631
THE STORAGE OF GAS
when radial rollers alone are used, one-quarter of the columns must be considered as directly loaded. As stated above, however, the remainder of the framework will, indirectly, be taking its share of the load.
As the vertical columns are fixed. at one end only (t.e. to the top of the tank) the whole framing may be looked upon as a girder acting in th.e capacity of a huge cantilever. A simple cantilever when. loaded tends—
1. To shear off at the point of support.
2. To undergo distortion due to the bending moment. Hence, the vertical columns have a tendency—
1. To shear ofi at the base and overturn.
2. To bend and become distorted, thus causing some deformation of the true cylindrical form of the guide-framing.
The first condition applies to the frame structure as a whole, whilst the second affects each column individually in accordance with the wind pressure on any particular section.
The general proportions and scheme of design (such, as height, diameter, and probable number of standards) of the holder being known, the first step in the calculations is that of mum stress likely to standard.
determining the maxi-be met with. in each
Wind
Pressure
Consider a common form of gasholder ha,ving guide-framing extending to the full height of the bell.
Let L = total length. of columns, which in
this case is the same as the height of the bell (in feet).
Let D — mean diameter of bell (in feet).
„ P = Wind pressure (total force) in Ibs.
The wind pressure P acts at the centre of pressure of the beil, i.e. at a distance half-way up from the base. Therefore, the overtuming moment
-px|
but P = total wind pressure,
i.e. 16 Ib. per sq. foot on an area of L X D sq. feet,
P = 16 LD,
or, the overtuming moment — 16 LD X = 8 L2D.
This moment is resisted by the reactionary force R at the top of the guide-framing, and the moment of the reaction
f
II co r< to u