Annex 3-1 Guidance for the Direct Strength Assessment

1. Direct strength calculation of steel ships

(1) General

(A) Application

The application of direct stress analysis is governed by:

(a) In such cases where simplified formulations are not able to take into account special stress distributions, boundary conditions or structural arrangements with sufficient accu- racy, direct stress analysis may be required.

(b) In some cases direct stress calculations may give reduced scantlings, especially when op- timization routines are incorporated.

(2) Plating

Normally direct strength analysis of laterally loaded plating is not required as part of rule scan- tling estimation.

(3) Stiffeners

(A) General

Direct strength analysis of stiffeners may be requested in the following cases:

(a) stiffeners affected by supports with different deflection characteristics

(b) stiffeners subject to large bending moments transferred from adjacent structures at sup- ports

(B) Calculation procedure

(a) The calculations are to reflect the structural response of the 2 or 3 dimensional structure considered. Calculations based on elastic beam element models or finite element analyses may normally be applied, with due attention to:

- boundary conditions

- shear area and moment of inertia variations

- effective flange

- effects of bending, shear and axial deformation

- influence of end brackets

(b) Plastic deformation is to be taken into account for the end parts of stiffeners attached on the watertight bulkheads.

(C) Loads

(a) The local lateral loads are to be taken as specified in Pt 3, Ch 3, Sec. 4 to Sec 8.

(b) For double bottom and other cofferdam type structures, a cofferdam bending moment and a shear force inducing stiffener bending moment may have to be considered at the same time.

(D) Allowable stresses

(a) Allowable stress level for stiffeners is given in Table 3.1.

(b) Stiffeners are in no case to have web and flange thickness less than given in Pt 3, Ch 3, 601.

Table 3.1 Allowable Stresses for Stiffeners

(4) Girders

(A) General

(a) For girders in 2- or 3-dimensional structural system, a complete structural analysis amy have to be carried out to demonstrate that the stresses are acceptable when the structure is loaded as described in (C).

(b) Calculations may have to be carried out for:

- bottom structures

- side structures

- deck structures

- bulkhead structures

- transverse frame structures in monohull craft supporting deckhouses, containers and other permanent or cargo masses subject to tripping

- strength of deck along wide hatches

- other structures when deemed necessary by the Society

(B) Calculation methods

(a) Calculation methods or computer programs applied are to take account the effects of bending, shear, axial and torsional deformations. The calculations are to reflect the struc- tural response of the 2- or 3-dimensional structure considered, with due attention to boundary conditions. For systems consisting of slender girders, calculations based on beam theory (frame work analysis) may be applied, with due attention to:

- shear area variation

- moment of inertia variation

- effective flange

(b) For rise of floor bottoms,

shear in the bottom plating will resist vertical deflection of

the keel, eith a releasing effect on the longitudinal girder, which may be taken into account.

(c)

For deep girders, bulkhead

panels, bracket zones, etc. where results obtained by applying

the beam theory are unreliable, finite element analysis or equivalent methods are to be applied.

(C) Design load conditions

(a)

The calculations are to be based on the loads at design level as given in Pt 3, Ch 2.

(b) For sea-going conditions realistic combinations of external and internal dynamic loads and inertia forces are to be considered.

(c)

The mass of deck structures may be neglected when less than 5 % of the applied loads are in the vertical direction.

(D) Allowable stresses

(a) The equivalent stress is defined as:

: normal stress in x-direction

: normal stress in y-direction

: Shear stress in the xy-plane

(b) For girders in general, the following stresses given in Table 3.2 are normally acceptable.

(c)

For girders subjected to hull girder stresses, the following additional requirement applies.

N mm

plus maximum allowable longitudinal stress according to Pt 3, Ch 3, Sec 403.

or maximum allowable transverse stress according to Pt 3, Ch 3, Sec 406.

(d) The actual longitudinal or transverse stress in the girder is taken from the calculation in

Pt 3, Ch 3.

(E) Allowable deflections

(a) Requirements for minimum moment of inertia or maximum deflection are to be consid- ered for hatchways, doors or special cases.

(b) Designers are to pay due attention to deflections in general.

(c)

For weather deck hatch coamings, the horizontal deflection at weather tightening level should not exceed 25 mm , unless tightness at a greater deflection may be proved. For

weathertight and watertight hatches and doors, the relative deflection of cover and hull coamings in the pressure direction should not result in leakage due to loss of packing pressure.

(d) Deflection limits of girders and coamings of covers and doors

Pt 3, Ch 4 in terms of a moment of inertia requirement.

themselves are found in

Table 3.2 Allowable Stresses for Girders

2. Direct strength calculation of aluminium alloy ships

(1) General

(A) Application

The application of direct strength analysis is governed by:

(a) In such cases where simplified formulations are not able to take into account special

stress distributions, boundary conditions or structural arrangements with sufficient accu- racy, direct strength analysis may be required.

(b) In some cases direct strength calculations may give reduced scantlings, especially when

optimization routines are incorporated.

(2) Plating

(A) Normally direct strength analysis of laterally loaded plating is not required as part of rule scantling estimation.

