· · · Institutional Repository

concept v. 29-07-2011

Poster. In the Netherlands, 60% of the existing bridges were built before 1975, while the traffic volumes and loads have increased over time. The results of a first assessment of the existing bridges showed that particularly the shear capacity of reinforced concrete solid slab bridges is often lower than the resulting shear stresses due to dead loads and traffic loads.

The capacity of existing solid slab bridges in the Netherlands is under discussion for two reasons: 1) the increased traffic loads and volumes and 2) the fact that the majority of the existing bridges were built before 1976, and are thus reaching the end of their original life span. Upon assessment according to the governing codes, a large number of slab bridges are found to be shear-critical. However, the shear capacity as prescribed by the codes is based on experiments on beams in shear. Slabs subjected to concentrated loads (such as truck wheel load) are assumed to have additional capacity as a result of transverse load redistribution. This thesis studies the capacity of slabs under concentrated loads close to supports. A literature review, resulting in a slab shear database with 215 experiments from the literature, is used to study the mechanisms at work in one-way and two-way shear. For this research, 156 experiments were carried out on 38 half-scale bridge deck specimens. The experimental results are studied by means of a parameter analysis. To determine the capacity of slabs in shear subjected to concentrated loads, two methods are proposed in the thesis: 1) the Modified Bond Model, a new theory to determine the capacity of slabs subjected to concentrated loads; and 2) by using a code extension proposal that results from probabilistic calculations following the safety philosophy of the Eurocodes. Finally, the link to the assessment practice is made by formulating recommendations, improving the Quick Scan assessment tool of the Ministry of Infrastructure and the Environment, and then applying this to cases of existing solid slab bridges.

In slabs subjected to concentrated loads, the shear strength checks are conducted for two limit states: 1) shear over an effective width, and 2) punching shear on a perimeter around the point load. In current practice, the shear strength at the supports is determined with models that do not consider the transverse redistribution of load that occurs in slabs, which results in underpredictions for the actual slab shear capacity. Currently, an experimental program is being conducted at Delft University of Technology to determine the shear capacity of slabs under point loads near to the support. This paper presents the results of the tests conducted in continuous slabs and slab strips. In addition to studying the influence of the slab width, the specimens are tested with two types of reinforcement (ribbed and plain bars). The results of the experiments are compared to strength predictions from current design models. Also, recommendations for the support effective width and an enhancement factor for considering the effect of transverse load redistribution are given.

Several existing reinforced concrete solid slab bridges in the Netherlands do not meet the criteria for shear when calculated according to the recently implemented Eurocodes. The shear capacity is assessed by comparing the design beam shear resistance to the design value of the applied shear force due to the dead load, permanent load and live load. Transverse load redistribution which occurs in slabs is not taken into account. To evaluate a large number of slab bridges, a first round of assessments is necessary to determine which bridges need a more detailed shear analysis. A series of 26 slabs and 12 slab strips are tested until shear failure. The results of these experiments are compared to the state-of-the-art in beam shear research to compare the shear behavior of beams nd slabs. Recommendations for the shear assessment of slabs are formulated, and used to verify the shear capacity of 10 cases of slab bridges. This “Quick Scan” approach is compared to the AASHTO provisions, which are found to be less conservative. However, the underlying target reliability index is significantly smaller for the AASHTO provisions. For the existing bridges in the Netherlands, the proposed method can analyze a large number of cross-sections and thus help prioritize the efforts f the owners such that cases which need a more detailed shear analysis are identified.