Hot-rolled flat bars with a rounded bulb along one edge, welded by the plain edge to steel plate as stiffeners. Bulb flats are the standard longitudinal stiffener in ship hulls and decks, and are also used in offshore structures, bridge decks and lock gates.
Also called
HP bulb flats
Holland profile
Bulb plates
Bulb bars
Ship stiffeners
Flat bulb steel
AR
مسطحات ببصلة (بلب فلات)
ZH
球扁钢
DE
Wulstflachstahl
FR
Plats à boudin
ES
Llantas con bulbo
IT
Piatti a bulbo
TR
Bulb lama (gemi profili)
RU
Полособульб
Main formsHP (Holland profile) bulb flats; Japanese-type bulb plates
Typical sizesWidth 80–430 mm, thickness 5–20 mm; lengths 6–18 m
Main standardsEN 10067, ISO 657-19, JIS G3192, classification society rules
Key gradesShipbuilding A, B, D, E · AH32, DH36, EH36 · S235JR, S355J2
A bulb flat is a flat bar with a bulb rolled along one edge. Welded by its plain edge to a plate, it stiffens the plate against bending and buckling. The bulb puts extra steel at the free edge, where it adds the most stiffness, so a bulb flat stiffens a plate almost as well as an angle of the same weight while having only one weld, no hidden corner where corrosion starts and surfaces that are easy to paint and inspect.
The international series is the Holland profile (HP), designated width × thickness — HP 200 × 10 — and rolled from about 80 to 430 mm wide. It is standardised in EN 10067 and ISO 657-19, and the same sizes appear in Japanese and Chinese standards. The Japanese bulb plate, with an angled bulb on one side, is a different and less common shape.
Because almost all bulb flats go into ships and offshore structures, they are normally ordered in shipbuilding grades certified by a classification society — grade A for ordinary hull steel, AH32 or AH36 for higher-strength hulls, and the D and E qualities where toughness at low temperature is required — with the society's mark on each bar.
Demand in the Middle East comes from ship repair and new building yards, offshore fabrication yards building platforms and jackets, and marine structures. Bulb flats are rolled by a small number of specialist mills in Europe and East Asia and are imported.
Forms & sizes
The size is width × web thickness in millimetres. Each width is rolled in two or three thicknesses; the bulb height grows with the width.
HP bulb flats
Holland profile, 80 × 6 to 430 × 17 mm, the international shipbuilding series
Bulb plates
Japanese type with an angled bulb, 180 × 9.5 to 250 × 12 mm
Lengths
Usually 6–12 m; up to about 18 m to order, matched to the ship's frame spacing
Surface
Mill finish, or shot-blasted and shop-primed for shipyards
The steel grade for a ship comes from the classification society's rules (ABS, Lloyd's Register, DNV, BV and others), which define grades A to EH36 in the same way.
