S21

S21(1997)(cont)(Rev.1
2002)(Rev.2
Nov 2002)(Rev.3
April 2003)(Rev.4
July 2004)(Corr.1
Oct 2004)(Rev.5
May 2010)(Rev.6
Jan 2023
Complete
Revision)

Requirements concerning
STRENGTH OF SHIPS
Evaluation of Scantlings of Hatch Covers andHatch Coamings and Closing Arrangements ofCargo Holds of Ships
TABLE OF CONTENTS
1
APPLICATION AND DEFINITIONS .......................................................................... 4
1.1 Application ......................................................................................................................... 4
1.2 Definitions.......................................................................................................................... 4
1.2.1 Hatch cover types ........................................................................................................ 4
1.2.2 Positions ..................................................................................................................... 5
1.3 Material .............................................................................................................................. 5
1.4 General requirements ......................................................................................................... 5
1.5 Net scantling approach ....................................................................................................... 52
HATCH COVER AND COAMING LOAD MODEL ................................................. 6
2.1 Vertical weather design load .............................................................................................. 6
2.2 Horizontal weather design load ......................................................................................... 8
2.2.1 General horizontal weather design load ..................................................................... 8
2.2.2 Horizontal weather design load applicable to coamings of Type-2 ships ............... 10
2.3 Cargo loads ...................................................................................................................... 11
2.3.1 Distributed loads ...................................................................................................... 11
2.3.2 Point loads ................................................................................................................ 11
2.4 Container loads ................................................................................................................ 11
2.4.1 General ..................................................................................................................... 11
2.4.2 Corner loads for ship in upright condition ............................................................... 11
2.4.3 Corner loads for ship in heel condition .................................................................... 12
2.4.4 Load cases with partial loading................................................................................ 13
2.4.5 Mixed stowage of 20’ and 40’ containers on hatch cover ........................................ 14
2.5 Loads due to elastic deformations of the ship's hull ......................................................... 14
3
HATCH COVER STRENGTH CRITERIA ............................................................... 15
3.1 Permissible stresses and deflections ................................................................................ 15
3.1.1 Yield strength ........................................................................................................... 15
Page 1 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

3.1.2 Deflection ................................................................................................................. 16
3.2 Local net plate thickness .................................................................................................. 16
3.2.1 Local net plate thickness of hatch covers for wheel loading ................................... 17
3.2.2 Lower plating of double skin hatch covers and box girders .................................... 17
3.3 Net scantling of stiffeners ................................................................................................ 17
3.4 Net scantling of primary supporting members................................................................. 19
3.4.1 Primary supporting members ................................................................................... 19
3.4.2 Edge girders (Skirt plates) ....................................................................................... 19
3.5 Strength calculations ........................................................................................................ 19
3.5.1 General requirements for FEM calculations ............................................................ 20
3.6 Buckling strength of hatch cover structures ..................................................................... 20
3.6.1 General ...................................................................................................................... 20
3.6.2 Slenderness requirements.......................................................................................... 21
3.6.3 Buckling requirements .............................................................................................. 21
4
DETAILS OF HATCH COVERS ................................................................................ 24
4.1 Container foundations on hatch covers ............................................................................ 24
4.2 Weather tightness ............................................................................................................. 24
4.2.1 Packing material (General) ...................................................................................... 24
4.2.2 Dispensation of weather tight gaskets...................................................................... 24
4.2.3 Drainage arrangements ............................................................................................ 25
5
HATCH COAMING STRENGTH AND LOCAL DETAILS................................... 25
5.1 Local net plate thickness of coamings ............................................................................. 25
5.2 Net scantling of stiffeners of coamings ........................................................................... 26
5.3 Coaming stays .................................................................................................................. 27
5.3.1 Coaming stay section modulus and web thickness .................................................. 27
5.3.2 Coaming stays under friction load ........................................................................... 28
5.4 Further requirements for hatch coamings ........................................................................ 28
5.4.1 Longitudinal strength ............................................................................................... 28
5.4.2 Local details ............................................................................................................. 29
5.4.3 Stays ......................................................................................................................... 29
5.4.4 Extend of coaming plates ......................................................................................... 29
5.4.5 Drainage arrangement at the coaming ..................................................................... 29
6
CLOSING ARRANGEMENTS ................................................................................... 30
6.1 Securing devices .............................................................................................................. 30
6.1.1 General ..................................................................................................................... 30
6.1.2 Rod cleats ................................................................................................................. 30

Page 2 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

6.1.3 Hydraulic cleats ....................................................................................................... 30
6.1.4 Cross-sectional area of the securing devices ........................................................... 30
6.1.5 Anti lifting devices ................................................................................................... 31
6.2 Hatch cover supports, stoppers and supporting structures ............................................... 31
6.2.1 Horizontal mass forces ............................................................................................. 31
6.2.2 Hatch cover supports................................................................................................ 32
6.2.3 Hatch cover stoppers ................................................................................................ 33
7
CORROSION ADDITION AND STEEL RENEWAL .............................................. 34
7.1 Corrosion addition for hatch covers and hatch coamings ................................................ 34
7.2 Steel renewal .................................................................................................................... 35

Notes:
1. The “contracted for construction” date means the date on which the contract to build thevessel is signed between the prospective owner and the shipbuilder. For further details
regarding the date of “contract for construction”, refer to IACS Procedural Requirement (PR)No. 29.
2. Rev. 3 of this UR applies to ships contracted for construction on or after 1 January 2004.
3. Till Rev. 5 of this UR, it only applies to Type-2 ships as defined in 1.1.
4. Rev.6 of this UR applies to both Type-1 and Type-2 ships as defined in 1.1 which arecontracted for construction on or after 1 July 2024. UR S21A originally applicable to Type-1
ships is superseded by this UR since 1 July 2024.
Page 3 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

1

Application and definitions

1.1

Application

These requirements apply to all ships except CSR bulk carriers, and are for all cargo hatch
covers and coamings on exposed decks.
As specified in this UR, parts of the requirements are for some specific ship types as
categorized below:
 Type-1 ships, including all ships except bulk carriers, self-unloading bulk carriers, ore
carriers and combination carriers, as defined in UR Z11.
 Type-2 ships, including all bulk carriers, self-unloading bulk carriers, ore carriers andcombination carriers, as defined in UR Z11.
The strength requirements are applicable to hatch covers and hatch coamings of stiffened plate
construction and its closing arrangements.
This UR is applicable to hatch covers and coamings made of steel. In case of alternative
materials and innovative designs the approval is subject to the individual class society.
This UR does not apply to portable covers secured weathertight by tarpaulins and battening
devices, or pontoon covers, as defined in ICLL Regulation 15.
Hatch covers and hatch coamings of fishing vessels are to comply with the requirements of theindividual class society.
These requirements are in addition to the requirements of the ICLL.
1.2

Definitions

ICLL Where ICLL is referred to in the text, this is to be taken as the International Convention
on Load Lines, 1966 as amended by the 1988 protocol, as amended in 2003.
1.2.1

Hatch cover types• Single skin cover

A hatch cover made of steel or equivalent material that is designed to comply withICLL Regulation 16. The cover has continuous top and side plating, but is open
underneath with the stiffening structure exposed. The cover is weathertight and fittedwith gaskets and clamping devices unless such fittings are specifically excluded.
• Double skin cover

A hatch cover as above but with continuous bottom plating such that all the stiffening
structure and internals are protected from the environment.
• Pontoon type cover

Page 4 of 36

IACS Req. 1997/Rev.6 2023

S21

A special type of portable cover, secured weathertight by tarpaulins and battening
devices. Such covers are to be designed in accordance with ICLL Regulation 15 andare not covered by this UR.

S21(cont)

Clarification note:
Modern hatch cover designs of lift-away-covers (also called lift-on/lift-off, or just simply
LoLo covers) are in many cases called pontoon covers. This definition does not fit to thedefinition above. Modern lift-away hatch cover designs should belong to one of the twocategories-single skin covers or double skin cover.
1.2.2

Positions

The hatchways are classified according to their position as follows:
Position 1

Upon exposed freeboard and raised quarterdecks, and upon exposed
superstructure decks situated forward of a point located a quarter of ship’s
length from forward perpendicular.

Position 2

Upon exposed superstructure decks situated abaft a quarter of the ship’s lengthfrom the forward perpendicular and located at least one standard height of thesuperstructure above the freeboard deck.
Upon exposed superstructure decks situated forward of a point located a quarterof the ship’s length from the forward perpendicular and located at least two
standard height of the superstructure above the freeboard deck.

1.3

Material

Hatch covers and coamings are to be made of material in accordance with the definitions of
UR S6. Material class I is to be applied for top plate, bottom plate and primary supporting
members.
1.4

General requirements

Primary supporting members and stiffeners of hatch covers are to be continuous over thebreadth and length of hatch covers, as far as practical. When this is impractical, sniped end
connections are not to be used and appropriate arrangements are to be adopted to provide
sufficient load carrying capacity.
Generally, the spacing of primary supporting members parallel to the direction of stiffeners is
not to exceed 1/3 of the span of primary supporting members. If sufficient strength based on
FE analysis can be verified, this requirement may be waived.
Stiffeners of hatch coamings are to be continuous over the breadth and length of hatchcoamings, as far as practical.
1.5

Net scantling approach

Unless otherwise quoted, the thicknesses t of the following sections are net thicknesses.
Page 5 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)The net thicknesses are the member thicknesses necessary to obtain the minimum net
scantlings required by 3 and 5.
The required gross thicknesses are obtained by adding corrosion additions, tc, given in Tab.8
in 7.1.
Strength calculations using FEM are to be performed with net scantlings.
2

Hatch cover and coaming load model

Structural assessment of hatch covers and hatch coamings is to be carried out using the design
loads, defined in this chapter.
Definitions
L
LLL
x
Dmin

= length of ship, in m, as defined in UR S2= freeboard length of ship, in m, as defined in ICLL Regulation 3= longitudinal coordinate of midpoint of assessed structural member measured fromaft end of length L or LLL, as applicable= the least moulded depth, in m, as defined in ICLL Regulation 3ℎ𝑁𝑁= standard superstructure height in m

2.1

Vertical weather design load

= 1,05 + 0,01𝐿𝐿𝐿𝐿𝐿𝐿 , 1,8 ≤ ℎ𝑁𝑁 ≤ 2,3

The pressure PHC, in kN/m², on the hatch cover panels is given by ICLL. This may be takenfrom Tab.1. The vertical weather design load needs not to be combined with cargo loads
according to 2.3 and 2.4.
In Fig.1 the positions 1 and 2 are illustrated for an example ship.
Where an increased freeboard is assigned, the design load for hatch covers according to Tab.1
on the actual freeboard deck may be as required for a superstructure deck, provided thesummer freeboard is such that the resulting draught will not be greater than that corresponding
to the minimum freeboard calculated from an assumed freeboard deck situated at a distance atleast equal to the standard superstructure height hN below the actual freeboard deck, see Fig.2.
For hatch covers of cargo holds designed for carriage of ballast or liquid cargo, the internallateral pressures are also to be considered according to the Rules of individual societies.

Page 6 of 36

IACS Req. 1997/Rev.6 2023

S21

Tab.1 Design load PHC of weather deck hatches

S21(cont)

Design load PHC [kN/m2]
Position𝑥𝑥≤ 0.75
𝐿𝐿𝐿𝐿𝐿𝐿0.75 <

for 24 m ≤ LLL ≤ 100 m

9.81(1.5𝐿𝐿𝐿𝐿𝐿𝐿 + 116)
76
for LLL > 100 m

on freeboard deck
9.81
𝑥𝑥
�(4.28𝐿𝐿𝐿𝐿𝐿𝐿 + 28)− 1.71𝐿𝐿𝐿𝐿𝐿𝐿 + 95�
76
𝐿𝐿𝐿𝐿𝐿𝐿

upon exposed superstructure decks located at least one superstructure
standard height above the freeboard deck
9.81
(1.5𝐿𝐿𝐿𝐿𝐿𝐿 + 116)
76
on freeboard deck for type B ships according to ICLL
𝑥𝑥
9.81 �(0.0296 ⋅ 𝐿𝐿1 + 3.04) ⋅− 0.0222 ⋅ 𝐿𝐿1 + 1.22�
𝐿𝐿𝐿𝐿𝐿𝐿1

9.81× 3.5

for 24 m ≤ LLL ≤ 100 m

2

𝑥𝑥
≤ 1.0
𝐿𝐿𝐿𝐿𝐿𝐿on freeboard deck for ships with less freeboard than type B
according to ICLL𝑥𝑥
− 0.1089𝐿𝐿1 + 9.89�
9.81 �(0.1452𝐿𝐿1 − 8.52)
𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿1 = 𝐿𝐿𝐿𝐿𝐿𝐿 but not more than 340 m

upon exposed superstructure decks located at least one superstructure
standard height above the freeboard deck
9.81 × 3.5
9.81(1.1𝐿𝐿𝐿𝐿𝐿𝐿 + 87.6)
76

for LLL > 100 m

9.81 × 2.6

upon exposed superstructure decks located at least one superstructure standard height above thelowest Position 2 deck
9.81 × 2.1

Page 7 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

Freeboard Deck

2**
2
1

1

2**

2**
1

2
1*

1*

0.85Dmin
0.25 LLL
Length LLL* reduced load upon exposed superstructure decks located at least one
superstructure standard height above the freeboard deck** reduced load upon exposed superstructure decks of vessels with LLL > 100 m
located at least one superstructure standard height above the lowest Position 2
deck
Fig.1 Positions 1 and 2

Actual Freeboard Deck
Assumed Freeboard Deck

2**
2**
2

2

2**

2**
1*

2**
2

2

≥ hN

0.25 LLL

0.85DminLength LLL* reduced load upon exposed superstructure decks located at least one superstructure
standard height above the freeboard deck** reduced load upon exposed superstructure decks of vessels with LLL > 100 m
located at least one superstructure standard height above the lowest Position 2 deck
Fig. 2 Positions 1 and 2 for an increased freeboard
2.2

Horizontal weather design load

2.2.1

General horizontal weather design load

The horizontal weather design load 𝑃𝑃𝐴𝐴 , in kN/m², for determining the scantlings of outer edge
girders (skirt plates) of weather deck hatch covers and of hatch coamings is:Page 8 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)Cw=

𝑃𝑃𝐴𝐴 = 𝑓𝑓𝑛𝑛 𝑓𝑓𝑐𝑐 (𝑓𝑓𝑏𝑏 𝑐𝑐𝐿𝐿 𝐶𝐶𝑤𝑤 − 𝑧𝑧)

L
+ 4.1
25

for L < 90 m
1.5

300 − L 
= 10.75 −  100 = 10.75

1.5

L − 350 = 10.75 − 
 150 

cL

=

L
90

= 20 +

L1
12

for unprotected front coamings and hatch cover skirt plates.

