Plated girders have been extensively used in steel bridge construction since the 19th century. Over the past century, the effective width method was developed to take the nonlinearities resulting from shear lag and plate buckling into account. This method leads to having a cross-section with a reduced area in which the stress can linear be considered. The section properties of the reduced cross-section can be used for determining the resistance and the longitudinal stress in the plated girder. The reduction of the cross-section area takes place in two steps. First, the section area is reduced in local scale for subpanels that have class 4, and local effective section properties will be determined. Second, the local effective cross-section is reduced to consider the overall buckling, which can be plate-like buckling, column-like buckling, or their interaction.
The longitudinal compressive stress that can be determined from effective section properties in a box girder with pure bending moment will be higher since the effective section includes a reduced second moment of inertia and an increased distance between the neutral line and the compression flange. This master thesis aims to assess whether effective section properties accurately determine the compression stress magnitude in box sections with pure bending moment that remain unaffected by local buckling and experience overall buckling in the form of pure column-like buckling in the compression flange.
The findings indicate that if the subpanels of the isolated compression plate are stocky, and the overall buckling failure has the form of pure column-like buckling, the stress, and behavior remain within the elastic region up to column-like buckling failure. There does not exist any sudden increase in compression stress magnitude in the plate. The analytical method that can address the maximum magnitude of the compression stress in the plate has contribution of the gross section area of the plate.
If such a stiffened plate with pure column-like buckling failure and stocky subpanels serves as the compression flange of a box girder, the same conclusion was reached regarding determining the first moment of inertia. However, in the presence of the slender web in the cross-section, the effective area of the web, in combination with the gross section area of compression flange should be considered for determining the section properties with which the maximum longitudinal stress can be determined.

The increasing number of vehicles does not stop, axle loads get higher and wheel contact surfaces have decreased causing bridges to show significant fatigue damage. Demolishing and building a new bridge is out of the picture due to the current ideas on sustainability and environmental impact. Therefore renovation is necessary, as much of the existing structures should be reused in the design of a new structure. This results in massive challenges especially in the design of new movable bridge leaves which should reuse the pillars and foundation of the current bridge. The weight of the new movable bridge is limited to the same or even less than the old one, but should outperform it by many years. Parametric models have been created for both bridge types, orthotropic and hybrid, using finite element software RFEM to quantify this added. Both bridge models have been exposed to the same load cases stated in the Eurocode for traffic loads and fatigue loads. Maximum deflection and stresses have been verified in the steel of both bridges for the traffic loads. The sandwich panel has been verified for buckling and fatigue using a local model. The fatigue load cases have been used to verify the bridges steel frame. Global fatigue details in the connection between main girder and crossbeams have been investigated. Results showed a clear favour towards the hybrid bridge for all of the spans and widths investigated. The difference in weight of the hybrid bridges showed a decrease of 15 to 30% as opposed to the orthotropic steel bridge. This decrease in weight was mainly caused by the difference in deck structure. The sandwich panel, with a weight of 85 kilograms per square meter, shows far less weight contribution in the hybrid bridge than the stiffeners and deck plate, with a combined weight of 256 kilograms per square meter, have in the steel orthotropic bridge. A cost comparison showed that the OSD bridge and hybrid bridge are competitive in pricing in an early design stage. However there are many uncertainties that could result in one of the bridge types being significantly more expensive than the other. More experience with FRP and the hybrid interaction is needed to create better cost indications in such an early design stage. The environmental impact showed a difference ranging from 10% to 30% in favour of the hybrid bridge for both the CO2 impact analysis and the life cycle analysis by GWW. This difference was obtained when considering the production of the movable bridge including the counterweight and span. The difference is mainly due to the large amount of steel needed for the bridge leaf of the steel orthotropic bridge which also means that more material is needed for the counterweight. Although further research is necessary, this report shows the added value of hybrid bridges with full hybrid interaction and creates a step towards more use of hybrid steel and fibre reinforced polymer structures.