An Orthotropic Steel Deck (OSD) is suspectable to fatigue and this forms an important design criterium. In this research the rib-to-crossbeam connection with a cope hole is studied using Finite Element Methods (FEM). The fatigue assessment is performed with the hot spot stress method making use of surface stress extrapolation. Two fatigue cracks are investigated: the crack initiating from lower weld end in the rib, and the crack which appears at the weld toe of the rib-to-crossbeam connection and propagating in the crossbeam at some distance from the soft toe of the cut-out.

Engineering firms widely use shell elements to model the structure, instead of solid elements. The modelling and computational time is limited when compared to the solid element. When a comparison is made between the results from the solid and shell FE models, a scatter in results is often seen. This thesis studies the difference of shell and solid element modelling in the hot spot stress calculation of OSDs. Improvement of the shell element modelling is given based on the FE analysis using various shell element modelling techniques and compared with solid element models which represent as a reference.
Three different weld modelling techniques are applied to shell FE model: IIW, Eriksson and a combination of the first two approaches. The weld modelling techniques for the shell FE model are first applied on a simple single sided fillet welded transverse stiffener connection. In this study both the deformations and hot spot stress are compared.

A small parametric analysis is performed to investigate the difference in hot spot stress for pure in-plane and out-of-plane rotation of the crossbeam. In this study the different weld modelling techniques for the shell FE models are included. In the extensive parametric analysis two different studies are performed: one based on the influence of the loading positions, the second study is based on different thicknesses of the parts of the OSD. Based on the loading positions study the critical loading positions are further investigated.

From both the fillet welded transverse stiffener specimen and the full OSD specimen it is concluded that the shell FE model in which use is made of the combined weld modelling technique, gives the largest improvement. The ratio between shell and solid FE models of the obtained hot spot stress is reduced from a factor 1.25 to a factor 1.05 for the lower weld toe crack in the rib.

For the crack in the trough, the mean value of the hot spot stress ratios is equal to 1.04 for both the old and new OSD variant and the coefficient of variation (CV) is equal to 0.8% and 2.2% respectively. For the crack in the crossbeam, the mean values are equal to 0.80 and 0.76 for the old and new OSD variant respectively. The CV is equal to 1.9% and 2.0% for the old and new OSD variant respectively. Furthermore, the stress gradient and the stress at 40mm from the weld toe show similar results when compared with the solid FE models. The coefficient of variation for both investigated cracks is low, i.e. below 5%, thus the ratio between obtained hot spot stress of the shell with combined weld and solid FE models is consistent.

The phenomenon of fatigue in orthotropic steel deck (OSD) bridges is a predominant problem because of complexity of the prediction methods. In the past, many researchers have studied the fatigue behaviour of various details in OSDs via both experiments and Finite Element Modelling (FEM). In the present research, the connection of open stiffener to crossbeam at the location of cope hole in OSDs has been be studied. Structural hot-spot stress method using surface stress extrapolation has been used to investigate the cracks in stiffener in the longitudinal direction and cracks in crossbeam.

FEM is extensively used for analysing OSDs. In engineering applications, 2D shell elements are widely used instead of 3D solid elements for analysis due to less computational cost. The welds are generally not modelled with shell elements for fatigue assessment of welded structures. In this study, large difference of SHSS is obtained by shell and solid elements for both simple and complex fillet welded details and also for the OSD. This difference in structural hot-spot stress (SHSS) is reduced by the application of three weld modelling techniques with shell elements: (i) the IIW approach, (ii) the Eriksson’s approach and (iii) a combination of IIW and Eriksson’s approaches. All the three methods are based on increasing the thickness of shell elements at the weld region which are easy to be applied in practice. The dependence of SHSS on mesh size and element type is also investigated in this thesis.

A parametric study is performed first on simple and then on complex fillet welded details to check whether the weld modelling technique can be applied to different geometries, loading and boundary conditions. The solid element model of the complex detail is first validated with experimental strain measurements. Then, SHSS values from other numerical models are compared with the solid model. Representative load cases are investigated initially followed by load combinations. The weld modelling method with shell elements gives good consistency in the ratio of hot-spot stress compared to the solid element model for these details. The deformations are also investigated for all load cases and load combinations. The combined weld modelling technique (iii) with shell elements replicated the weld stiffness of the solid model for both in-plane and out-of-plane load cases.

As a final step in checking the consistency of SHSS ratios between shell elements with welds and solid elements in the application of OSD, a parametric investigation is performed. This study involved two geometric variants of OSD with different load positions. These two variants were based on the design of existing bridges in The Netherlands with relatively thin plate and newly designed ones with thicker plates. The parametric study is divided into two parts. The first part is based on representative load cases. The second part is based on SHSS influence lines for determination of critical loading positions having maximum and minimum hot-spot stress. For both these studies, the weld modelling approaches with shell elements gave a good match of SHSS compared to the solid models. The SHSS results from the shell model with weld are more consistent compared to the regular shell model without weld. From the preliminary parametric study on OSD, it is found that after weld modelling with shell elements using the combined approach of IIW and Eriksson, less scatter is observed in the SHSS ratios. The coefficient of variation (CV) in SHSS ratio for crossbeam is 6.8% and that for stiffener is around 5.1% which is low. The SHSS values are computed based on the stress perpendicular to weld toe. From the detailed parametric study, the mean value of SHSS ratio is 1.07 (range: 0.99-1.15) for the crossbeam and 1.02 (range: 0.98-1.10) for the stiffener. The CV of SHSS ratio is 5.4% for the crossbeam and 4% for the stiffener. The stress profiles are also investigated at the critical locations of OSD. The shell model with the combined weld modelling approach is in good agreement with not only SHSS but also with the stress at a distance far away from the stress concentration when compared to the solid model. The deformations are also very similar for both the numerical models. Thus, it is concluded that the combined weld modelling technique using the IIW and the Eriksson’s approach with shell elements could be used for accurate fatigue life assessment using hot-spot stress method where the measure of accuracy is with respect to the solid element model.