ABSTRACT
INTRODUCTION The classic period of iron bridge truss development was the third quarter of the 19th century. This
paper deals with the pre-history to 1850 by which time several distinct types had evolved; they are considered here as
i, the tied arch and bowstring truss ii, the inverted bowstring iii, the lenticular truss iv, the lattice v, the Warren truss vi, iron versions of the Howe and Pratt trusses
THE TIED ARCH AND BOWSTRING TRUSS The idea of counteracting the thrust of rafters or masonry arches in buildings with metal tie-bars is
extremely ancient. There are 15 ft long examples in the Church of St. Sophia, Constantinople (6th century)! and 50ft examples in some Russian monastery refectories (17th century), the latter requiring intermediate hanging supports. The application of the same principle to bridge arches to eliminate thrust on the abutments can be traced back at least as far as Leonardo da Vinci (1452-1519) and Faustus Verantius (1551-1617) who both provided illustrations (Figs. 1 and 2, Page 69 ).2.3
Timber trusses with suspended deck ("through-trusses") also go back many centuries although the f!rst yet confirmed as having truly arched ribs is the aqueduct over the Fontanka at St. Petersburg built by Harman van Boles (1683-1764) in the late 1720s (two 80ft spans, Fig. 3).4 The first world-famous example was the Grubenmanns' Wettingen bridge over the Rhine (c.200 ft span, 1764-6, Fig. 4).5 The framing of some of these early through-trusses, with longitudinal members assembled from several timbers continuously scarfed together, was such that the deck acted to some extent as a tie and they were relatively springy.6 (See page 69 for Figs 3 and 4. Ed).
