Abstract When the shear resistance of prestressed beams with stirrups is determined with the current Eurocode, no distinction is made between regions with and without flexural cracks. This while it may be expected that a region without flexural cracks will have a higher shear resistance. This is due to the lower longitudinal strains and the narrow crack widths, resulting in a higher contribution of aggregate interlock. Also, the Eurocode does not take into account that in regions without flexural cracks, a significant part of the shear force is transferred through the uncracked flanges. This article proposes therefore a shear resistance model, based on Modified Compression Field Theory (MCFT), that does consider the low longitudinal strains and shear transfer through the uncracked flanges. From a comparison it was found that the proposed model can determine shear resistance as accurately as the most comprehensive level III approach of the Model Code 2010. However, the proposed model was found to be much easier to use in engineering practice as no iterations are necessary.
<p> The shear problem is typically studied by testing small, heavily reinforced, slender beams subjected to concentrated loads, resulting in a beam shear failure, or by testing slab-column connections, resulting in a punching shear failure. Slabs subjected to concentrated loads close to supports, as occurring when truck loads are placed on slab bridges, are much less studied. For this purpose, the Bond Model for concentric punching shear was studied at first. Then, modifications were made, resulting in the Modified Bond Model. The Modified Bond Model takes into account the enhanced capacity resulting from the direct strut that forms between the load and the support. Moreover, the Modified Bond Model is able to deal with moment changes between the support and the span, as occurs near continuous supports, and can take into account the reduction in capacity when the load is placed near to the edge. The resulting Modified Bond Model is compared to the results of experiments that were carried out at the Stevin laboratory. As compared to the Eurocodes (NEN-EN 1992-1-1:2005) and the ACI code (ACI 318-11), the Modified Bond Model leads to a better prediction.</p>
In the Netherlands, the assessment of existing prestressed concrete slab-between-girder bridges showed that the thin, transversely prestressed slabs may be critical for static and fatigue punching when evaluated using the recently introduced Eurocodes. On the other hand, compressive membrane action increases the capacity of these slabs and changes the failure mode from bending to punching shear. To improve the assessment of the existing prestressed slab-between-girder bridges in the Netherlands, two 1:2 scale models of an existing bridge, the Van Brienenoord Bridge, were built in the laboratory and tested monotonically as well as under cycles of loading. The result of these experiments is: 1) the static strength of the decks, showing that compressive membrane action significantly enhances the punching capacity, and 2) the Wöhler curve of the decks, showing that compressive membrane action remains under fatigue loading. The experimental results can then be used for the assessment of the most critical existing slab-between-girder bridge. The outcome is that the bridge has sufficient punching capacity for static and fatigue loads, and thus that the existing slab-between-girder bridges in the Netherlands fulfil the code requirements for static and fatigue punching.
Abstract In Anbetracht steigender Verkehrslasten und geringer angesetzter Tragfähigkeiten in den vor Kurzem eingeführten Eurocodes steht in den Niederlanden die Querkrafttragfähigkeit vieler bestehender Brücken mit Stahlbetonmassivdecke zur Debatte. In den Normvorschriften werden jedoch günstige Wirkungsmechanismen, wie die Querkraftverteilung in den Platten, nicht berücksichtigt. Man benötigte Forschungsergebnisse, um das Verhalten von Stahlbetonplatten unter Einzellasten in der Nähe von Auflagern zu verstehen. Für diese Studie wurden Versuche an Platten und Balken durchgeführt. Sie zeigten, dass Platten unter konzentrierter Querkraftbelastung eine größere Tragfähigkeit haben als Balken. Das bedeutet, dass für Plattenbrücken ein höherer Belastungswert angesetzt werden kann, sodass weniger Brücken als bisher vermutet einer weiteren Prüfung bedürfen. Shear capacity of slabs close to supports – Overview of experiments The shear capacity of many of the existing reinforced concrete solid slab bridges in The Netherlands is subject to discussion due to the higher traffic loads and lower prescribed shear capacities in the recently implemented Eurocodes. However, the code provisions do not take into account beneficial mechanisms such as transverse load redistribution in slabs. Research was needed to understand the behavior of reinforced concrete slabs under concentrated loads close to supports. For this study, experiments on slabs and beams were carried out. The experiments showed that slabs under concentrated loads in shear have a larger capacity than beams. This implies that a higher capacity can be contributed to slab bridges upon assessment, so that less bridges will be identified as needing further study.
