A Treatise On The Principles And Practice Of Harbour Engineering

CHARTER IV. PILING. Use of Framework in Maritime Structures—Association of Piling therewith—Varieties— Bearing Piles—Sheeting Piles—Materials for Piles—Timber—Varieties—Destructibility and Preservation—Metal—Concrète—Reinforced Concrète — Typical Systems—Pile- driving—Sustaining Power—Varions Data. Structural Principles.—Maritime structures are, generally speaking, based on one or other of two distinct systems of construction. First, there is the compact, solid mass, capable of withstanding the attacks of the elements by means of sheer intrinsic inertia, and, secondly, there is the framework structure, composed of an association of members or parts, all slender in themselves, but so contrived and connected as to afford one another mutual support, and at the same time able to discharge special individual functions. Typical of the former system is the breakwater, mole, or quay, built as a solid mound or mass of rubble, masonry, or concrete, or a combination of these, the ideal being a homogeneous monolith, without break or joint. This type is really an adaptation of nature’s own system exemplified in rugged cliff and massive headland. The framework structure, on the other hand, is a strictly scientific design, utilising the minimum of material to the maximum advantage. It is based on the same theoretical considérations as those which govern the synthesis of all trusses, whether in the form of bridges, roofs, or other openwork. Its principal source of weakness lies in the jointing together of the various parts, for under the violent alternations of impact and recoil, which are characteristic of marine forces, there is every disposition for the joints to become loosened through excessive vibration. There is, moreover, another disadvantage attending those structures which are composed of unprotected metal and timber, viz., their liability to corrosion and decay. Both these considérations militate greatly against the realisation of any great degree of durability and permanence, and render structures of the second class inferior in certain respects to those founded on the former system, while, at the same time, they obviously involve much greater expenditure in the way of maintenance and repair. There are, however, circumstances under which framework structures become inevitable, and many others where they are undoubtedly désirable. 60