_TIMBER_
=Classification and Structure.=—A short study of the
classification and structure of wood will be useful, as it will enable
the scaffolder to use material free from the inherent defects of its
growth.
The trees used by the scaffolder are known as the exogens or
outward-growing trees.
The cross section of an exogenous tree shows, upon examination, that
the wood can be divided into several parts. Near the centre will
be seen the pith or medulla, from which radiate what are known as
the medullary rays. These in the pine woods are often found full of
resinous matter.
Next will be noted the annual rings forming concentric circles round
the pith. They are so called because in temperate climates a new ring
is added every year by the rising and falling of the sap. As the tree
ages, the first-laid rings harden and become what is known as duramen
or heartwood. The later rings are known as alburnum or sapwood. The
distinction between the two is in most trees easily recognised, the
sapwood being lighter and softer than the heartwood, which is the
stronger and more lasting. The bark forms the outer covering of the
tree.
=Defects in the living tree.=—Shakes or splits in the interior of
the wood are the most common defects in the living tree, and are known
as _star_ or _radial_ and _cup_ or _ring shakes_.
The cause of these defects is imperfectly understood. They are rarely
found in small trees, say those of under 10 inches diameter. Stevenson,
in his book on wood, puts forward the following reason, which, up to
now, has not been refuted by any practical writer. 'In the spring, when
the sap rises, the sapwood expands under its influence and describes
a larger circle than in winter. The heartwood, being dead to this
influence, resists, and the two eventually part company, a cup or ring
shake being the result’ (see fig. 55, where the cup shake is shown in
its commonest form).
[Illustration: FIG. 55]
[Illustration: FIG. 56]
The star or radial shake is a variant of the same defect. In this case
the cohesion between the sapwood and the heartwood is greater than the
expansive forces can overcome, the result being that the heartwood
breaks up into sections as shown in fig. 56.
The star shake may have two or more arms. More than one cup shake, and
sometimes both cup and star shakes are found in a single tree. The
radial shake is probably the most common.
The branches suffer in this respect in like manner as the trunk, the
same shakes being noticeable throughout
The development of these defects is the forerunner of further decay in
the tree, giving, as they do, special facilities for the introduction
of various fungi, more especially that form of disease known as the
rot. Wet rot is found in the living tree and occurs where the timber
has become saturated by rain.
Other authorities believe that these defects are caused by severe
frost, and their idea receives support from the fact that in timber
from warmer climates this fault is less often seen. This may be so for
the reason that it would not pay traders to ship inferior timber from a
great distance.
_Loose hearts_: in the less resinous woods, as those from the
White Sea, the pith or medulla gradually dries and detaches itself,
becoming what is known as a loose heart. In the more resinous woods,
such as the Baltic and Memel, this defect is rare.
_Rind galls_ are caused by the imperfect lopping of the branches,
and show as curved swellings under the bark.
_Upsets_ are the result of a well-defined injury to the fibres
caused by crushing during the growth. This defect is most noticeable in
hard woods.
It will be of no practical use to follow the living tree further in its
steady progress towards decay, but take it when in its prime, and study
the processes which fit it for the purpose of scaffolding.
=Felling.=—The best time to fell timber, according to Tredgold,
is mid-winter, as the vegetative powers of the tree are then at rest,
the result being that the sapwood is harder and more durable; the
fermentable matters which tend to decay having been used up in the
yearly vegetation. Evelyn, in his 'Silva,’ states that: 'To make
excellent boards and planks it is the advice of some, that you should
bark your trees in a fit season, and so let them stand naked a full
year before felling.’ It is questionable if this is true of all trees,
but it is often done in the case of the oak. The consensus of opinion
is that trees should be felled in the winter, during the months of
December, January and February, or if in summer, during July. Winter
felling is probably the better, as the timber, drying more slowly,
seasons better.
The spruce from Norway and the northern fir are generally cut when
between 70 and 100 years old. When required for poles, spruce is cut
earlier, it having the advantage of being equally durable at all ages.
Ash, larch and elm are cut when between 50 and 100 years old.
=Conversion.=—By this is meant cutting the log to form balks,
planks, deals, &c. It is generally carried out before shipment. A log
is the trunk (sometimes called the stem or bole) of a tree with the
branches cut off.
