the abutments and piers, it would be much more rigid than the hinged bridge.

The following example of a stiffened suspension bridge1 will illustrate the method of determining the most important of the stresses in this class of structure.

The bridge, illustrated in Plates VI., VIa, and VIb, is 775 feet long, in three spans of 150, 500, and 125 feet respectively, and 28 feet wide in the clear. It consists of steel-wire cables suspended from masonry towers and supporting steel lattice stiffening girders and steel cross-girders.

The cables, which are six in number, three on each side of the bridge, consist each of seven ropes formed with plough-shear steel wire. The central deflection of the cables is 38 feet 6 inches, and they are continued over the main towers on each side of the central span, and pass below the ground, through inclined shafts excavated in the rock, to the anchorages.

The sectional area of steel wire in each rope is 2 square inches, and the total sectional area in the six cables is 84 square inches. The arrangements for anchoring the cables to the rock are shown in Plate VIb., and consist of six hollow caststeel beams, 15 inches in diameter, and 2 inches thick, arranged three in each of two chambers excavated in the rock. An inclined rectangular shaft gives access to each of the chambers referred to from the ground level, and the cables pass down these to cast-iron thimbles, around which the ropes are coiled, and the ends of the wires are spread out in conical holes, where they are firmly secured by driving in round steel taper pins. The width between the cables over the piers is 48 feet centre to centre, and in the centre of the bridge 32 feet centre to centre. The towers are 112 feet high, and the cables discharge their weight upon expansion rollers 3 feet in diameter, which rest upon granite bed-stones, from which the pressure is distributed uniformly over the towers.

The stiffening girders, which are designed to prevent the main cables altering their curvature when the bridge is traversed by a moving load, are hinged at the centres, and spaced 15 feet centre to centre across the bridge.

The main girders are 12 feet 6 inches effective depth, and the cross-girders are 2 feet deep. The method of connecting the cross-girders to the main girders, and the cables, and 1 North Sydney Suspension Bridge. Engineers, J. E. F. Coyle and the author.

the sections of these girders are sufficiently illustrated in Plates VIa, VIʊ.

Calculations.-The total dead load, including cables, girders, deck, bracing, handrails, etc., on central span is 0.93 ton per lineal foot.

The live load for a crowd of people, at 40 lbs. per square foot of deck, is 0.50 ton per lineal foot.

The live load for a tram motor and three loaded cars is equivalent to a load of say 05 ton per foot run over 150 feet.

The cables must be strong enough to carry the total live load of 0·93 + 0·5 = 1·43 ton per foot lineal; say 1·5 ton.

Hence the maximum intensity of working stress in the cables is

This factor of safety will be slightly reduced by a fall in temperature, which diminishes the central deflection, and increased by a rise in temperature.

It can easily be shown, by equating the moment of resistance of the hollow steel girders at the anchorages with the bending moment produced by the maximum pull on the cables, that the sizes mentioned are sufficient for their purpose.

Again, the stability of the towers against a wind pressure of 30 lbs. per foot can easily be shown to be ample.

The stiffening girders receive their maximum stresses during the passage of a tram motor with three loaded cars, and, as the tram line is shown on one side, so as to leave room for a carriage way on the other, the stiffening girder nearest the centre of the tram line will receive twice as great a load as the other.

Let a the distance from the central hinge to the point where the bending moment is a maximum.

1 = the half-span.