Reinforced concrete short-span solid-slab bridges are used to compare Dutch and North American practices. As an assessment of existing solid-slab bridges in the Netherlands showed that the shear capacity is often governing, this paper provides a comparison between Aashto (American Association of State Highway and Transportation Officials) practice and a method based on the Eurocodes, and recommendations from experimental research for the shear capacity of slab bridges under live loads. The results from recent slab shear experiments conducted at Delft University of Technology indicate that slabs benefit from transverse force redistribution. For ten selected cases of straight solid-slab bridges, unity checks (the ratio between the design value of the applied shear force and the design beam shear resistance) are calculated according to the Eurocode-based method and the Aashto method. The results show similar design shear forces but higher shear resistances in the North American practice, which is not surprising as the associated reliability index for Aashto is lower.
Proof load testing of existing reinforced concrete bridges is becoming increasingly important as the current bridge stock is ageing. In a proof load test, a load that corresponds to the factored live load is applied to a bridge structure, to directly demonstrate that a bridge fulfils the code requirements. To optimize the procedures used in proof load tests, it can be interesting to combine field testing and finite element modelling. Finite element models can for example be used to assess a tested structure after the test when the critical position could not be loaded. In this paper, the case of viaduct De Beek, a four-span reinforced concrete slab bridge, is studied. Upon assessment, it was found that the requirements for bending moment are not fulfilled for this structure. This viaduct was proof load tested in the end span. However, the middle spans are the critical spans of this structure. The initial assessment of this viaduct was carried out with increasingly refined linear finite element models. To further study the behavior of this bridge, a nonlinear finite element model is used. The data from the field test (measured strains on the bottom of the concrete cross-section and in the steel reinforcement, as well as measured deflection profiles) are used to update the nonlinear finite element model for the end span, and to improve the modelling and assessment of the critical mid spans of the structure. Similarly, an improved assessment based on a linear finite element model is carried out. The approaches shown for viaduct De Beek should be applied for other case studies before recommendations for practice can be formulated. Eventually, an optimized combination of field testing and finite element modeling will result in an approach that reduces the cost of field testing.
The paper investigates the effect of various geometrical and material parameters on the bearing (punching shear) capacity of transversely prestressed concrete deck slabs by numerical methods. Experiments on a 1:2 scale model of such a bridge were carried out in the laboratory and a 3D nonlinear finite element (FE) model was developed in the finite element analysis software package TNO DIANA (2012) to study the structural behavior in punching shear. A comparison of the experimental and numerical ultimate loads show that the non-linear FE models can predict the load carrying capacity quite accurately with a standard deviation of 0.1 and the coefficient of variation of only 10%. The effect of varying the transverse prestressing level, the presence and size of the ducts, size of the loading plate and the concrete class is also described as part of the parametric study. It was observed that sufficient saving in cost could be made if calibrated numerical models are employed to investigate existing structures rather than doing expensive experimental studies.
As the existing bridge stock is aging, improved assessment methods such as proof load testing become increasingly important. Proof load testing involves large loads, and as such the risk for the structure and personnel can be significant. To capture the structural response, extensive measurements are applied to proof load tests. Stop criteria, based on the measured quantities, are used to identify when further loading in a proof load test is not permitted. For proof load testing of buildings, stop criteria are available in existing codes. For bridges, recently stop criteria based on laboratory tests on beams reinforced with plain bars have been proposed. Subsequently, improved stop criteria were developed based on theoretical considerations for bending moment and shear. The stop criteria from the codes and the proposed stop criteria are compared to the results from field testing to collapse on the Ruytenschildt Bridge, and to the results from laboratory tests on beams sawn from the Ruytenschildt Bridge. This comparison shows that only a small change to the stop criteria derived from laboratory testing is necessary. The experimental evidence strengthens the recommendation for using the proposed stop criteria in proof load tests on bridges for bending moment, whereas further testing to confirm the stop criteria for shear is necessary.
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