Skewed slab highway bridges

Abstract

In situations with spatial limitations where a bridge is desired, a skewed bridge is a solution. The bridge deck of a skewed bridge has the shape of a parallelogram, in which the angle that remains in the acute corners is defined as the skew angle. This thesis focuses on the design of reinforced concrete skewed slab highway bridges that are simply supported on discrete bearing pads. A parametric tool is created which, based on a set of values for input parameters, generates and analyzes bridge models created in a finite element method (FEM) software program. After analysis, results are summarized, which allows for a parametric study. The goal of this study is to develop knowledge for decision-making in early stages of projects. The influence of main geometric parameters (skew angle, bridge span length, bridge road width) on the load distribution in a skewed bridge is investigated, as well as the influence of FEM mesh element size, applied plate theory, traffic load configuration and support configuration. Another important parameter, being the bridge deck height, is set to be dependent to the span length at first. Skewed slab bridges tend to span from support to support in the shortest direction. This means that loads will concentrate in the obtuse corner. Resultant quantities, mainly bending moments and shear force in the obtuse corner, are studied on their relation with the different parameters. The obtuse corner load concentrations can require the bridge height to become larger than desired. A powerful measure is to add additional triangular sections (ATS) to the side of the bridge, which increases its width. This way, the road skew angle no longer equals the bridge skew angle. Load concentrations in the obtuse corners are reduced, which can result in a lower deck height, less reinforcement, or both. The effect of ATS addition on resultants is studied: great reduction is observed, even for relatively small ATS addition. In order to relate the bridge deck height to bridge geometry, a reinforcement study is conducted. A procedure is described for a certain cross-section, in which the resultants are known from models generated by the parametric tool. Starting point is the longitudinal bending reinforcement based on the crack width criterion, which is usually governing in bridge design. Next, required shear reinforcement is calculated. Following, the ultimate bending moment capacity is checked, which is found to be always sufficient for crack-width-based reinforcement design. Finally, optimization (different bar diameters, add or remove a reinforcement layer, deck height reduction) is investigated and applied if possible. Using the procedure described above, a case study is conducted. A highly skewed bridge (road skew angle of 20◦) is taken. Starting with 0◦, ATS is added in steps of 5◦. For each geometry, a model is created with the tool after which resultants are used for the reinforcement design. For a bridge span of 13.7 m and a road width of 8.3 m, the first bridge (no ATS, skew angle of 20◦) was found to lie beyond the limits of reinforced concrete. Two layers of 40 mm bars required fatigue-sensitive couplers, while detailing and proper anchorage would prove to be unconstructible. Increasing the cross-section also increased governing moments due to concrete self-weight and therefore is not an option. Addition of a 5◦ ATS resulted in a 900 mm high bridge deck, which seems possible to construct. Addition of 10◦ resulted in a 750 mm high deck, 15◦ into 600 mm and further ATS addition reduced deck height even more. Although it should be noted that ATS addition increases deck surface, a significant reduction in deck height can be obtained. ATS addition is shown to be a powerful measure. Depending on project situation, it can provide optimization and cost savings; the parametric tool is proven to be useful in investigating this.