= 10 +

L1
12

for unprotected front coamings and hatch cover skirt plates, where

L1
15
L
x'
= 7 + 1 −8
100
L
= 5+

fb

for 350 m ≤ L ≤ 500 m

for L ≥ 90 m

= 5+

L1

for 300 m ≤ L < 350 m

for L < 90 m

=1
fn

for 90 m ≤ L < 300 m

L1
x'
−4
100
L

the distance from the actual freeboard deck to the summer load line
exceeds the minimum non-corrected tabular freeboard according to
ICLL by at least one standard superstructure height hN.
for side and protected front coamings and hatch cover skirt plates.
for aft ends of coamings and aft hatch cover skirt plates abaft
amidships.
for aft ends of coamings and aft hatch cover skirt plates forward ofamidships.
= L, need not be taken greater than 300 m.
2
 x' − 0.45 x'< 0.45= 1.0 +  L
for

+
C
0.2
L
 B



2

 x'

 L − 0.45 x'≥ 0.45= 1.0 + 1.5 ⋅  forL CB + 0.2 
0.6 ≤ CB ≤ 0.8 , when determining scantlings of aft ends of coamings and aft hatch cover skirt
plates forward of amidships, CB need not be taken less than 0.8.
x'
= distance in m between the transverse coaming or hatch cover skirt plate considered
and aft end of the length L. When determining side coamings or side hatch coverskirt plates, the side is to be subdivided into parts of approximately equal length, notexceeding 0.15L each, and x ' is to be taken as the distance between aft end of thelength L and the centre of each part considered.
Page 9 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)z= vertical distance in m from the summer load line to the midpoint of stiffener span,or to the middle of the plate field.

fc= 0.3 + 0.7b'= breadth of coaming in m at the position considered.

B'= actual maximum breadth of ship in m on the exposed weather deck at the position
considered.

b'B'b'/B' is not to be taken less than 0.25.
The design load PA is not to be taken less than the minimum values PA-min given in Tab.2.
Tab.2 Minimum design load PA-min
L
≤ 50
> 50
< 250
≥ 250

2.2.2

PA-min in kN/m² forunprotected fronts

elsewhere

30

15

25 +

𝐿𝐿
10

50

12.5 +
25

𝐿𝐿
20

Horizontal weather design load applicable to coamings of Type-2 ships

The pressure 𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 , in kN/m2, on the No. 1 forward transverse hatch coaming is given by:
𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 220, when a forecastle is fitted in accordance with UR S28= 290 in the other cases
The pressure 𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 , in kN/m2, on the other coamings is given by:

𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 220

Note:
The horizontal weather design loads 𝑃𝑃𝐴𝐴 and 𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 need not be included in the direct strength
calculation of the hatch cover, unless it is utilized for the design of substructures of horizontal
support according to 6.2.3.

Page 10 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

2.3

Cargo loads

2.3.1

Distributed loads

The load on hatch covers due to distributed cargo loads PL , in kN/m², resulting from heave andpitch (i.e. ship in upright condition) is to be determined according to the following formula:
𝑃𝑃𝐿𝐿 = 𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 (1 + 𝑎𝑎𝑉𝑉 )where:

PCargo = uniform cargo load in kN/m²
𝑎𝑎𝑉𝑉= vertical acceleration addition as follows:v
F = 0.11 0Lm

𝑎𝑎𝑉𝑉 = 𝐹𝐹 ∙ 𝑚𝑚

x
≤ 0.2
L
x
= 1.0
for 0.2 < ≤ 0.7
Lx
m +1  x= 1+ 0− 0.7  for 0.7 < ≤ 1.0
L
0.3  L
= m0 − 5 (m0 − 1)xL

for 0 ≤

m0

= 1.5 + F

v0

= maximum speed at summer load line draught, v0 is not to be taken less than
knots

2.3.2

L in

Point loads

The load P, in kN, due to a concentrated force 𝑃𝑃𝑆𝑆 , in kN, except for container load, resulting
from heave and pitch (i.e. ship in upright condition) is to be determined as follows:
𝑃𝑃 = 𝑃𝑃𝑆𝑆 (1 + 𝑎𝑎𝑉𝑉 )2.4

Container loads

2.4.1

GeneralThe loads defined in 2.4.2 to 2.4.4 are to be applied where containers are stowed on the hatch
cover.
2.4.2

Corner loads for ship in upright condition

The load P in kN, applied at each corner of a container stack, and resulting from heave andpitch (i.e. ship in upright condition) is to be determined as follows:
𝑃𝑃 = 9.81

𝑀𝑀(1 + 𝑎𝑎𝑉𝑉 )4Page 11 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

where:
𝑎𝑎𝑉𝑉= acceleration addition according to 2.3.1.
𝑀𝑀= maximum designed mass of container stack in t.
2.4.3

Corner loads for ship in heel condition

The loads, in kN, applied at each corner of a container stack, and resulting from heave, pitch,and the ship's rolling motion (i.e. ship in heel condition) are to be determined as follows, (seealso Fig.3):
𝑀𝑀ℎ𝑚𝑚(1 + 𝑎𝑎𝑉𝑉 ) �0.45 − 0.42 �
2
𝑏𝑏
𝑀𝑀
ℎ𝑚𝑚
𝐵𝐵𝑧𝑧 = 9.81 (1 + 𝑎𝑎𝑉𝑉 ) �0.45 + 0.42 �2𝑏𝑏
𝐵𝐵𝑦𝑦 = 2.4𝑀𝑀
𝐴𝐴𝑧𝑧 = 9.81

where:
𝑎𝑎𝑉𝑉= acceleration addition according to 2.3.1.
𝑀𝑀= maximum designed mass of container stack in t.
ℎ𝑚𝑚

zi
Wi
b
Az, BzBy= ∑ Wi= designed height of centre of gravity of stack above hatch cover top in m,
may be calculated as weighted mean value of the stack, where the centre of gravityof each tier is assumed to be located at the centre of each container,= ∑ ( zi ⋅ Wi ) M= distance from hatch cover top to the centre of i-th container in m.
= weight of i-th container in t
= distance between midpoints of foot points in m.
= support forces in z-direction at the forward and aft stack corners.
= support force in y-direction at the forward and aft stack corners.

Values of AZ and BZ applied for the assessment of hatch cover strength are to be shown in thedrawings of the hatch covers.
Note:
It is recommended that container loads as calculated above are considered as limit for footpoint loads of container stacks in the calculations of cargo securing (container lashing).

Page 12 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

Fig.3 Forces due to container loads
2.4.4

Load cases with partial loading

The load cases defined in 2.4.2 and 2.4.3 are also to be considered for partial nonhomogeneous loading which may occur in practice, e.g. where specified container stack places
are empty. For each hatch cover, the heel directions, as shown in Tab. 3, are to be considered.
The load case partial loading of container hatch covers can be evaluated using a simplified
approach, where the hatch cover is loaded without the outermost stacks that are locatedcompletely on the hatch cover. If there are additional stacks that are supported partially by thehatch cover and partially by container stanchions then the loads from these stacks are also to
be neglected, refer to Tab.3. Partial loading of container hatch covers.
In addition, the case where only the stack places supported partially by the hatch cover andpartially by container stanchions are left empty is to be assessed in order to consider the
maximum loads in the vertical hatch cover supports.
It may be necessary to also consider partial load cases where more or different container stackplaces are left empty. Therefore, a classification society may require that additional partial
load cases be considered.

