These we will therefore use.

The strut AD has but 13,100 pounds' compression upon it, and is only seven feet long; so that, for this, two three-inch channels placed back to back will be amply sufficient.

For the ties we will use angle-irons. The greatest strain in the main tie is 74,500 pounds, which requires only seven square inches of cross-section; and hence, if we use for our main tie two light four-inch by six-inch angles, we shall have ample strength under all circumstances.

For the tie FI we will use two bars three inches by one-half inch, giving a total cross-section of three square inches.

For the tie DE we will use two bars two inches by three-eighths of an inch.

The joints of the truss will be formed by riveting a thick piece of iron plate between the channels, and riveting the struts and ties to that, after the method shown in Figs. 16 and 17, Chap. XXVII.

With this example we leave the subject of roof-trusses. As we have stated before, the method of finding the strains due to wind pressure alone we have not given, because, in the trusses which come especially before the architect or builder, the methods here given we believe sufficiently accurate. Any one wishing to learn the method of drawing the diagram of strains due to wind pressure alone will find it fully explained in Green's "Graphical Analysis of Roof-Trusses." 1

1 Published by John Wiley & Sons, New York.

CHAPTER XXIX.

JOINTS.

THE stability of any piece of frame-work depends in a very great measure upon the manner in which the joints are made. It is therefore very important, that in drawing trusses, or framework of any kind, the designer should have a good knowledge of the fundamental principles upon which every joint should be constructed, and of the most approved methods of forming the principal joints found in frame-work.1

Joints are the surfaces at which the pieces of a frame touch each other. They are of various kinds, according to the relative positions of the pieces and to the forces which the pieces exert on each other.

Joints should be made so as to give the largest bearing-surfaces consistent with the best form for resisting the particular strains which they have to support, and particular attention should be paid to the effects of contraction and expansion in the material of which they are made.

In planning them the purpose they are to serve must be kept in mind, for the joint most suitable in one case would oftentimes be the least suitable in another.

JOINTS IN TIMBER-WORK.

In frames made of timber, the pieces may be joined together in three ways, by connecting them;

-

1. End to end;

1 As the author could think of no better way in which to present the subject, he has taken, by permission of Professor Wheeler and of the publishers, the following chapter on joints from the text-book, on Civil Engineering, prepared by Professor Wheeler for the use of the cadets of the United-States Military Academy, and published by John Wiley & Sons of New York. The author heartily recommends Professor Wheeler's work to the architect or builder who wishes to obtain a thorough knowledge of construction and the materials employed therein.

a

9

2. The end of one piece resting upon or notched into the face of another; and

3. The faces resting on, or notched into each other.

I. Joints of Beams united End to End, the axes of the beams being in the same straight line.

First, Suppose the pieces are required to resist strains in the direction of their length.

Fig. 1.

Represents the manner in which two beams, a and b, are fished by side-pieces, c and d,

bolted to them.

с

Fig. 2.

Represents a joint to resist extension, iron rods or bars being used to connect the beams, instead of wooden fish-pieces.

This case occurs, when, in large or long frames, a single piece of the required length cannot be easily procured.

The usual method of lengthening is in this case by fishing or scarfing, or by a combination of the two.

Fish-Joints. - When the beams abut end to end, and are connected by pieces of wood or iron placed on each side, and firmly

e

d

Fig. 3.

Represents a fished joint in which the side-pieces c and d are either let into the beams, or secured by keys e, e.

e

bolted to the timbers, the joint is called a fish-joint, and the beam is said to be fished.

This joint is shown in Fig. 1, and makes a strong and simple connection.

When the beams are used to resist a strain of compression, the fish-pieces should be placed on all four sides, so as to prevent any ateral movement whatever of the beams.

If the strain be one of tension, it is evident that the strength of the joint depends principally upon the strength of the bolts, assisted by the friction of the fish-pieces against the sides of the timber.

The dependence upon the bolts may be much lessened by notching the fish-pieces upon the beams, as shown on the upper side of the piece in Fig. 3; or by making use of keys or blocks of hard wood inserted in shallow notches made in both the beam and fishpiece, as shown on the lower side of the piece in the same figure.

d

2

d

Care should be taken not to place the bolts too near the ends of the pieces. The sum of the areas of cross-sections of the bolts should not be less than one-fifth that of the beam.

Scarf-Joints. — In these joints the pieces overlap each other, and are bolted together. The form of lap depends upon the kind of strain to which the beam is to be subjected.

Fig. 6.

Represents a scarf-joint secured by iron fish-plates, c, c, keys, d, d, and bolts.

Represents a scarf-joint for a cross-strain, fished at bottom by a piece of timber c.

Fig. 4 is an example of a simple scarf-joint that is sometimes used

when the beam is to be subjected only to a slight strain of extension. A key or folding wedge is frequently added, notched equally in both beams at the middle: it serves to bring the surfaces of the joint tightly together.

This joint is often made by cutting the beams in such a manner as to form projections which fit into corresponding indentations. A good example, in which two of these notches are made is shown in Fig. 5.

a