151
SECT. VIII.]
nal pieces should also be inserted between the road timbers, as lateral motion should as faras possible be prevented.
268._In larger spans, that is, in spans exceeding 50 feet, there will be difficulty in ob-
taining timber deep enough for>the ribs ; therefore they should be built the contrary way,and bent to the required curve, so as to increase the depth. The beams forming the ribsshould be scarfed at the joinings; the form of the scarf should be such as would resisteither pressure or tension (see Sect . IX. art. 295), and the scarfs should be kept as distantfrom one another as possible. The number of thicknesses in each rib must depend on thesize required for the span, and the dimensions of the timber that can be procured; and thewhole should be well bolted together. The supports of the roadway and diagonal bracesto be placed as described in the last article.
Fig. 112, Plate XIX. represents a bridge designed for a 200 feet span : fig. 113 is a sec-tion across at CD to a larger scale. This bridge is sustained by four ribs, each rib 18 inchesthick and 4 feet deep ; the ribs to be two thicknesses in width, and either three or four indepth, according to the size of the timber : the lengths of timber should he disposed so asto cross the joints as much as possible, and the joints should be scarfed. One of the mostsimple scarfs will be the best adapted for that purpose. The pieces composing a rib mustbe well bolted together, and keys in the joints would be a further means of preventing anysliding of the parts.
The vertical pieces which support the roadway are intended to be put on in pairs, notchedto the ribs, and bolted together, and not more than 18 feet apart. And at each pair a doubletie is intended to cross both the back and the under side of the ribs, notched on to theribs and bolted to the vertical pieces.
Between the timbers which carry the joists of the roadway diagonal braces should beframed so as to secure the bridge from lateral motion. A series of braces for the same pur-pose might be framed over the back of the ribs: but one of these methods, if well executed,will be sufficient.
The bridge is intended for a gravel or paved roadway, and is calculated to sustain twoloaded waggons at its weakest point without injury.
This kind of bridge is adapted to any span that is usual in bridge building ; the ribs shouldnot be more than 8 feet apart, and their dimensions may be easily calculated for any spanby the help of art. 274 and 275 of this Section, and art. 128 of Sect. II.
As the beams are intended to be bent to the form of the rib, it will be prudent to ascer-tain whether such a degree of curvature may he given to the beams without impairing theirelastic force. The curvature to be given to the beams will be sensibly uniform, and the de-gree of uniform curvature which may be given to a beam is inversely as its depth, or the ra-dius of curvature will be directly as the depth. But it has been shewn by Dr. Young, thatthe deflexion of a beam uniformly curved is to that of one bent, by a load placed in the