Page 13 of 36

IACS Req. 1997/Rev.6 2023

S21

Tab.3 Partial loading of container hatch covers

S21(cont)

Heel direction

Hatch covers supported by
the longitudinal hatchcoaming with all container
stacks located completelyon the hatch cover

Hatch covers supported by
the longitudinal hatchcoaming with the outermost
container stack supported
partially by the hatch coverand partially by container
stanchions

Hatch covers not supportedby the longitudinal hatchcoaming (center hatch
covers)2.4.5

Mixed stowage of 20’ and 40’ containers on hatch coverIn the case of mixed stowage (20'+40' container combined stack), the foot point forces at thefore and aft end of the hatch cover are not to be higher than resulting from the design stackweight for 40’ containers, and the foot point forces at the middle of the cover are not to behigher than resulting from the design stack weight for 20’ containers.
2.5

Loads due to elastic deformations of the ship's hull

Hatch covers, which in addition to the loads according to 2.1 to 2.4 are loaded in the ship'stransverse direction by forces due to elastic deformations of the ship's hull, are to be designed
such that the sum of stresses does not exceed the permissible values given in 3.1.1.

Page 14 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

3

Hatch cover strength criteria

3.1

Permissible stresses and deflections

3.1.1

Yield strength

All hatch cover structural members are to comply with the following formulae
𝜎𝜎𝑣𝑣𝑣𝑣 ≤ 𝜎𝜎𝑎𝑎 for shell elements in general.

𝜎𝜎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 ≤ 𝜎𝜎𝑎𝑎 for rod or beam elements in general.
where:𝜎𝜎𝑎𝑎: Allowable stress as defined in Tab. 4.

ReH: Specified minimum yield stress, in N/mm2, of the material.

𝜎𝜎𝑣𝑣𝑣𝑣: Von Mises stress, in N/mm², to be taken as follows:

𝜎𝜎𝑥𝑥

𝜎𝜎𝑦𝑦

2
𝜎𝜎𝑣𝑣𝑣𝑣 = �𝜎𝜎𝑥𝑥 2 − 𝜎𝜎𝑥𝑥 𝜎𝜎𝑦𝑦 + 𝜎𝜎𝑦𝑦 2 + 3𝜏𝜏𝑥𝑥𝑥𝑥

: Normal stress, in N/mm2, in x-direction.
: Normal stress, in N/mm2, in y-direction.

𝜏𝜏𝑥𝑥𝑥𝑥 : Shear stress, in N/mm2, in the x-y plane.

𝜎𝜎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 : Axial stress in rod or beam elements, in N/mm2.

Indices x and y are coordinates of a two-dimensional Cartesian system in the plane of theconsidered structural element.
In case of finite element(FE) calculations using shell (or plate) elements, the stresses are to beread from the centre of the individual element. It is to be observed that, in particular, at flangesof unsymmetrical girders, the evaluation of stress from element centre may lead to nonconservative results. Thus, a sufficiently fine mesh is to be applied in these cases or, the stressat the element edges shall not exceed the allowable stress. Where shell elements are used, thestresses are to be evaluated at the mid plane of the element.
For steels with a minimum yield stress of more than 355 N/mm2, the value of ReH to be applied
throughout this requirement is subject to the individual classification society but is not to be
more than the minimum yield stress of the material.
Stress concentrations are to be assessed to the satisfaction of the individual classificationsociety.
Tab. 4 Allowable stresses
Members ofSubject toPage 15 of 36

𝜎𝜎𝑎𝑎 , in N/mm2
IACS Req. 1997/Rev.6 2023

S21

S21

Hatch cover
structure(cont)

3.1.2

External pressure, as defined in
2.1

0.80 ReH

Other loads, as defined in 2.2 to
2.5

0.90 ReH for static+dynamic loadcase
0.72 ReH for static load case

Deflection

The vertical deflection of primary supporting members due to the vertical weather design load
according to 2.1 is to be not more than 0.0056lg , where lg is the greatest span of primary
supporting members.
Note:
Where hatch covers are arranged for carrying containers and mixed stowage is allowed, i.e.,a 40'-container stowed on top of two 20'-containers, particular attention should be paid to thedeflections of hatch covers. Further the possible contact of deflected hatch covers within holdcargo has to be observed.
3.2

Local net plate thickness

The local net plate thickness t, in mm, of the hatch cover top plating is not to be less than:𝑡𝑡 = 0.0158𝐹𝐹𝑝𝑝 𝑠𝑠�

𝑃𝑃
0.95𝑅𝑅𝑒𝑒𝑒𝑒

and to be not less than 1% of the spacing of the stiffener or 6 mm if that be greater.
𝐹𝐹𝑝𝑝= factor for combined membrane and bending response= 1.5 in general𝜎𝜎
𝜎𝜎= 1.9 𝜎𝜎 , for 𝜎𝜎 ≥ 0.8 for the attached plate flange of primary supporting members𝑎𝑎𝑎𝑎

where:
𝑠𝑠= stiffener spacing in mm
𝑃𝑃= pressure PHC and PL, in kN/m2, as defined in 2.
𝜎𝜎= maximum normal stress, in N/mm2, of hatch cover top plating, determined according
to Fig.4
𝜎𝜎𝑎𝑎= as defined in Tab. 4
For flange plates under compression sufficient buckling strength according to 3.6 is to be
demonstrated.

Page 16 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

Fig.4 Determination of normal stress of the hatch cover plating
3.2.1

Local net plate thickness of hatch covers for wheel loading

The local net plate thickness of hatch covers for wheel loading have to be derived from theindividual classification society’s Rules.
3.2.2

Lower plating of double skin hatch covers and box girders

The thickness to fulfill the strength requirements is to be obtained from the calculationaccording to 3.5 under consideration of permissible stresses according to 3.1.1. When thelower plating is taken into account as a strength member of the hatch cover, the net thickness,in mm, of lower plating is to be taken not less than 5 mm. When project cargo is intended tobe carried on a hatch cover, the net thickness must not be less than:𝑡𝑡 = 6.5𝑠𝑠 × 10−3 mm

where:s= stiffener spacing in mm
Note:
Project cargo means especially large or bulky cargo lashed to the hatch cover. Examples areparts of cranes or wind power stations, turbines, etc. Cargoes that can be considered as
uniformly distributed over the hatch cover, e.g., timber, pipes or steel coils need not to beconsidered as project cargo.
When the lower plating is not considered as a strength member of the hatch cover, thethickness of the lower plating should be determined according to the individual class society’s
rules.
3.3

Net scantling of stiffeners

The net section modulus Z and net shear area 𝐴𝐴𝑠𝑠ℎ𝑟𝑟 of uniformly loaded hatch cover stiffeners
constraint at both ends must not be less than:
Page 17 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

𝑃𝑃𝑃𝑃𝑙𝑙2

𝑍𝑍 = 𝑓𝑓

𝑏𝑏𝑏𝑏 𝜎𝜎𝑎𝑎, in cm³,

𝐴𝐴𝑠𝑠ℎ𝑟𝑟 =8.7𝑃𝑃𝑃𝑃𝑃𝑃
10−3 , in cm²,
𝜎𝜎𝑎𝑎s

= stiffener spacing in mm

Pfbc= pressure PHC and PL, in kN/m2, as defined in 2.
= boundary coefficient of stiffener, taken equal to:
fbc = 8, in the case of stiffener simply supported at both ends or simply supported at
one end and clamped at the other end
fbc = 12, in the case of stiffener clamped at both ends.
= allowable stress as defined in Tab.4

where:
l= stiffener span, in m, to be taken as the spacing, in m, of primary supporting
members or the distance between a primary supporting member and the edge
support, as applicable. When brackets are fitted at both ends of all stiffener spans,the secondary stiffener span may be reduced by an amount equal to 2/3 of theminimum brackets arm length, but not greater than 10% of the unsupported span,for each bracket

