A Treatise On The Principles And Practice Of Dock Engineering

386 DOCK ENGINEERING. 4. That where water lodges to the exclusion of air, as frequently occurs at the back of ridge laps or the lower end of bottom sheets, or under sky- light aprons, the white efflorescence is transformed into a light umber paste which attacks the zinc in annular forms, and speedily rots it. 5. That where steam power is largely used under a roof, and the dis- charge from the furnace chimney is delivered within the shed, the zinc covering in the immediate neighbourhood, and especially at the openings through which the smoke makes its exit, shows signs of wear corresponding to the effect produced by a fine sand blast. The zinc also becomes soft and loses its elasticity. Galvanised and Corrugated Iron Sheets are open to all the objections enumerated with regard to zinc, with the additional drawback that, as soon as the thin coating of zinc is perforated, the galvanic action set up between the two metals enormously increases the rate of decay. This is particularly in evidence at bolt-holes and fastenings, where the leather washers usually employed afford but imperfect protection from the access of moisture. In many instances, sheds constructed with galvanised sheets have had to be coated with tar or black varnish to preserve them from further ravages. Weight of Shed Roofs—In order to afford a rough estimate of the weight to be carried by columns and side walls, the following approximate data will be found useful. The weights are given in Ibs. per square foot of horizontal area, covered by roofs of from 20 to 60 feet span :_ TABLE XXXI. Timber trusses, ineluding purlins, ..... Si to 71 Iron .. » Common rafters, 51 3 si Battens, 2 Boards, } to IJ inches thick, 28 to 4t Slates, 5 14 Tiles, 13 20 Lead covering, ....... 6 9 Zinc ,, 1t 2 Corrugated iron covering, Allowance for snow, 21 5 3t „ „ wind, 20 , J 40 From a brief but detailed consideration of some of the more salient features of shed construction, we now pass to a succinct review of the types to be met with at various ports. Tilbury Dock Sheds, London.* ^ico “ The twenty-four berths in the branch docks are each provided with a quay shed 301 feet long and 120 feet wide, giving a floor area of over acre, the roofs being constructed with timber storey-posts, iron roof- * Scott on “ Construction of Tilbury Docks,” Min. Proc. Inst. C.E., vol. cxx.