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tanks, the riveting for the lowermost strakes becomes difficult owing to the large number of rivets for which accommodation must be found. Sometimes the diameter of the rivets is increased in order to reduce their number; and this, within reasonable limits, is good practice, since the shearing resistance of a rivet varies as the square of its diameter. Rivets more than 1 in. in diameter are apt to be very troublesome to close properly, however, especially out in the field; and it is by no means certain that any real saving is effected by their- use.
With regard to the suggestion sometimes put forward that the vertical seams should be lap joints, it will be seen that such a course would be the reverse of advantageous in practice. Since the
resistance of each rivet would be practically halved by reason of its being placed in single shear, the total number of rivets in the joint would probably not be less than with the double-covered butt joint; while the slight saving through the omission of the butt covers would almost certainly be more than cancelled by the necessity for thinning and setting the plate corners —an extremely troublesome matter with such heavy plates and very long laps, and likely to cause trouble with such high pressures.
Cases may arise in which a saving in the plate thickness of a strake is obtainable by arranging the rivets on some such lines as those indicated in Fig. 160. The increased pitch at the first row of rivets past the edges of the butt covers causes less reduction in the plate section, and thus may render permissible the use of a plate thickness which might otherwise be difficult to justify. It will be clear, however, that the calculated increase in the plate resistance obtainable by such means cannot be very great; while the distribution of loading among the rivets, as well as over the plate section, is highly problematical. Such methods should be used very sparingly; and it is an unfortunate fact that the only cases in which they might be of real assistance are just
those in which the need for reliability is greatest, and the consequences of some relatively slight deficiency likely to be most serious.
The use of a pitch much less than three times the rivet diameter is undesirable not only because it involves a large reduction of the
plate resistance (thus necessitating the use of thicker plates), but also because it tends to increase the difficulties of riveting by reducing the clearance spaces for the tools.
Trouble may be saved in designing by the construction of a diagram such as that shown in Fig. 161.
The principle on which the diagram is constructed will be clear from the following notes. Since
_ HD _ H D X 1000 _ H -p. / 25 \
h« ~ 270 - hm> ~ 270 x 16 _ n U V1o8>
•-. ^1000 = H (^qs) per ^oot °* tank diameter. Where
^1000 = I0> H = I0^2g 10 = 43 ft- nearly. Total tension in strake
= (6^5HDx_5\ tons.
Rivet area required = A = H D ( 5 X —z—*) sq. in.
^ \2 x 2240 x 9/625/ H
hundredths of a sq. in. per foot of tank diameter. Where
. u 20 X 17,248 _ .
Alon = 20, H = — = 28 ft. nearly.
Modifications, refinements and approximations will doubtless suggest themselves, both for simplifying the construction of the diagram and for increasing its usefulness.
Since the rivet areas required have been estimated on the basis of shearing resistance in double shear, riveting provisionally designed from Fig. 161 requires investigation with regard to bearing resistance; while appropriate increase in the rivet areas must be made if the rivets are to act in single shear.
ELEVATED CYLINDRICAL TANKS
58. Elevated Cylindrical Tanks.—An elevated cylindrical tank, supported upon a substructure of braced steelwork, may have either a flat floor or a dished bottom, as indicated in Fig. 162.
The flat floor needs a system of deck beams to support it, and this fact is often put forward as a disadvantage of the flat floor
by those who favour the dished bottom. It will be seen, however, that with the flat floor the problems of design are simple, the type of work involved in manufacture comparatively straightforward and cheap, and the fabricated material free from special difficulties as regards transport and erection—all factors tending towards economical and rapid production of the structure as a whole.
The dished bottom adds to the capacity of a tank, and may be considerably lighter than the flat floor with its deck beams; but its action is likely to be somewhat uncertain with changes in the head of the contained liquid, while the work involved is obviously expensive, both in manufacture and erection. Besides the troublesome and costly work of bending and riveting the plates, two rather serious practical difficulties are introduced by the dished bottom. First, the connection of the bottom with the cylindrical wall of the tank proper is awkward, and requires great care in design and fitting to ensure adequate strength and tightness against leakage; and second, the support of the tank as a whole becomes I complicated through the interference of
^ I the suspended bottom.
"T^V I /: - 59- ^lat-Floored Tanks.—-The roofs
/ j STANCMION8 j \^ an(j wans 0f elevated cylindrical tanks
I I I \ with flat floors may be of design and
T~! I T j construction similar to those for tanks
\! sTANcLoNs I / which stand upon a flat base at or near I ground level.
The floor-plates may be arranged on
a lay-out similar to those suitable for 1IG It>3- ordinary tanks, as already described, but
the plates must be of such thickness as will permit of their spanning between the supporting joists without exceeding the permissible limits of stress. The methods of treatment from this point of view are, however, not materially different from those already described in connection with the flat floors of elevated rectangular tanks; and hence, no further discussion is necessary beyond pointing out that the joists supporting the floor plates should be arranged to lie at right angles to the main (long) seams, and the transverse seams placed where they will not be subjected to stresses likely to cause them to open sufficiently to become leaky.
Elevated tanks are seldom more than 40 ft. (usually they are in the neighbourhood of 20 ft.) in diameter; and hence the author is of opinion that only four stanchions are necessary for their sup port, arranged at the corners of a square, as indicated in Fig. 163 By this means, the supporting structure may be braced to form a square tower, with great strength and stiffness in resisting the