III.

PARTICULARS OF THE CONSTRUCTION OF A CAST-IRON BRIDGE OVER THE LARY NEAR PLYMOUTH, BY JAMES M. RENDEL, CIVIL ENGINEER; MEMBER OF THE ROYAL INSTITUTION OF CIVIL ENGINEERS; AND OF THE PLYMOUTH INSTITUTION,

THE Construction of bridges of large dimensions has, in every civilized age and country, been considered an object of general interest. Nor is this to be wondered at, when we regard their utility, or the difficulties to be encountered in their erection. It is not however my intention to attempt the history of this interesting species of architecture-It has already been amply discussed in several valuable treatises-All that I propose in this paper, is to describe a work of great local importance, of difficult construction, and, in some respects, of novel arrangement.

The Lary bridge is constructed over an estuary, from which it derives its name, and is distant from Plymouth about one mile and a quarter.

It connects a large agricultural district, on the southern shores of the county, with the populous and improving neighbourhood of Plymouth.

The Earl of Morley, whose property at Saltram forms the southern bank of the Lary, early perceived the importance of a more direct and commodious communication than the circuitous route by Plympton; and, in the year 1807, engaged Mr. Alexander, an engineer of eminence, to survey and report, on the practicability of erecting a bridge for that purpose. That gentleman reported that, in consequence of the unfavourable nature of the bed of the river, the erection of such a structure would, (if at all practicable,) be attended with enormous expense. The idea of a bridge was therefore abandoned; but, fully convinced of the importance of the communication, his Lordship, being proprietor of the neighbouring antient Ferry between Oreston and Catdown, was enabled to establish a Ferry Boat of an improved character. By means of this boat, which, from its peculiar construction and accommodation was called a 'Flying Bridge', carriages of every description, with their horses attached, were ferried across the river with much greater safety aud convenience than by any ferry boat of common construction. The success of this establishment sufficiently proved its utility to the public; but although superior to all other ferries in the neighbourhood, it was liable to interruption in bad weather and spring tides, and in proportion as the public became practically acquainted with its advantages, these interruptions were felt and regretted.

In September 1822, having projected a Bridge of Suspension across the Tamar at Saltash, I waited on the Earl of Morley to solicit his support. With that quickness, which in all matters of business characterizes his Lordship, he suggested the applicability of the principle to a bridge over the Lary, and directed me to turn my attention to a design for that purpose.

The drawings being approved, an Act of Parliament was obtained in the session of 1823 for carrying the plan into effect. Subsequently however, circumstances occurred to occasion the abandonment of the site first proposed, and the one on

which the present bridge is built, being unfavourable to the erection of a bridge on the principle of suspension, the original intention was relinquished. In the session of 1824, another act was therefore obtained, repealing the act of 1823, so far as related to the Suspension Bridge, and extending its powers to meet the erection of the present structure.

Aware of the opinion of Mr. Alexander, I was anxious to acquaint myself with every circumstance that could be of importance, and therefore made a survey of the river. In the strait where the bridge is built, the waters are confined to a channel of about 550 feet wide (at high water,) formed by abrupt lime rock cliffs, which disappear immediately above the bridge, and leave the Lary a basin of considerable magnitude; receiving the waters of the river Plym, the Tavy, and minor

streams.

On boring in several places, it was found that the substratum was schistos or slate rock, lying nearly horizontal, at a depth of 80 feet below high water. The superstratum consists of a mixture of granite sand, deposited by the Plym, and alluvial matter brought in by the tide, which, having accumulated to a depth of 60 feet, on an average, a considerable portion of the higher parts of the basin is, at low water, left dry.

On minutely examining the ground at the site of the bridge, it was discovered that, from the boundary of high water on the northern shore, the rock dipped at an angle of 80°, and on the southern, at an angle of 35°:-that the chasm was filled with the above described deposits, to a depth of 70 feet, to which we bored, and that the maximum surface velocity* of

* Having obtained the surface velocity in the middle of the stream, the velocity at the bottom is easily ascertained; for it has been found, by experiment, that if from the square root of the surface velocity expressed in inches per second, unity be subtracted, the square of the remainder is the velocity at the bottom. If, therefore, the former velocity bev, the velocity at the or the mean velocity = v — √ v + §.

bottom=v2√o+l.

spring tides, through this comparatively narrow channel, was three feet six inches per second, and their perpendicular rise from eighteen to nineteen feet.

It further appeared that inasmuch as the matter composing the bed of the river, was loose to a considerable depth, whatever tended, by narrowing the channel, to increase the velocity of the current, (such for instance as piers of a bridge,) would at the same time, deepen it. To avoid such obstructions, as much as possible, we determined on having the arches as large as a due attention to economy would admit. But as large stone arches are very ponderous, the expense of preparing adequate artificial foundations for their piers and abutments, on such ground, would have been much greater than it was deemed prudent to incur. And as cast-iron combines durability and strength, with lightness, it appeared the material best fitted to meet the several necessities of the case.

The value of iron, as a material for bridge building, is fully displayed, in many magnificent arches constructed in this country. Its application to such purposes, was an idea worthy of English artisans; and in point of boldness of design, and usefulness, is nearly allied to the invention of the steam-engine.

The Colebrook Dale company, had the honour of taking the lead in the construction of cast-iron bridges. In the year 1777, that company constructed the first over the Severn, immediately below their works. This bridge is one hundred feet span, and forty-five feet rise. As a first attempt, it may be considered a bold and ingenious design.-It is, however, defective in combination; but this derogates nothing from the merits of the inventors, who, observing the comparative incompressibility of iron, and the facility with which it could be moulded into any form, first saw, and demonstrated, its fitness for the construction of large arches.

Since the completion of the Colebrook Dale bridge, the advantages of iron, for such purposes, have occupied the attention of our most celebrated engineers; and the result has been the employment of it in the construction of some of the finest specimens of bridge architecture. We need only mention the one at Sunderland, the Southwark bridge, and one lately built over the Severn, near Tewkesbury.

To prove the superiority of iron, it need only be stated that the specific gravity of cast-iron, of the quality termed gun metal, is to the hardest description of granite as 2,75 to 1; but its power of resisting pressure, is in the proportion of 33 to 1. Hence it would appear that a pillar of the hardest granite would crush at its base, when raised to the height of 9600 feet-while a pillar of cast-iron would bear an elevation of 115,200 feet, before its base would give way.

It is this superiority of strength in proportion to its weight, which makes cast-iron of such value in the construction of large arches; and it is pleasing to reflect that, by a judicious combination of parts, we have the means of extending the mechanic bow, far beyond what could have been done with any other

material.

In the construction of frame-work of every description, it should be a first care to avoid complexity and mutilation in the several parts. It should also be recollected, that quantity of material does not constitute strength, unless every part is accurately proportioned and adjusted to the several strains to which it is exposed. It is the due observance of this principle that stamps the value of every piece of framing, but more especially of that now under consideration.

In the construction of iron arches, (after having secured adequate abutments) let each segment of a circle, or any other curve composing a rib, be formed of pieces, as long as can