A Treatise On The Principles And Practice Of Dock Engineering

EFFECTIVE WEIGHT. 303 As regards the relative advantages of wood versus iron gates, the following points may be noted :— 1. Dead Weight.—For a given width of entranoe, wooden gates are considerably the heavier. Greenheart is the wood now most extensively adopted in this country, but in spite of the fact that its specific gravity, though high for timber, is considerably less than that of either wrought iron or steel, being only 1-1 to 1'2 as compared with 7'6 and 7'8 for the metals respectively, yet it outweighs them both by reason of the excessive bulk required to offer an equal resistance to stress. This disparity in strength is still further emphasised in the case of the lighter woods, such as oak and pitch pine, considerably in vogue at Continental ports. And it must also be observed that no inconsiderable addition is made to the weight of a pair of gates by the unavoidably extensive use of metal fittings and connections. The weight of a pair of iron gates, 25^ feet deep, at Dublin, for a 70-foot entrance is stated to be 90 tons. A similar pair of steel gates at Limerick are about the same weight, while a 69-foot lock at Dunkirk possesses iron gates, 24 feet deep, weighing 88 tons. As against these fairly representative values for metal gates may be set the weight, 204 tons, of the wooden gates (48 feet deep) to a 70-foot lock at Avon- mouth. These gates are mainly framed in pitch pine and memel, the heelposts and mitreposts alone being of greenheart. The weight of the iron fittings, including a cast-iron roller path, amounts to 42 tons. At the south lock of Buenos Ayres Harbour, the waterway is 5 feet less in width and 13 feet less in depth, but the gates weigh as much as 206 tons, owing to their entire construction in greenheart. For entrances of greater width, wooden gates attain enormous figures, as, for example, the green- heart gates (44 feet deep) at a 90-foot passage at Liverpool, which weigh no less than 330 tons. It is quite safe to assert that a pair of metal gates of the same size would not exceed half that amount. 2. Effective Weight.—-Not only is the dead weight of wooden gates necessarily much in excess of that of iron gates, but the practicability of forming watertight compartments in the latter, constitutes a means of still further reducing the actual working load, since the flotation power thus obtained may be arranged so as to practically counterbalance the weight of the gates, leaving only a small margin for stability. By this means the power required for opening and closing the gates is reduced to a minimum. Even in localities where there is very great tidal range, and where anything like an exact counterbalance would be attended with much difficulty and some danger, the reduction in weight which can be safely made is far from negligible. At Dunkirk there were, some short time back, two similar entrances, 69 feet wide, one fitted with iron and the other with wooden gates. When immersed at mean sea level, the weight of the iron gates was reduced from 98 to 16 tons, to which 16 tons of water ballast was added making 32 tons in all. The wooden gates, when immersed, weighed just double this last amount. They have now been replaced by iron gates.