E.O.L. Lantsoght

Shear in One-Way Slabs under Concentrated Load Close to Support

One-way slabs under concentrated loads are generally designed for shear by checking the beam shear resistance and the punching shear resistance over an effective width. Only a small number of test data regarding the shear resistance of one-way slabs subjected to concentrated loads is currently available. To be able to better evaluate the shear resistance of one-way slabs, a series of experiments was carried out on continuous one-way slabs (5 m x 2.5 m x 0.3 m [16.4 ft x 8.2 ft x 11.8 in.]) subjected to concentrated loads close to the supports, in which the load position, transverse reinforcement ratio, and concrete strength were varied. The test results are compared with code provisions and a method developed by Regan. The results show a different behavior in shear for slabs under concentrated loads than for beams.

Recommendations for the shear assessment of reinforced concrete slab bridges from experiments

Abstract Upon assessment of existing reinforced concrete short-span solid slab bridges according to the recently implemented Eurocodes that include more conservative shear capacity provisions and heavier axle loads, a number of these structures were found to be shear-critical. The results from recent experimental research on the shear capacity of slabs indicate that slabs benefit from transverse load distribution. Recommendations for the assessment of solid slab bridges in shear are developed on the basis of these experiments. A load spreading method for the concentrated loads is proposed and the applicability of superposition of loading is studied. The resulting most unfavourable position for the design trucks is provided and implemented in the so-called Dutch “Quick Scan” method (QSEC2). Cases of existing bridges are studied with the previously used QS-VBC as well as with the QS-EC2 that includes the recommendations. As a result of the assumed transverse load distribution, the shear stress to be considered at the support based on the recommendations becomes smaller.

One-Way Slabs Subjected to Combination of Loads Failing in Shear

SUMMARY AND CONCLUSIONS For the assessment of existing solid slab bridges subjected to dead loads and live loads, it is necessary to analyze the combination of the shear stresses from concentrated and distributed loads. A first series of experiments on the shear capacity of slabs only considered concentrated loads close to supports. Recommendations for the effective width in shear resulted from these tests.The current series of experiments studied the combination of loads. The hypothesis of superposition of concentrated loads over their respective effective width and distributed loads over the full slab width was tested. A unique series of 26 experiments on eight slabs subjected to a combination of a concentrated load close to the support and a line load was conducted. It was found that the hypothesis of superposition is valid and conservative. Typically, larger shear capacities were found for the case of combined loading.The influence of the distance between the concentrated load and the support and the shear capacity was confirmed for the case of combined loading. Slabs supported by a line of elastomeric bearings showed a less brittle failure mode than slabs supported by a line of steel bearings. There is no experimental evidence for treating direct load transfer differ -ently depending on the support material (as prescribed by NEN-EN 1992-1-1:2005).Experiments were carried out with the concentrated load close to the simple and continuous support. A slightly larger shear capacity was found at the continuous support. This increase was smaller than predicted by α

Collapse test and moment capacity of the Ruytenschildt reinforced concrete slab bridge

Abstract A large number of existing reinforced concrete solid slab bridges in the Netherlands are found to be insufficient for shear upon assessment. However, research has shown additional sources of capacity in slab bridges, increasing their total capacity and possibly changing their failure mode. Previous testing was limited to half-scale slab specimens cast in the laboratory. To study the full structural behaviour of slab bridges, testing to failure of a bridge is necessary. Research on load testing is carried out in order to develop load testing guidelines. In August 2014, a bridge was tested in two spans. The bridge was load tested, and additional cycles until yielding occurred in the reinforcement were added to the experiment. Though calculations with current design provisions showed that the bridge could fail in shear, the field test showed failure in flexure before shear. The unity check for flexure was determined. The experiment shows that the methods for rating of existing reinforced concrete slab bridges are conservative.

