188. It is the work of the carpenter to raise and inclose the frame of a building, to construct its floors and roofs, and to complete all parts which give stability to the structure; the joiner makes the doors and windows, erects the stairs, and provides such interior woodwork as will finish the building as a habitation. A single mechanic may perform almost every kind of work required in the construction of a building, thus eliminating this distinction of trades; but for convenience in classification we may imagine the work of the carpenter and that of the joiner to be quite distinct.

It will be understood that neither carpentry nor joinery is confined to house-building. While all bench work may properly be classed as joinery, it involves forms and principles that are the logical outgrowth of carpentry. For this reason, in the following consideration of joints, there are presented, first, those belonging to carpentry, which will include such as are used in uniting timbers, as in a frame for a building; and, secondly, those belonging to joinery, which will include such as are used in joining small planks or boards. This classification cannot be rigidly adhered to, but it will serve the purpose of the following pages.

1 Tredgold's "Carpentry," and "Notes on Building Construction," published by Rivingtons, have furnished many of the facts presented under Carpentry and under Joinery.

189. Any two timbers may be united in the direction of their length, or they may be united at an angle.

Timbers united in the direction of their length are usually subject to compressional stress, which has a tendency to reduce their length, as indicated by Fig. 220; or tensional stress, which has a tendency to increase their length, Fig. 221; or transverse stress, which has a tendency to bend them, Fig. 222; or to two of these stresses at the same time.

Fig. 220

Fig. 221

190. A Timber subjected to transverse stress must always bend. The fibers forming that surface which is convex or has a tendency to become so (as the lower surface, A, Fig. 222) will be subject to tensional strain, while the fibers forming the opposite surface will be brought under compressional strain This is shown by Fig. 223, A representing a straight timber, and B the same timber bent. It follows, then, that somewhere between the compressed surface and the extended surface there will be a line which is subject to neither compressional nor tensional strain; such a line is called the neutral axis of a timber, and will be located with sufficient accuracy for the purposes of this work, if drawn midway between the upper and lower surfaces, as shown by the dotted line CD, Fig. 223.

Fig. 222

Fig. 223

In the timber that has been forced into a curved form, Fig. 223, the fibers within the neutral axis are under no strain excepting that required to hold the compressed portion to the extended portion; but the conditions are found to change rapidly as the examination extends to fibers more and more remote from this axis. In other words, the strength of such a timber increases rapidly as its depth increases. For example, if Fig. 222 represents a 2" x 4" timber (2" wide and 4" deep) supported at B, B, and capable of sustaining 200 pounds at C, it can be shown that, if the depth is doubled, leaving the width the same, by substituting a 2" X 8" timber, it will sustain four times the original load, or 800 pounds; while if the width is doubled, leaving the depth the same, by substituting a 4" x 4" timber, it will sustain only twice the original load, or 400 pounds. The law is that the strength of timbers subject to transverse stress varies as the width and as the square of the depth.1

191. Rankine has given five principles to be observed in designing joints and fastenings. They are as follows: 1. "To cut the joints and arrange the fastenings so as to weaken the pieces of timber that they connect as little as possible."

1 By what has been given it will be seen that in any body of material the portions most affected in resisting transverse stresses are those lying near the upper and lower surfaces (Fig. 222). In view of this fact, parts that are to receive a transverse stress, especially if of iron, are, in important structures, formed to present a large amount of material near these surfaces. A railroad rail or an I-beam are simple illustrations; a bridge truss is an elaboration of this principle.

2. "To place each abutting surface in a joint as nearly as possible perpendicular to the pressure which it has to transmit."

3. "To proportion the area of each surface to the pressure which it has to bear, so that the timber may be safe against injury under the heaviest load which occurs in practice, and to form and fit every pair of such surfaces accurately, in order to distribute the stress uniformly."

4. "To proportion the fastenings so that they may be of equal strength with the pieces which they connect."

5. "To place the fastenings in each piece of timber so that there shall be sufficient resistance to the giving way of the joint by the fastenings shearing or crushing their way through the timber."

Complicated forms of joints are likely to violate Rule 3.

Joints connecting Timbers in the Direction of their Length.

192. A Lapped Joint, shown by Fig. 224, fastened either by straps A or bolts B, is clumsy, but very strong.