646 MODERN GASWORKS PRACTICE
practice, however, the cliief danger of buckling lies, not so much in the stresses pro-duced by wind and top pull, but from the possibility of the bell tilting during its movement on the guide stanchions. In this direction a heavy drift of snow might give rise to danger.
For the purpose of withstanding any buckling effort which might occur, the dome curb in holders of any size should preferably be designed on the girder principle, or be provided with suitable gussets. Various forms of top curbs, which. have been adopted in practice, and some of which are not to be recommended, are shown in Fig. 394. Cripps Jias pointed out that if these curbs are of sufficient strength to resist the compressive stress, then there will be little likelihood of buckling. Common designs for bottom curbs, that is the lower curb of the bottom lift, are shown in Fig. 395, while Fig. 397 iHustrates the rollers and bottom curbs on. the three lifts of a telescopic holder.
Fig. 397.—Bottom Cukbs and Rollers, 3-Lift Holder.
COMPRESSION IN THE TOP CURB
The compressive stress in the top curb and hence its necessary dimensions may be oalculated as follows :—
Let D be the diameter of the top lift, in feet.
,, R ,, ,, rise ,, ,, dome, in feet.
,, P ,, ,, effective pressure thrown by the holder in 1b. per square foot.
As regards pressure it will be seen that if the total weight of the holder bell
W 1b
is W 1b., then the nominal pressure thrown would be ---------—. It is neces-
area of bell
sary tö consider, however, the upward pressure of the gas on the dome sheeting, which actually detracts from the total weight of the dome.