Modern Gasworks Practice

THE STORAGE OF GAS 637 Choice should fall on the lightest section which fulfils the required condition as to stability. Section A would, therefore, be decided upon, for the very slight deficiency would be more tlian compensated for by the metal in the web. Some designers might not care about a section so deep as 18 inches for a holder of this size, and if such is the case, that under heading D, thougli slightly heavier, might be conveniently employed. It should be mentioned that with a rolled standard of this description some engineers base the required flange area on the figure given by the formula relating to the bending moment on the single bay only, delegating the remainder (i.e. the area required to resist the capsizing of the holder as a whole) to the web of the section. It is necessary to emphasize, however, that when a composite section of the lattice-braced type is adopted, no reliance must be placed in. the web. For this reason, and also owing to the faet that a maximum stress of 6 tons per square inch. has been taken, the airthor prefers to deal with all sections in the manner indicated above, so that when the web is of assistance a small inerease is obtained in the gross factor of safety. TYPES OF STANDARDS The ultimate design of the standards must primarily be inflnenced by the calcu-lation for area as shown above. Keeping this figure in view and bearing in mind generally accepted working precedents, it is possible to evolve a section combining a minimum of metal with a maximum of stability. For instance, the metal compos-ing the flanges may (within certain limits) be reduced in proportion to the inerease in depth of the section ; in other words, the weight per foot run may be decreased as the depth. becomes greater. So far as the plain rolled steel joist is concemed, this possesses the disadvantagfe that it cannot be reduced in cross-section towards the top of the columns, thus considerable wastage of metal occurs. In spite of this, however, a joist will be found the most economical section for holders up to about million cubic feet capacity, owing to the cornparativeJy small amount of labour involved, and in contrast to the expenditure under this item in connection with standards of the built-up description. Also the plain gilder will weather Letter and offers less chance of deterioration. So far as the old-fashioned cast-iron columns are concemed, it may be noted that gasholders designed on these lines. cannot be treated as a braced cantilever, as is the case with the modern structure. In the old pattern each standard forms a cantilever in itself, and although. horizontal gilders, and in some instances, light diagonal ties, coimect the columns together, it cannot be assumed that the framework is of such elasticity as to distribute the loading through out the whole. Being of the rigid type each column takes the loading in accordance with the direction in which this may be imposed, and rigidity is obtained by employing a superabimdance of dead weight. Fig. 389 shows the elevation of a number of forms of gasholder columns, both steel and cast iron. The cast-iron designs with pulleys for the chains of counterbalance weights are now obsolete, except in special circumstances.