A balk is a log squared. Masts are the straight trunks of trees with a
circumference of more than 24 inches. When of less circumference they
are called poles.
According to size, timbers are classed as follow:—
Balks 12 in. by 12 inches to 18 in. by 18 inches
Whole timber 9 in. by 9 inches to 15 in. by 15 inches
Half timber 9 in. by 4½ inches to 18 in. by 9 inches
Quartering 2 in. by 2 inches to 6 in. by 6 inches
Planks 11 in. to 18 inches by 3 in. to 6 inches
Deals 9 inches by 2 in. to 4½ inches
Battens 4½ in. to 7 inches by ¾ in. to 3 inches
When of equal sides they are termed die square. In conversion the pith
should be avoided, as it is liable to dry rot.
When the logs are to be converted to whole timbers for use in that
size, consideration has to be given as to whether the stiffest or the
strongest balk is required. The stiffest beam is that which gives most
resistance to deflection or bending. The strongest beam is that which
resists the greatest breaking strain. The determination of either can
be made by graphic methods.
[Illustration: FIG. 57]
To cut the stiffest rectangular beam out of a log, divide the diameter
of the cross section into four parts (see fig. 57). From each outside
point, A and B, at right angles to and on different
sides of the diameter, draw a line to the outer edge of the log. The
four points thus gained on the circumferential edge, C,
D, E, F, if joined together, will give the
stiffest possible rectangular beam that can be gained from the log.
[Illustration: FIG. 58]
To cut the strongest beam:—In this case the diameter is divided into
three parts. From the two points A and B thus marked
again carry the lines to the outer edge as before. Join the four points
C, D, E, F, together, and the
outline of the strongest possible rectangular beam will result (see
fig. 58).
Boards—a term which embraces planks, deals, and battens—should be cut
out of the log in such a manner that the annual rings run parallel to
the width of the board. This method of conversion allows the knots,
which are a source of weakness, to pass directly through the board, as
A, fig. 59, and not run transversely across as other sections
(B) allow.
[Illustration: FIG. 59]
=Seasoning.=—Poles and scantlings, after conversion, are next
prepared for seasoning.
The drying yard, where the timbers are seasoned, should give protection
from the sun and high winds, although a free current of air should be
allowed. It should be remembered that rapid drying tends to warping and
twisting of the timbers.
The ground on which the timbers are stacked should be drained and kept
dry.
The method of stacking is as follows: The timbers are laid upon
supports a few inches above the ground at sufficient intervals to allow
of a free circulation of air between them, and on these others are laid
at right angles. It is usual to put long strips of wood about ¾ inch
square between each row to prevent the timbers touching, as, after a
shower, the timbers cannot dry for a long time if one is resting on the
other.
Planks, deals, &c. can be stacked by laying them in one direction
throughout, provided that the space which is left between the boards
occurs in one layer immediately over the centre of the boards beneath
(see fig. 60).
In many cases very little drying takes place before shipment, but the
same methods of stacking should be observed whenever it takes place.
Stacking poles by placing them on end is not recommended, as they may
warp from insufficient support. This point is more important when the
poles are required for ladder sides.
[Illustration: FIG. 60]
Timber may be considered to be sufficiently seasoned for rough work
when it has lost one-fifth of its weight by evaporation.
[Illustration: FIG. 61]
The poles from St. Petersburg have a narrow strip of bark removed in
four equidistant longitudinal lines throughout their entire length.
This treatment assists drying, and tends to prevent dry rot.
Weather shakes sometimes form on the outside of the wood while
seasoning (see fig. 61).
They arise owing to the sapwood contracting more when drying than the
heartwood. Unless they extend to a considerable depth they do not
affect the quality of the wood. Balk timbers, where the sapwood is
uncut, and whole timbers principally suffer in this manner.
_Water seasoning_—that is, having the timbers completely immersed
in water for a short time before drying. This is a common practice. It
is frequently carried out at the docks, where the balks may be seen
floating about on the surface of the water. This is a bad method, as
the wood is at the same time under the influence of water, sun and air.
