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Transition between Shear and Punching in Reinforced Concrete Slabs: Review and Predictions with ACI Code Expressions
One-way slabs under concentrated loads may fail by one-way shear, two-way shear, flexure, or a combination of these modes. This paper reviews shear and punching shear-failure mechanisms of one-way slabs under concentrated loads tested from the literature and investigates the accuracy of different approaches to predict the ultimate capacity for such slabs using the ACI code expressions. A database with 160 test results was evaluated. Shear and concentrated loads measured at failure were reviewed according to parameters such as the load position, slab width, and reinforcement ratios. The load position and slab width play a marked influence on the failure mechanism and tested loads. The analyses improved the understanding of the main parameters influencing the behavior of one-way slabs under concentrated loads. Finally, the proposed effective shear width expression enables accurate shear capacity predictions using the ACI code expressions.
Journal Article
Alternative Punching Shear Design Methods for Concrete Slabs on Rectangular Columns
Worldwide punching shear design provisions for interior slabcolumn connections subjected to concentric shear differ greatly in how to account for column rectangularity (aspect ratio). In some, a reduced nominal shear capacity along the critical perimeter is assumed, whereas an effective or reduced critical perimeter is assumed in others. In this paper, three alternative methods to estimate the concentric punching shear capacity of interior rectangular slab-column connections without shear reinforcement, which implicitly account for the influence of column rectangularity and the ratio of the minimum column dimension to the effective slab depth, are presented. The accuracy of the proposed methods is studied through comparisons to 76 nonlinearfinite element models and 86 experiments. The predicted punching capacities from the proposed methods andACI 318-19 are also compared.
Journal Article
Effectiveness of Glass Fiber-Reinforced Polymer Stirrups as Shear Reinforcement in Glass Fiber-Reinforced Polymer- Reinforced Concrete Edge Slab-Column Connections
Recent years have seen a great interest in testing concrete slab-column connections reinforced with glass fiber-reinforced polymer bars (GFRP-RC). Yet, current fiber-reinforced polymer (FRP) codes and guidelines have not addressed the design of slab-column connections with FRP shear reinforcement. Results from an experimental investigation aimed at evaluating the effectiveness of glass fiber-reinforced polymer (GFRP) stirrups as shear reinforcement in edge slab-column connections reinforced with GFRP bars are presented. Four full-sized slabs with and without stirrups as shear reinforcement were tested to failure under combined vertical load and unbalanced moment. The effect of the GFRP stirrup type and extension on the punching shear response of the tested slab-column connections are analyzed and discussed. In addition, simplified design provisions to predicate the ultimate shear capacity of the tested specimens are proposed. The test results revealed that the presence of GFRP shear reinforcement as either closed or spiral stirrups within the slab around the column perimeter improved the punching-shear response of the tested connections. The results also indicated that the performance of the spiral stirrups was equivalent to or better than that of the closed stirrups in reducing the brittleness of the tested specimens with the same amounts of flexural and shear reinforcement. The proposed design provisions as extensions to those in CSA S806 design code yielded good, yet conservative predictions with an average [V.sub.tes]/[V.sub.pred] of 1.28 [+ or -] 0.24 for test specimens with FRP shear stirrups, as well as others with different types of FRP shear reinforcement found in the literature. This represents a step forward for engineers in designing two-way concrete slabs reinforced with FRP stirrups. Keywords: design codes; edge slab; flat slab; glass fiber-reinforced polymer; parking garages; punching shear; shear reinforcement; stirrups; unbalanced moment.
Journal Article
The Effect of Voids on Flexural Capacity of Reinforced Concrete Slabs
The voided reinforced concrete slab system is mainly produced with polyester foam placed mostly at the bottom of the slab. The aim of the voids is to reduce the weight of the slab. In this paper behavior of the voided reinforced concrete slabs in which voids placed at the mid-height of the slab cross-section, is examined analytically. A series of models were created to come up with a lightweight slab. Two distinct slab models were analyzed using the ABAQUS software. In the first group, slabs had three layers, in which bottom and top layers were of solid reinforced concrete, but the mid layer was of voided unreinforced concrete. In the second layer, in order to increase the contact between top and bottom layers of the slab, crossties were utilized, and the mid layer was reinforced accordingly. Since all the layers were 5 cm thick, the total thickness of the slabs were 15 cm. Slabs were 100 cm wide and 200 cm long. They were simulated the three-point bending test. Concrete damaged plasticity material model (CDPM) for concrete and elastoplastic material model for steel was selected. From the results it was found that moment capacity decreased with the increase in the volume of the voids. There was a sudden decrease in strength after reaching the yield strength in voided slab without a crosstie. In addition, crossties enabled the reduction of the weight of the slabs without significant decrease in moment capacity.
