The other type is that in which the vertical members are spaced further apart, and are of such dimensions and weight as may reasonably be expected to resist the shocks they may be called upon to endure, assisted by a heavily framed floor, and such bracing as it may be practicable to introduce. This type is of more rigid and unyielding character, and its individual members will probably derive less mutual assistance than the other, and in consequence the separate columns must, as before said, be of considerable weight and solidity. Such a type is commonly associated with supporting columns of cast-iron cylinders of considerable diameter, and which may at times attain the dimensions of bridge piers.

In either type it will, of course, be necessary to consider not only the amount of deck load to be carried, but the numerous accessories to be provided for on the deck of every commercial wharf or jetty, such as foundations for fixed cranes, the roadways for travelling cranes, the attachments of bollards or fairleads, while in jetties for the loading or unloading of coal special consideration will be necessary in the provision of coaling appliances, such as hoists, coal tips, coal spouts or hoppers, and the like. A railway system will be found frequently associated with such structures, and must be provided for in the design of the deck, including possibly framing for turntables, while the necessary requirements of electric, hydraulic, compressed air, or steam power will each demand separate consideration in connection with the conveyance of such power by pipes or cables, in troughs or otherwise, from a central station to the points at which the power is to be applied.

Commercial jetties or wharves, of which the general requirements are as above sketched, are usually found in waters more or less sheltered, and the questions of sea-exposure and the "fetch" and dimensions of waves are of less importance.

But in the case of jetties or piers on exposed coasts, such as the familiar type of the promenade pier at our fashionable watering places, conditions other than those above described prevail, and it becomes of vital importance to consider the maximum height of wave likely to be experienced, and the minimum height above high water of spring tides at which the promenade deck should be laid to secure safety from the wrecking and lifting power of the crest of the wave, the security of the structure as a whole against wave action being derived principally from the small dimension of the vertical supports or columns, and the correspondingly small resistance offered by them to the advancing wave, a security which, however, may become grievously imperilled if the wave bears with it anything in the shape of wreckage, floating logs, or the like.

In such structures, the decking arrangements are of a simple character, the purpose of the pier being less commercial, the size and weight of the craft alongside being possibly not more than that of a loaded excursion steamer, the condition of stress arising from shocks or bumps are less severe (except so far as they may be affected by sea exposure), although it is customary to relieve the slender structure of such piers from this kind of stress by providing a totally independent landing jetty, so arranged as to take the shock of steamers coming alongside, to the relief of the more slender structure behind.

But whatever may be the type of jetty, wharf, or pier to be adopted, the details of the supporting piles or columns will always be largely influenced by the nature of the strata of the sea-bottom into which they are to be driven, screwed, or by other means brought down to their foundation level.

The subject of the measures to be adopted to force down the supporting piles or columns of a jetty, or the piers of bridgework, and the various precautions to be adopted in meeting all the contingencies arising from very variable qualities of strata, with widely differing resisting and bearing powers, and of varying degrees of Watertightness, is of far too wide a scope than to be more than hinted at here.

The procedure may include ordinary pile-driving as applied to iron piles, the use of screw piles, either with or without the water jet, the combination of screws with a serrated cutting edge, the forcing down by dead weight loaded up on the top of the pile or cylinder, combined with the removal of the soil inside either by means of a "miser" or in larger cylinders by a grab, the use of the "disc" piles with a water jet, - these and other methods, including the use in important cases of the compressed air system, will each and all take their share in the final results, while the application of any one of them will depend in all its details upon the variable nature of the local conditions which may have to be met, and in relation to which no general law can be laid down as to the best system to be adopted in any one particular case.

Thus, for example, the adoption of the screw-pile system will at once bring to the forefront the design of the screw-blade to be adopted, its diameter, scantlings, pitch, and methods of attachment to the body of the pile. For soft soils, where a considerable bearing area of blade is required, the diameter will be of as large dimensions as is consistent with safe screwing and handling, although in such cases it is very essential that the strength of the blade section is sufficient, considered as a cantilever, to prevent its breaking off from the body of the pile, or becoming sheared either by vertical stress or torsion.

On the other hand, in certain hard soils the blade diameter may have to be reduced to comparatively small dimensions in order that the labour of screwing may be kept within the bounds of what can be practically applied, and that the connections of the pile and screwed portion may not be sheared off, as sometimes has happened, by excessive torsional stresses.

In certain cases of exceptionally soft soils it has been found necessary to supplement the bearing area of the screw-blade, or the frictional resistance of the pile itself, by timber "platforms so constructed as to distribute the pile load over a considerable area.