648 MODERN GASWORKS PRACTICE
and W must be horizontal, otherwise the holder bell would not be in equilibrium, but would tend to be always moving upwards or downwards in its framing.
The resultant of T and W is, therefore, represented by the line DO (Fig. 398).
. •. Resultant = T cos. a
D2P D2P
=—-------X cos. a —----- X cotan. a.
8 sin. a 8
The angle « may be readily found as follows :—
D2 - 4R2 cos. «= , ■ .
D2 + 4 R2 where D = diameter of holder in feet, and R = rise of dorne in feet.
Internal Pressure of Gas on Sides
Cripps’ method of calculating the efiects of this force is to consider the ring tension in the side sheets, and to allow a definite proportion of this as transmitted to the curbs. Thus ring tension
__ fe X D X where d — depth of lift in feet.
Then of the above is taken for holders ha ving vertical stays attached through-out their length, and for stays attached at top and bottom only. The efl'ect oi the internal gas pressure is to counteract in some degree the external wind pressure and bne,kling effect. The extent to which this efiect is feit is, at all events, an uncertain factor ; and, in the light of practical observations, the author considers that it is quite reasonable to allow for it by deductmg 55 per cent, from the usual figure taken for maximum wind pressure on a cylindrical surface; that is to say, to assume external effective wind pressure at 12 1b. instead of 26 1b. per square foot. This, of course, applies to those cases in which the vertical stays are attached throughout their length. Such, an allowance would be representative of the efiect in the majority of holders now erected, and in the case of the very large holders any slight error would be on the right side.
Objection. might be raised to this method owing to the fact that the gas pressure, the magnitude of which is easily computed, is taken as constant in all holders. But as the whole treatment must necessarily be one of some conjecture, the author considers that bis assumption, owing to its simplifikation of formulæ, is perfectly justifiable.
Effective wind pressure, therefore,
— D X d X 12 1b. per square foot.
But this is distributed to both. upper and lower curbs of the lift and is also withstood by the two diametrically opposite sections of the curb. Accordingly, the total force on any section of the curb
= 1 X D X d X 12 = 3 DcZ.
4
By a Summation of the above force and the resultant of the tangential and