Lesson 3FRAMING SYSTEMS As we have seen earlier, the shell (ship’s bottom and side) and deck plating of a ship has to be stiffened internally to prevent it from collapsing. The stiffeners are welded internally to the plating and are arranged in longitudinal and transverse directions perpendicular to each other. Longitudinals run along the length of the ship and the transverse frames run along the transverse section i.e; perpendicular to the fore-and-aft centreline, horizontal along the beam and vertical along the ship’s side. History Historically, early iron and steel vessels were built with transverse framing as this was the tried and tested configuration used for wooden ship building. The structural design requirements used for wooden ships were copied over to iron ships, featuring very heavy keel structures and relatively light decks. As ships got larger the limitations of thin transversely framed decks were observed and understood, although the industry was slow to adapt. One notable exception to this was the ‘Great Eastern’ (1858) which was a very early example of a scientifically designed ship. Isambard Kingdom Brunel, a civil engineer, used beam theory in the structural design of this vessel which was based on a cellular system of longitudinal framing. With a tonnage five times greater than any other vessel of the time, this remarkable ship boasted many other innovative features and despite her lack of commercial success, the structure performed well throughout her 31 year life. Although the technical benefits of longitudinal framing were known in the 19th Century it was not until the British naval architect Joseph Isherwood introduced his longitudinal framing method in 1906, that interest was revived. His system used longitudinal stiffeners and deep transverse web frames in the same way that modern arrangements do. The benefit was primarily a lighter structure, which for commercial vessels equated to increased deadweight for a given displacement, and hence a more profitable ship. This was particularly true for oil tankers where the increased web frame depth did not affect cargo stowage volume. The first ship using this system, the tanker ‘Paul Paix’, was built in 1908 to Lloyd’s Register class. By 1918, over 1000 ships had been built using the Isherwood framing system. Description The stiffening generally is provided by primary supporting and secondary supporting members. Primary supporting members Primary supporting members are the big members or metaphorically speaking the big bones having large scantlings and spaced wide apart. They provide most of the longitudinal bending strength to the ship. The word ‘scantlings’ denotes the cross- sectional dimensions of structural members, as distinct from their lengths. Thus an angle bar whose scantlings are 200 x 150 x 12.5 mm has one 200 mm leg and one 150 mm leg and is 12.5 mm thick, regardless of its length. Steel plate scantlings normally refer only to the plate thickness regardless of its length or width. BSW Ship Co Lecture Notes4 Framing Systems Page 1 of 8 centreline and side girders are fitted. or between girder and side frame. and can be divided into three parts: Bottom transverse This is the first part at the bottom and is represented by a solid floor or a plate floor in ships with double bottom construction. if this vertical distance is 10 meters. Rolled sections of bulb plates or unequal angles are normally used for those secondary supporting members TYPES OF FRAMING SYSTEMS Based on the direction of provision of secondary framing the framing system may be classified into Longitudinal Framing system or Transverse framing system. e. stringers and deck girders Web or transverse rings Longitudinal Girders. Transverse Web rings The transverse web rings hold the main longitudinal girders together and also provide the transverse strength. Web frame The second part on the side is known as the web frame. the depth of the web will be 1250 mm. BSW Ship Co Lecture Notes4 Framing Systems Page 2 of 8 . The depth of the deck transverse varies depending on the span between supporting girders or bulkheads. On the shipside. horizontal girders known as side stringers (or stringers) are fitted. The necessity for providing the secondary frames arises because the primary stiffeners are wide apart and the plate panel between them needs to be stiffened further as it would otherwise deform because of water pressure. SECONDARY SUPPORTING MEMBERS Secondary supporting members are the small bones of the ship which are arranged within the space available between primary stiffeners. On the bottom a centre girder is provided which is also called as the ‘centreline vertical Keel’.5 m.g. Depending on the beam of the ship at bottom there may be one or more additional longitudinal girders provided by the side of the CVK which are also called as ‘bottom side girders’. under the deck of a ship. The depth of the side web frame is about 0. They are fitted at intervals of about 3 to 4 meters apart along the length of the ship. The height of the solid floor is similar to that of the centreline girder.The primary supporting members or stiffeners consist of Main longitudinal girders at the bottom. Deck transverse The third part on the deck is known as deck transverse. The number of side stringer depends on the depth of the ship. They are much smaller in size as compared to the primary supporting members.125 times the vertical distance from the tank top to the deck above. They are fitted usually at spacing of less than 1000 mm apart. Side girders are of smaller scantling. Up to beam of 10 m minimum one side girder is provided on either side of the vertical keel and thereafter they are provided at a spacing of about 2. The vertical keel runs continuously from the fore peak to aft peak. It may be remembered here that the Primary stiffeners are a part of both these systems. have same depth as centre girder but they are neither watertight nor continuous. Similar to the bottom. These are fitted either in the longitudinal or in the transverse direction. principally to carry the loads imposed by keel-blocks during dry-docking. closely spaced longitudinals supporting the plating directly and being supported in turn by a few large.Transverse Framing System In the transverse framing system. Very deep. deck girders and Web rings Secondary: bottom longitudinals (secondary). the hold frames are fitted transversely. and deck longitudinals respectively). The transverse system therefore consists of following stiffeners Primary: Centre girder and other bottom longitudinals. The bottom member is known as an open floor or bracket floor (for