At the Admiralty's insistence, any bridge would have to permit passage of the strait by a fully rigged man-of-war. Stephenson therefore intended to cross the strait at a high level, over , by a bridge with two main spans of , rectangular iron tubes, each weighing , supported by masonry piers, the centre one of which was to be built on the Britannia Rock. Two additional spans of length would complete the bridge, making a continuous girder. The trains were to run inside the tubes (inside the box girders). Up until then, the longest wrought iron span had been , barely one fifteenth of the bridge's spans of . As originally envisaged by Stephenson, the tubular construction would give a structure sufficiently stiff to support the heavy loading associated with trains, but the tubes would not be fully self-supporting, some of their weight having to be taken by suspension chains.