32 ft. long, as nearly uniform in quality throughout their lengths as possible were selected. The following is taken from the bulletin:

One-half of each 32-ft. piece was tested in a green condition, and the other half tested after air-seasoning. The average moisture content of the air-seasoned material was 16.4%. The average ultimate strength in bending of the green material was 5440 lb. per sq. in., while the same value for the air-seasoned timber was 6740 lb. per sq. in., or an increase of 24%. The corresponding values for the elastic limit were 3740 and 5478 lb. per sq. in., showing an increase in strength due to seasoning of 47%.

A number of tests were made on various grades of Douglas fir stringers seasoned from 6 to 8 months; the grades select, merchantable, the seconds, being those defined in the export grading rules of the Pacific Coast Manufacturers Association adopted in 1903. In this group of stringers the fiber-stress at elastic limit and the modulus of rupture, in the case of select material, was increased, respectively, 8% and 5% by seasoning, the modulus of elasticity remaining practically unchanged. In the merchantable material the increase in these functions was respectively 19%, 33%, and 6%. In the seconds, the fiber-stress at elastic limit increased 6%, while the modulus of rupture and modulus of elasticity showed, respectively, a decrease of 12% and 2%. The failures in seasoned Douglas fir stringers and car-sills were similar to those in green material, except that failures in horizontal shear were more

common.

Failure in horizontal shear is more common in seasoned than in green timbers, because the net areas resisting shear along the neutral plane is often considerably decreased by checks. It seldom occurs in weak, low-grade material, which fact is doubtless due to the dowelling-pin action of the knots invariably associated with low-grade timbers.

4. Methods of Seasoning Timber. The most satisfactory method of seasoning is airseasoning, or the exposure of the green timber to the drying action of natural air currents, the timber being protected from the weather. Lumber to be thus properly seasoned must be portected from rain, piled on level, firm foundations well off the ground, supported at frequent intervals in its length, and "stuck"--that is, each layer of boards separated from the adjoining layers by strips of timber. The boards in each layer should be separated from one another. This method of piling allows the air to circulate freely around each stick of timber.

Air-seasoning takes time-usually many months in order to thoroughly season the timber. For this reason certain classes of lumber, as flooring, ceiling, etc., are commonly kiln-dried, that is, exposed to high temperatures in a drying kiln. Kiln-drying must be properly done, else the timber will be brittle. Careful regulation of the heat, avoidance of extremely high temperatures, prevention of draughts of outside air, and the use of steam baths before drying are factors tending to reduce the tendency to brittleness.

5. Effect of Defects on Strength of Timber.-Defects, such as knots, cross-grain, wind shakes, etc., dcrease the strength of timber. Knots are always co-existent with cross or diagonal grain in the immediate vicinity of the knot. In former years, the effect of a knot was considered most serious only when the timber was subjected to tension. It has been proved, however, that knots vitally weaken the timber in compression, due to the variation of the timber fibers around the knot from their normal direction.

Minor defects, such as pitch pockets, sap stain, etc., are of no consequence structurally. 6. Deterioration of Timber.-Mechanical deterioration of timber occurs from continued use. Flooring will wear out, railway spikes will become loose in the ties, and screws subject to continued "working" will enlarge their holes in the wood and pull out. The probable deterioration due to wear and tear can be fairly accurately foreseen.

6a. Deterioriation Due to Age.-In building construction, timber properly protected from the weather and from dry rot will not deteriorate with age, and in many cases increased strength may result.

66. Deterioriation Due to Decay.—The decay of all timber in building construction may be said to be due to the presence and action of bacteria or fungi. Without going into a discussion of the nature and action of such fungi, the conditions conducive to decay should be realized. These are the presence of a certain amount of air, heat, and moisture. Timber so situated that it will be subject to a free circulation of air at all times will not rot. Timber completely and permanently immersed in water or timber buried deeply without access to any air will not decay. On the other hand, timber alternately wet and dry will be subject to decay. The danger of dry-rot in timber joists so set in masonry that free circulation of air cannot occur is well known. Timber in contact with the soil and set in close proximity to the ground in exterior walls will decay. In this connection, the danger of infection of otherwise sound timber from diseased timber should be recognized. In many instances infected timber is brought upon the job from the lumber yard. Only the most careful inspection can prevent such a circumstance, and the only recourse should be the rejection and removal of each affected

stick unless by cutting off to safely remove all vestige of "date" the shortened stick made available.

Certain timbers, as cedar and redwood, are very resistant to the action of fungi.

