Section 2 External blast
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Naval Ships, January 2023 - Volume 1 Ship Structures - Part 4 Military Design and Special Features - Chapter 2 Military Load Specification - Section 2 External blast

Section 2 External blast

2.1 General

2.1.1 Structures and their response to air blast loadings, can be considered to fall into two categories:

  • Diffraction-type structures.
  • Drag-type structures.

2.1.2 In a nuclear type explosion, the diffraction-type structures would be affected mainly by diffraction loading and the drag-type structures by drag loading.

2.1.3 Large flat sided structures, with few openings, will respond mainly to diffraction loading because it will take an appreciable time for the blast wave to engulf the structure and the pressure differential between front and rear exists during the whole of this period. A diffraction-type structure is primarily sensitive to the peak over-pressure in the shock wave to which it is exposed.

2.1.4 If structures are small, or have numerous openings, the pressures on different areas of the structure are quickly equalised; the diffraction forces operate only for a very short time. The response of this type of structure is then mainly due to the dynamic pressure (or drag forces) of the blast wind. This is typical of masts and funnels. The drag loading on the structure is determined not only by the dynamic pressure but also by the shape of the structure. The drag coefficient is less for rounded or streamlined structures than for irregular or sharp edged structures.

2.1.5 The relative importance of each type of loading in causing damage will depend upon the type of structure as well as the characteristics of the blast wave.

2.2 Threat level determination

2.2.1 Ships complying with the requirements of this Section will be eligible for the notation EB1, EB2, EB3 or EB4 as defined in Vol 1, Pt 4, Ch 2, 2.3 Notation assessment levels and methodology.

2.2.2 External blast loading can come from a variety of threats the two main ones are far field from nuclear or fuel air type threats and near field from detonation by close in weapon systems. This part of the Rules is concerned only with the far field explosions.

2.2.3 The actual threat level used in the calculation of performance and the areas of the ship to be protected by this design method are to be specified by the Owner and will remain confidential to LR.

2.3 Notation assessment levels and methodology

2.3.1 Design to withstand increasing levels of blast pressure needs to employ increasing sophistication and complexity of analysis method if the structure is to be kept lightweight.

2.3.2 An EB1 assessment method may utilise the simple design methodology suggested in Vol 1, Pt 4, Ch 2, 2.8 Conventional explosive pressure loads for structural assessment. The design criteria should ensure that the structure behaves in an elastic perfectly plastic manner with small displacements when subjected to the proposed blast level.

2.3.3 An EB2 assessment method may utilise an extension of simple design methodology suggested in Vol 1, Pt 4, Ch 2, 2.8 Conventional explosive pressure loads to look at the elasto-plastic behaviour for the structural assessment. The structure is to be designed such that maximum displacements experienced by all structure does not compromise the structural integrity, water or gas-tight integrity or functioning of critical items of equipment required for operation of the ship and systems that is attached or adjacent to the structure.

2.3.4 An EB3 assessment method should employ a failure criterion based on an elasto-plastic methodology which considers the following structural responses:

  • Local response of the plating, here the plating can be represented as a 2D plate strip and a large displacement, elasto-plastic dynamic response analysis carried out using a beam-column approach.
  • Local bending response of stiffened panels, the preferred model will be to evaluate the non-linear dynamic response of a single stiffener with an attached strip of plating modelled as a beam-column with the appropriate boundary conditions under blast pressure.
  • A lumped parameter model can be employed to look at ‘overall sidesway’ response of a ships superstructure.

The structure is to be designed such that maximum displacements experienced by all structure does not compromise the structural integrity, water or gas-tight integrity or functioning of critical items of equipment required for operation of the ship and systems that are attached or adjacent to the structure.

2.3.5 An EB4 assessment method should employ a full non-linear analysis using finite element methods to predict the structural response. Using this methodology it is assumed that the ship must survive, this implies the need to retain primary hull structural integrity, water and gas-tight integrity or functioning of critical items of equipment required for operation of the ship and systems that is attached or adjacent to the structure.

2.3.6 For EB3 and EB4 notations, the assumptions made for initial deformations are to be submitted. Where these differ for normal ship building practice, the details are to be recorded on the approved plan.

2.4 Definitions

2.4.1 Atmospheric pressure P o is to be taken as 101,3 kN/m2.

2.4.2 The dimensions of superstructure blocks are given in Figure 2.2.1 Superstructure definitions.

Figure 2.2.1 Superstructure definitions

2.5 Blast pressure loads

2.5.1 For explosions of different magnitude, the range at which the peak blast incident and dynamic pressures occur can be scaled using the following equation.

where

D i = incident distance
D n = the distance at which the pressure occurs, in metres
W = the equivalent weight of TNT for the explosive, in kg.

2.5.2 Similarly for weapons of a different magnitude, the duration tp+ of a blast can be scaled using the scaling equation

where

t i = incident duration
t n = duration the pressure occurs, in seconds
W = the equivalent weight of TNT for the explosive, in kg.