Water seasoning may make the wood more suitable for some purposes, but
Duhamel, while admitting its merits, says: 'Where strength is required
it ought not to be put in water.’
=Description.=—Pine or northern fir (_Pinus sylvestris_) is
light and stiff, and is good for poles and scaffolding purposes, but
only the commonest of the Swedish growths are used for this purpose.
The hardest comes from the coldest districts. It has large red knots
fairly regularly placed, inclined to be soft, and starting at acute
angles. It grows chiefly in Northern Europe.
White spruce or white fir (_Abies excelsa_).—The knots are small
and irregularly placed, are dark in colour, and start at an obtuse
angle, showing an absence of clean wood. It is used principally for
scaffold poles and ladder sides. It will snap under live loads, and is
not so strong as pine. It also chiefly grows in Northern Europe.
Larch (genus _Larix_) is imported from Northern Europe and
America. It is yellow in colour, tough, and is suitable for poles,
very durable, free from knots, but warps easily.
Elm (genus _Ulmus_) grows in the British Isles. The colour varies
from the reddish-brown of the heartwood to yellowish white of the
sapwood. It bears considerable pressure across the grain, and is most
useful in balks, as it is liable to warp when in smaller sizes. Is
suitable where bolts and nails have to be used.
Birch (_Betula alba_).—Light brown in colour, hard, even grain,
which enables it to be easily and readily split in the direction of its
length. It is not considered durable. It is used chiefly for putlogs.
Is exported from Europe and America.
Ash (_Fraxinus excelsior_).—Very light brown in colour, extremely
tough, and makes excellent rungs for ladders. It is found in Europe.
Oak (genus _Quercus_), also known as the common British oak, is a
native of all parts of Europe, from Sweden to the Mediterranean. The
wood has often a reddish tinge; and the grain is fairly straight, and
splits easily. It is generally free from knots, and is most suitable
where a stiff, straight-grained wood is desirable. It also offers
considerable resistance to pressure across the grain.
=Selection.=—The importation of scaffolding timber commences in
March. In June the Russian timbers are on the market, and the arrivals
continue until October.
Poles are selected from spruce, pine, and larch trees. Balk timbers are
of elm and fir and spruce. The putlogs are from the birch.
Poles are known in two qualities—'prime’ and 'brack.’ These terms refer
to their straightness of grain, freedom from knots, regularity of
taper, which should be slight, and condition as to seasoning.
Buyers take them usually unbarked, as they rise from the stack, and
sort them afterwards for their different purposes.
As soon as possible the bark should be removed, as it holds the water
and insects, and encourages the growth of a white fungus which is the
precursor of dry rot.
The St. Petersburg poles are the most generally used for scaffolding.
They are weaker than the Christiania poles, breaking shorter under
cross strains, are whiter in colour, have a smoother bark, are
straighter in grain, and therefore make better ladder sides.
Christiania poles are more yellow in colour, and break longer under a
cross strain than other poles. They are only to be obtained early in
the year, and are soon bought up when on the market.
To test a pole remove the bark, then prise across the grain with a
penknife. If the fibres break up short and brown, the pole is decayed
and useless.
To test a pole for any local weakness lift one end, leaving the other
on the ground. Two or three sharp jerks will cause undue bending at any
spot that may be seriously defective.
Balk timbers ring, if in a sound condition, when struck with a hammer.
A fresh-cut surface should be firm, shining, and somewhat translucent.
A dull, chalky, or woolly appearance is a sign of bad timber.
Poles, unlike most converted timbers, are not branded. Those known as
_prime_ should certainly be used for scaffolding purposes. The
brack are rough and irregular in growth.
Balk timbers and smaller scantlings are, in the finest qualities,
branded, the different countries from which they are exported being
Russia, Norway and Sweden.
Russian woods are generally hammer branded, no colour being used.
Norwegian woods are marked a blue colour, and Swedish wood a red colour.
Straight-grained timber should be chosen. Twisting, which may occur
to the extent of 45°, is not so apparent in young trees. The beam is
thereby weakened as regards tension and compression.
Large knots should be avoided. They weaken the beam for tension and
cross strains, but serve good purpose when in compression.
