Søgning i bogen
Den bedste måde at søge i bogen er ved at downloade PDF'en og søge i den.
Derved får du fremhævet ordene visuelt direkte på billedet af siden.
Digitaliseret bog
Bogens tekst er maskinlæst, så der kan være en del fejl og mangler.
64
HARBOUR ENGINEERING.
strong wood, extremely durable under favourable circumstances, and more
particularly in dry and open situations. Piles, 100 feet to 120 feet long and
20 inches square, have been obtained in Tasmania. Stringy bark, according
to some authorities, weighs about 70 Ibs. per cubic foot, and blue gum about
77 Ibs. ; others place the figures at 60 and 65 Ibs. respectively. Some
variation of weight in different specimens is, of course, inévitable. The
transverse strength of unit beams (see p. 63) may be taken at anything
from 450 to 850 Ibs.
It will be noted that all the timbers in this group have a very high
spécifie gravity, and this property is found to be very useful in connection
with driving piles in water of any depth. The lighter kinds of wood have
necessarily to be weighted at the lower ends, in order to cause them to assume
an upright position suitable for driving.
As regards durability in marine situations, it cannot be claimed that any
of the foregoing timbers are absolutely immune from the attacks of insects.
On the contrary, there is distinct evidence that boring has occurred in each
kind of wood, though it is apparent that there is no great attraction in these
timbers when others are present in the neighbourhood. Greenheart appears
to be least susceptible, possibly on account of the poisonous oil which it
contains. At certain ports it exhibits no sign of any depredation whatever,
but this may be due to the absence of the inimical agencies. Altogether as
a class, the timbers are the least vulnerable of any which can be applied to
marine work, and in many instances they have demonstrated extremely high
resisting powers.
The second group includes timbers which, though durable enough in
themselves, are much more subject to insect attack.
Teak (Tectona grandis) is a native of India, Burmah, Siam, and Java.
It is a firm, durable wood, fine and straight in grain, and easily worked,
though possessing a tendency to splinter. It contains an aromatic oil of a
resinous nature, which, on exposure, coagulates to such a degree of hardness
as to spoil the cutting edges of tools. The tree often attains a height of over
100 feet and sometimes a girth of 10 feet. It is usually imported in logs
from 25 to 40 feet long and from 10 to 20 inches square. The weight of teak
varies from 41 to 52 Ibs. per cubic foot, and the transverse strength of a
unit beam lies between 600 and 700 Ibs.
Oak (Quercus) is found on both the European and American continents,
as also—less commonly—elsewhere. The best is grown in Great Britain. The
wood is firm, with a fine, straight grain, comparatively free from knots, and
it is readily cleavable. Logs vary from 10 to 40 feet long and from 10 to 24
inches square. The longer logs corne from America. Oak is heavier than
teak, weighing from 49 to 61 Ibs. per cubic foot; but it is not quite so
strong — -about 50 to 100 Ibs. less in ultimate transverse strength. Oak
contains an acid which corrodes iron, and is therefore destructive of bolts
and other fastenings.
Both the above timbers are admittedly assailable by insects, but they