6c. Deterioration Due to Animal Life.-In timber exposed to sea water, deterioration may occur from marine life, as the teredo and the lim noria; in the case of land structures, certain wood borers, as the beetles and termites, destroy timber. Protection against destruction from marine life must be given by the designer of timber structures; protection of timber from deterioration due to land life is properly the function of the forester and horticulturist.

7. Treatment of Timber to Prevent Decay.—The prevention of decay in timber should be given as careful consideration by the structural designer as is the strength of a joint. Details of design should be so drawn that the timber is protected from the weather and from moisture, has no contact with the ground, and has access to circulation of air. Unseasoned timber should not be painted. The presence of an air-excluding film of paint around a green stick of timber is almost certain to result in dry-rot. The underpinning of a building should be well ventilated, Green lumber should not be used for flooring or other tightly driven work. The ends of floor joists when embedded in masonry walls should have space around them to give access to air. Contact faces of timber to timber, and timber to metal, may well be painted with a good wood preservative.

When further measures than good design are required to prevent decay of timber, treatment by preservatives must be resorted to. Two methods are used: (1) impregnation of the timber with the preservative, and (2) brush or surface treatment of the timber.

The most common wood preservatives used are creosote, zinc chloride, copper sulphate and mercury chloride, and combinations of these. Of the above, creosote and zinc chloride are probably used most extensively.

In the impregnation process, the timber is first treated by drying or steaming, and then subjected to a bath of the preservative under pressure. The impregnation process is much more effective than surface treatment, and is ordinarily done at specially designed plants.

The surface treatment of timber with wood preservative is accomplished by painting the timber with the preservative, or by dipping the timber in an open bath of the preservative. This work is done on the job. Timber to be thus treated should be seasoned and dry, otherwise the treatment is more or less ineffective. The preservative should be heated. Dipping is far preferable to painting. All timber dipped should remain in the hot liquid at least 15 min. An advantage is claimed by some experimenters for an alternate soaking in hot and cold preservative. For brush treatment, at least two surface treatments with hot preservative are necessary, and three are to be advised.

8. Sawing of Timber.-Boards and planks are ordinarily cut tangentially,—that is, tangent to the annual rings of the log. This method is known as flat sawing, and the boards or planks are said to have flat grain, bastard grain, or to be slash cut.

In quarter-sawing the logs are cut into quarters, and then sawed across the annual rings. This method is also known as rift sawing, and the material thus cut is said to have edge grain. Quarter-sawing gives the beautiful graining seen in quarter-sawed oak and similar timbers. Edge-grained lumber also shrinks and warps less than flat-sawed lumber, does not sliver, and gives greater resistance to wear, as in vertical-grain flooring.

Timbers in which the heart of the tree is enclosed in the cross section are said to be "boxed-hearts." Such timbers will check deeply on all sides, with radial checks extending deeply into the section.

9. Classification of Lumber. The following extracts from the General Timber Specifications adopted by the American Society for Testing Materials, August 1915, indicate the nature of defects, number and character of those allowed, also the specifications for the various grades of lumber, including the definition of the "density" rule for Southern yellow pine.

Knots.-Knots shall be classified as round and spike in form; and for quality, as sound, encased, loose, and unsound. Knots are also classed as to size.

A sound knot is one which is solid across its face and which is as hard as the wood surrounding it; it may be either red or black, and is so fixed by growth or position that it will retain its place in the piece. A loose knot is one not firmly held in place by growth or position. A pith knot is a sound knot with a pith hole not more than 1/4 in. in diameter. An encased knot is one whose growth rings are not intergrown and homogeneous with the growth rings of the piece it is in. The encasement may be partial or complete; if intergrown partially or so fixed by growth or position that it will retain its place in the piece, it shall be considered a sound knot; if completely intergrown on one face, it is a water-tight knot. An unsound knot is one not as hard as the wood it is in. A pin knot is a sound knot not over 1⁄2 in. in diameter. A standard knot is a sound knot not over 11⁄2 in. in diameter. A large knot is a sound knot, more than 11⁄2 in. in diameter. A round knot is one which is oval or circular in form. A spike knot is one sawn in a lengthwise direction; the mean or average width shall be considered in measuring these knots.

Wane.-Wane is bark, or the lack of wood from any cause, on edges of timbers.

Shakes. Shakes are splits or checks in timbers which usually cause a separation of the wood between annual rings. Ring shake is an opening between the annual rings. Through shake is a shake which extends between two faces of a timber.

Shakes not hereinbefore described unless known to have extensive penetration shall not be considered a defect under this classification.