Probabilistic prediction of the failure mode of the Ruytenschildt Bridge

In the Netherlands, the shear capacity of a large number of existing reinforced concrete solid slab bridges is subject to discussion, as initial assessments indicated that their capacity was insufficient. In certain cases, the deterministic value of the moment capacity is larger than the deterministic value of the shear capacity. However, when the variability of the material properties, and of the capacity models themselves are factored in, a probability of a certain failure mode can be calculated. Here, a method is introduced to calculate the chance that a cross-section fails in shear before it fails in bending. The method that is derived here is applied to the Ruytenschildt Bridge. This case study is a reinforced concrete solid slab bridges that was tested to failure in two spans during the summer of 2014. The relative probability of failure in shear of the bridge was determined. The predictions indicated a smaller probability of a shear failure than of a bending moment failure. In the first tested span, failure was not reached, but indications of flexural distress were observed. In the second span, a flexural failure was achieved, in line with the probabilistic predictions. The presented method can be used in the assessment of existing bridges to determine which failure mode is most probable, taking into account the variability of materials and capacity models.

Beam Experiments on Acceptance Criteria for Bridge Load Tests

Loading protocols and acceptance criteria are available in the literature for load tests on buildings. For bridges, proof load tests are interesting when crucial information about the structure is missing, or when the uncertainties about the structural response are large. The acceptance criteria can then be applied to evaluate if further loading is acceptable, or could lead to permanent damage to the structure. To develop loading protocols and acceptance criteria for proof loading of reinforced concrete bridges, beam experiments were analyzed. In these experiments, different loading speeds, constant load level times, numbers of loading cycles, and required number of load levels were evaluated. The result of these experiments is the development of a standard loading protocol for the proof loading of reinforced concrete bridges. Based on these limited test results, recommendations for acceptance criteria are also proposed.

Development of Stop Criteria for Proof Loading

Proof loading of bridges is an option to study existing bridges when crucial information is lacking. When proof loading is chosen, the question arises which maximum load should be attained during the test to demonstrate sufficient capacity, and which criteria, the “stop criteria”, based on the measurements during the test, would indicate that the test needs to be aborted before reaching the maximum desired load. A review of the literature identifies the stop criteria in currently used codes and guidelines. Beams sawn from the Ruytenschildt bridge were tested in a controlled way in the laboratory and analyzed with regard to the stop criteria from the literature. Recommendations are given for the future development of stop criteria for flexure and shear. These recommendations will form the basis for a guideline on proof loading of existing concrete bridges that is under development in The Netherlands.

Case study on aggregate interlock capacity for the shear assessment of cracked reinforced-concrete bridge cross sections

AbstractA 55-year-old bridge showed large cracking in the approach bridge caused by restraint of deformation and support settlement. After repair, it was uncertain at which crack width the traffic loads on the bridge should be further restricted. The shear capacity was calculated by counting on the aggregate interlock capacity of a supposedly fully cracked cross section. An aggregate interlock relationship between shear capacity and crack width based on an unreinforced section was used to find the maximum allowable crack width. Limits for crack widths at which load restrictions should be imposed were found. The large structural capacity of the cracked concrete section shows that the residual bearing resistance based on the aggregate interlock capacity of reinforced concrete slab bridges with existing cracks is higher than expected. This expected capacity could be calculated with the inclined cracking load from the code provisions. The procedure outlined in this paper can thus be used for the shear assessm...

Predicting the shear capacity of reinforced concrete slabs subjected to concentrated loads close to supports with the modified bond model

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.

Shear assessment of reinforced concrete slab bridges

The capacity of reinforced concrete solid slab bridges in shear is assessed by comparing the design beam shear resistance to the design value of the applied shear force due to the permanent actions and live loads. Results from experiments on half-scale continuous slab bridges are used to develop a set of recommendations for the assessment of slab bridges in shear. A method is proposed allowing to take the transverse force redistribution in slabs under concentrated loads into account, as well as a horizontal load spreading method for the concentrated loads. For selected cases of existing straight solid slab bridges, a comparison is made between the results based on the shear capacity according to the Dutch Code NEN 6720 and from the combination of the Eurocode (EN 1992-1- 1:2005) with the recommendations, showing an improved agreement.