506

HISTORY OF ENGINEERING.

Book I.

Fig. 509.

ARCH OF LARY BRIDGE.

that the courses of the arch, except a few at the springing, should stand square, with theoutside of the bridge ; all these difficulties are, however, overcome by iron ribs, applied asin the present example.

Lari/ Bridge, near Plymouth, completed in 1827, is of cast-iron, and is built over theestuary of the river Plym, and connected with Plymouth Sound by the Catwater. Thewidth of the estuary is from500 to 600 feet; the tiderushes with a velocity of3 feet 6 inches per second,and flows on an average16 feet perpendicular.

There are five arches; thatin the centre spans 100 feet,and rises 14 feet 6 inches;the next are 95 feet span,and rise 13 feet 3 inches; theother arches are 81 feet span,and rise 10 feet 6 inches;the piers are 10 feet and9 feet 6 inches ; the road-way between the abutmentsis 24 feet wide, supportedupon five cast-iron equi-distant ribs, each at thespringing 2 feet 6 inchesdeep, and at the apex 2 feet2 inches. The masonry cost13,365/., and the iron-work27,126/.

Cast Iron Bridge overthe Avon, 7 miles north ofTewkesbury , over which theBirmingham and Gloucesterrailway passes, was com-pleted in the year 1840.

This bridge has three arches,each of 57 feet span, and aversed sine of 5 feet 2 inches; the distance between the centres of the piers is 66 feet6 inches, the breadth of the cutwaters 8 feet 6 inches, the clear water-way is 173 feet6 inches.

The two piers arc formed of cast-iron caissoons, filled in with solid masonry and concreteto the height of 12 feet; after which hollow masonry is used to support a capping plate ofiron, on which is placed eight columns over each pier, supporting the entablatures andarches, all which are of cast-iron. The caissoons are 41 feet 6 inches in length at the base,and 16 feet in width ; their ends are semicircular, and they taper upwards for 12 feet allround, the dimensions at the top being 34 feet 6 inches by 8 feet 6 inches, after whichthey ris* 1 perpendicularly 8 feet 9 inches. They have cast-iron flanched plates, of about ^ ofan inch in thickness, put together with screw-bolts and iron cement; each caissoon whenput together contains about 28 tons of iron. The total weight of the iron work in thisbridge is 520 tons, and the cost is stated at about 10,000/.

Cast-iron Bridge at Portumna, over the Shannon, which unites the counties of Galway andTipperary. The total length is 558 feet 6 inches, exclusive of the island; the width betweenthe railing is 17 feet.

The abutments are of masonry, of Portumna limestone, built with hydraulic mortar ; thesheeting piles are of beech and larch, whilst the main piles and waling pieces which supportthe roadway girders are of JVIemel timber.

There are thirteen openings, each of 18 feet 6 inches span, between the Tipperary shoreand the island, and twelve openings of a similar span between the island and the outer pierof the swivel bridge, which adjoins the Galway shore, and is 40 feet 6 inches span.

The cast-iron girders are 20 feet in length, 17 inches deep, and 1^ inch thick, with aflanch at the top 8 inches wide, to receive the plates which carry the roadway ; there is alsoanother flanch at the bottom 4 inches in width. The roadway plates are | of an inch inthickness, secured by bolts and plates put together with iron cement. On the outsidegirders, cast-iron plates are affixed, which carry the railing of wrought-iron; each girderwas proved by passing along it a weight of 12 tons, whilst it was mounted on two piers at20 feet apart. The swivel bridge has two leaves, with a clear opening of 40 feet.

The total cost of the bridge, including abutments, cofferdams, and superintendence, was24,131/., it was executed under the superintendence of Mr. Rhodes.

SECTION THROUGH PIER.