Part 18 (1/2)

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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.