This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Lapped Joint. - This joint is of a very clumsy appearance, and is only used in work of a temporary nature or in small timber framings hidden from view. It is formed by overlapping the ends of two pieces of timber, and fixing them by means of nails or bolts as at A, or by straps as at B, Fig. 250. Nails are used when the timbers are small. It has been found also in practice that a bolted joint is more suitable for resisting a tensile stress than a strapped joint, the latter being a more satisfactory joint when the timbers are in a state of compression.
When forming a joint, experience is usually relied upon as a guide in proportioning the various parts, and when experience is lacking they can be calculated. The timbers in Fig. 250 are in a state of tension, which causes a stress in the sections of the bolts along the line AB, similar to that caused by a pair of shears in cutting a piece of paper. Such a stress is called a Shear, and the power of a bolt to resist it is directly proportional to its sectional area. To form a joint of uniform strength it is necessary that the ultimate resistance to shear of the metal in the bolts shall be equal to the ultimate resistance to tension of the timber.
Labbed Joint Botted.
Fig. 250. Lapped Joint, Strapped.
Suppose the timbers at A, Fig. 250, to be Baltic fir, then by the table at the end of this Chapter it is seen that this material has an ultimate resistance to tension of 4000 lbs. per square inch. The bolts are made of wrought iron, and offer an ultimate resistance to shearing of 44,800 lbs. per square inch So that for a joint of uniform strength the total sectional area of the bolts must be 4000/44800, (roughly), of the effective sectional area of the timber. Now the question arises as to the size of the bolts, which is usually determined by some such rule of thumb as the following: For timbers of 10 square inches effective section use 1/2-inch bolts, and add 1/16 inch to the diameter of the bolts for every additional 5 square inches to the section of the timbers. At A, Fig. 250, the effective sectional area of the timbers is something less than 64 square inches, so that the bolts required will be \\ inch in diameter.
The effective sectional area of the timbers, i.e. the area left after a bolt hole has been made, is 8 x (8 - 1 1/8) = 55 square inches, and the area of metal required in the bolts is 44 = 5 square inches.
Now,the area of each bolt = π x diamerter2 / 4 sq. inches.
= 22 x 9 x 9 / 7 x 4 x 8 x 8 sq. inches.
Therefore the number of bolts required = 5x7x4x8x8 / 22x9x9
= 5 nearly.
One bolt hole should be bored and the bolt inserted and tightened up before the other holes are bored, in order to ensure that the holes in the various timbers coincide exactly. This applies to all bolted joints.
The distance between the bolts should not be less than 5 inches, although a greater distance is preferable, as it lessens the chance of the bolts cutting through the timbers, or the shearing out of the portion of the timber against which the bolts thrust. Where the width of the timber permits, the bolt should not be arranged in one line, but should be zigzagged as at A, Fig. 250, as this presents a greater area of wood to resist shear.
In this method of uniting timbers corresponding portions are cut away from the ends of each, and the remaining portions are overlapped and fitted together. This joint is used when appearance is the main object, for which reason great care should be taken in its designing and cutting, so that it may be as imperceptible as possible.
A good form of scarfed joint for resisting a tensile stress is shown in elevation at A, Fig. 251, and in isometric projection at B, Fig. 251. It will be seen that the timbers are considerably weakened by scarfing, the effective area in the case of A being that across either AC or BD. It is obvious also that if there is a tensile stress in the sections AC and BD, the section AB will be in a state of compression, and there will be a tendency for the timber to shear along the line BE. Now, from the table at the end of this Chapter - assuming this joint is formed of Baltic fir - the ultimate resistance to tension and compression are 4000 and 5000 lbs. per square inch respectively, then the distances AC, AB, and BD must be in the proportion of 5000: 4000: 5000, or 5: 4: 5. Therefore CA and BD are each equal to 5/14d, while AB = 4/14d. Again, the ultimate resistances of Baltic fir to tension and shear are 4000 and 600 respectively. Hence BE = 7 AC, nearly.
There is no fear of the joint failing at the wedges, as these are formed of good pieces of oak, which has a much higher crushing resistance than Baltic fir.
