other codes, aligning with approximately 85% of experimental data in ideal performance quadrants. Close spiral pitch (≤75 mm [2.95 in.]) and low axial loads were critical to achieving drift ratios ≥3% and moment capacity ratios (Mmax/Mo) exceeding 2.0. Replacing steel spirals with GFRP spirals did not result in substantial variation in the seismic performance of columns. Columns with GFRP longitudinal bars exhibited comparable ductility and observed substantial increase in moment capacity (Mmax) compared to the unconfined nominal moment capacity (Mo) due to delayed bar buckling under effective confinement. However, columns with GFRP longitudinal bars observed a softer response, and the determination of the probable moment to calculate the shear demand remains questionable and requires more analytical investigations.

This paper compares the flexural performance of post-tensioned slabs with steel fiber reinforcement to post-tensioned slabs with conventional bonded reinforcement in terms of behavior at service and factored loads and flexural strength. A total of five one-third scale models of a two-way post-tensioned flat plate were tested under uniformly distributed loads. All slabs had the same dimensions and were designed to meet allowable stresses per ACI 318. The specimens varied with respect to the post-tensioning tendon layouts (banded-uniform vs banded-banded), post-tensioning type (unbonded vs bonded), and nonprestressed reinforcement (conventional bonded reinforcement vs distributed steel fibers). Each nine-bay specimen was loaded to service and factored load multiple times, then loaded to failure. The performances of all specimens were very similar at service and factored loads. All specimens with unbonded tendons achieved similar failure loads, while the specimens with bonded post-tensioning tendons achieved a considerably higher load at failure.