Journal Article
Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab-slab contact and overlap on seismicity
We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1‐D velocity model. Then, traveltime tomography is carried out to estimate three‐dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130–140 km without a gap, even to the northwest of the Izu collision zone. We subsequently define the lateral extent of the contact zone between the bottom of the PHS slab and the upper surface of the PAC slab (PHS‐PAC interface) and reveal that the slab contact zone underlies a wider area beneath Kanto in harmony with the Kanto plain. The downdip limit of interplate (thrust‐type) earthquakes on the PAC slab is deepened by ∼30 km locally under the slab contact zone. This deepening is probably caused by a lower‐temperature environment in the PAC slab, resulting from the overlap with the PHS slab subducting above and consequent thermal shielding by the PHS slab from the hot mantle wedge. We detect an extremely low‐velocity anomaly in the easternmost portion of the PHS slab, which is probably attributable to serpentinization of mantle peridotite. Interplate earthquakes are almost absent along the PHS‐PAC interface overlain by the serpentinized mantle in the PHS slab, suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine.
Journal Article
Reconstructing the Alps–Carpathians–Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion
Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust–mantle decoupling during Adria–Europe collision. In particular, the switch from south-directed European subduction to north-directed “wrong-way” Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench–trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps–Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust–mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.
Journal Article
Dynamics of slab detachment
We investigate the dynamics of slab detachment around the detachment zone and evaluate the amount of time necessary for slabs to detach. The study combines results of two‐dimensional (2D) state‐of‐the‐art thermomechanical numerical simulations and a 1D analytical solution for viscous necking under gravity. We show that the dominant deformation mechanism during slab detachment is viscous necking, independent of the depth of slab detachment. When the slab dip is moderate (35–70°), slab detachment is partly affected by localized simple shearing in the colder parts of the slab. Brittle fracturing (breaking) plays a minor role during slab detachment. Our 2D thermomechanical models indicate that the duration of slab detachment, quantified from the onset of slab thinning until the actual detachment (i.e. vanishing of slab‐pull force), is relatively short (<5 Ma) and can occur in less than 0.5 Ma. No clear correlation between the depth and the duration of slab detachment was observed. The simulations suggest that even deep slab detachment (>250 km) can occur within a short time interval (<1 Ma) which has implications for geodynamic interpretations using slab detachment as explanation for processes such as melting, exhumation or surface uplift. The thinning of the slab during detachment, observed in 2D simulations, agrees well with predictions from a 1D analytical solution indicating that the 1D solution captures the first‐order features of the detachment process. We also evaluate the impact of shear heating on the duration of slab detachment. The predictions of a simple semi‐analytical solution, based on dimensionless parameters, agree well with our and previously published results. Key Points We show that slab detachment mostly involves viscous deformation Slab detachment is the result of pure (necking) and simple shear deformation The duration of slab detachment is geologically fast (less than 5 My)
Journal Article
Combined Beam-Slab Collapse Mechanism in Isolated Reinforced Concrete Beam-Slabs—Strength Design and Load Testing
In the conventional method of strength design of reinforced concrete (RC) beam-slab systems, it is assumed that if the beams are adequately stif, the slab and beams can be analyzed and designed separately under factored gravity loads. This paper demonstrates, through yield line analysis and load testing of isolated beam-slab systems, that such a design, which tacitly assumes a 'slab alone failure' mechanism, is irrational and overconservative (failing at a load level much higher than expected). The actual collapse of the conventionally designed beam-slab system invariably involves a combined beam-slab failure mechanism. It is therefore more rational and economical to design explicitly for such a collapse mechanism, accounting for plastic hinge formation in the beams along with yield lines in the slab. The proposed method suggests provision of minimum slab steel (as prescribed by the design code), and then designing the beams aiming for a combined two-way beam-slab failure. Experimental load testing establishes that the collapse occurs as planned and that the proposed economical design has the desired code-specified safety margins. Keywords: beam-slab system; combined beam-slab failure; rational design methodology; slab alone failure; yield line analysis.
Journal Article
Finite element analysis of flexural performance of reinforced truss hollow composite concrete slabs
Combining the advantages of cast-in-place hollow slabs and prefabricated reinforced truss composite concrete slabs, a novel hollow composite slab is proposed, characterized by the inclusion of hollow thin-walled boxes without reinforcement at the edges, referred to as the hollow composite slab. To further investigate the flexural performance and critical design parameters of the hollow composite slab, numerical simulations were conducted using the finite element software ABAQUS. Based on the actual specimen fabrication and test results, the rationality of the finite element modeling was validated. Using the finite element model, a parametric analysis of key parameters for the specimens was conducted. The results showed that the finite element model could effectively simulate the crack distribution, flexural performance, and deformation characteristics of hollow composite slabs. The influence of concrete strength and the longitudinal dimension of hollow thin-walled boxes on the flexural performance of hollow composite slabs was minimal, with ultimate bearing capacities changing by only 4.63% and 0.91%, respectively. In contrast, changes in slab thickness and span had a significant impact on the flexural performance, with ultimate bearing capacities changing by 20.46% and 42.09%, respectively. The bearing capacity of hollow composite slabs increased significantly with increasing slab thickness but decreased markedly with increasing span.
Journal Article