A Treatise On The Principles And Practice Of Dock Engineering

IRON COLUMNAR PIERS. 287 the nature of a temporary structure ; indeed, it is doubtful whether it is applicable to other than the particular localities in which it has been devised and practised, admittedly with success, where timber is plentiful and cheap, and where present requirements outweigh considérations of future contingencies. Cribs are box-shaped frames of timber (pine, cedar, ash, tamarac, or elm), constructed in open-work, with numerous compart- ments formed by means of transverse and longitudinal ties. They range from 30 to 50 feet in length and are never narrower than the total height, with a minimum in the shallowest cases of 20 feet. The main timbers åre 12 inches square throughout, except in the lowermost course, or grillage, where they are 12 inches by 18 inches. The transverse and longitudinal ties are about 10 inches by 12 inches, and the structure is held firmly together by Ig-inch wrought-iron bolts. This method of construction will be tolerably clear from an inspection of fig. 224. The préparation of the site for the cribs is a matter of importance. A sandy bottom is not very suitable, giving rise to unequal settlement. A mound of rubble has been found to answer the purpose best. The cribs are framed on a sheltered beach, within easy reach of a draught of 10 or 12 feet of water. After three or four courses have been bolted together the structure is launched, and additional courses put on until the height is several feet greater than the depth of the jetty site. The crib is then towed into position and weighted with stone until it sinks, after which it is filled level with the top. After the final settle- ment, all the cribs are levelled up with wedges, and a roadway of planking is laid at a height of 5 or 6 feet above water level. The cost of crib work in 24 feet of water at Chicago, in 1871, amounted to about £30 per lineal foot. Iron Columnar Piers form light, ornamental structures, and they are often adopted where the traffic is mainly in passengers. The open columns also cause practically no interférence with the movements of the sea, and consequently the type is a suitable one in situations where there is a littoral current which it is inadvisable to deflect in any way. The columns are either piles themselves or are bolted to the heads of piles, unless the bottom surface be rock, in which case there is no need for piling. Screw piles are very generally employed, on account of the broad bearing afforded by the surface of the screw. The columns are arranged in bays, and are connected just below the decking by longitudinal and transverse girders, the depth and design of which will depend upon the distance apart of the columns. There is so much scope for individual taste and opinion that it is impossible to lay down any rules, of a general nature, in regard to the design of iron columnar piers. Two examples will suffice by way of illustration. At the port of Soukhoum, in the Black sea, there is an iron pier (figs. 225 and 226), about 154 feet long, constructed in 1889. The bays are each 14 feet in extent, with one of 7 feet at the end. There is also a further