It is now known that decay in wood is caused by fungi or low forms of
plant life which cannot live without a certain amount of water, food,
heat and air.
A fence post decays first at the place where it enters the ground,
because at that point the conditions are most favourable. If wood can
be kept entirely under water, one item--air--is lacking, so the fungous
growths cannot exist and the wood will last indefinitely. This has been
proved in many instances. One of the old Viking ships was raised from
the bottom of the Christiania Fjord, Norway, after having been under
water for a thousand years and it was found to be in a perfect state of
preservation. Even the rudder oar or steerboard and wooden shields were
intact.
As soon as it was brought into the air the process of decay began, and
it became necessary to coat it with preservative. It stands to-day, 103
feet long, in the museum at Christiania.
Many other instances of under-water preservation might be mentioned.
The other extreme is also true. Wood which is kept perfectly dry will
last indefinitely, as in the case of woodwork taken from the pyramids
of Egypt, 3,000 years old, which is found to be perfectly preserved.
[Illustration: Fig. 233. A pile]
But when wood is alternately wet and dry it decays rapidly. A pile
driven into the bottom of a tidal river is a good illustration. If such
a pile be divided into four sections (see Fig. 233), _a_ is always in
the ground, _b_ is always in the water, _c_ is alternately in air and
water, _d_ is always in the air. Sections _a_ and _b_ may be considered
to be under the same conditions and should last the longest; _c_ should
decay first: _d_ would last indefinitely if the atmosphere were always
perfectly dry; but humidity and rain, air and heat combined finally
bring about decay, and although this part of the pile will last longer
than _c_ it will in time decay. Section _c_ should be coated with a
preservative.
Various woods under the same conditions act very differently and
according to no well understood law. For example, in contact with the
soil black locust is our most durable wood. It is very hard, and its
life under these conditions is estimated at from ninety to a hundred
years. Red cedar comes next, though it is soft wood. Oak decays in a
few years; chestnut, much softer, lasts two or three times as long. Our
approaching timber famine has induced a study of this subject, since
the preservation of wood is becoming an absolute necessity.
It has been found that certain materials put on the wood before it is
placed in the ground prolong its life. Coal tar, wood tar, paint, and
creosote all help, but creosote has so far proved to be the best. It is
one of the by-products of coal tar and is being used extensively by the
railroad companies for prolonging the life of ties.
Experiments with creosote have brought out some very interesting facts.
It has been found that after being treated with the hot creosote all
woods resist decay alike, regardless of their hardness or softness.
Consequently, a treated cheap wood will last as long as an originally
valuable one. This is a great gain, as it allows us to make use of wood
like the poplar which would otherwise be practically of no value.
The various coatings we put on wood, such as paint, varnish, oil,
etc., are intended not only to beautify but to preserve it, which they
do by filling up the pores and excluding moisture, preventing fungous
growths, etc. All of these coatings should be put only on dry wood,
else they prevent evaporation of the sap and may hasten decay.
[Illustration: Fig. 234]
Drying lumber increases its strength, as it has been found by
experiment, even as much as 400 per cent. if no checking occurs. When
this happens it counteracts much of the gain, and if the wood absorbs
moisture once more the strength will decrease. The strength of the
various timbers varies greatly, and sap wood is usually weaker than
heart wood.
The strains that may be brought to bear on timbers are illustrated in
Fig. 234, the arrows indicating the directions in which the forces
operate. At _a_ the wood is under tension, the forces at work on it
tending to pull it apart. At _b_ the piece is under compression, the
forces tending to reduce its length by forcing its fibres together.
A pillar supporting a weight is under compression. At _c_ the weight
tends to bend the beam. The upper part is under tension, the lower part
under compression. This is known as beam action, and depends on whether
the beam is supported at one end, as shown, or on both ends. Also it is
important to know whether the beam has a uniformly distributed load or
whether the weight is at one point only. The problems relative to beam
action are largely of an engineering character and involve considerable
mathematics.
Shearing is the sliding of one part of the timber along the grain. If a
piece of wood is cut to the form shown at _d_ and a weight applied at
_e_, the tendency will be for this upper part to slide down as shown at
_f_. When this occurs, shearing has taken place.
The strength of wood differs in resisting these various strains,
the tensile strength being greater than the crushing or compressive
strength. Ash, for example, has a tensile strength of 16,000 pounds
to the square inch, but its crushing strength is only 6800 for the
same size. The tensile strength of dry white pine is 10,000 pounds,
its crushing strength 5400 pounds, and its shearing strength varies
from 250 pounds to 500 pounds, showing that its weakest point is along
the grain. If the young woodworker becomes ambitious enough to think
of designing a bridge or large building he can find these figures
in any engineers' hand-book. There are so many important factors to
be considered that the amateur will do well to go ahead with great
caution. Knots and other defects reduce the proportionate strength of
large beams greatly, so that it would not be safe to assume that a beam
6 inches square would be 36 times as strong as a piece 1 inch square.
In upright posts of considerable length, not alone the crushing
strength must be considered, but a bending action enters into the
problem. Wherever the question of danger to life enters, as in a bridge
or a house, it is wise to leave a large margin for safety. We realize
this fully when we read of a grand stand holding hundreds or thousands
of people collapsing under the weight. The architect has also to reckon
with still other elements, such as wind pressure and vibration.