2.5.3 When a pressure shock front strikes a solid surface placed normal to the direction of shock travel there is an instantaneous rise in pressure above that of the shock front itself. The total pressure referred to as the reflected pressure is given by:

when P i << P o (small charge at large stand off) P r may be taken as 2Pi similarly when P i >>P o (large charge at short range) P r may be taken as 8P i

where P i = peak blast incident over-pressure in kN/m2 from Figure 2.2.2 Blast parameters for TNT and nuclear explosions

Figure 2.2.2 Blast parameters for TNT and nuclear explosions

2.5.4 The reflected pressure, P r can be assumed to diminish linearly until it reaches the stagnation pressure P s at time t s where

where
d = is the lesser of h or ⋉/2 in metres, see Figure 2.2.1 Superstructure definitions
U = shock front velocity in m/s
=
U o = speed of sound in air in m/s
= 332+0,6T o
T o = ambient air temperature in °C.

2.5.5 The passage of the blast is immediately followed by a transient ‘blast wind’ that exerts a supplementary dynamic pressure which is given by:

where
P i = peak blast incident over-pressure in kN/m2 from Figure 2.2.2 Blast parameters for TNT and nuclear explosions

The duration of the dynamic pressure, t q+, can be determined from Figure 2.2.2 Blast parameters for TNT and nuclear explosions.

2.5.6 The stagnation pressure, P s, is determined for the front of the superstructure block by

and for the top, sides and rear by

where
P i = peak incident pressure from Figure 2.2.2 Blast parameters for TNT and nuclear explosions
q i = the dynamic pressure from Vol 1, Pt 4, Ch 2, 2.5 Blast pressure loads 2.5.5
C D = the drag coefficient of the structure from Table 2.2.1 Drag coefficients.

Table 2.2.1 Drag coefficients

Structure Drag coefficient, C D
Ship sides +1,0
Front face +1,0
 
Top and sides  
0–170 kN/m2 +0,4
170–340 kN/m2 +0,3
340–930 kN/m2 +0,2
   
Masts and funnels +0,75

2.5.7 For the top and sides of the superstructure the peak pressure will occur at time t t which is given by:

where
b = superstructure breadth in m, see Figure 2.2.1 Superstructure definitions
U = shock front velocity in m/s, see Vol 1, Pt 4, Ch 2, 2.5 Blast pressure loads 2.5.4.

2.5.8 For the rear of the superstructure the peak pressure will occur at time t r which is given by:

where
d = is the lesser of h or ⋉/2, in metres, see Figure 2.2.1 Superstructure definitions
b = superstructure breadth, in metres, see Figure 2.2.1 Superstructure definitions
U = shock front velocity, in m/s.

2.5.9 Pressure distributions for the faces of the superstructure block are given in Figure 2.2.3 Pressure distribution, together with the overall pressure acting on the block which is obtained by subtracting the forces on the rear face from those on the front.

Figure 2.2.3 Pressure distribution

2.6 Nuclear threats

2.6.1 An atmospheric nuclear explosion is most likely to occur at some height above ground level at a location known as ground zero, which may be optimised to produce maximum damage effects. The blast wave is reflected from the surface and at a certain distance from ground zero, primary reflected waves combine to form a vertical ‘mach’ front or stem that propagates outwards from ground zero with diminishing intensity. The peak blast incident over pressure P i can be determined from Vol 1, Pt 4, Ch 2, 2.3 Notation assessment levels and methodology.

2.7 Fuel air pressure loads

2.7.1 In general a structure designed to resist a moderate degree of nuclear blast will also have a reasonable resistance to fuel air threats and calculations is not normally required.

2.7.2 Where there is a risk of fuel air explosions, and for ships for which there is no nuclear threat position required, consideration needs to be given to the blast wave characteristics of such explosions, see also Vol 1, Pt 4, Ch 2, 3.1 General.

2.7.3 The effects of temperature on the material of the structure due to fuel air threats are to be considered using the structure surface temperature.

2.8 Conventional explosive pressure loads

2.8.1 Blast waves caused by free field explosions in air are dependent upon the mass shape and type of explosive, the distance from the target and the height of the burst. As blast waves travel through air, rapid variations occur in pressure, density, temperature and particle velocity.

2.8.2 For a given high explosive of an equivalent TNT mass at a direct distance from the target Section Vol 1, Pt 4, Ch 2, 2.3 Notation assessment levels and methodology can be used to determine the blast parameters.

2.9 Structural assessment

2.9.1 The rules for the EB1 and EB2 structural assessment are based on the assumption that the structure can be idealised as a single degree of freedom system. They assume that there is no significant loading on the superstructure or ship’s sides at the time of the blast. In cases where there are significant lateral loadings or concentrated point loads or fluids, the natural frequency and strength of the structure will be specially considered.

2.9.2 The acceptance criteria based contained in this section assume that the structure is loaded beyond its elastic limit but not such that significant deformations result.

