Carpentry for Boys Part 18

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The simplest form known is called the _Scissors Beam_, ill.u.s.trated in Fig. 289. This has been utilized for small s.p.a.ces, and steep pitches.

Each rafter (A) has an angled beam or brace (B), springing from its base, to the opposite rafter (A), to which it is joined, midway between its ends, as at C.

Where the two braces (B) cross each other they are secured together, as at D. As a result, three trusses are formed, namely, 1, 2, 3, and it possesses remarkable strength.

[Ill.u.s.tration: _Fig. 289. Scissors Beam._]

BRACED COLLAR BEAM.--This is a modification of the last type, but is adapted for thick walls only. The tie rod braces (A, A) have to be brought down low to give a good bracing action, and this arrangement is capable of considerable ornamentation.

The steeper the pitch the higher up would be the inner and lower brace posts (B, B) which were supported by the top of the wall. This form is not available for wide spans, and is shown to ill.u.s.trate how the development was made into the succeeding types.

[Ill.u.s.tration: _Fig. 290. Braced Collar Beam._]

THE RIB AND COLLAR TRUSS, Fig. 291, is the first important structural arrangement which permitted the architect to give full sway to embellishment. The inwardly-projecting members (A, A) are called _Hammer Beams_. They were devised as a subst.i.tute for the thick walls used in the Braced Collar Beam Truss, and small brackets (B, B) were placed beneath as supports.

[Ill.u.s.tration: _Fig. 291. Rib and Collar Truss._]

The short tie beam (C), near the apex, serves as the member to receive the thrust and stress of the curved ribs (D, D). It forms a most graceful type of roof, and is capable of the most exquisite ornamentation, but it is used for the high pitched roofs only.

[Ill.u.s.tration: _Fig. 290-1/2. Hammer Beam Truss._]

The acme of all constructions, in which strength, beauty, and capacity for ornamentation are blended, is the _Hammer Beam Truss_. Here the hammer beam projects inwardly farther than in the preceding figure, and has a deeper bracket (B), and this also extends down the pendant post (C) a greater distance.

The curved supporting arch (D), on each side, is not ribbed, as in the Rib and Collar Truss, but instead, is provided with openwork (not shown herein), together with beadings and moldings, and other ornamental characteristics, and some of the most beautiful architectural forms in existence are in this type of roof.

What are called Flying b.u.t.tresses (E) are sometimes used in connection with the Hammer Beam Truss, which, with heavy roofs and wide spans, is found to be absolutely necessary.

CHAPTER XX

ON THE CONSTRUCTION OF JOINTS

In uniting two or more elements, some particular type of joint is necessary. In framing timbers, in making braces, in roof construction and supports, in floor beams, and in numerous other places, where strength is required, the workman should have at his command a knowledge of the most serviceable methods.

Ill.u.s.trations can most forcibly convey the different types; but the sizes must be determined by the character of the material you are working with. Our aim is to give the idea involved, and the name by which each is known.

[Ill.u.s.tration: _Fig. 292. Bridle Joints._]

Reference has been made in Chapter X, to certain forms of scarfing and lapping pieces. This chapter has to do with a variety of other structural forms, but princ.i.p.ally with such as are used in heavy building work, and in cases where neither fish plates nor scarfing will answer the purpose.

[Ill.u.s.tration: _Fig. 293. Spur Tenon._]

[Ill.u.s.tration: _Fig. 294. Saddle Joints._]

BRIDLE JOINTS.--This is a form of joint where permanency is not desired, and where it is necessary to readily seat or unseat the vertical timber.

It is also obvious that the socket for the upright is of such a character that it will not weaken it to any great extent.

SPUR TENON.--This tenon can be used in many places where the regular one is not available. This, like the preceding, is used where the parts are desired to be detachable, and the second form is one which is used in many structures.

