reduce the available section below one-third of the whole.

It is proper to observe with regard to this splice, and also the succeeding one, that the power being applied upon the reversed shoulder, or hook, out of the line of the unbroken fibres which resist the power, the tendency is to throw the ends outward, and produce a degree of lateral action, which weakens the timber to a somewhat greater degree than in proportion to the amount of fibres severed.

FIG. 57.

کے

With a double lock splice, as in Fig. 57, one-half of the section is available. This requires a lap of 10 times the thickness of the timber.

of the

By three lockings upon the same principle, fibres may be utilized for tension, with a lap of 12 thicknesses (or 12t.), and, by a lap 133t, we make twothirds of the fibres available. Finally, by a lap of 201. and an infinite number of lockings the whole crosssection would be available.

But this, of course, is a point not attainable in practice. From to say an average of, is as much as can be reckoned on, and about as much as can usually be made available for tension, at the end connections of a single timber.

Splicing may also be effected by a plain scarf, with bolting, pinning and spiking, as indicated in Fig. 58. With bolts, pins and spikes properly arranged and proportioned, a strong splice may be formed in this

manner, with a less lap than what is required in the lock splice. In this case the fastenings should pass

FIG. 58.

through at right angles with the plane of the joint, that they may not be slackened by a slight yielding of the timber to pressure, in the holes. This, however, is a device which will probably, seldom be resorted to in bridge construction.

Timbers may also be shackled together end to end by iron bolts and straps, as shewn in Fig. 59. The aggregate cross-section of straps should be about 1 square inch to each 10 to 15 thousand pounds of strain which the splice is intended to bear; and the diameter of bolts fastening the straps, about one-fifth of the thickness of timber, to secure the greatest effect for the amount of section destroyed in cutting the bolt hole.

FIG. 59

To connect two timbers 10x12 inches, so as make half of the fibres available for tension, we may take 6 straps 2 feet long from hole to hole, and containing a cross-section of about 1 square inch, each. Also 6 bolts of 2" in diameter, and arrange the straps and bolts as shown in the figure, the straps being placed upon the 12" sides. This will cost, say for 170lbs. of iron at 7cts., $11.00.

The expense of a double lock splice (Fig. 57), will

be about 7 cubic ft. of waste timber,.... 40lbs. of iron bolts, washers and plates,... Labor in fitting the timbers, say,........

Total,.....

$3.50 2.80

1.

$7.30.

showing the shackle connection to be from 4 to 5 dollars the more expensive.

CONSTRUCTION OF WOODEN TRUSSES.

CLVII. With a thorough comprehension of the power of timber to resist the various kinds of strain to which it may be liable in bridges, and other timber structures, and of the general principles of forming connections in timber work, as attempted to be explained and set forth in the last few preceding pages; and a knowledge of the general forms of arrangement for the several members in bridge trusses, or girders, and of the manner of computing the stresses to which the several parts are liable to be subjected, as treated of in the first 100 pages or so, of this work, the details of practical construction of wooden truss bridges may be intelligently entered upon.

Nothing more elaborate will be here undertaken, than a reference to general forms of trussing suitable for wooden bridges of different spans, and a description of what seem to be the most feasible methods of forming connections at peculiar and specific points.

The method pursued will be, to proceed from the shorter spans, and more simple combinations, to structures of greater length, and requiring a greater number and a more complex arrangements of parts.

Two PANEL TRUSSES.

CLVIII. The form presented in Fig. 3, with rafter braces ad and de, and a tie or chord ac, together with an iron tension member db (in 1 or 2 pieces), is probably the best adapted to bridges from 20 to 25 feet in length. The braces should meet with a vertical joint at d (Fig. 3), and toe into the chord tie with two headings, and one or two small bolts, as in Fig. 60.

FIG. 60.

12 18

e

Assuming the brace to be capable of sustaining a thrust of 500lbs. to the inch of section, and the heading 1,000lbs, to the inch, the aggregate depth of heading, af, and de, should be one-half the depth cb, of the brace; and, the point f, should fall below the point d, by ad, so as to give a length of cleavage fh, 10af or 10 dh. The shoulder de, then, should be,

(1),... de cbad, = rad, = cb = 1cbfab + i db. We here speak of adb as a straight horizontal line, not shown. This is regarding af as equal to the vertical depth of cut at af; which will be sufficiently near the truth for our present purpose, provided the brace De not very steep.

But (2),...de db. sin. dbe, db. sin. cab.

and, putting this value of de equal to the one above, and changing vulgar to decimal fractions, we have, (3), ..db. sin. cab 0.5 cb 0.1ab + 0.1db.

=

Then, transposing, and uniting co-efficients of db. (4.)...(sin. cab 0.1) db = 0.5cb-0.1ab, whence,

(5),...db

0.5 cb0.1 ab

sin. cab-0.1

Now, from equation (2) we derive db

value of db being substituted in equation (5), we have

8. cab

=

1

sin. cab

26°33′, which is regarded

Making the angle cab as a suitable inclination for the brace, being one, vertical, and two, horizontal reach, sin. cab 0.447, which substituted in (8), gives de = .356 cb, aud af = .144cb.

=

This, it will be recollected, is deduced upon the supposition that the brace will sustain a compression of 500lbs. to the inch, and no more; which will depend upon the length as compared with the least diameter. If the brace be capable of bearing with safety, more or less than 500lbs. to the inch, the heading, or butting surface should be more or less than half the area of cross-section, in like proportion. For, if unnecessarily large, it requires too much cutting of the chord, and if too small, the pressure upon abutting surfaces becomes too great.

With the inclination of brace above assumed, the compression upon the brace obviously equals the weight