A Treatise On The Principles And Practice Of Dock Engineering

COLUMNS AND PIERS. 381 In calculating the strength of a floor, due regard must be paid to the weight which is likely to be placed upon it. This may be estimated from the weights of the various items of which an average cargo is composed. Table xxix. gives a series of values obtained from actual observation, but it is necessary to point out that the figures can only be regarded as approximately exact, there being frequently a considerable divergence in the extremes from which the average has been computed. It will probably be found sufficient in ordinary cases to provide for an average pressure of 3 tons to the square yard on a quay floor, and of 30 cwts. to the square yard on an upper floor, exclusive of the weight of the shed structure itself. Care should be taken to see, by official inspection, that wharfingers and others do not stack or pile goods to a height inconsistent with the weight allowed for. This is more important in the case of heavy ores, kentledge, and metal goods, which exert a vastly augmented pressure per unit volume,, compared with bulkier articles. Columns and Piers.—To avoid roofs of excessive span in single storey sheds, and upper floors of undue weight in sheds of more than one storey, intermediate supports are generally introduced in both cases. These usually take the form of metal columns or brick piers connected longitudinally by girders. Brick piers are bulky ; they occupy a good deal of valuable space and obstruct light to a considerable extent. Columns, either of cast iron or steel, are better adapted to the conditions obtaining in dock sheds. Cast-iron columns are commonly circular in section and in one piece with planed bearing surfaces for the seats of the upper connecting girders. The bases may, however, be cast separately. Steel columns åre usually built by rivetting together marketable forms into a rectangular or I section, bases and bearings being formed by plates with gusset stays. Hollow columns have the advantage of forming suitable duets for rain water from the roof to the ground drain. All columns, piers, doorway jambs, and the like should have their bases protected by metal bumpers or (granite) guard stones to a height of about 2 feet above the floor. These are designed to ward off concussions with passing vehicles. For columns, hollow castings of an approximately ellip- soidal or spherical form, bolted together in two segments and filled with concrete, will be found most suitable. Occasionally, wisps of straw have been wound round the column prior to the insertion of the concrete, in order to still further diminish the shock, but the précaution is of dubious value. On account of the unsatisfactory behaviour of ironwork under the heat of a conflagration, columns of concrete strengthened by a hearting of metal have been proposed as a substitute for the ordinary type of iron and steel columns. It will certainly be found expedient to leave no metal surface exposed, and one valuable safeguard is to encase metal columns with external fireclay cylinders. These may be obtained in lengths of 2 feet or less; they are generally about 1 inch thick and exceed the diameter