ships with double bottoms). bottom longitudinals and deck girders. the side member is known as the hold frame (or main frame) and the deck member is known as the deck beam. Longitudinal deck girders support the transverse deck beams. Longitudinal Framing System In this system. all the secondary supporting members are fitted the longitudinal direction and are known as longitudinals (bottom longitudinals. is extra large and heavy. heavy transverse structures called transverse webs or web frames are constructed at BSW Ship Co Lecture Notes4 Framing Systems Page 3 of 8 . widely spaced longitudinals. stringers. deck girders and Web rings Secondary: Open or bracket floors. by the deck plating by a number of large. stringers and deck girders The bottom longitudinal on centerline. or center girder. The longitudinal system therefore consists of following stiffeners Primary: Centre girder and bottom longitudinals. hold frames and deck beams Longitudinal strength in a transversely framed ship is provided by: the center girder. The system consists of many small.g. widely spaced longitudinal members e. stringers. stringers or side longitudinals. the shell plating and inner bottom plating. Longitudinal stiffeners spaced about 600 to 900 mm apart give direct support to the plating of the deck. heavy transverse structures called transverse webs or web frames are constructed at intervals of about 3 to 5 meters These heavy transverse webs consist of deck. and may be resisted effectively by having the plating longitudinally stiffened. side. The secondary logitudinals also contribute to the longitudinal strength of the ship. on longer ships tests and experience have shown that there is a tendency for the inner bottom and bottom shell and deck to buckle if welded transverse framing is adopted. bottom. making the longitudinal framing system more structurally efficient than the transverse system. Very deep. This is the main system used for larger tankers where longitudinal strength is a major consideration.intervals of about 3 to 5 meters to support the longitudinals and to provide transverse strength. and bulkheads. sides. The explanation of this is that. This buckling occurs as a result of the longitudinal bending of the hull. The side transverse and vertical web on the bulkhead are usually tied together with one to three horizontal struts or cross ties. Where a ship's length exceeds 120m it is considered desirable to adopt longitudinal framing. and bottom transverses plus a vertical web on each longitudinal bulkhead. and adoption of any other system will result in unacceptably large hull weight. BSW Ship Co Lecture Notes4 Framing Systems Page 4 of 8 . The hydrostatic pressure on each successive longitudinal and its associated plating increases with its depth below the waterline or below the surface of the liquid in the tank. For this reason. The spacing of the longitudinals around the girth of the ship can be maintained constant so long as the girth does not change. Resistance to Buckling. in which most of the plating stiffeners are disposed in the fore and aft direction. that is. Longitudinally stiffened plating is more resistant to buckling between longitudinal stiffeners. BSW Ship Co Lecture Notes4 Framing Systems Page 5 of 8 . Advantages of the longitudinal framing system over transverse framing system: Strength to weight ratio. and at the same time they contribute to the ship's resistance to longitudinal bending (which transverse frames do not). As the hull narrows toward bow and stem. a longitudinally framed ship has superior longitudinal strength to a transversely framed ship of equal size and structural weight. when the deck or bottom (especially prevalent in a ship's upper deck) is subjected to compressive stresses as it bends in a seaway than it would be if stiffened transversely. the girth necessarily reduces and the longitudinals become closer together. Another difficulty arises in the structural arrangements near the ends of the ship. Graduated size.TWO SYSTEMS COMPARED Although the transversely framed ships were structurally sound and satisfactory in service. of achieving the required strength for the least weight. Therefore. however. The longitudinal system. has superior structural efficiency. This "graduated size" configuration cannot be done effectively with transverse frames. which as a result are overly heavy at their upper ends. thus achieving an efficient use of the structural material. The transverse framing system was not optimal from the standpoint of structural efficiency. Longitudinal stiffeners supporting side shell and bulkhead plating are subjected to variable pressures from the sea or from liquid cargo. Each longitudinal can be sized to withstand the maximum pressure associated with its depth in the ship. Constructional difficulty. Difficulties in construction arise when they converge so closely that some longitudinals have to be eliminated. transverse framing is usually resorted to at the bow and stem of longitudinally framed ships. Many longitudinal stiffeners serve dual purposes: they support the shell plating against local loading caused by water pressure and cargo loads (just as transverse frames in the transverse system do). Disadvantages of the longitudinal framing system: The intrusion of deep webs into prime cargo spaces for ships carrying packaged cargo is an disadvantage. the fact that most of their stiffeners contributed nothing to the ship's resistance to longitudinal bending. The transverse framing members are the deep. More closely spaced. where the advantages of extra longitudinal strength and resistance to compressive plate buckling are most needed. Combination systems are employed in many types of ships. smaller transverse side frames support the side shell plating between the deep transverses. Longitudinal framing is used in the bottom and decks. BSW Ship Co Lecture Notes4 Framing Systems Page 6 of 8 . containerships and RO/RO ships. precluding the need for deep webs that might inhibit efficient cargo stowage.COMBINED OR MIXED FRAMING SYSTEM In this system. and transverse framing is used in the sides. including general dry cargo ships. a combination of both transverse and longitudinal framing is adopted in the same cross section of the ship. widely spaced transverses that support the longitudinals in the decks and bottom. Longitudinal Framing Transverse sectional view Longitudinal Framing Longitudinal sectional view BSW Ship Co Lecture Notes4 Framing Systems Page 7 of 8 . Ordinary Frame Web Frame Transverse Framing Transverse Framing .Longitudinal sectional view BSW Ship Co Lecture Notes4 Framing Systems Page 8 of 8 .