Section 4 Requirements for section analyses

4.1.1 Although recognising that the selected concrete Code or Standard will have requirements for acceptance of design and detailing for the various limit states, the additional items outlined in this Section should also be complied with.

4.2.1 The material partial factor, , for reinforcement and pre-stressing strand should not be less than 1,15, irrespective of the Code or Standard selected.

4.2.2 In assessing panel members for buckling, adequate allowance is to be made for local and global geometric tolerances. Panels are to be assessed for a hydrostatic head based on the maximum still water draught together with the maximum wave pressures.

4.2.3 When considering shear close to supports, favourable arch effects are to be ignored when fluid pressure is acting in the cracks.

4.2.4 Where the shear failure mechanism is not well defined, the design is to be based on principal tensile stresses.

4.2.5 It is acceptable to include the positive effects of both compressive axial load and pre-stress when calculating shear resistance. However, it is considered that shear cracking prior to the ULS should be avoided and the appropriate method of calculation is to be adopted.

4.2.6 Where in-plane deformation forces (excluding pre-stressing) enhance the transverse shear capacity, they should be neglected. This may necessitate performing shear checks both with and without certain deformation loads, e.g. temperature.

4.2.7 Where temperature effects are significant and/or where lightweight concrete is used, the coefficient of temperature expansion, ∝, should be obtained by testing.

4.2.8 If the loading pattern of the cargo can result in significant torsion, these effects should be considered in the design.

4.3.1 Particular attention is to be given to design, detailing and construction of the large concrete areas in the splash zone.

4.3.2 The following crack width limits assume a formula similar to CEB/FIP recommendations. Equivalence should be demonstrated where the method of calculating crack widths is significantly different from that assumed.

4.3.3 Based on the normal serviceability condition (as defined in Table 2.3.1 Load factors and combinations for use with characteristics loads in Pt 9, Ch 2 Loads and Load Combinations) the calculated crack widths should satisfy the requirements in Table 3.4.1 Zonal crack width limits. External to the hull, the splash zone should be considered to extend from 3,0 m below the lightship draught up to the deck level. For units subject to green seas on deck and frequent sea spray, the top deck surface should also be considered as the splash zone. The interior of ballast tanks are also to be designed on the same basis as the splash zone.

4.3.4 Allowance is to be made in the crack width calculations for deformation strains (temperature) to be concentrated at the cracked face of sections and increase the concrete crack width. The practice of using a strain twice the elastically calculated strain is acceptable.

4.3.5 For construction, transportation and installation, the crack widths shall not exceed 0,6 mm.

4.3.7 In areas of the structure adjacent to the sea which are intended to be watertight/oiltight, through thickness cracks are to be avoided under normal serviceability conditions. In general, this is to be achieved by strictly maintaining a ‘no tension’ criterion for in-plane membrane forces for this condition.

4.3.9 Details of minimum cover requirements are given in Pt 9, Ch 4, 2.7 Concrete cover reinforcement.

4.4.1 All stress variations imposed on the structure during its design life are to be considered in the fatigue evaluation. Account should be taken of the range of operating draughts and cargo filling/emptying cycles if significant.

4.4.2 A fatigue evaluation is to be carried out for the critical areas of the structure. It is expected this will be based on linear cumulative damage (Palmgren – Miner’s Rule). The material partial factors and characteristic fatigue strength relationships (S-N curves) are to be appropriate for the selected Code or Standard, and should account for air and water locations, stress state and reinforcement diameter.

4.4.3 The dynamic behaviour of the unit is to be investigated to determine whether the increase in load effects due to dynamic amplification is important. If dynamic effects are considered significant then a response analysis is to be carried out.

4.4.4 The fatigue life factors of safety required are given in Table 6.5.1 Design load cases for deck stiffening and supporting structure in Pt 4, Ch 6 Local Strength and range from 1 to 10, depending on location in the unit, the ability to inspect or repair and the consequences of failure. The factors chosen are to be agreed for areas assessed.

4.4.5 Where large compression or compression/tension stress ranges occur (e.g. hull bottom), consideration is to be given to appropriate design and detailing. Confinement reinforcement is to be provided to ensure ductile behaviour. As far as practicably possible, cycling into the tension range should be avoided.

4.4.6 It should be demonstrated that the design and detailing of penetrations, openings and access ways consider the increased cyclic nature of loading on floating concrete units compared to fixed offshore structures.

4.5.1 In general, for accidental or abnormal loads, it should be documented that the strength or the ductility of the structure is sufficient for the applied loads.

4.5.2 For impact and explosive loads, account can be taken of increased material strength and modulus in accordance with the selected Code or Standard.