Diagonal Bracing. In order to stiffen a bridge, it should have the two Trusses braced together at the Lower Chords always, at the Upper Chords when practicable—and in case of a deck bridge, where the roadway is supported on the upper chords, it is as well to have rods for vertical diagonal braces, their planes being perpendicular to the axis of the bridge. The more usual form is similar to the web members of the Howe Truss—the rods from ¾" to 1" in diameter, and the braces of 6" x 7" scantling, footed on wooden blocks, usually. It is more usual to have the tie rods of the horizontal diagonal bracing, and the braces themselves, meet in a point about midway of a Truss panel on the centre line, nearly, of the chord. This will generally give a half panel of diagonal bracing near each end of the truss—and it is very usual to have the diagonals foot at their intersection there against a cross timber interposed between the trusses, while the tie rod prevents any spreading.

Floor Timbers. The general dimensions of the transverse floor beams, when about 3 feet apart, from centre to centre, are 8" x 14", the largest dimension being the depth. The stringers should be notched to the floor beams about 1" or 2", and should be about 10" or 12" x 14". The cross ties should be 18" to 24" apart, from centre to centre, and be 3½" x 6".

Large, heavy bridges require no fastening to connect them with their seats, but light bridges should be fastened, as the spring on the sudden removal of a load, (as when the last car of a train has passed,) may move it from its proper position.

Splices. As the upper and lower chords have to be made in several lengths, securely fastened to each other, and, in order to weaken the built beam as little as possible, it is necessary to adopt some form of splicing whereby the greatest amount of tensional strength may be retained in the chord with the least amount of cutting, and yet have a secure joint. Such a splice is shown in Pl. II, Fig. 4, and below is a table from Vose's Hand-book, giving reliable dimensions.

Span.
Feet.		AC
Feet.		BB
Inches.		CD
Feet.
501.001½1.50
1001.2522.00
1501.752½2.25
2002.0032.75

This manner of splicing requires the back of the splice block to be let into the chord stick, against which it lies, about ¾ of an inch. To show how the various Engineers differ, as to their estimates of the sizes of the several parts of bridges, I subjoin two Tables—one by Prof. G.L. Vose, a well known Engineer, and one by Jno. C. Trautwine, an Engineer of note also—and I would premise that a bridge built according to either would be amply strong.

TABLE FOR DIMENSIONING A HOWE TRUSS BRIDGE.
G.L. VOSE.

Span.Rise.Panel.Chords.End
Braces.		Centre
Braces.		End
Rods.		Centre
Rods.
501072—8 x 107 x 75 x 51—1⅛2—1
751292—8 x 108 x 85 x 52—1½2—1
10015112—8 x 108 x 9 6 x 62—1¾2—1
15020134—8 x 1210 x 106 x 73—23—1
20025154—8 x 1612 x 127 x 75—25—1

TABLE FOR DIMENSIONING A HOWE TRUSS BRIDGE.
JNO. C. TRAUTWINE, C.E.

An Upper Chord.A Lower Chord.An End Brace.A Centre Brace.Counter.End Rod.Centre Rod.
Clear Span in feet. Rise in feet.No. of panels.No. pieces. Size.No. pieces. Size.No. pieces. Size.No. pieces. Size.No. pieces. Size.No. rods. Size.No. rods. Size.
256 83 4 x 53 4 x 102 4 x 62 5 x 51 4 x 52 1-5/162 ⅞
509 93 6 x 73 6 x 102 6 x 72 5 x 61 5 x 62 1⅝ 2 1-1/16
7512 103 6 x 93 6 x 112 6 x 82 6 x 61 6 x 62 1⅞2 1-3/16
10015 113 6 x 103 6 x 122 8 x 92 6 x 81 6 x 82 2-3/162 1-5/16
12518 124 6 x 104 6 x 132 8 x 102 6 x 91 6 x 92 2⅝2 1⅜
15021 134 8 x 104 8 x 143 9 x 103 6 x 92 6 x 93 2⅜3 1-3/16
17524 144 10 x 124 10 x 153 9 x 113 8 x 82 8 x 83 2⅝3 1¼
20027 154 12 x 124 12 x 163 9 x 123 8 x 102 8 x 103 2⅞3 1⅜