1350
THEORY AND PRACTICE OF ENGINEERING.
Rook II.
Trajan's Bridge over the Danube is shown in bas-relief upon the triumphal column erectedto that emperor in Rome : its construction was timber resting on stone piers, and so framedthat it was both solid and durable ; three rows of concentric arches united by bindingpieces formed each division ; theseabutted upon timbers radiatingwith the curve, which wereframed into heads and sills,again strengthened by bracesand struts ; the joists which car-ried the floor traversed the bridge,and rested on strong plates whichwere laid on the backs of thetimber arches. The parapets wereformed of perpendicular pieceswith a St. Andrew’s cross betweenthem, united at the top and bottomby a longitudinal sill and head.
This construction is admirable, though more strength might have been introduced in thecarpentry, which rests on the piers and forms the abutments to the concentric arches whichspan the river ; they would then have resisted more perfectly the thrust to which they are sub-jected ; horizontal timbers driven between the struts, particularly between the heads of theshorter, would have added materially to the strength, and given those timber abutmentsgreater power of resistance to compression.
The Pontus Sullicius at Rome was an earlier w’ooden bridge, and constructed 500 yearsbefore the Christian aera, under the reign of Ancus Marcius , one of the first kings : allthat we know of it is that it could be taken to pieces, and that neither iron nor nails wereused in its construction; the timber was united by solidly framing it together, usingwooden pins to secure it.
Sublicius is derived from sublica , a pile, which indicates that the platform or roadway wassupported by them, driven at regular distances from each other : this bridge is mentionedas that so valiantly defended by Horatius Codes. Most of the timber bridges that havebeen constructed since the fall of the Roman empire exhibit in their design one or other ofthe twelve principles which we are about to describe.
The first principle that we can adopt is that of a single beam, or one composed of manythicknesses: if the iower, which spans from A to B, is laid flat, the others, as P', C', P, C,
Fig. 2173.
A!
In
Fig. 2174.
FIRST PRINCIPLE.
may be cut tapering, and as they are laid upon each other either bolted or strapped togetherto make one entire mass : by this means the requisite increase of depth towards the centremay be given, to support any weight for which it is destined.
The second principle, as adopted at the bridges of Schaffhausen , