Framing Systems & Components
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SHIP STRUCTURAL FRAMING SYSTEMS
& COMPONENTS
UNIT
1 Introduction
In addition to being the largest moving structures designed by man, the design of vessels differs
considerably from that of other structures for several reasons. Unlike many fixed land-based
structures to which known loads from wind, snow and ice, static loads, etc., can be applied with
is additional to the known loads but its magnitude is uncertain. Furthermore, the sea loading is
made even more complex by the motions of the vessel itself in response to the sea state. A
ical terms.
Most of the structure consists of broad expanses of plating, stiffened by a variety of structural
profiles, whereas other large structures contain little plating. Optimum distribution of material is
a matter of higher priority in vessel design than in many other structures.
An additional important requirement is that the structure must provide for the containment of
liquids within and the exclusion of water from without, besides having sufficient strength to
withstand the pressures imposed by these liquids. Furthermore, allowance must be made for the
imprecise yet significant way all the foregoing requirements are affected by the corrosive nature
of the marine environment in which the structure operates. A design meeting the requirements of
global and local loadings then has to be reconciled with the resistance considerations for the
immersed hull and aesthetic requirements.
2 Functions of Hull Structural Elements
The strength deck, bottom and side shell of a vessel act as a box girder in resisting bending and
other loads in addition to forming a watertight envelope to provide essential buoyancy. The
remaining structure contributes directly or indirectly to these functions by maintaining the
position and integrity of these main members and enabling their efficient function.
2.1 Bottom Plating (incl. inner bottom)
The bottom plating is a principal longitudinal member constituting the lower flange of the hull
girder and being part of the watertight envelope is subject to the local hydrostatic pressure. In
the forward region it must withstand the additional dynamic pressure associated with slamming.
When fitted, the inner bottom makes a significant contribution to the strength of the lower flange.
Inner and outer bottom plating, together with bottom girders and floors, function as a double-
plate panel to distribute secondary bending effects (caused by external hydrostatic, internal fluid
and cargo loads) to main supporting boundaries, i.e., bulkheads and side shell.
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2.2 Decks
One or more strength decks form the principal members of the hull girder upper flange and
usually the upper watertight boundary and may be subject to local water, cargo and equipment
loadings. Other decks, depending upon longitudinal extent, vertical distance from the
neutral axis, and their effective attachment, contribute to a lesser extent in resisting longitudinal
bending. Locally, internal decks are subject to the loads imposed by cargo, machinery, stores,
and liquid pressure if forming a tank boundary or barrier against progressive flooding.
2.3 Shell Plating
The side shell provides the webs for the main hull girder and is an important part of the
watertight envelope, being subject to static water pressures and dynamic loadings due to wave
action and vessel motion, particularly impact loadings (slamming, berthing and tug landings). In
the stern region, extra plate thickness is beneficial in way of rudder and shaft strut mountings and
stern tubes, for increased strength and panel stiffness and for the reduction of vibration. In ice-
capable vessels the ice belt plating is required to withstand ice loadings and abrasion.
2.4 Bulkheads
Bulkheads are one of the major components of internal structure. Their function in the hull
girder depends on their orientation and extent. Main transverse bulkheads act as internal
stiffening diaphragms for the girder and resist in-plane torsion (racking) loads but do not
contribute directly to longitudinal strength. Longitudinal bulkheads, if extending more than
about 10% of the hull length, do contribute to longitudinal strength and may be as effective as the
side shell itself.
Bulkheads generally serve other structural functions such as tank boundaries, deck support,
superstructures and major load-inducing installations (e.g., crane pedestals), and add rigidity to
reduce vibration. Transverse watertight bulkheads additionally provide subdivision to prevent
progressive flooding, and both transverse and longitudinal bulkheads provide fire integrity
forming divisions between fire zones.
2.5 Double Bottom Construction
Cargo vessels of gross tonnage 500 tons and greater, and passenger vessels (other than high-
speed light craft) require a double bottom construction, in most cases between collision and aft
peak bulkheads. The inner bottom, other than contributing to the strength of the lower flange of
the hull girder, provides improved watertight integrity and protection against flooding in the
event of bottom damage. The double bottom is given a cellular construction which enables the
enclosed volume(s) to be utilised for ballast and fuel storage.
Vertical plating connects the bottom shell and inner bottom. Those fitted transversely are called
floors and those fitted longitudinally are centre girders or side girders, as appropriate. These
vertical orthogonally arranged plates, if watertight, may form the boundaries of tanks, and
irrespective of watertight integrity additionally provide the main points of support for the vessel
during dry-docking. Floors and girders are stiffened vertically, usually employing flat bar
stiffeners.
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2.6 Stiffening of Major Elements
The foregoing structural elements are essentially large plate sections whose thicknesses are very
small compared with their other dimensions and which generally carry both in-plane and normal
loads. These large planar or curved plate sections must be stiffened appropriately in order to
perform their required function efficiently. Stiffening usually involves the welding of sections of
a selected sectional profile to the plate, forming an orthogonal grillage. Corrugations in bulkhead
plating may however be employed to provide stiffening in the primary direction of loading.
The various stiffening members have several functions. For example, in deck structure,
longitudinal frame stiffeners provide stiffness to the plating, transverse beams support the
stiffeners, and girders (longitudinal) in turn support the transverse beams transferring the load to
transverse bulkheads. In a transversely-framed side shell, the transverse frames (vertically
orientated) stiffen the side shell plating and support the ends of transverse deck beams and the
transverse side frames are supported by decks and stringers.
2.7 Interaction of Structural Components
Stiffening members do not act independently of the plating to which they are attached. A portion
of the plate either side of the stiffener serves as one flange of the stiffener and properties such as
section modulus and moment of inertia used in strength analysis must reflect this. Stiffening
members serve 2 functions, depending upon their loading. In the case of loads normal to the
plate (e.g., hydrostatic loading on a transverse bulkhead) the stiffeners provide edge restraint for
the plate. In the case of in-plane loads [e.g., compressive loads imposed on deck structure due to
bending (sagging) of the hull girder] longitudinal stiffeners serve to resist buckling and hence
maintain the deck plating in its designed shape. Longitudinal stiffeners sustain the same bending
stress as the plating and contribute substantially to the hull girder strength.
Decks, side shell, inner and outer bottoms, and bulkheads interact to provide overall edge
restraint for each other. For example, the ultimate support for a transverse bulkhead is provided
by the side shell, decks and bottom. Simultaneously, the bulkhead provides edge restraint for
the large stiffened plate panels of the decks, side shell, longitudinal bulkheads and bottom which
span major transverse elements such as bulkheads. Pillars may be used to support deck girders
or deck transverses. These supports, in addition to carrying local loadings from cargo, machinery
installations, etc., serve to maintain geometric separation of decks and bottom during longitudinal
bending of the hull girder and hence may sustain significant buckling (axial compressive) loads.
This interaction between structural components creates complex stress patterns at intersections of
stiffened plate elements, and between pillars and adjacent components. All structural elements
act together; both distributing and contributing static and dynamic loads and hence thorough and
adequate structural analyses are required to ensure a structurally sound vessel.
3 Systems of Framing
The term framing used in this context may, more accurately, refer to the method (orientation) of
stiffening the hull shell plating, i.e., deck, side and bottom plating. There are 3 systems available
to the designer, however, only 2 are ever considered in contemporary design due to the
inefficiencies of the third (transverse) system. The framing system adopted is primarily driven
by vessel length and type.
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3.1 Transverse System of Framing
The transverse system of hull framing may be considered as merely a continuation of the
traditional method of framing used in wooden vessels adapted to vessels of all-metal construction
with the introduction of iron-
Transverse framing may be used quite successfully in small pleasure craft, inshore and harbour
and coupled with low to moderate sea-states longitudinal bending stresses (and the associated
buckling stresses) may considered insignificant.
Essentially the transverse system of framing consists of a series of closely spaced ribs encircling
the hull. These ribs, comprising of vertical side frames, horizontal deck beams and floors in the
bottom, provide the stiffening of the shell and deck plating upon which the longitudinal strength
of the vessel primarily depends. The encircling ribs and their integral components also provide
support of hydrostatic and local loadings and maintain the geometric integrity of the hull.
deck plating deck transverse
(integral to web frame)
beam knee
(or bracket) web frame deck girder
deck girder deck beam
side stringer side stringer
deck transverse
(integral to web frame)
beam knee
(or bracket) deck girder deck beam deck girder
side plating
side stringer side stringer
side frame (main frame)
web frame
tank side bracket floor inner bottom plating
centre girder
side girder bottom plating
(intercostal)
Fig. (a) Fig. (b)
[ordinary transverse frame] [web frame]
Figure 12.1 Transverse framing system showing an ordinary frame, and web frame.
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Side stringers and deck girders may be employed where deck spacing and beam span
respectively necessitate support of side frames and deck beams. Web frames, or alternatively
deep frames (frames of intermediate size) are fitted every 2 or 4 frame spaces to support stringers
and girders with the side stringers and deck girders fitted intercostally. Spacing of transverse
frames is rarely permitted to be greater than 1000 mm in larger vessels and in smaller craft may
be as little as 300 mm. Floors in the bottom structure (whether single or double) should be
aligned immediately below the side frames to provide support and structural continuity.
In larger vessels, the transverse system of framing provides insufficient resistance to buckling of
deck and bottom plating induced by axial (in plane) compression arising from the sagged or
hogged conditions respectively. As the combined loadings of the stillwater and wave bending
moments and shear forces are frequently those of greatest magnitude to be sustained by the hull,
the transverse system of framing has been superseded by the longitudinal and combined systems
of framing.
deck girder deck plating deck beam (transverse)
bracket stringer
web frame side frame
deck girder deck beam (transverse)
bracket
web frame side frame transverse bulkhead
stringer
tank side bracket inner bottom plating reverse frame
(inner bottom)
20 25 30
bracket floor plate floor bottom plating side girder (intercostal) bottom frame
Figure 12.2 Longitudinal section of a transversely framed hull showing side structure.
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3.2 Longitudinal System of Framing
Although a number of early iron- and steel-hulled vessels of the late 19th century used a
longitudinal system of framing it did not gain prominence until 1906 with the introduction of the
Isherwood system; the forerunner of the contemporary longitudinal system.
When the frames which stiffen and support the shell (and inner bottom when adopted) and
members which stiffen and support the decks are run longitudinally instead of transversely, and
are made effectively continuous through transverse bulkheads, they contribute significantly to the
section modulus of the hull girder and hence assist in resisting the longitudinal bending of the
hull. Where the primary plating is subject to high in-plane compressive stress, longitudinal
stiffeners also increase the critical buckling strength of the plating to which they are attached. It
is in this regard that the longitudinal system is considerably more structurally efficient than the
transverse system and hence is used exclusively in vessels such as tankers and bulk carriers and
in vessels over 100 m in length unless the combination system of framing (refer to 2.3) is
preferable.
deck beam
deck plating (transverse)
deck stiffener deck girder
side transverse deck beam
(transverse)
side plating
deck stiffener deck girder
side transverse
side shell stiffener
inner bottom stiffener inner bottom plating
centre girder
side girder bottom stiffener floor (intercostal) bottom plating
12.3 Longitudinal framing system.
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The longitudinal system of framing comprises longitudinal stiffeners attached to deck, side-shell,
bottom and inner bottom plating. The longitudinal deck stiffeners are supported by transverses
(beams) and the side-shell stiffeners are supported by side transverses. These side transverses are
normally significantly deeper than a web-frame used in the transverse framing system. Both
deck- and side-transverses are spaced at intervals not exceeding 3800 mm. Bottom (and when
adopted, inner bottom) stiffeners are supported by intercostal floors.
Where spans of transverses are large, deck girders (or longitudinal bulkheads) provide support,
spanning intervals between transverse bulkheads. The longitudinal system of framing, being the
more efficient of the 2 alternatives is generally employed in naval combatants and high-speed
commercial craft where strength and weight saving is of paramount importance and the system is
invariably used in oil tankers.
deck girder deck longitudinal deck plating deck transverse (beam)
bracket side longitudinal side transverse
bracket
deck longitudinal
deck girder
bracket
side transverse bracket
side longitudinal
transverse bulkhead
inner bottom longitudinal
floor stiffener inner bottom plating side girder
20 25 30
transverse bilge bracket bottom longitudinal plate floor bottom plating bracket
Figure 12.4 Longitudinal section of a longitudinally-framed hull showing side structure.
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deck deck transverse deck plating longitudinal sheer strake
longitudinal bulkhead
vertical web vertical web web stiffener
WING BALLAST CENTRE WING
TANK CARGO TANK CARGO TANK
horizontal cross tie side shell longitudinal
girder
hull shell plating
inner hull DOUBLE BOTTOM
longitudinal BALLAST TANK
inner hull plating
inner bottom inner bottom
plating longitudinal plate floor
HOPPER TANK bottom centre girder duct keel keel plate bottom bottom shell bilge strake
longitudinal side girder
Figure 12.5 Transverse section of a double-hulled oil-tanker
showing longitudinal framing system.
3.3 Combination System of Framing
Longitudinal framing is so efficient that it might be expected to have become standard practice.
For many types of commercial vessel, the deep side transverses required to support the
longitudinal side framing can have serious disadvantages.
