Section 3 Cross-deck strength

3.1.1 The requirements for transverse strength of trimarans are contained within this Section.

3.1.2 Transverse strength calculations are to be carried out for all vessels. If the vessel's transverse deadweight distribution varies significantly over the range of load and ballast conditions proposed, then transverse hog and sag still water conditions must be specified to be applied in conjunction with the hogging and sagging wave splitting moments. If the transverse deadweight distribution remains fairly constant, one transverse still water distribution may be used.

3.2.1 The primary stiffening members of the cross-deck structure are to provide sufficient strength to satisfy the stress criteria in Table 3.3.1 Cross-deck component stresses.

Table 3.3.1 Cross-deck component stresses

Component stress type	Nominal stress (N/mm2)
Direct stress induced by the sag splitting moment, M sps, and the hog splitting moment M sph, as defined in Vol 1, Pt 5, Ch 4, 3.1 Splitting moment
Shear stress induced by the splitting shear force, Q sps and Q sph, as defined in Vol 1, Pt 5, Ch 4, 3.2 Splitting shear force
Bending stress induced by the transverse torsional moment, M tt, as defined in Vol 1, Pt 5, Ch 4, 3.3 Transverse torsional moment
Shear stress induced by the transverse torsional moment, M tt, as defined in Vol 1, Pt 5, Ch 4, 3.3 Transverse torsional moment
where K = R = 2t dk S bhd d cd for the deck R = 2t bhd S bhd d cd for the transverse bulkhead Z cd = is to be taken as the lesser of Z cdb and Z cdt = σsps, σsph, Z cdb, Z cdt, σtt, n bhd, S p, τsps, τsph, A cd, τtt, d cd, A wi, z i, I i, t dk and t bhd are defined in Vol 1, Pt 6, Ch 1, 1.3 Symbols and definitions = M sps, M sph, M tt, y o, y I, Q sps and Q sph are defined in Vol 1, Pt 5, Ch 1, 1.4 Symbols and definitions.

3.2.2 When calculating the stresses due to the transverse torsional moment M _tt the following is assumed:

1. The cross-deck structure is a series of 'beams' formed by transverse bulkheads and decks.

2. Transverse bulkheads are equally spaced.

3. The beams are treated as built-in at each hull, i.e. the hulls have much greater stiffness than the cross-deck.

4. The cross-deck is symmetrical forward and aft of a transverse axis at its mid-length.

5. The hulls move vertically only, rotating about their pitch axis, but do not rotate about their own longitudinal axis.

3.2.3 Cross-deck designs other than those described in Vol 1, Pt 6, Ch 3, 3.2 Cross-deck component stresses 3.2.2 may require an alternative analysis to demonstrate compliance with the combined stress criteria.

3.2.4 Section properties are to be calculated using an effective breadth of plating to be determined in accordance with Vol 1, Pt 6, Ch 2, 2.2 Effective width of attached plating.

3.2.5  Z _cdt, Z _cdb and A _cd, defined in Vol 1, Pt 6, Ch 1, 1.3 Symbols and definitions, are to be calculated using the effective section at the appropriate load calculation point, see Figure 3.3.1 Cross-deck section.

Figure 3.3.1 Cross-deck section

3.3.1 The total stresses are to be evaluated for four different load cases (a) to (d) as given in Table 3.3.2 Cross-deck primary member stress criteria.

Table 3.3.2 Cross-deck primary member stress criteria

Stress type	Component stresses	Allowable stress level, N/mm2
Total direct stress, σcd	(a) σsps	0,72f hts σyd
	(b) σsph
	(c) σtt + 0,6σsps
	(d) σtt + 0,6σsph
Total shear stress, τcd	(a) τsps	0,72f hts τyd
	(b) τsph
	(c) τtt + 0,6τsps
	(d) τtt + 0,6τsph
Equivalent stress, σeq (see Note 1)		0,90f hts σyd
Note 1. When calculating equivalent stress, the component stresses for direct and shear stress are to correspond to the load case considered, i.e. for load case (a) Note 2. f hts is defined in Vol 1, Pt 6, Ch 2, 1.4 Higher tensile steel