“The roadway of the bridge is entirely supported by the steelwork of the superstructure … which does the whole of the useful work, although for architectural reasons it is hidden by the masonry which envelopes it.” JE Tuit, Sir William Arrol & Co, contracting engineers
Not many people know that the two towers of Tower Bridge are made of steel – only if you have visited inside will you be able to remark on the scale of the columns and cross struts. If you stripped away the external stone cladding then you would be able to see how this works structurally.
In effect the whole bridge is supported by the two sets of immense chains that hang from the south and north towers and are attached to each other across the top and to the abutments on the riverbanks. This produces a form of structural equilibrium illustrated in the below metal model aimed at primary/elementary school children.
“It has been argued that the manner in which the steelwork has been concealed by the masonry superstructure is similar to the way in which the skeleton of the human body is enveloped and hidden by the flesh ..” JE Tuit, Sir William Arrol & Co, contracting engineers
The tower columns were first attached to the two enormous concrete and stone piers that had been built to support the whole structure and withstand any movement of the London Clay below the Thames riverbed. While a lot of riveting of steel plates was done at Arrol’s Glasgow works, much was also done on site often using specially made tools. Getting the pair of bascule leaves in place was also delicate work. Each leaf weighed 400 tons and had to balance on an edge so the counterweight would allow it to swing easily up and back down.
Sir John Wolfe Barry spoke about the cladding in his public lecture on Tower Bridge published in 1894. He said that stone had been preferred to cast iron for aesthetic and preservation reasons and pushed aside the objections of ‘old architects’ to the final appearance, noting that Wren had built the dome of nearby St Pauls Cathedral with internal brickwork which couldn’t be seen.