4/5 in to 1 ft

• 1791-2

Pen, pink, blue and sepia washes, pencil on thin buff laid paper (575 x 630)

Dance

The relevant structural parts are carefully washed in. Pale blue represents wrought iron while the beams have dabs of pink wash that may represent concrete, masonry (brickwork) is plain pink and voids are sepia washed.

Dance's large banking hall without internal columns, straddling an alley and with three storeys above it, posed a structural problem. The solution lay in the use (Kalman pp.231-2) of 'long cast iron trussed girders composed of perforated plates and narrow strips. The design is remarkably advanced for the year 1792. The girder combines the stiffness of a beam with the lightness and strength of a truss, and could be assembled from easily fabricated sections. As actually built, the banking hall was located on the ground floor and measured only about half the intended length. Nevertheless the girders and trusses spanned across the room and probably should still have been needed.'

It is not certain that the iron members were executed. The surviving accounts (Barclays Bank Group Archives) include payment of £500 to Larkins, Eade & Company, smiths and ironmongers, but the list of items for which they were paid is missing.

Dr David Yeomans (correspondence, 24 August 2001) wrote 'It seems clear from the different colour washes that these trusses are a combination of cast and wrought iron', that is, cast-iron plates with holes, stiffened by wrought iron, which is similar to Kalman's line of thought.

James Sutherland, consulting engineer (correspondence, 3 October 2001), has suggested another possibility. 'My feeling when I saw the large-scale cross-section was this showed wrought iron floor trusses supporting a stiff, secure and "fireproof" floor of some form of "concrete" made lighter by large diameter tubes of some undefined material. For Britain [at this time] this seem a very advanced form of construction ....Others have supported the idea that the concentric circles were some form of tube to lighten concrete. The trusses I feel are fairly slender wrought iron, compatible with French practice in the 1780s and later except that the whole truss and in particular the long sloping struts would tend to buckle unless restrained if surrounded by concrete - or whoever drew it did not know his "structures" .... If tubes [then] what were they made of, and how long? ...The important point is that, apart from mills with cast iron beams, you have unearthed a much earlier form of iron floor - real or just intended - than anything I have seen or read about in Britain.'

Dance's letter to Soane of about 1796 spoke of an experimental project the he wishes to share concerning 'incombustible floors' and 'iron trusses' of 'at least a 20 ft. span', revealing an enthusiastic interest in new materials and methods.

Drawing

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