At C, Fig. 251, another very good form of scarfed joint is shown, which is more easily worked than the one shown at A. In this case the joint has to be designed so that the total resistance to shear along AB and CD may be equal to the resistance to tension in EF. The wedges WW are also in shear, and their width is usually made about equal to EF, and inserted with the grain at right angles to the pressure. According to the table, the safe resistance to shear in oak across the grain is 3000 lbs. per square inch, but this is for larger-sized average timbers, while wedges are carefully selected pieces which can be made smaller in consequence than the size demanded by theoretical calculations. Squared joggles of oak are sometimes used, but wedges are preferable, as they can be tightened, thus ensuring the assistance of every portion of the joint.
The ends of the scarfed joints are sometimes cut as shown at D, with the object of preventing lateral motion in the timbers, but it is very doubtful whether such cutting has any compensating advantages over the labour expended upon it.
The reason for forming the projecting portions P, as shown at A, B, and C, Fig. 251, is to prevent the joints from opening as at E, but their place is often taken by bolts, as at F. When bolts are used the scarf can be made much shorter, and the wedges need only be deep enough to draw the joint up tightly before the bolts are inserted.
When it is desirable to join two timbers without reducing their section to the extent shown in Fig. 251, a joint known as a Fished Joint, and formed as shown in Fig. 252, is used. The simplest form of fished joint is that shown at A, Fig. 252. It is formed by butting the ends of the timbers and bolting wooden fish-plates on to two parallel sides.
The distances EF and GH should together be equal to FG, so that the combined sectional area of the plates may be equal to that of the joined timbers.
The size of the bolts may be taken from the same rule as was given for lapped joints.
It will be noted that at A, Fig. 252, the bolts tend to shear along the two planes AB and CD, for which reason they are said to be in double shear. It is clear, therefore, that this joint requires half the number of bolts that would be used for a lap joint in the same timbers, on each side of the line EF; or, which amounts to the same thing, this joint requires the same number of bolts as a lapped joint. Thus if A, Fig. 250, and A, Fig. 252, be compared it will be seen that the lapped joint has five bolts, while fished joint has six, the total being increased by one in the latter case to equalise the number on each side of the joint.
Sometimes the fish-plates are tabled, i.e. they have projecting portions formed on them which fit into corresponding indents in the timbers joined. The use of such tables is shown at B, in which case it is found by calculation that it would require three and a quarter 1-inch bolts approximately. Instead of using four bolts, three only are employed on each side of the joint, and the strength of the odd quarter bolt is supplied by the resistance to shear along the plane CD. Keys and wedges are sometimes used for the same purpose as tables, corresponding indents being formed in the jointed timbers, and the fish-plates, as shown at C and D, Fig. 252. In jointing small timbers it is very doubtful whether much real advantage is obtained by the use of such wedges, but in very large timbers with long fish-plates, especially when subjected to a transverse strain, wedges add considerably to the stiffness and strength of the joint. By using iron plates a less clumsy joint may be formed, as shown at E. Sometimes the ends of iron fish-plates are turned down into corresponding indents, as shown at F, the object of such a joint being the same as that obtained with a tabled wood fish-plate.
Halved Joint. - The most simple joint for connecting two portions of a wall-plate is shown at A, Fig. 253. It is made by cutting away half the section of the adjoining ends, or Halving, as it is technically termed. The methods of forming this joint when the two portions of the plate meet or cross at an angle are shown respectively at B and C.
A more secure joint is formed by bevelling the adjoining surfaces of the halvings, thus producing a joint known as a Bevelled Halved Joint, and shown at D, Fig. 253. In these halved joints it is always desirable to place a joist or bring weight to bear upon them, as this checks any tendency of the plates to move either longitudinally or transversely.
A bevelled halved joint between two plates meeting at an angle is shown at E.
For joining plates at an angle the dovetailed halved joint shown at F is efficient, on account of its simplicity of formation and its power of resisting the action of dislodging forces. It should be formed of well-seasoned timber, as any shrinkage in the width of the dovetail loosens the joint.
Sometimes the plates are jointed by means of a hard-wood key formed as shown at G, and drawn tight by means of wedges. Although this joint serves its purpose in a satisfactory manner it is unnecessarily expensive. It is used to a great extent in joinery work.
This joint, which is shown at H, like the keyed joint, is rarely used in carpenter's work, as it is unnecessarily complex for the simple function it has to perform.
A very useful joint is formed by butting the ends of the plates and securing them by means of a hand - rail screw, as at K.
Fig. 254. Bearing Joints