bulb tee

A bulb tee is a structural steel T-beam where the lower flange has been thickened into a bulbous or enlarged cross-section. The vertical web connects to this bulged base, creating a shape that resembles the letter T from the side. The bulb acts as a reinforced foot, giving the beam extra stiffness and bearing capacity at its bottom edge without proportionally increasing weight across the entire member.

This shape is standard in marine and shipbuilding applications, where bulb tees form the longitudinal stiffeners (stringers) that run along the length of hulls. The bulb sits on the inner face of the shell plating and resists buckling from hydrostatic pressure and bending loads. Common sizes range from small frames with bulb dimensions of 40 by 8 millimetres up to heavier sections exceeding 100 millimetres in width and depth. Steel grades typically run 235 to 355 megapascals tensile strength, depending on service temperature and impact requirements.

The bulb concentrates material where it resists bending most efficiently. A traditional T-beam with a uniform flange width must make the flange thick if it needs to carry shear at the junction with the web; this adds dead weight everywhere. The bulb tee achieves the same second moment of inertia with less total mass because the enlargement occurs only where needed. In ship hulls, this economy reduces displacement while maintaining stiffness.

Fabrication of bulb tees involves hot rolling the steel section to shape in the mill. Some grades arrive from stock; others are custom-rolled to specification. Connection and installation depend on the application. In ships, bulb tees are often fillet-welded or adhesively bonded to the plating. Inspection for laminations, surface defects, and dimensional accuracy is critical, as the bulb region is prone to lamellar tearing if welding parameters are not controlled.

The term bulb tee is primarily North American and marine industry usage. European and other traditions may call the same shape a bulb angle or bulb profile depending on context. Confusion sometimes arises with bulb angles, which are L-shaped sections with a bulb at the toe rather than a full T-shape, so verify cross-sectional geometry when ordering or specifying.