Ship Structure - Frames & Floors — Most Asked Questions
14 questions asked about ship structure - frames & floors across the General and Fn 3 and Fn 4b papers, heaviest first.
Learn the system first
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1 Types of floors ?
Also asked: “Different types of floors”
Answer
Double-bottom floors are solid plate where loads are heavy and open bracket floors elsewhere — solid takes the load, brackets save the steel.
- Solid plate floors: full transverse plates tying bottom shell to inner bottom, fitted at every frame in the engine room, pounding areas, under bulkheads and where grabs discharge.
- Elsewhere solids may space out to 3.0 m with bracket floors — short transverse plate brackets at centre girder and tank sides — at the frames between.
- Bracket breadth at least 75% of the local depth, flanged for stiffness; both types carry the inner bottom and shell loads into the girders.
- Transversely framed bottoms lean on floors plus transverse frames; longitudinally framed bottoms lean on floors plus longitudinals.
Surveyor follow-up: why is every frame a solid floor in the engine room but not in the midship holds?
Full theory: Double Bottom Structure and Bottom Framing -
2 In these ships, the plate floors (see the figure to understand the context of ‗Floors‘ in shipbuilding) themselves act as the stiffening members of the bottom shell plating. Plate floors (as shown in the figures) constit
Also asked: “In these ships, the plate floors (see the figure to understand the context of ‗Floors‘ in shipbuilding) themselves act as the stiffening members of the bottom shell plating. Plate floors (as shown in the figures) constitute transversely running plates at every frame spacing”
Answer
Plate floors are the transverse bottom strength members: solid plates tying shell to tank top, with brackets between them.
- Solid plate floors run transversely from keel to tank top in the double bottom, stiffening the bottom shell directly.
- Between solids run bracket floors — lighter plate brackets doing the same job with less steel.
- Spacing follows service: every frame in machinery and pounding zones, wider with brackets elsewhere.
- Girders run fore-and-aft between floors; intercostal girders stop at floors while continuous girders pass through.
Surveyor follow-up: point at the double bottom and name the floor, the girder and the bracket — then say which is intercostal.
Full theory: Double Bottom Structure and Bottom Framing -
3 It is applicable for all the ships above 400 GT and platform and drilling rigs engage in FG voyages
Also asked: “• It is applicable for all the ships above 400 GT and platform and drilling rigs engage in FG voyages”
Answer
Garbage rules bite at 400 gross tons and on rigs and platforms in foreign voyages: record book, placards and plan aboard.
- Every ship of 400 gross tons and up plus certified 15-person ships on international runs carries the record book.
- Fixed and floating platforms in exploration and exploitation carry it too.
- Each discharge or incineration logged with date, position and category, each page signed by the master.
- Entries in English, French or Spanish with placards and management plan to match.
Surveyor follow-up: 399 gross tons with 16 persons aboard internationally — record book or not?
Full theory: MARPOL 73/78: The Convention & Its Six Annexes -
4 Vent for double bottom.and location
Answer
Double-bottom tanks breathe through air pipes at the high far end, led above the bulkhead deck with soundings nearby.
- Air pipes sit opposite the filling pipe and at the highest point so trapped air escapes as the tank fills.
- Every double-bottom, deep tank to the ship’s side, or tank run up from sea gets its pipe above the bulkhead deck.
- Oil, cofferdam and pumpable-tank vents rise to open deck clear of oil and vapour danger with Load Line closing gear.
- Sounding pipes serve the same tanks close by so levels are read where filling and venting truly act.
Surveyor follow-up: air pipe at the same end as the filling pipe on a double bottom — vented or air-locked, and why?
Full theory: Double Bottom Structure and Bottom Framing -
5 What is hopper tank
Also asked: “What is Hopper tank ( don't know”
Answer
Hopper tanks are the sloped lower side tanks of a bulk carrier: ballast wings that trim the hold self-clearing.
- Sloped plating joins double bottom to side shell, shedding grab and grain back to the hold centre.
- Work as wing ballast tanks in ballast passage and empty self-trimming volume in laden holds.
- Stiffened against cargo and sea pressure with smooth faces that resist hold damage and speed discharge.
- Survey by ballast soundings, coating condition and drain paths — cracked hoppers flood holds silently.
Surveyor follow-up: hopper ballast found in a laden hold with grain aboard — trimming aid or stability breach?
Full theory: SOLAS Chapter XII — Additional Safety Measures for Bulk Carriers -
6 Check diaphragm and frame near affected part
Answer
Check the diaphragm and the frame around the damage: local buckling and cracks spread from the dent, not from the impact alone.
- Examine the shell plating around the damage for deformation, cracking and coating breakdown on both sides of the impact.
- The frame and the girder at the damaged span are checked for buckling and for any crack running out of the doubler.
- Measure the indentation against the permissible limit, and check the doubler and the connection for movement.
- A dented plate with a cracked frame behind it is a fracture path, not a cosmetic defect — that is the surveyor’s point.
Surveyor follow-up: plating indented with the frame behind it cracked — repair the plate only, or the frame too?
Full theory: Ship Stresses and Longitudinal Hull Strength -
7 Difference between girder and frames
Answer
Frames run fore-and-aft supporting the shell; girders run athwartships carrying the floors, and the two are transverse to each other.
- A frame is a longitudinal member on the centreline of a shell frame, stiffening the plating and carrying the transverse floors.
- A girder is a heavier athwartships member that supports the floors and concentrates their loads into the shell and the centre girder.