Sizes. All rough timber, except No. 1 Common, must be full size when green. 4 in. shall be allowed for each side surfaced.

Lengths. Standard lengths are multiples of 2 ft., 8 to 20 ft. inclusive; extra lengths are multiples of 2 ft., 2 ft. and longer. When lineal average is specified, standard of lengths shall be multiples of 1 ft.

Heart Timbers. All timber specifications, except "Merchantable" and "Select Structural Timbers" specifying heart requirements, shall be considered as a special contract, and shall specify whether the heart requirements refer to surface or girth measurements in each piece.

No. 1 Common Timbers.-May be either dense or sound pine. admit any amount of sapwood.

Unless otherwise specified, this grade will

Common timbers, rough, 4 X 4 and larger, may be in. scant in either or both of its dimensions, shall be well manufactured and may have 11⁄2 in. wane on one corner, the length of the piece, or its equivalent on two or more corners, the wane measured on its face.

Timbers 10 X 10 may have 2 in. wane as above; the larger sizes may have wane as above in proportion to sizes. Common timbers may contain sound knots and pith knots, provided that the diameter of any one knot shall not exceed the following in size:

In sizes not mentioned the diameter of knots admissible will increase or decrease in proportion to the size of the timbers on same basis as above specified.

In determining the size of knots, mean or average diameter shall be taken, or the equivalent of the above in grouped knots at any one point.

Will admit shakes extending 16 the length of the piece, round or ring shakes, unsound knots 11⁄2 in. or less in diameter, a limited number of pin worm holes, well scattered, sap stain, and seasoning checks. Unless otherwise specified, this grade will admit any amount of sap stain.

Square-edge and Sound Timbers.-May be either dense or sound pine. Unless otherwise specified, this grade will admit any amount of sapwood.

Square-edge and sound timbers shall be well manufactured and shall be free from defects such as injurious ring or round shakes and through shakes that extend to the surface, unsound and loose knots and knots in groups that will materially impair the strength, and shall be free from wane. Seasoning checks and sap stain shall not be considered defects.

Merchantable Timbers.-May be either dense or sound pine. All merchantable timbers shall be well manufactured and shall be free from defects, such as injurious ring and round shakes and through shakes that extend to the surface, unsound and loose knots, and knots in groups that will materially impair the strength. Seasoning checks and sap stain shall not be considered defects.

Sizes under 9 in. on the largest dimension, shall show 33 or more heart on both of the wide faces. When sticks are square, the face showing the most heart shall govern the inspection on sizes under 9 in., and the two faces showing the most heart shall govern the inspection when 9 in. and over. Heart showing the full length, even if not 35 of the area as above, shall meet the requirements of this quality.

Wane not exceeding of the dimension of the face and 4 of the length of the piece on one corner, or the equivalent on two or more corners or not to exceed 10% of the pieces, shall be admitted.

Southern Yellow Pine.-This term includes the species of yellow pine growing in the Southern States from Virginia to Texas-that is, the pines hitherto known as longleaf pine (Pinus palustris), shortleaf pine (Pinus echinata), loblolly pine (Pinus taeda), Cuban pine (Pinus heterophyla) and pond pine (Pinus serotina).

Under this heading two classes of timber are designated: (a) dense Southern yellow pine, and (b) sound Southern yellow pine. It is understood that these two terms are descriptive of quality rather than of botanical species. (a) Dense Southern yellow pine shall show on either end an average of at least 6 annual rings per inch and at least summer wood, or else the greater number of rings shall show at least summer wood, all as measured over the third, fourth, and fifth inches of a radial line from the pith. Wide-ringed material excluded by this rule will be acceptable, provided that the amount of summer wood as above measured shall be at least one-half.

The contrast in color between summer wood and spring wood shall be sharp and the summer wood shall be dark in color, except in pieces having considerably above the minimum requirement for summer wood.

In cases where timbers do not contain the pith, and it is impossible to locate it with any degree of accuracy the same inspection shall be made over 3 in. on an approximate radial line beginning at the edge nearest the pith in timbers over 3 in. in thickness and on the second inch (on the piece) nearest to the pith in timbers 3 in. or less in thickness.

In dimension material containing the pith but not a 5-in. radial line, which is less than 2 X 8 in. in section or less than 8 in. in width, that does not show over 16 sq. in. on the cross-section, the inspection shall apply to the sec

ond inch from the pith. In larger material that does not show a 5-in. radial line the inspection shall apply to the three inches farthest from the pith.

