Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011


Boundary conditions are defined in a way that the aft side cross-section is fixed in all degrees of freedom, while the fore side cross-section is rigid, so that sections may remain in plane after the load is applied (sectional moments). In this way symmetry of the load and boundary conditions is satisfied.


The loading applied to the global model consists of sectional design moments, transverse pressure loads on decks and water induced pressure load on the hull for both hogging and sagging loading condition.


The supporting (or backing-reinforcement) strip plate connects the two longitudinals - it is bent so that it fits precisely under the longitudinal and serves as a support during welding. The supporting plate is 40mm wide and the weld root gap between longitudinals is 5 mm wide. During the welding process a supporting plate is welded to the longitudinal and acts as its reinforcement. The trapezoidal longitudinal reinforcement is entirely connected to the deck plate by spot welds.


Spot welds along the trapezoidal longitudinal are 20 mm wide and distanced 80 mm from each other. Alternatively, continuous


welding is used and the longitudinal is continuously connected to the deck plate. The spot-weld model stress


The distance between transverse frames is equal to the length


of the sub-model, 2,400 mm. The two


neighbouring transverse frames represent the boundaries of the sub-model. The minimum width of the sub-model is 700 mm, which corresponds to transverse distance between trapezoidal longitudinals. The sub-model height is 158 mm (see Fig. 2 to 4).


The two local models, spot-weld and all-weld model, are generated using volume finite elements (20-node solid and 15-node solid) so that the weld geometry may be taken properly into account (see Figure 2 and Figure 3). The local model, or sub-model, consists of 9,502 finite elements 19,466 nodes and 58,398 degrees of freedom.


Near the supporting plate, i.e. at the middle of the longitudinal span, the neighbouring finite element width varies from 4mm to 7.5mm wide. In this way, it does not conform strictly with the usual “txtxt” requirement of the hotspot


stress evaluation procedure, where t is the


thickness of the plating. Finite element width variation was made during precise modelling of support plate and surrounding welds and geometry.


The all-weld model has the same finite element mesh size as the spot-weld model, except the weld geometry.


The loading of the local model consists of prescribed displacements from the global model, both in hogging and sagging loading conditions, and a concentrated force of 48.75 kN (car-breaking load) acting at the middle of the local model span. It is considered that breaking load is acting only during the boarding.


distribution is shown in


Figure 2, where three types of highly stressed areas may be distinguished (hotspots 1, 2 and 3).


Three areas of high stress concentration (Hotspot 4, 5 and 6) may be observed for the all-weld finite element model, Figure 3. The hot spot 4 on the all-weld model corresponds to the hotspot 2 of the spot-weld model.


The hot spot 6 in the all-weld model corresponds to the hotspot 3 in the spot-weld model and it is presented with red arrow in Figure 3. High stress is affecting both the deck plate and the trapezoid longitudinal, so the weld toe becomes a subject for the hot spot stress evaluation.


Figure 2 – Hot spot 1 to 3 for spot-weld model


The concentrated force represents a truck tire load that is acting at the middle of


the longitudinal span.


Concentrated forces are acting on four neighbouring spot welds. The truck tire contact surface is relatively small and if the model force acts directly to the deck plate it would produce very large stresses.


Figure 3 – Hot spots 4 to 6 for all-weld model


©2011: The Royal Institution of Naval Architects


A-233


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