2 Stiffener-Frame Connections

2.1 Design standard A

2.1.1

Designs for cut outs in cases where web stiffeners are omitted or not connected to the longitudinals are required to adopt tight collar or the improved design standard “A” as shown in Table 1 or equivalent, for the following members:

Side shell below 1.1T_sc.
Bottom.
Inner hull longitudinal bulkhead below 1.1T_sc.
Topside tank sloping plating below 1.1T_sc.
Hopper.
Inner bottom.

2.1.2

Designs that are different from those shown in Table 1 are acceptable subject to demonstration of satisfactory fatigue performance, e.g. by using comparative finite element analysis. The comparative FE analysis is to be performed following the modelling guidance given in Figure 1.

Figure 1 : Finite element model for verification of equivalent design

2.2 Equivalent design of stiffener-frame connections

2.2.1

If the required designs for stiffener-frame connections in [2.1] are not followed, the alternative design is to be verified to have equivalent fatigue strength to the design standard “A” or to be verified to have satisfactory fatigue performance. The alternative design is to be verified according to the procedure given in [2.2.2] to [2.2.5] and documentation of results is to be submitted to the Society.

2.2.2 The procedure of [2.2.3] and [2.2.4] is provided to verify the alternative design to have equivalent fatigue strength with respect to any position in the transverse ring, i.e. double bottom and double side. The hot spot stress of the alternative design and that of the required design is to be compared to the critical hot spots in way of the cut-out. The critical hot spots depend on the detail design and are to be selected in agreement with the Society. The hot spot stress is to be derived according to Ch 9, Sec 5, [3.1] and Ch 9, Sec 5, [3.2]. It is to be noted that welded hot spots at the free edge are classified as hot spot type “b”. Example of typical hot spots for checking is shown in Ch 9, Sec 2, [2].

Table 1 : Design standard A – stiffener-frame connection

Cut outs for longitudinals in transverse webs where web stiffeners are omitted or not connected to the longitudinal flange
Design standard A
1	2

3	4

Note 1: Soft toes marked ‘*’ are to be dimensioned to suit the weld leg length such that smooth transition from the weld to the curved part can be achieved. Maximum 15 mm or thickness of transverse web/collar plates/lug plates whichever is the greater. Note 2: Configurations 1 and 4 indicate acceptable lapped lug plate connections.
Critical location	Locations around cut-out with high stress concentration and locations in way of weld terminations.
Detail design standard	Improved slot shape to avoid high stress concentrations in transverse webs due to shear loads and local pressure loads transmitted via welded joints.
Building tolerances	Ensure alignment of all connecting members and accurate dimensional control of cut-outs according to IACS Recommendation No. 47.
Welding requirements	A wraparound weld, free of undercut or notches, around the transverse web connection to longitudinal stiffener web.

2.2.3

The very fine mesh finite element models are made to analyse the behaviour in way of double side or double bottom. The models should have an extent of 3 stiffeners in cross section, i.e. 4 stiffener spacings, and the longitudinal extent is to be one half frame spacing in both forward and aft direction. A typical model is shown in Figure 1. No cut-outs for access openings are to be included in the models. Connection between the lug or the web-frame to the longitudinal stiffener web, connections of the lug to the web-frame and free edges on lugs and cut-outs in web-frame are to be modelled with elements of net plate thickness size (t_n50 × t_n50). The mesh with net plate thickness size should extend at least five elements in all directions. Outside this area, the mesh size may gradually be increased in accordance with the requirements in Ch 9, Sec 5, [2]. The eccentricity of the lapped lug plates is to be included in the model. Transverse web and lug plates are to be connected by eccentricity elements (transverse plate elements). The height of eccentricity element is to be the distance between mid-layers of transverse web and lug plates having a thickness equal to 2 times the net thickness of web-frame plate t_w-n50. Eccentricity elements representing fillet welds are shown in Figure 2.

Figure 2 : Modelling of eccentric lug plate by shell elements

2.2.4 Three load cases are to be applied to the models of the design standard and alternative designs:

External pressure of unit value, fixed boundary conditions at top and bottom of model.
Shear stress by prescribed unit displacement at the model top and fixed boundary conditions at the model bottom.
Axial load by prescribed unit displacement at the model top and fixed boundary conditions at the model bottom.

The forward and aft part of the model should have symmetry condition describing the behaviour in a double hull structure. Load application and boundary conditions are provided in Figure 3.

2.2.5

The alternative design may also be verified to have satisfactory fatigue performance using sub-modelling technique where a very fine mesh model of the alternative design located at the actual position of the stiffener-frame connection is analysed. The alternative design is considered acceptable if the fatigue acceptance criterion of Ch 9, Sec 1 is achieved. The fatigue acceptance criterion is checked by applying the methodology described in Ch 9, Sec 1, Ch 9, Sec 3 and Ch 9, Sec 5. The alternative design is considered acceptable only for the particular position where it is analysed.

Figure 3 Load application and boundary conditions – FE model for verification of alternative design