686 MODERN GASWORKS PRACTICE
bars mto position, also in ensuring that the concrete forms a homogeneous mass round bars which must of necessity be closely-pitched. The factor of safety must depend largely on these two conditions, for if the tank is carelessly constructed the tensional stresses will be falling upon the concrete instead of on the steelwork. The result is that cracks and leaks will make their appearance. One of the great draw-backs to this type of tank is the time taken in construction, this heilig-—from the time of commencement until the tank is ready for service—nearly twice that roquired with a similar tank made from Steel. Probably the largest reinforced tank in existence is that erected at New York. This tank is 300 feet in diameter and 48 feet 3 inches deep.
It should be mentioned that reinforced concrete tanks develop minute hair cracks^ when the stress m the tensile members becomes excessive, in fact when the stress m the steel reaches 6,0001b. per square inch. The New York City Board, as a result of investigations with reinforced concrete cylinders, have Hmited the iniit stress in the reinforcement of water-retaining structures to 8,0001b. per square inch.
THE COST OF GASHOLDERS
A large holder is much cheaper per unit of volume than is one of smaller dimen-
Holder Tanks.
and Tank Cüst in £ Holder Capacity, (Thousands). in Thousands.
Fig. 428.—Chart showing Pre-warcosts OF ERECTING GäSHOLDERS AND TANKS.
sions. So far as the actual weight of metal is concerned, the holder of light construction is not necessarily the cheapest, for there will in all probability be additional labour expenditure which will more than absorb any saving in material. Increasing the number of lifts, moreover, actually renders the holder bell more costly ; but., in this case, there would be a reduced expenditure on tank and guide-framing, so that a three-lift holder will usually be cheaper than a two-lift holder of equal capacity. For very small holders a single lift is, as a general rule, the cheapest form of construction.
Per ton of metal the average cost of a gasholder in pre-war times varied between £19 and £22, this figure including the whole stru.ctu.re and steel tank. To-day the cost is so much as £60 to £70. As regards the individual items, much, of course, depends on size and design, but the total outlay will be accounted for somewhat as follows
Bell, complete . 35 to 45 per cent, of total
Guide-framing . 18 to 23 „ „
Tank (steel) . 35 to 45 „ „