Finite element analysis of the closed stiffener to crossbeam connection in OSDs using the hot spot stress approach

Abstract

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.