2.9.3 For plating the thickness is not to be less than:

where
l = the length of the plate panel, in metres
s = width of the panel, in mm (short span length)
σo = yield stress of the material, N/mm2
f p = plate aspect ratio factor, see Table 2.2.2 Plate factors
fσ = stress factor
= 1,3 for σo ≤ 300 N/mm2
= 1,2 for σo > 300 N/mm2
P p = the peak pressure, P r, for the front of the superstructure, or P s for the top sides and rear, as defined in Vol 1, Pt 4, Ch 2, 2.5 Blast pressure loads, in KN/m
f DLF = dynamic load factor to be determined from Vol 1, Pt 6, Ch 2, 5 Dynamic loading:
= for superstructure front and ship sides using a linearly decreasing load with initially:
= t 1 = P r t s/P s seconds
= if t m determined from Vol 1, Pt 6, Ch 2, 5 Dynamic loading is greater than 1,1 P r t s/P s then f DLF, is to be recalculated such that:
=
= For superstructure top, sides and rear using a triangular load with:
= t 1 = 2t t seconds
= t 1 = 2t r seconds as appropriate.
= where
Pr = peak reflected pressure as defined in Vol 1, Pt 4, Ch 2, 2.5 Blast pressure loads 2.5.3
Ps = stagnation pressure, as defined in Vol 1, Pt 4, Ch 2, 2.5 Blast pressure loads 2.5.6
tm = time at which maximum deflection occurs
tp+ = positive blast pulse duration
ts = corresponding time at stagnation pressure, P s.

Table 2.2.2 Plate factors

Aspect ratio (A R) f p
1,0 1000
0,9 916
0,8 858
0,7 817
0,6 775
<0,5 750

2.9.4 The minimum edge through thickness area of the plate is not to be less than:

where
= t, σo, l, s are given in Vol 1, Pt 4, Ch 2, 2.9 Structural assessment 2.9.3
τo = shear yield stress in N/mm2
P tm = Pressure at the time of maximum displacement, t m, in kN/m2 based on assumed pressure distribution.
f p1,f p2 = shear load factors, given in Table 2.2.3 Plate shear factors.

Table 2.2.3 Plate shear factors

Aspect ratios/ short span sides long span side
  f p1 f p2 f p1 f p2
1,0 0,18 0,07 0,18 0,07
0,9 0,16 0,06 0,20 0,08
0,8 0,14 0,06 0,22 0,08
0,7 0,13 0,05 0,24 0,08
0,6 0,11 0,04 0,26 0,09
0,5 0,09 0,04 0,28 0,09

2.9.5  The stiffener and plate combination is considered to be satisfactory if the plastic modulus of the beam plate combination is greater than:

where

P p, f DLF, f σ and σ o are given in Vol 1, Pt 4, Ch 2, 2.9 Structural assessment 2.9.3

Z p = plastic section modulus of the stiffener and attached plate, in cm3
e = effective length of the beam, in metres
= the length of the beam, in metres
fbz = beam support factor
= 12 for fully fixed
= 8 for simply supported
s = spacing of the beams, in mm.

2.9.6 The maximum elastic deflection given by:

is not to be greater than

where

P p, fDLF, s, l and l e are given in Vol 1, Pt 4, Ch 2, 2.9 Structural assessment 2.9.3

I = second moment of inertia cm4
fbd = beam support factor
= 384 for fully fixed
= 76,8 for simply supported.

2.9.7 The shear area of the stiffener web is not to be less than:

where
= Z p, σo, le, l, s are given in Vol 1, Pt 4, Ch 2, 2.9 Structural assessment 2.9.3
= τo, P tm are given in Vol 1, Pt 4, Ch 2, 2.9 Structural assessment 2.9.4
= f s 1, f s 2 = shear load factors, given in Table 2.2.4 Beam shear factors.
= f b z is given in Vol 1, Pt 4, Ch 2, 2.9 Structural assessment 2.9.5.

Table 2.2.4 Beam shear factors

Beam type Location f s1 f s2
Simply supported Both ends 0,39 0,11
Fixed ends Both ends 0,36 0,14
Simple and fixed Fixed end 0,43 0,12
  Simple support 0,26 0,19

2.9.8 Direct calculations or analyses based on the elastoplastic or plastic response of structure using a dynamic load factor or finite element approach will be specially considered. The designers’ calculations are to be submitted for approval.

2.9.9 In addition to the assessment of plating and stiffeners, the global capability of superstructure and above water structure are to be assessed. The designers' calculations are to be submitted.

2.10 Design considerations

2.10.1 To minimise the effects of external blast, protrusions from the superstructure are to be kept to a minimum.

2.10.2 Re-entrant corners are to be avoided, where this is impractical they are to be covered by a blast deflecting plate, or be constructed such that the included angle between orthogonal faces is to be as large as possible.

2.10.3 Where the clear air gap between superstructure blocks is less than 0,1L R, the interaction under external blast loading will be specially considered.


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