(B) Laterally loaded local plate fields may be subject to direct stress analysis applying general

3-dimensional plate theory or finite element calculations. The calculations should take ac- count the boundary conditions of the plate field as well as membrane stresses developed

during deflection of the plate.

(C) Allowable stresses

(a) When

stress

combining the calculated local bending stress with in-plane stresses the equivalent

in the middle of a local plate field is not to exceed 240 N m m The lo-

cal bending stress in the same point is in no case to exceed 160 N mm

sum of local bending stress and in-plane stresses in the -direction

: sum of local bending stress and in-plane stresses in the -

direction shear stress in the -plane

(b) The final thickness is not, however, to be less than the minimum thickness structure in question.

for the

Guidance Relating to the Rules for the Classification of High Speed and Light Crafts 2015 17

(3) Stiffeners

(A) General

Direct strength analysis of stiffeners may be requested in the following cases:

(a) stiffeners affected by supports with different deflection characteristics

(b) stiffeners subject to large bending moments transferred from adjacent structures at sup- ports

(B) Calculation procedure

(a) The calculations are to reflect the structural response of the 2 or 3 dimensional structure considered. Calculations based on elastic beam element models or finite element analyses may normally be applied, with due attention to:

- boundary conditions

- shear area and moment of inertia variations

- effective flange

- effects of bending, shear and axial deformation

- influence of end brackets

(b) Plastic deformation is to be taken into account for the end parts of stiffeners attached on the watertight bulkheads.

(C) Loads

The local lateral loads are to be taken as specified in Pt 3, Ch 2.

(D) Allowable stresses

Allowable stress level for stiffeners is given in Table 3.3.

Table 3.3 Allowable Stresses for Stiffeners

(4) Girders

(A) General

(a) For girders in 2- or 3-dimensional structural system, a complete structural analysis amy have to be carried out to demonstrate that the stresses are acceptable when the structure

is loaded as described in (C).

(b) Calculations may have to be carried out for:

- bottom structures

- side structures

- deck structures

- bulkhead structures

- transverse frame structures

- other structures when deemed necessary by the Society

(B) Calculation methods

(a) Calculation methods or computer programs applied are to take account the effects of bending, shear, axial and torsional deformations. The calculations are to reflect the struc-

tural response of the 2- or 3-dimensional structure considered, with due attention to boundary conditions. For systems consisting of slender girders, calculations based on

beam theory (frame work analysis) may be applied, with due attention to:

- shear area variation

- moment of inertia variation

- effective flange

(b) For deep girders, bulkhead panels, bracket zones, etc. where results obtained by applying the beam theory are unreliable, finite element analysis or equivalent methods are to be

applied.

(C) Design load conditions

(a) The calculations are to be based on the loads at design level as given in Pt 3, Ch 2.

For sea-going conditions realistic combinations of external and internal dynamic loads and inertia forces are to be considered. The mass of deck structures may be neglected when less than 5% of the applied loads are in the vertical direction.

18 Guidance Relating to the Rules for the Classification of High Speed and Light Crafts 2015

(b) For transverse web frame analysis, the following combinations of load apply.

- sea pressure on all elements

- slamming pressure on bottom

(c) If twin hull, the following three conditions are to be added:

- slamming pressure on bottom from outside and sea pressure on hull outer side

- slamming pressure on bottom from inside and sea pressure on tunnel side tunnel top

- slamming pressure on tunnel top and sea pressure on tunnel side and bottom from in- side

- For all load cases, deck load pressure from cargo, passengers etc. is to be added.

(D) Allowable stresses

(a) The equivalent stress is defined as:

normal stress in x-direction

: normal stress in y-direction

Shear stress in the xy-plane

(b) The longitudinal combined stress taken as the sum of hull girder and longitudinal bot- tom, side or deck girder bending stresses, is normally not to exceed 190 N mm .

(c)

For girders in general, the following stresses given in Table 3.4 are normally acceptable.

Table 3.4 Allowable Stresses for Girders

3. Direct strength calculation of FRP ships

(1) General

(A) Direct calculation using the full stiffness and strength properties of the laminates in all di- rections will be accepted based on the criteria given below.

(a)

Laminates are dimensioned in accordance with the Tsai-Wu composite strength criterion.

(b) The failure strength ratio, , for a ply in the Tsai-Wu failure criterion is expressed as:

, =

(c)

Where ≤ 1 indicates ply failure.

The terms in the failure criterion are defined in the notes in Table 3.5

All relevant load combinations for the laminate panel are to be considered.

(2) Allowable stress and deflections

(A) For direct calculations in accordance with (1) (A) (b), the failure strength ratio, , is not

to be less than the values given in Table 3.5. Core shear stresses in sandwich panels shall be in accordance with Pt 3, Ch 5, Sec 5. Panel deflections shall not be greater than

in Pt 3, Ch 5, Sec. 5 and Sec 6.

Guidance Relating to the Rules for the Classification of High Speed and Light Crafts 2015 19

Table 3.5 Failure Strength Ratio,

20 Guidance Relating to the Rules for the Classification of High Speed and Light Crafts 2015