- In a transversely framed double bottom the floors sit on the centre and side girders, and the girders pass the load on to the frames.
- So the girder is what a floor pushes against, and the frame is what carries that into the hull — neither replaces the other.
Surveyor follow-up: a floor spanning to the shell with no girder under it — is that a design, a detail, or a defect?
Full theory: Double Bottom Structure and Bottom Framing -
8 Double bottom safety and constrction
Answer
The double bottom is the ship’s spare buoyancy and its ballast tank: a watertight bottom over a cellular space with a flat top.
- The double bottom consists of the shell plating, the inner bottom or tank top, and the side girders and floors between them.
- It gives reserve buoyancy if the bottom is damaged and saves the ship from sinking by a single plate leak.
- It is divided into compartments by transverse girders so one flooded cell cannot flood the whole space.
- It also carries ballast water, so the same structure must be tight for sea water as it is for cargo.
Surveyor follow-up: double bottom with a weeping crack in one cell — can the ship sail, and what proof is required?
Full theory: Double Bottom Structure and Bottom Framing -
9 In double bottom tank, procedure of removing slugde
Answer
Sludge comes out the same way it went in — through the sounding pipe with a pump, or by gravity into a portable tank with the tank pumped down first.
- Pump the double bottom down to a level that leaves the sludge exposed, using the sounding and the bilge suction with non-return valves proved.
- Open the sludge cock or the sounding rod opening and pump the sludge into drums or a portable tank on deck.
- Never leave the tank open to air with oil vapour above it — the space is kept gas-free with the covers and vents controlled.
- Recovered sludge is landed to reception facilities and the entry into the record book; it is waste, not a bilge.
Surveyor follow-up: sludge removal on a tank that cannot be pumped down to expose it — pump out, gas-free and enter, or wait for a shore gang?
Full theory: MARPOL Annex I — Prevention of Pollution by Oil -
10 What all double bottom checks are to made during ILLC survey
Answer
The double bottom is examined inside and out: structure, plating thickness, corrosion pattern and the tightness of the tank top.
- Thickness measurements are taken before or with the close-up survey — every deck plate, three transverse sections and each bottom plate in the cargo area.
- Structural members subject to close-up are measured for general assessment and for recording the corrosion pattern.
- Suspect areas, all wind and water strakes within the cargo area, and internal structure in the fore and aft peaks are all checked.
- The survey looks for wastage, pitting, deformation and loose rivets, and for leakage into the space from the tank top above.
Surveyor follow-up: double bottom with heavy pitting but thickness within limits — is it corrosion allowance, coating or the floor span?
Full theory: Double Bottom Structure and Bottom Framing -
11 What are plate floors, girder, types of girder, what is intercostal girder
Answer
A plate floor is a solid plate web; a girder is the deeper member spanning between them, web and flange; an intercostal girder spans transversely.
- The floor is the transverse strength member, and in its simplest form it is a solid flat plate welded between the shell and the centreline girder.
- A girder is a deeper beam with a web and flanges, built up where a plain plate is not stiff enough to span.
- The types are the flat bar, the deep or heavy girder, the corrugated or stiffened girder, and the lightening-hole girder to save weight.
- An intercostal girder runs transversely across the ship at floor level, as distinct from a longitudinal girder or a side girder.
Surveyor follow-up: girder found with corrosion under every lightening hole — why does the hole matter?
Full theory: Double Bottom Structure and Bottom Framing -
12 What is frame
Answer
A frame is the transverse stiffener that turns shell plating into a structure: a web with flanges, carrying the floors and the shell’s edge loads.
- It runs vertically from the keel to the deck, spaced at a frame spacing, and supports the shell plating between the frames.
- It is the member that keeps the shell from plate into a structure able to take water pressure and cargo loads.
- At the bottom it carries the floors and at the deck it carries the deck beams, passing both loads into the shell.
- Frames are doubled at the ends, at the engine room, at hatches and where heavy concentrated loads land.
Surveyor follow-up: frame web found with sharp-cornered cut-outs at the lightening holes — is that a detail or a stress raiser?
Full theory: Double Bottom Structure and Bottom Framing -
13 What is frame floor and types?
Answer
A frame floor is the transverse plate closing the double bottom at each frame; the two types are solid and bracket, chosen by tank or hold beneath.
- Solid floors are complete plates with lightening holes, used where the space below must stay watertight as a tank.
- Bracket floors are open floors stiffened with angle brackets, used where there is no tank to be formed.
- Plate floors are further stiffened with flat bar, and the girders act as their supporting members.
- Where the frame spacing needs strengthening, solid floors go in at alternate frame spaces in the pounding region.
Surveyor follow-up: bracket floor found over a double-bottom tank — is it a defect, or is the tank line drawn on the wrong plan?
Full theory: Double Bottom Structure and Bottom Framing -
14 Where solid floors is used?
Answer
Solid floors go where the load is worst and the space below must be a tank: the engine room, the pounding region and the ends of a double bottom.
- In the engine room region all the frames are provided with plate floors and no bracket floors are used.
- In the pounding region of a longitudinally framed double bottom, solid plate floors are fitted at alternate frame spaces.
- Intercostal side girders follow at a maximum spacing of three times the transverse floor spacing in that region.
- And everywhere the double bottom is a tank, the floors must be solid so the tank holds water when the ship is upright.
Surveyor follow-up: solid floor spacing doubled in the forward holds — is the design, the draught or the rule first at fault?
Full theory: Double Bottom Structure and Bottom Framing
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