𝜎𝜎𝑎𝑎

For stiffeners of lower plating of double skin hatch covers, requirements mentioned above are
not applied if there are no lateral loads. For double skin hatch covers of holds designed forballast or liquid cargo, the stiffeners on lower plating are to be strengthened according toindividual class society rules.
The net thickness, in mm, of the stiffener (except U-type/trapeze stiffeners) web is to be taken
not less than 4 mm.
The net section modulus of the stiffeners is to be determined based on an attached plate widthassumed equal to the stiffener spacing.
Stiffeners parallel to primary supporting must be continuous at crossing primary supporting
member and may be regarded for calculating the cross-sectional properties of primary
supporting members. It is to be verified that the combined stress of those stiffeners induced by
the bending of primary supporting members and lateral pressures does not exceed thepermissible stresses according to 3.1.1. The requirements of this paragraph are not applied tostiffeners of lower plating of double skin hatch covers if the lower plating is not considered asstrength member.
For hatch cover stiffeners under compression sufficient safety against lateral and torsional
buckling according to 3.6.3 is to be verified.
For hatch covers subject to wheel loading or point loads stiffener scantlings are to be
determined under consideration of the permissible stresses according to 3.1.1 or are to be
determined according to the individual class society’s rules.

Page 18 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

3.4

Net scantling of primary supporting members

3.4.1

Primary supporting members

Scantlings of primary supporting members are obtained from calculations according to 3.5
under consideration of permissible stresses according to 3.1.1.
For all components of primary supporting members sufficient safety against buckling must be
verified according to 3.6.
The net thickness, in mm, of webs of primary supporting members shall not be less than:𝑡𝑡 = 6.5𝑠𝑠 × 10−3 in mm
t min = 5 mm
where:
s = stiffener spacing in mm
3.4.2

Edge girders (Skirt plates)

Scantlings of edge girders are obtained from the calculations according to 3.5 underconsideration of permissible stresses according to 3.1.1.
The net thickness, in mm, of the outer edge girders exposed to wash of sea shall not be lessthan the largest of the following values:
𝑡𝑡 = 0.0158𝑠𝑠�

𝑃𝑃𝐴𝐴
0.95𝑅𝑅𝑒𝑒𝑒𝑒

𝑡𝑡 = 8.5𝑠𝑠 × 10−3 in mm
t min = 5 mm

where:𝑃𝑃𝐴𝐴= horizontal pressure as defined in 2.2.1
𝑠𝑠= stiffener spacing in mm

For the required moment of inertia of edge girders, refer to 6.1.4.
3.5

Strength calculations

The stresses in hatch covers are to be determined by FE analysis.
The stress calculation model in this section is to be used for both yielding and bucklingstrength assessments in accordance with 3.1 and 3.6, respectively.
The net scantlings as defined in 1.5 are to be used.

Page 19 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

3.5.1

General requirements for FEM calculations

For the strength assessments of hatch covers by means of FE analysis, the hatch covergeometry shall be idealized as realistically as possible. In no case shall element width be larger
than stiffener spacing. In way of force transfer points and cutouts the mesh is to be refinedwhere applicable. The ratio of element length to width shall not exceed 3.
The element size along the height of webs of primary supporting member is not to exceed onethird of the web height. Stiffeners, which support plates subjected to lateral pressure loads, areto be included in the FE model idealization. Stiffeners may be modelled by using beam
elements, or shell/plate elements. Buckling stiffeners may be disregarded for the stresscalculation.
Hatch covers fitted with U-type stiffeners as shown in Fig. 5 are to be assessed by means ofFE analysis.The geometry of the U-type stiffeners is to be accurately modelled usingshell/plate elements. Nodal points are to be properly placed on the intersections between thewebs of a U-type stiffener and the hatch cover plate, and between the webs and flange of the
U-type stiffener.

Fig.5: Example of hatch cover fitted with U-type stiffeners

Wherever applicable the following boundary conditions are to be applied to the FE model:Boundary nodes in way of a bearing pad on the hatch coamings are to be fixed against
displacement in the direction perpendicular to the pad.
Lifting stoppers are to be fixed against displacements in the direction determined by
the stoppers.
For a folding type hatch cover, the FE nodes connected through a hinge are to havethe same translational displacement in the direction perpendicular to the hatch cover
top plating.

3.6

Buckling strength of hatch cover structures

3.6.1

General

Buckling strength of all hatch cover structures is to be checked. Buckling assessments are tobe performed in compliance with the requirements in UR S35 for the conditions specified in
3.6.2 and 3.6.3.
The net scantlings as defined in 1.5 are to be used for buckling check.

Page 20 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

3.6.2

Slenderness requirements

The slenderness requirements are to be in accordance with those specified in UR S35/Sec 2.
The slenderness requirements need not be applied to the lower boundary of double skin hatch
covers unless the cargo hold is designed for carriage of ballast or liquid cargo.
The breadth of the primary supporting member flange is to be not less than 40% of their depth
for laterally unsupported spans greater than 3.0 m. Tripping brackets attached to the flange
may be considered as a lateral support for primary supporting members.
3.6.3

Buckling requirements

3.6.3.1 Application

These requirements apply to the buckling assessment of hatch cover structures subjected tocompressive and shear stresses and lateral pressures. The buckling assessment is to be
performed for the following structural elements:

Stiffened and unstiffened panels, including curved panels and panels stiffened with Utype stiffeners.

Web panels of primary supporting members in way of openings.

The buckling strength assessment of coaming parts is to be done according to the individualSociety’s rules.
For rule application, the panel types and assessment methods, the applied lateral pressure andstresses, safety factors and buckling check criteria are defined in 3.6.3.2, 3.6.3.3, 3.6.3.4 and3.6.3.5, respectively. The procedure and detailed requirements for buckling assessment are
given in UR S35/Sec4, including idealization of irregular plate panels, definition of reference
stresses and buckling criteria.
Unless otherwise specified, the symbols used in 3.6.3 are defined in UR S35.
3.6.3.2 Panel types and assessment methods

The plate panel of a hatch cover structure is to be modelled as stiffened panel (SP) or
unstiffened panel (UP) as defined in UR S35/Sec 4, [2]. Assessment Method A (-A) andMethod B (-B) as defined in UR S35/Sec 1, [3] are to be used in accordance with Tab. 5, Fig.
6 and Fig. 7. For a web panel with opening, the procedure for opening should be used for its
buckling assessment.
For a hatch cover fitted with U-type stiffeners, the additional buckling assessment
requirements specific for panels with U-type stiffeners in UR S35/Sec 5, [2.5] are also to befollowed.

Page 21 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

Tab.5 Structural members and assessment methods
Structural elements

Assessment
method(1)(2)

Normal panel definition

Hatch cover top/bottom plating structures, see Fig. 6
Hatch cover top/bottom plating
Irregularly stiffened panels

SP-A

Length: between transverse girders
Width: between longitudinal girders

UP-B

Plate between local stiffeners/PSM

Hatch cover web panels of primary supporting members, see Fig. 7
Web of transverse/longitudinal
girder (single skin type)

UP-B

Plate between local stiffeners/face plate/PSM

Web of transverse/longitudinal
girder (double skin type)SP-B(3)

Length: between PSM
Width: full web depth

Web panel with opening

Procedure for
opening

Plate between local stiffeners/face plate/PSM

Irregularly stiffened panels

UP-B

Plate between local stiffeners/face plate/PSM

Note 1: SP and UP stand for stiffened and unstiffened panel respectively.
Note 2: A and B stand for Method A and Method B respectively.
Note 3: In case that the buckling carlings/brackets are irregularly arranged in the web of transverse/longitudinal girder, UP-B method may be
used.