SADDLE JOINT.--This is still another manner in which a quickly detachable joint can be constructed. The saddle may be mounted on the main base, or cut into the base piece. An infinite variety of forms of saddles are made, most of them being used in dock work, and for framing of that character where large timbers are used, as in the building of coal chutes, and the like.

[Ill.u.s.tration: _Fig. 295. Joggle Joints._]

[Ill.u.s.tration: _Fig. 296. Framing Joints._]

JOGGLE JOINT.--This joint is used almost exclusively for brace work where great weight must be supported. The brace has a tenon, and the end must also be so arranged that it will have a direct bearing against the upright, which it braces and supports, or it may have two faces, as in the second figure, which is an exceedingly strong construction.

[Ill.u.s.tration: _Fig. 297. Heel Joints._]

[Ill.u.s.tration: _Fig. 298. Stub Tenon._]

FRAMING JOINTS.--These are the simplest form in which two members are secured together. They are used almost wholly in rafter work, and have very few modifications. The depth of the cut, for the toe of the rafter, depends on the load to be carried, and also on the distance the end of the rafter is from the end of the horizontal member on which the rafter rests.

HEEL JOINTS.--This is by far the most secure of the framing type of joints. This, if properly made, is much better than the construction shown in the previous ill.u.s.tration, but the difficulty is to make the rafter fit into the recesses properly. This is no excuse for failure to use, but it is on account of inability to make close fits that is accountable for lack of use. It will be seen that in case one of the heels rests against the recess, and the others do not, and the pressure is great, there is a liability to tear out the entire joint.

[Ill.u.s.tration: _Fig. 299. Tusk Tenon._]

STUB TENON.--This is another form of tenon which is made and designed to be used where it is in close proximity to another tenon, or where the mortises, if made full size, will weaken the member. The long tusk can be shortened, to suit the place where it projects, and the stub tenon on each side of the tusk may be made very short, and one side longer than the other if necessary.

TUSK TENON.--Two forms of tusk construction are given. Any number of forms have been devised, all for special purposes, and designed for different kinds of woods. These shown are particularly adapted for soft woods, and the princ.i.p.al feature that is valuable lies in the fact that they have a number of shoulders within the mortise, each of which, necessarily adds to the strength. It should be observed that in the construction of the tusk tenon, the greatest care must be taken to have it fit the mortise tightly, and this has reference to the bottom and shoulder ends as well.

[Ill.u.s.tration: _Fig. 300. Double Tusk Tenon._]

DOUBLE TUSK TENONS.--The distinguishing difference between this and the preceding is in the tusk, which in this form of construction goes through the upright member, and is held by a cross key. The double tusk is intended for hard woods, and it is regarded as the finest, as well as the strongest, joint known.

COGGED JOINTS.--This differs from the regular tenoning and mortising methods, princ.i.p.ally because the groove or recess is in the form of an open gain. It is used where the member is to be inserted after the main structure is put together.

[Ill.u.s.tration: _Fig. 301. Cogged Joints._]

[Ill.u.s.tration: _Fig. 302. Anchor Joint._]

ANCHOR JOINT.--This form of connection is designed for very large timbers, and where great care must be taken in making the parts fit together nicely, as everything depends on this. This style is never used where the angles are less than 45 degrees, and the depth of the gain in the timber receiving the brace is dependent on the thrust of the brace.

[Ill.u.s.tration: _Fig. 303. Deep Anchor Joint._]

The Deep Anchor Joint is an extension of the tongue of the Anchor tenon, so that it affords a greater support for the end thrust. To clearly distinguish between this and the preceding form, it might be said that the Anchor Joint is one designed to protect the member containing the gains, while the Deep Anchor Joint favors the brace, by giving it a greater power.

CHAPTER XXI

SOME MISTAKES, AND A LITTLE ADVICE IN CARPENTRY

In the mechanical arts, workers are as likely to learn from the mistakes committed as through correct information imparted. Advice, therefore, might be considered superfluous. But there are certain things which are easily remembered and may be borne in mind while engaged in turning out any work.