Transcript
Lesson 3 FRAMING SYSTEMS As we have seen earlier, the shell (ship’s bottom and side) and deck plating of a ship has to be stiffen stiffened ed internal internally ly to preven preventt it from from col collap lapsin sing. g. The stiffen stiffeners ers are welded welded intern internall ally y to the platin plating g and are arrang arranged ed in longit longitudi udinal nal and trans transve verse rse direct direction ions s perpen perpendic dicula ularr to each each other other.. Longit Longitudi udinal nals s run run alo along ng the length length of the ship ship and the transverse frames run along the transverse section i.e perpendicular to the fore!and!aft centreline, hori"ontal 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 keel struct structure ures s and relati relativel vely y light light decks. decks. As ships ships got larger larger the limita limitatio tions ns of thin thin transversely framed decks were observed and understood, although the industry was slow to adapt. One notable exception to this was the !reat "astern# $%&'&( which was a very early example of a scientifically designed ship. )sambard *ingdom +runel, a civil engineer, used beam theory in the structural design of this vessel which was based on a cellular system of longitudinal framing. ith a tonnage five times greater than any other vessel of the time, this remarkable ship boasted many other innovative features and despite despite her lack of commercial commercial success, the structure structure performed performed well throughout throughout her -% year life. Although the technical benefits of longitudinal framing were known in the %th /entury / entury it was not until the +ritish naval architect Joseph Isherwood introduced his longitudinal framin framing g method method in %01, %01, that that intere interest st was revive revived d . His system system used longitudina longitudinal l stiffeners and deep transverse web frames in the same way that modern arra arrang ngeme ement nts s do. do. The The benef benefit it was was prima primaril rily y a light lighter er struc structu ture, re, whic which h for for commercial vessels equated to increased deadweight for a given displacement, and and hence hence a more more profit profitab able le ship. ship. This This was particula particularly rly true for oil tankers tankers where the increased web frame depth did not affect affect cargo stowage stowage volume. T volume. The he first ship using this system, the tanker 2aul 2aix#, was built in %0& to 3loyd#s 4egister class. +y %%&, over %000 ships had been built using the Isherwood the Isherwood framing system sy stem.. Description The stiffening generally is provided by primary supporting and secondary supporting memers! "rimary supporting memers "rimary supporting memers are the big members or metaphorically speaking the ig ones havi having ng larg large e sca scant ntli ling ngs s and and space spaced d wide wide apart apart.. They They prov provid ide e most most of the the longitudinal bending strength to the ship. The word word scantl scantling ings# s# denote denotes s the cross5secti cross5sectiona onall dimens dimension ions s of struct structura ural l members, as distinct from their lengths. Thus an angle bar whose scantlings are 600 x %'0 x %6.' mm has one 600 mm leg and one %'0 mm leg and is %6.' mm thick, regardless of its length. 7teel plate scantlings normally refer only to the plate thickness regardless of its length or width.
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The primary supporting members or stiffeners consist of • •
#ain longitudinal girders at the bottom, stringers and deck girders $eb or transverse rings
Longitudina# Girders. %n the bottom a centre girder is provided which is also called as the ¢reline vertical 'eel’. The vertical keel runs continuously from the fore peak to aft peak. epending on the beam of the ship at bottom there may be one or more additional longitudinal girders provided by the side of the *' which are also called as &bottom side girders’. +p to beam of - m minimum one side girder is provided on either side of the vertical keel and thereafter they are provided at a spacing of about ./ m. 0ide girders are of smaller scantling, have same depth as centre girder but they are neither watertight nor continuous. %n the shipside, hori"ontal girders known as side stringers (or stringers) are fitted. The number of side stringer depends on the depth of the ship. 0imilar to the bottom, under the deck of a ship, centreline and side girders are fitted. Trans$erse %e rings The transverse web rings hold the main longitudinal girders together and also provide the transverse strength. They are fitted at intervals of about 1 to 2 meters apart along the length of the ship, and can be divided into three parts3 •
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4ottom transverse This is the first part at the bottom and is represented by a so#id &#oor or a p#ate &#oor in ships with double bottom construction. The height of the solid floor is similar to that of the centreline girder. $eb frame The second part on the side is known as the 'e &rame! The depth of the side web frame is about -./ times the vertical distance from the tank top to the deck above, e.g. if this vertical distance is - meters, the depth of the web will be /- mm. eck transverse The third part on the deck is known as deck transverse. The depth of the deck transverse varies depending on the span between supporting girders or bulkheads, or between girder and side frame.