SP-A

Fig.6 Hatch cover top/bottom plating structures

Page 22 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

UP-B

Fig.7 Hatch cover webs of primary supporting members
3.6.3.3 Applied lateral pressure and stresses

The buckling assessment of hatch covers is based on the lateral pressure as defined in S2.1 and
2.2, and stresses obtained from FE analysis, refer to 3.5.
3.6.3.4 Safety factors

For all hatch cover structural members, safety factor S=1.0 is to be applied to both of theplating and stiffener buckling capacity formulas as defined in UR S35/Sec5/2.2 and UR
S35/Sec5/2.3, respectively.
3.6.3.5 Buckling acceptance criteria

A structural member is considered to have an acceptable buckling strength if it satisfies thefollowing criterion:ηact ≤ ηallwhere:

ηact: Buckling utilisation factor based on the applied stress, as defined in UR S35 /Sec1[3.2.2] and UR S35/Sec4, and calculated per UR S35/Sec5.

ηall: Allowable buckling utilisation factor, taken as given in Tab. 6.
Tab.6 Allowable buckling utilisation factors
Structuralcomponent

Plates and stiffeners
Web of PSM

Subject toηall, Allowable buckling utilisation
factorExternal pressure, as defined in
2.1

0.80

Other loads, as defined in 2.2 to
2.5

0.90 for static+dynamic load case
0.72 for static load case

Page 23 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

4

Details of hatch covers

4.1

Container foundations on hatch covers

Container foundations are to be designed to the satisfaction of the individual class society. The
substructures of container foundations are to be designed for cargo and container loads
according to 2, applying the permissible stresses according to 3.1.1.
4.2

Weather tightness

Further to the following requirements IACS Rec. 14 is applicable to hatch covers.
4.2.1

Packing material (General)

The packing material is to be suitable for all expected service conditions of the ship and is tobe compatible with the cargoes to be transported. The packing material is to be selected withregard to dimensions and elasticity in such a way that expected deformations can be carried.
Forces are to be carried by the steel structure only.
The packings are to be compressed so as to give the necessary tightness effect for all expected
operating conditions. Special consideration shall be given to the packing arrangement in ships
with large relative movements between hatch covers and coamings or between hatch coversections. The specification or grade of the packing material is to be indicated on the drawings.
4.2.2

Dispensation of weather tight gaskets

For hatch covers of cargo holds solely for the transport of containers, upon request by theowners and subject to compliance with the following conditions the fitting of weather tightgaskets according to 4.2.1 may be dispensed with:• The hatchway coamings shall be not less than 600 mm in height.
• The exposed deck on which the hatch covers are located is situated above a depth H(x).

H(x) is to be shown to comply with the following criteria:

H ( x ) ≥ T fb + f b + h in m

T fb = draught, in m, corresponding to the assigned summer load linef b = minimum required freeboard, in m, determined in accordance with ICLL Reg. 28
as modified by further regulations as applicablex≤ 0.75h = 4.6 m forLLLx> 0,75= 6.9 m for
LLL• Labyrinths, gutter bars or equivalents are to be fitted proximate to the edges of eachpanel in way of the coamings. The clear profile of these openings is to be kept as small
as possible.
• Where a hatch is covered by several hatch cover panels the clear opening of the gap in

between the panels shall be not wider than 50mm.

Page 24 of 36

IACS Req. 1997/Rev.6 2023

S21
• The labyrinths and gaps between hatch cover panels shall be considered as unprotected

S21

openings with respect to the requirements of intact and damage stability calculations.

(cont)• With regard to drainage of cargo holds and the necessary fire-fighting system reference

is made to the sections Piping Systems, Valves and Pumps and Fire Protection and Fire
Extinguishing Equipment of the individual classification society’s rules.

• Bilge alarms should be provided in each hold fitted with non-weathertight covers.
• Furthermore, Chapter 3 of IMO MSC/Circ. 1087 is to be referred to concerning thestowage and segregation of containers containing dangerous goods.

4.2.3

Drainage arrangements

Cross-joints of multi-panel covers are to be provided with efficient drainage arrangements.
5

Hatch coaming strength and local details

5.1

Local net plate thickness of coamings

The net thickness of weather deck hatch coamings shall not be less than the larger of thefollowing values:(1) For Type-1 ships:
𝑃𝑃

𝑡𝑡 = 0.0142𝑠𝑠�0.95𝑅𝑅𝐴𝐴 in mm

t min = 6 +

L1
mm
100

𝑒𝑒𝑒𝑒

(2) For Type-2 ships:
𝑃𝑃

𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
𝑡𝑡 = 0.016𝑠𝑠�0.95𝑅𝑅
in mm,

𝑡𝑡𝑚𝑚𝑚𝑚𝑚𝑚 = 9.5 mm

𝑒𝑒𝑒𝑒

where:

PA = pressure, in kN/m2, as defined in S21.2.2.1
Pcoam = pressure, in kN/m2, as defined in S21.2.2.2
sL1= stiffener spacing in mm= L, need not be taken greater than 300 m

In addition, for both Type-1 and Type-2 ships, longitudinal strength aspects are to beobserved.

Page 25 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

5.2

Net scantling of stiffeners of coamings

The stiffeners must be continuous at the coaming stays. For stiffeners with both ends
constraint the elastic net section modulus Z in cm3 and net shear area 𝐴𝐴𝑠𝑠ℎ𝑟𝑟 in cm2, calculatedon the basis of net thickness, must not be less than:(1) For Type-1 ships:
𝑍𝑍 =𝑃𝑃𝐴𝐴 𝑠𝑠𝑙𝑙 2
𝑓𝑓𝑏𝑏𝑏𝑏 𝑅𝑅𝑒𝑒𝑒𝑒

𝐴𝐴𝑠𝑠ℎ𝑟𝑟 =𝑃𝑃𝐴𝐴 𝑠𝑠𝑠𝑠 −2
10
𝑅𝑅𝑒𝑒𝑒𝑒where:
𝑓𝑓𝑏𝑏𝑏𝑏 = 12 in general= 8 for the end spans of stiffeners sniped at the coaming corners𝑙𝑙= stiffener span, in m, to be taken as the spacing of coaming stays
𝑠𝑠= stiffener spacing in mm

Note that for sniped stiffeners of coaming at hatch corners shear area at the fixed support has
to be increased by 35%. The gross thickness of the coaming plate at the sniped stiffener end
shall not be less than:𝑃𝑃 𝑠𝑠(𝑙𝑙−0.0005𝑠𝑠)

𝑡𝑡𝑔𝑔𝑔𝑔 = 19.6� 𝐴𝐴 1000𝑅𝑅

𝑒𝑒𝑒𝑒

in mm

(2) For Type-2 ships:
𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑠𝑠𝑙𝑙 2
𝑍𝑍 = 1.21
𝑓𝑓𝑏𝑏𝑏𝑏 𝑐𝑐𝑝𝑝 𝑅𝑅𝑒𝑒𝑒𝑒

where:

𝑓𝑓𝑏𝑏𝑏𝑏 = 16 in general= 12 for the end spans of stiffeners sniped at the coaming corners𝑙𝑙s= span, in m, of stiffeners= spacing, in mm, of stiffeners