In certain cargo vessels, e.g., roll-on roll-off and refrigerated cargo vessels they may be regarded
as interfering with the stowage and movement of cargo. In large cruise vessels the deep side
transverses may not readily facilitate the preferred arrangement of accommodation outfit and its
integral joinery and deck girders may interfere with the transverse branches of piping and air-
conditioning ducts.
In such cases a practical solution may then be to longitudinally frame the bottom shell, inner
vessel types all decks may be longitudinally framed and only the side shell is transversely
framed. In either case, this hybrid system of framing is referred to as the combination system of
framing.
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Single-hull (side-shell) bulk-carriers employ the combination system (albeit modified to cater for
their topside and hopper tanks) because the transverse side framing does not trap the bulk cargo
as would longitudinal side frames.
deck plating deck transverse
(integral to web frame)
bracket
web frame deck girder
deck girder deck stiffener
side stringer side stringer
deck transverse
(integral to web frame)
bracket
deck girder
deck girder deck stiffener
side plating web frame
side stringer side stringer
side frame (main frame)
tank side bracket inner bottom stiffener inner bottom plating floor (intercostal)
transverse bracket
centre girder
side girder bottom stiffener bottom plating transverse bracket
Fig. (a) Fig. (b)
[section at transverse main frame] [section at web frame]
Figure 12.6 Combination framing system (Ro-Ro vessel).
Icebreaking vessels will adopt the combination system of framing as the transversely framed side
shell and the integral web frames most efficiently resist lateral ice pressure in the region of the
ice belt while the longitudinally framed bottom and deck structure provide efficient flexural
strengthening required for the very high loadings on the hull during ramming of pressure ridges.
The Swedish icebreaker Oden, completed in 1988, employed the combination system of framing
from the ice-belt to the keel, while above the ice belt the hull was framed utilising a longitudinal
system. This hybrid system of structure facilitated maximum efficiency in response to the full
envelope of hull loadings and could be considered in other vessel types.
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deck longitudinal deck plating deck beam (transverse)
bracket
stringer deck girder web frame side frame
deck longitudinal deck beam (transverse)
stringer deck girder web frame side frame transverse bulkhead
inner bottom longitudinal side girder inner bottom plating tank side bracket bracket
20 25 30
bottom longitudinal transverse bilge bracket floor stiffener plate floor bottom plating
Figure12.7 Longitudinal section of a combination-framed hull showing side structure.
Figure 12.8 illustrates the typical cross-section of a single-hull bulk carrier. Although a
specialised form of the combination system of framing (where longitudinal stiffening is
employed over the upper and lower regions of the side shell within the ballast tank spaces and
transverse frames are used in the cargo hold space) the system of framing has been the standard
structural arrangement for such vessels for 4 decades. Ostensibly the transverse system of
framing within the cargo holds was adopted to prevent side structure from trapping bulk cargo
material.
Recently, however, double-hulled bulk carrier designs are being promoted which enable a fully
longitudinal system of framing to be adopted. This facilitates improved structural efficiency,
reduced damage and corrosion to structural members within the hold (normally consequential of
discharge grab damage), improved accessibility to survey side-shell structure (from within the
combination system may still be adopted with double-hulled bulk carriers although the logic for
doing so may be questioned.
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sheer strake deck plating hatch coaming deck topside tank
longitudinal
topside tank topside tank cross deck topside tank
transverse vertical plate structure longitudinal
topside tank plating
side shell
longitudinal
side shell frame CARGO HOLD side shell plating
hopper side transverse hopper tank plating
inner bottom inner bottom plate floor hopper tank
plating centre girder longitudinal longitudinal
bilge strake bottom bottom shell duct keel keel plate bottom hopper tank
side girder longitudinal
Figure 12.8 Midship section of a single-hulled bulk carrier
showing the combined system of framing.
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References
Taggart, R. (Editor)
Ship Design & Construction
S.N.A.M.E., New York 1980
Hughes, O.F.,
Ship Structural Design
S.N.A.M.E., New Jersey 1988
Eyres, D.J.
Ship Construction
Heinemann, London 1978
IACS International Association of Classification Societies
Bulk Carriers: Guidelines for Surveys, Assessment & Repair of Hull Structure
Witherby, London 1995
IACS International Association of Classification Societies
Shipbuilding & Repair Quality Standard
IACS, London 1998
Rules & Regulations for the Classification of Ships
Parts 3 and 4
High Speed Light Craft Rules
Part 3, Ch. 9 Direct Calculation Methods
DNV, Høvik 1996
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