PASSAGE OF RIVERS.

81

j Plate V. figs. 11,• 12 .

Ribs.

Piers.

Plates VI. & VII.Trans. Inst. C.E.Foundations,land arches.Great arch.

Plate VI.

Plate VII,

Figs. 11,12, shew the form of an ice-breaker to be used in rivers subject to floatingice: it should be moored a short distance above the bridge, with the sharp point upstream.

SECTION IV.-IRON BRIDGES.

The principles of the construction of iron bridges are in many respects similar tothose of wooden ones; that is, they are composed of several ribs kept in positionand strengthened by distance-pieces, diagonal braces, &c.; and on these ribs areplaced spandril standards supporting the roadway bearers.

In small arches the spandril standards and the ribs may be cast together.

In some cases the ribs have been formed by crossing the joints of short plates ofdifferent widths, like the planks in wooden arches of small span; but usually theparts of the rib abut end to end, and are connected by flanges bolted firmly together.

The ends of the ribs are fixed to abutment plates bedded in the masonry skew backs.

The abutments and piers may be either constructed entirely of masonry, or else ofmasonry as high as the springing, and iron above. The masonry of the piers mightbe entirely dispensed with, and iron piles substituted, if some means were found bywhich they could be rendered incorrodible, which object may perhaps be attained bythe process of galvanism, as lately applied to that purpose. Iron has, however, thisdisadvantage, that if one of the piles were struck by a heavy body, it would probablybreak and cause great danger to the superstructure.

Plates VI. and VII. represent the bridge erected by Telford over the Severn atTewkesbury .

The arch is 170 feet span, 17 feet rise, and the roadway 24 feet in the clear,between the rail skirtings.

The foundations of the abutments are supported on piles with sills spiked downto them, and the interior spaces filled in with rubble stone well rammed and grouted.From the springing-course up to the roadway, the structure consists of six archeswith piers and pilasters,—the piers of masonry, the pilasters of iron.

The great arch is composed of six ribs laid at equal distances and placed on strongabutment plates, firmly bedded in masonry. These are secured in their places bygauge-pipes and connecting wrouglit-iron bolts, covered with grated plates fastened bymortises fitted to joggles in the main ribs, and by screwed flanges. On these thespandril standard crosses are placed, and secured by gauge-pipes and cross ties in themiddle, and tenons at top and bottom. On the top of these are the road bearers,one over each rib, and on them the road plates are secured.

Plate VI. Fig. 1 is an elevation of half the bridge.

Fig. 2 is apian of one of the abutments.

Fig. 3 is a plan of the platform of the abutment.

Fig. 4 is a section through the abutment and one of the land arches.

Plate VII. Fig. 1 is an elevation of the main rib, shewing the spandril crosses andskirting, railing, &c.

Fig. 2 is a plate for connecting the main ribs.

Fig. 3 shews the mode of connecting the pieces of the main ribs together. Therib flanges are 4 inches deep.

Fig. 4 is a section of one of the spandril crosses, taken at the middle. They areconnected by wrought-iron bolts, and kept apart by gauge-pipes.

Fig. 5 shews one of the main balusters.

Fig. 6 is a section of the hand-rail.

Fig. 7 shews the skirting.

Fig. 8 skew's the road plates, f inch thick, with flanges 3 inches high inside.

VOL. III. F