𝑃𝑃𝐴𝐴 = pressure in kN/m2 as defined in 2.2.1

𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = pressure in kN/m2 as defined in 2.2.2

𝑐𝑐𝑝𝑝 = ratio of the plastic section modulus to the elastic section modulus of the stiffeners withan attached plate breadth, in mm, equal to 40 t, where t is the plate net thickness= 1.16 in the absence of more precise evaluation

Page 26 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)In addition, for both Type-1 and Type-2 ships, horizontal stiffeners on hatch coamings, which
are part of the longitudinal hull structure, are to be designed according to the individualclassification society’s rules.
5.3

Coaming stays

Coaming stays are to be designed for the loads transmitted through them and permissible
stresses according to 3.1.1.
5.3.1

Coaming stay section modulus and web thickness

At the connection with deck, the net section modulus Z, in cm3, and the net thickness tw, in
mm, of the coaming stays designed as beams with flange (examples 1 and 2 are shown in
Fig.8) are to be taken not less than:𝑍𝑍 =𝑃𝑃𝑠𝑠𝑐𝑐 𝐻𝐻𝑐𝑐2

1.9𝑅𝑅𝑒𝑒𝑒𝑒

2𝑃𝑃𝑃𝑃 𝐻𝐻in cm³𝑡𝑡𝑤𝑤 = ℎ𝑅𝑅𝑐𝑐 𝑐𝑐
where:

𝑒𝑒𝑒𝑒

in mm

HC = stay height, in m
sc = stay spacing, in mmh= stay depth, in mm, at the connection with the deck

P = pressure on coaming, in kN/m2, taken as PA defined in 2.2.1 in general and as 𝑃𝑃𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶
defined in 2.2.2 for Type-2 ships.
For other designs of coaming stays, such as those shown in Fig. 8, examples 3 and 4, thestresses are to be determined through FEM. The calculated stresses are to comply with thepermissible stresses according to 3.1.1.

Coaming stays are to be supported by appropriate substructures. For calculating the section
modulus of coaming stays, their face plate area is to be taken into account only when it iswelded with full penetration welds to the deck plating and adequate underdeck structure isfitted to support the stresses transmitted by it.
Webs are to be connected to the deck by fillet welds on both sides with a throat thickness of
a = 0.44tw .
For Type-2 ships, toes of stay webs are to be connected to the deck plating with full or partial
penetration double bevel welds extending over a distance not less than 15% of the stay width.
For other ship types the size of welding for toes of webs at the lower end of coaming stays
should be according to the individual class society’s Rules.

Page 27 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

Example 1

Example 3

Example 2

Example 4

Fig.8 Examples of coaming stays
5.3.2

Coaming stays under friction load

For coaming stays, which transfer friction forces at hatch cover supports, fatigue strength is tobe considered according to individual class society’s rules, refer to 6.2.2.
5.4

Further requirements for hatch coamings

5.4.1

Longitudinal strength

Hatch coamings which are part of the longitudinal hull structure are to be designed according
to the requirements for longitudinal strength of the individual classification society.
For structural members welded to coamings and for cutouts in the top of coamings sufficientfatigue strength is to be verified.
Longitudinal hatch coamings with a length exceeding 0.1L m are to be provided with tapered
brackets or equivalent transitions and a corresponding substructure at both ends. At the end of
the brackets they are to be connected to the deck by full penetration welds of minimum 300
mm in length.

Page 28 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

5.4.2

Local details

If the design of local details is not regulated in 5, local details are to comply with theindividual classification society’s requirement for the purpose of transferring the loads on thehatch covers to the hatch coamings and, through them, to the deck structures below. Hatch
coamings and supporting structures are to be adequately stiffened to accommodate the loadingfrom hatch covers, in longitudinal, transverse and vertical directions.
Structures under deck are to be checked against the load transmitted by the stays.
Unless otherwise stated, weld connections and materials are to be dimensioned and selected inaccordance with the individual classification society’s requirements.
5.4.3

Stays

On ships carrying cargo on deck, such as timber, coal or coke, the stays are to be spaced not
more than 1,5 m apart.
5.4.4

Extend of coaming plates

Coaming plates are to extend to the lower edge of the deck beams or hatch side girders are to
be fitted that extend to the lower edge of the deck beams. Extended coaming plates and hatchside girders are to be flanged or fitted with face bars or half-round bars. Fig. 9 gives anexample.

Fig.9 Example for a hatch side girder
5.4.5

Drainage arrangement at the coaming

If drain channels are provided inside the line of gasket by means of a gutter bar or verticalextension of the hatch side and end coaming, drain openings are to be provided at appropriate
positions of the drain channels.
Drain openings in hatch coamings are to be arranged with sufficient distance to areas of stressconcentration (e.g. hatch corners, transitions to crane posts).
Drain openings are to be arranged at the ends of drain channels and are to be provided with
non-return valves to prevent ingress of water from outside. It is unacceptable to connect fire
hoses to the drain openings for this purpose.

Page 29 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

If a continuous outer steel contact between cover and ship structure is arranged, drainage fromthe space between the steel contact and the gasket is also to be provided for.
6

Closing arrangements

6.1

Securing devices

6.1.1

GeneralSecuring devices between cover and coaming and at cross-joints are to be installed to provide
weathertightness. Sufficient packing line pressure is to be maintained.
Securing devices must be appropriate to bridge displacements between cover and coaming due
to hull deformations.
Securing devices are to be of reliable construction and effectively attached to the hatchway
coamings, decks or covers. Individual securing devices on each cover are to haveapproximately the same stiffness characteristics.
Sufficient number of securing devices is to be provided at each side of the hatch coverconsidering the requirements of 3.4.2. This applies also to hatch covers consisting of several
parts.
The materials of stoppers, securing devices and their weldings are to be to the satisfaction theindividual class society. Specifications of the materials are to be shown in the drawings of thehatch covers.
6.1.2

Rod cleats

Where rod cleats are fitted, resilient washers or cushions are to be incorporated.
6.1.3

Hydraulic cleats

Where hydraulic cleating is adopted, a positive means is to be provided so that it remains
mechanically locked in the closed position in the event of failure of the hydraulic system.
6.1.4

Cross-sectional area of the securing devices

The gross cross-sectional area in cm2 of the securing devices is not to be less than:𝐴𝐴 = 0.28𝑞𝑞𝑆𝑆𝑆𝑆𝑆𝑆 𝑘𝑘𝑙𝑙

Correspondingly, the stiffness of edge girders is to be sufficient to maintain adequate sealingpressure between securing devices. The moment of inertia, in cm4, of edge girders is not to beless than:4
𝐼𝐼 = 6𝑞𝑞𝑆𝑆𝑆𝑆𝑆𝑆

where:q= packing line pressure in N/mm, minimum 5 N/mm.
SSD = spacing between securing devices in m, not to be taken less than 2 m.