The radial line chosen shall be representative. In case of disagreement between purchaser and seller, the average summer wood and number of rings shall be the average of the two radial lines chosen.

(b) Sound Southern yellow pine shall include pieces of Southern Pine without any ring or summer wood requirement.

10. Strength Values of Timber.-Probably the most comprehensive study ever made of the strength values of structural timber was that made by the American Railway Engineering Asssociation, through their Committee on Wooden Bridges and Trestles. The ultimate and working stresses recommended by that committee are given in the accompanying table. The table gives no working unit stresses for pure tension. The working unit resistance to tension may be taken the same as for bending.

The working unit stresses as given (increased 50% for buildings) are the highest that should be taken for any structural timber; and for timber not better than what is generally known as No. 1 Common, it is here recommended that unit stresses not exceeding 25% of those given in the table, be used in building construction.

WORKING UNIT STRESSES FOR STRUCTUAL TIMBER

Adopted by the American Railway Engineering Association

The working unit stresses given in the table are intended for railroad bridges and trestles. For highway bridges and trestles, the unit stresses may be increased 25%. For buildings and similar structures, in which the timber is protected from the weather and practically free from impact, the unit stresses may be decreased 50%. To compute the deflection of a beam under long continued loading instead of that when the load is first applied, only 50% of the corresponding modulus of elasticity given in the table is to be employed.

Unit stresses are for green timber and are to be used without increasing the live load stresses for impact. Values noted * are for partially air dry timbers.

In the formulas given for columns, l

=

length of column, and d = least side or diameter, in inches.

11. Sizes and Lengths of Framing Timbers.—The nominal sizes in which framing timbers may be purchased is indicated in the following table:

Other sizes, such as 2 × 3, 2 × 5, 2 × 7, 2 × 9, 3 × 4, and 3 × 5 are sawed by a few mills, but are not common. 2 X 3 in Douglas Fir is shipped into California by the northern mills. The largest sizes of timbers listed in the above table are not ordinarily carried in stock, and must be ordered under special contract.

Rail shipments of dimension lumber are generally shipped surfaced one side one edge; water shipments, in the rough. If rough sizes are desired, the shipment should be so ordered under special contract. The sizes of timbers surfaced one side one edge (written S181E) are shown in tables on pp. 104 to 114 incl.

The

12. Measurement of Lumber.-In the United States, all lumber, except as noted below, is cut and sold in even lengths, as 12 ft., 14 ft., 16 ft., 18 ft., etc. Timbers will usually overrun such lengths a small amount. Thus a 10 X 10 timber ordered as a 16-ft. length may actually be 16 ft. 4 in. long. However, this full excess length cannot be counted upon. excess length may be only sufficient to allow a full 16-ft. length after cutting the ends square. Any odd or fractional lengths of timbers required in a building must be purchased as the next higher even length. A study of the size of building and arrangement of walls, posts, story heights, etc. may sometimes result in a considerable saving in lumber by the elimination of cutting lengths of timbers that are slightly under standard lengths.

All lumber, except some finishing lumber, is sold by board-measure, the unit of which is the board foot. One board foot of lumber is 144 cu. in. Expressed in another way, a board foot is the equivalent of a 12-in. length of board, 1 in. thick and 12 in. wide. For example, a piece of timber 6 in. square and 14 ft. long has 42 board feet. To compute the board feet in any timber, divide the product of the sectional area of the stick and the length of the timber by the factor 12, and the result will be the number of board feet in the timber.

In making out a lumber list, the information to be given is the number of pieces, sectional dimensions, length, kind of timber, quality, and statement of whether the lumber is to be rough or surfaced.

This information is usually presented in the following manner:

The first number gives the number of pieces, the next two, the thickness and depth, respectively, and the fourth number the length. The smallest cross-sectional dimension is written first, thus, 2" X 4" rather than 4" X 2". The abbreviation S4S indicates that the timbers are to be surfaced four sides.

The board feet in the four sticks would be the product of the four numbers divided by 12, or 427 board feet. Many tables are published giving the board feet measure of commercial sizes of timbers. The accompanying table shows the contents of timbers in board-measure. However, with a little practice, one can compute the quantities almost as fast as by the use of a table by performing the simpler multiplication mentally, factoring where possible. Thus, in the example given above, 80 X 16 X 80 × 5% 427 board feet. The nominal cross

=

sectional dimensions are always used, i.e., 8" X 10", instead of the finished dimensions, 71⁄2′′ × 91⁄2”.

The following extracts are taken from the 1917 Standard Specifications for Grades of Southern Yellow Pine Lumber of the Southern Pine Association.