Sap in the wood is denoted by a blue stain, and betokens inferiority
as to strength and lasting qualities. This blueness is more noticeable
when the wood is wet. It must not be confounded with dark weather
stains.
=The Decay of Timber.=—Timber exposed to the constant changes of
weather tends to decay early. It has been noticed that wood when dried
after being exposed to dampness not only lost the moisture it had
absorbed, but also part of its substance. This loss occurs in a greater
degree when these changes take place a second time. It is therefore
apparent that scaffolding timber, exposed as it is to the vicissitudes
of the weather, without any protection such as would be gained by
painting, will soon from this cause show signs of decay.
Quicklime, when wet, has also a most destructive effect upon timber;
the lime, by abstracting carbon, helping the decomposition materially.
Scaffold boards suffer most in this respect from contact with fresh
mortar, the lime in which has not had time to become mild.
From a similar cause, the ends of putlogs which are inserted in a
newly built wall, as when used in a bricklayer’s scaffold, tend to
decay rapidly.
Scaffolding poles, when used as standards, have an increased tendency
to decay at their butt ends, owing to their being imbedded in the
ground to a distance of two or three feet.
The presence of sap in improperly seasoned wood is also conducive to
early decay.
=Preservation of Timber.=—Scaffolding timber being comparatively
cheap has little attention paid to it in regard to preservation.
When out of use the poles should be stacked, and a free current of air
around each ensured, not laid carelessly on the ground, which is too
frequently the case. The same remarks apply to the care of boards.
Nails which have been driven into the timber should, after use, be
carefully drawn. When left in the wood they rust, and set up a new
source of decay.
As before noted, the butt ends of standards tend to decay early. A
fairly effective method of preventing this is to bore the butt end
upwards to a distance of about 2 feet. The bore, which should be about
1/2 inch in diameter, should then be filled with pitch and plugged. The
pitch will find its way by capillarity into the pores of the wood and
tend to keep the dampness out. Coating the outsides of the butts with
pitch is also useful, and a combination of the two would no doubt be
effectual; this is especially recommended where the standards have to
remain in one place for a long time.
Ladders and fittings that are expensive are generally painted.
=The Durability of Wood.=—In Young’s 'Annals of Agriculture’ it is
stated that experiments were made on some 1½-inch planks of 30 to 45
years’ growth. They were placed in the weather for ten years, and then
examined, with the following result:
Larch—heart sound, sapwood decayed.
Spruce fir—sound.
Scotch fir—much decayed.
Birch—quite rotten.
This experiment, while useful to show the natural resistance of the
wood to the weather, does not take into account the effect of wear and
tear.
It is almost impossible, in fact, to arrive at any data from which the
life of scaffolding timbers can be gauged; but, roughly speaking, poles
may be expected to last from six to ten years according to the care
exercised. Balk timbers, being usually cut up after a time for other
purposes, have only a short life on a scaffold, and therefore seldom
decay while in use.
[Illustration: FIG. 62]
=The Use of Scaffolding Timber.=—Poles vary up to 40 and 50
feet in length and up to 8 inches in diameter at the butt end. As
decay, which usually commences at this end, sets in, the poles can be
shortened and made into sound puncheons or splicing pieces for the
ledgers. Balks are used up to 50 or 60 feet in length; and their period
of service on a scaffold is often an interval during which they become
well seasoned and suitable for other requirements. Putlogs are about 6
feet in length and 4 inches square in cross section, tapering sharply
to 2-1/2 by 3-1/2 inches at the ends where required for insertion in
the wall. Being of square section they are not liable to roll on the
ledgers. They should be split, not sawn, in the direction of their
length. The fibres are thus uncut and absorb moisture less easily.
This procedure it is found increases their durability. When treated in
this manner they are also stronger, as the fibres, being continuous,
give greater resistance to a load, the strength not depending wholly
on the lateral adhesion, but also upon the longitudinal cohesion. When
partly decayed they can be shortened and make good struts for timbering
excavations.
The boards are usually from 7 in. to 9 in. wide and 1½ in. to 2 in.
thick. Their length averages up to 14 feet. The ends are sawn as fig.
62 and strapped with iron to prevent splitting. When decayed they are a
source of danger for which there is only one remedy—smash them up.