Page 30 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

235 𝑒𝑒�𝑘𝑘𝑙𝑙 = �𝑅𝑅𝑒𝑒𝑒𝑒ReH is the minimum yield strength of the material in N/mm2, but is not to be taken greater than
0.7Rm, where Rm is the tensile strength of the material in N/mm2.
e= 0.75 for 𝑅𝑅𝑒𝑒𝑒𝑒 > 235 N/mm²= 1.00 for 𝑅𝑅𝑒𝑒𝑒𝑒 ≤ 235 N/mm²

Rods or bolts are to have a gross diameter not less than 19 mm for hatchways exceeding 5 m²
in area.
Securing devices of special design in which significant bending or shear stresses occur may bedesigned as anti-lifting devices according to 6.1.5. The packing line pressure q is to be
specified, and as load, q multiplied by the spacing between securing devices SSD is to be
applied.
6.1.5

Anti lifting devices

The securing devices of hatch covers, on which cargo is to be lashed, are to be designed forthe lifting forces resulting from loads according to 2.4, refer Fig.10. Unsymmetrical loadings,
which may occur in practice, are to be considered. Under these loadings the equivalent stressin the securing devices is not to exceed:
150
𝜎𝜎𝑣𝑣𝑣𝑣 = 𝑘𝑘 in N/mm²
𝑙𝑙

Note:
The partial load cases given in Tab. 3 may not cover all unsymmetrical loadings, critical for
hatch cover lifting.
Chapter 5.6 of IACS Rec. 14 should be referred to for the omission of anti-lifting devices.

Fig.10 Lifting forces at a hatch cover
6.2

Hatch cover supports, stoppers and supporting structures

6.2.1

Horizontal mass forces

For the design of hatch cover supports the horizontal mass forces Fh = m ⋅ a are to be
calculated with the following accelerations:Page 31 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

a X = 0.2 g

in longitudinal direction

aY = 0.5 g
in transverse directionm = Sum of mass of cargo lashed on the hatch cover and mass of hatch cover.
The accelerations in longitudinal direction and in transverse direction do not need to beconsidered as acting simultaneously.
6.2.2

Hatch cover supports

For the transmission of the support forces resulting from the load cases specified in 2 and of
the horizontal mass forces specified in 6.2.1, supports are to be provided which are to be
designed such that the nominal surface pressures in general do not exceed the following
values:𝑃𝑃𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 = 𝑑𝑑𝑃𝑃𝑛𝑛 in N/mm²d= 3.75 − 0.015L

d max

= 3.0

d min

= 1.0 in general= 2.0 for partial loading conditions, see 2.4.1

Pn= permissible nominal surface pressure, see Tab.7

For metallic supporting surfaces not subjected to relative displacements the nominal surfacepressure applies:
pn max = 3 pn in N/mm²
Note:
When the maker of vertical hatch cover support material can provide proof that the material issufficient for the increased surface pressure, not only statically but under dynamic conditions
including relative motion for adequate number of cycles, permissible nominal surfacepressure may be relaxed at the discretion of the individual classification society. However,realistic long-term distribution of spectra for vertical loads and relative horizontal motion
should be assumed and agreed with the individual classification society.
Drawings of the supports must be submitted. In the drawings of supports the permitted
maximum pressure given by the material manufacturer must be specified.

Page 32 of 36

IACS Req. 1997/Rev.6 2023

S21

Tab. 7 Permissible nominal surface pressure Pn

S21(cont)Pn [N/mm²] when loaded by
Support material
Vertical force

Horizontal force(on stoppers)

Hull structural steel

25

40

Hardened steel

3550

Lower friction materials

50–

Where large relative displacements of the supporting surfaces are to be expected, the use ofmaterial having low wear and frictional properties is recommended.
The substructures of the supports must be of such a design, that a uniform pressure
distribution is achieved.
Irrespective of the arrangement of stoppers, the supports must be able to transmit thefollowing force Ph in the longitudinal and transverse direction:

Ph = µ ⋅where:PvµPVd= vertical supporting force= frictional coefficient= 0.5 in general

For non-metallic, low-friction support materials on steel, the friction coefficient may be
reduced but not to be less than 0.35 and to the satisfaction of the individual class society.
Supports as well as the adjacent structures and substructures are to be designed such that thepermissible stresses according to 3.1.1 are not exceeded.
For substructures and adjacent structures of supports subjected to horizontal forces Ph, fatigue
strength is to be considered according to the individual classification society’s rules.
6.2.3

Hatch cover stoppers

Hatch covers shall be sufficiently secured against horizontal shifting. Stoppers are to beprovided for hatch covers on which cargo is carried.
The greater of the loads resulting from 2.2 and 6.2.1 is to be applied for the dimensioning of
the stoppers and their substructures.
The permissible stress in stoppers and their substructures, in the cover, and of the coamings is
to be determined according to 3.1.1. In addition, the provisions in 6.2.2 are to be observed.

Page 33 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)

Specifically for Type-2 ships, the following additional requirements are to be complied with:
Hatch covers are to be effectively secured, by means of stoppers, against the transverse forces
arising from a pressure of 175 kN/m2.
With the exclusion of No.1 hatch cover, hatch covers are to be effectively secured, by meansof stoppers, against the longitudinal forces acting on the forward end arising from a pressureof 175 kN/m2.
No. 1 hatch cover is to be effectively secured, by means of stoppers, against the longitudinal
forces acting on the forward end arising from a pressure of 230 kN/m2.
This pressure may be reduced to 175 kN/m2 when a forecastle is fitted in accordance with UR
S28.
The equivalent stress:
i. in stoppers and their supporting structures, andii. calculated in the throat of the stopper welds is not to exceed the allowable value of
0.8ReH.
7

Corrosion addition and steel renewal

7.1

Corrosion addition for hatch covers and hatch coamings

The scantling requirements of the above sections imply the following general corrosion
additions tc:

Page 34 of 36

IACS Req. 1997/Rev.6 2023

S21

Tab. 8 Corrosion additions tc for hatch covers and hatch coamings

S21(cont)

Application

Structure

Weather deck cargo hatchesof container ships, carcarriers, paper carriers,
passenger vessels

tc [mm]Hatch covers

1.0

Hatch coamings

according to individualclass society’s rules

Weather deck cargo hatches Hatch covers in generalof Type-2 ships
Top and bottom plating of double
skin hatch covers

2.0

Internal structure of double skinhatch covers

1.5

Hatch coamings and coaming stays

1.5

Hatch covers in general

2.0

Weather deck cargo hatchesof all other ship types
covered by this UR

7.2

Weather exposed plating and bottom
plating of double skin hatch covers
Internal structure of double skinhatch covers and closed box girders
Hatch coamings not part of thelongitudinal hull structure

2.0

1.5
1.0
1.5

Hatch coamings part of thelongitudinal hull structure

according to individualclass society’s rules

Coaming stays and stiffeners

1.5

Steel renewal

Steel renewal is required where the gauged thickness is less than tnet + 0.5 mm for•single skin hatch covers,•
the plating of double skin hatch covers, and•coaming structures the corrosion additions tc of which are provided in Tab. 8.
Where the gauged thickness is within the range tnet + 0.5 mm and tnet + 1.0 mm, coating(applied in accordance with the coating manufacturer’s requirements) or annual gauging may
be adopted as an alternative to steel renewal. Coating is to be maintained in GOOD condition,
as defined in UR Z10.2.1.2.
For the internal structure of double skin hatch covers, thickness gauging is required when
hatch cover top or bottom plating renewal is to be carried out or when this is deemednecessary, at the discretion of the individual class society’s surveyor, on the basis of theplating corrosion or deformation condition. In these cases, steel renewal for the internal
structures is required where the gauged thickness is less than tnet.
For corrosion addition tc = 1.0 mm the thickness for steel renewal is tnet and the thickness forcoating or annual gauging is when gauged thickness is between tnet and tnet + 0.5 mm.

Page 35 of 36

IACS Req. 1997/Rev.6 2023

S21

S21
(cont)For coaming structures, the corrosion additions tc of which are not provided in Tab. 8, steel
renewal and coating or annual gauging are to be in accordance with the individualclassification society’s requirements.
End ofDocument

Page 36 of 36

IACS Req. 1997/Rev.6 2023