Say, 55 square inches. From formula (51) we have-it being noted that the flanges are symmetrical about the centre of section that is, 2.84 tons per square inch on the outer flange and 4 lbs. per square inch on the inner flange. 3 Fig. 275. Now, take the rib at a depth of 12 feet 9 inches, and deal with section 3, shown in fig. 275. In this case the inner plating is the waterbearing surface, whose radius = 51 feet, nearly. Say, 45 square inches. In this case the flanges are not symmetrical about the centre of section. Accordingly, we must find the stress in each flange It will be observed that all the foregoing stresses are well within the safe limits for mild steel. The thickness of the lowermost plating works out as follows: Such is a very condensed outline of the calculations entailed in connection with the design of dock gates. Gate Fittings. - We now turn our attention to some of the more prominent details connected with gate construction, leaving aside for the present those matters which relate to the working of the gates. These will be more advantageously dealt with in the chapter on Working Equipment. Rollers and Roller Paths.-Gates may be entirely hung upon a pivot or axis at the heel-post, or they may derive partial support from truck wheels, or rollers, placed under them at one or more points. There is much conflict of opinion among engineers as to the value or otherwise of the latter method. On the one hand, it is urged that rollers add unnecessarily to the weight and expense of the gates, that they are liable to get out of order, that they are difficult to adjust and repair, and that, generally, they are a source of much anxiety and inconvenience. On the other hand, it is contended that they are a valuable means of support, that they reduce the friction on the heel-post and relieve the stress on the anchor blocks, and that they can be maintained in a state of efficiency with very little trouble. Generally speaking, Continental (more especially Dutch) practice inclines to the former view, English practice to the latter, but there is no absolute uniformity in either case. Rollers have been, and are being, dispensed with at Hull, while on the Mersey, the Manchester Ship Canal, and elsewhere they are still the invariable rule. It may, however, be fairly conceded that for small wooden gates and for iron gates with buoyancy chambers, rollers are not absolutely essential. Heavy wooden gates of large span certainly do gain in steadiness by the attachment of rollers to their outer extremities. Types of rollers in use at various ports are illustrated in figs. 276 to 278, 279, and 294. Clapping Sills.-The facing of the lowermost horizontal member of a gate, forming a watertight joint with the dock sill, is almost invariably of wood, in wood and iron gates alike. Indiarubber, as a watertight material, Back Elevation Side Elevation Figs. 276, 277, and 278.-Gate Rollers at Liverpool. is not employed to any noticeable extent, though there is no apparent objection to its more extended use. An arrangement proposed by |