SE()NDARY S*"")RTING MEM+ERS Secondary supporting memers are the sma## ones of the ship which are arranged within the space available between primary stiffeners. They are much smaller in si"e as compared to the primary supporting members. The necessity for providing the secondary frames arises because the primary stiffeners are wide apart and the plate panel between them needs to be stiffened further as it would otherwise deform because of water pressure. They are fitted usually at spacing of less than --- mm apart. These are fitted either in the longitudinal or in the transverse direction. 5olled sections of bulb plates or une6ual angles are normally used for those secondary supporting members TY"ES )F FRAMING SYSTEMS 4ased on the direction of provision of secondary framing the framing system may be classified into Longitudinal 7raming system or Transverse framing system. 8t may be remembered here that the 9rimary stiffeners are a part of both these systems. BSW Ship Co Lecture Notes4
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Trans$erse Framing System 8n the transverse framing system, the hold frames are fitted transversely. The bottom member is known as an open floor or bracket floor (for ships with double bottoms), 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. The transverse system therefore consists of following stiffeners "rimary3 entre girder and other bottom longitudinals, stringers, deck girders and $eb rings Secondary3 %pen or bracket floors, hold frames and deck beams Longitudina# strengt, in a transversely framed ship is provided by3 the center girder, the shell plating and inner bottom plating, by the deck plating • by a number of large, widely spaced longitudinal members e.g bottom longitudinals • and deck girders. •
Longitudina# Framing System 8n this system, all the secondary supporting members are fitted the longitudinal direction and are known as longitudinals (bottom longitudinals, stringers or side longitudinals, and deck longitudinals respectively). The system consists of many small, closely spaced longitudinals supporting the plating directly and being supported in turn by a few large, widely spaced longitudinals. The longitudinal system therefore consists of following stiffeners "rimary3 entre girder and bottom longitudinals, stringers, deck girders and $eb rings Secondary3 bottom longitudinals (secondary), stringers and deck girders The bottom longitudinal on centerline, or center girder, is e:tra large and heavy, principally to carry the loads imposed by keel!blocks during dry!docking. *ery deep, heavy transverse structures called transverse webs or web frames are constructed at BSW Ship Co Lecture Notes4
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intervals of about 1 to / meters to support the longitudinals and to provide transverse strength. Where a ships length e!ceeds "#$m it is considered desirable to adopt longitudinal framing. The e:planation of this is that, on longer ships tests and e:perience have shown that there is a tendency for the inner bottom and bottom shell and deck to buckle if welded transverse framing is adopted. This buckling occurs as a result of the longitudinal bending of the hull, and may be resisted effectively by having the plating longitudinally stiffened. The secondary logitudinals also contribute to the longitudinal strength of the ship, making the longitudinal framing system more structurally efficient than the transverse system. This is the main system used for larger tankers where longitudinal strength is a ma8or consideration, and adoption of any other system will result in unacceptably large hull weight. 3ongitudinal stiffeners spaced about 100 to 00 mm apart give direct support to the plating of the deck, sides, bottom, and bulkheads. 9ery deep, heavy transverse structures called transverse webs or web frames are constructed at intervals of about - to ' meters These heavy transverse webs consist of deck, side, and bottom transverses plus a vertical web on each longitudinal bulkhead. The side transverse and vertical web on the bulkhead are usually tied together with one to three hori:ontal struts or cross ties.
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T%) SYSTEMS ()M"ARED Although the transversely framed ships were structurally sound and satisfactory in service, the fact that most of their stiffeners contributed nothing to the ship;s resistance to longitudinal bending. The transverse framing system was not optimal from the standpoint of structural efficiency, that is, of achieving the re6uired strength for the least weight. The longitudinal system, in which most of the plating stiffeners are disposed in the fore and aft direction, has superior structural efficiency. Ad$antages of the longitudinal framing system over transverse framing system3 Strengt, to 'eig,t ratio. #any longitudinal stiffeners serve dual purposes3 they support the shell plating against local loading caused by water pressure and cargo loads (graduated si"e> configuration cannot be done effectively with transverse frames, which as a result are overly heavy at their upper ends. Disad$antages of the longitudinal framing system3 T,e intrusion o& deep 'es into prime cargo spaces for ships carrying packaged cargo is an disadvantage. (onstructiona# di&&icu#ty. Another difficulty arises in the structural arrangements near the ends of the ship. The spacing of the longitudinals around the girth of the ship can be maintained constant so long as the girth does not change. As the hull narrows toward bow and stem, however, the girth necessarily reduces and the longitudinals become closer together. ifficulties in construction arise when they converge so closely that some longitudinals have to be eliminated. 7or this reason, transverse framing is usually resorted to at the bow and stem of longitudinally framed ships.
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()M+INED )R MI/ED FRAMING SYSTEM
8n this system, a combination of both transverse and longitudinal framing is adopted in the same cross section of the ship. Longitudina# &raming is used in t,e ottom and dec-s0 ',ere t,e ad$antages o& e1tra #ongitudina# strengt, and resistance to compressi$e p#ate uc-#ing are most needed0 and trans$erse &raming is used in t,e sides0 prec#uding t,e need &or deep 'es t,at mig,t in,iit e&&icient cargo sto'age. The transverse framing members are the deep, widely spaced transverses that support the longitudinals in the decks and bottom. #ore closely spaced, smaller transverse side frames support the side shell plating between the deep transverses. ombination systems are employed in many types of ships, including general dry cargo ships, containerships and 5%?5% ships.
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Longitudinal Framing Transverse sectional view
Longitudinal Framing Longitudinal sectional view
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Ordinary Frame
Web Frame Transverse Framing
Transverse Framing - Longitudinal sectional view BSW Ship Co Lecture Notes4
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