tubular bridge

A tubular bridge is a type of bridge that is primarily supported by steel tubes or is constructed as a rectangular tube made of plate girders. This engineering innovation emerged during the 19th century with the advancement of materials like wrought iron and steel, driven by the need for stronger railway bridges capable of handling heavier loads.12

A notable example is Robert Stephenson's Britannia Bridge, completed in 1850. It was the first to utilize a hollow box girder design, which provided the deck with extra stiffness similar to a truss but was simpler to construct. This design allowed trains to pass through the hollow, wrought-iron boxes.2