PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 903 



in two distinct parts — the sustaining truss and the ro^idway — Ijoth 

 acting independent of each otiier. The principal dimensions are 

 as follows : Clear width of each roadway 16 feet; of each foot 

 w^alk 6 feet; verse sine, or deflection of chains 45 feet; centre to 

 centre of panels 14 ' 09 ", and of which there are 44. The curve 

 of the chains is a true parabola, since that is the uniformly loaded 

 condition assumed by the catenary, the curve of tlio freely sus- 

 pended cord. The greatest load that ever could come upon one 

 span, such as a dense crowd, together with the weight of the span 

 itself, would be about 3,650 tons, of which the side trusses must 

 support 1,150 tuns each, leaving 1,350 tons for the duty of the 

 middle truss. This loading would induce a horizontal strain in the 

 chains of 2,075 and 2,437 tons, respectively, for each side and the 

 middle truss, which would increase at the piers to 2,158 and 

 2,664 tons. This, then, tells us what provision to make in the 

 chains, and having before taken 15 tons per square inch as a safe 

 limit, we require a sectional area of 140 square inches at tlie cen- 

 ter of the side chains, swelling proportionately to an area of 144 

 square inches at the ends, while the middle truss requires at the 

 centre 162, and at the ends 177 square inches. Ten links, there- 

 fore, each ly^g-" X 12" are required to resist the horizontal ten- 

 sion of the side trusses, as 3^ou see in the diagram, while the mid- 

 dle truss requires for the centre chains, 12 links, each 1|" x 12''. 

 These links are 15 feet long, more or less, according to inclination, 

 the libres composing Avhich are continuous, that is, the heads are 

 not formed by welding, but are swelled out the proper size, the 

 holes for the eye-bolts being afterwards drilled very accurately to 

 size. The bolts themselves are turned carefully, fitting the aljove' 

 drilled eyes very truly. These piers being exposed to a shearino- 

 stress at each set of opposite links, the diameters required would be 

 4^ and 5^ inches for the two sides and middle trusses, respectively. 

 The force which the chains exert at the abutments and pier may be 

 decomposed into two, the one vertical, the other horizontal. Of 

 course the vertical is counteracted by the support of masonr}', while 

 the horizontal one is taken up by the chords, as you perceive in the 

 diagram shown b^^ the arrows. These chords are composed of 

 rolled segments of a cylinder, thirty inches in diameter, with lips, 

 between which are inserted radial pieces, the whole being riveted 

 together. The area required is of course ^\ the horizontal strain 

 (since we have taken twelve tons per square inch, as the unit of 

 safety for compressive strains). These chords are continuous, 



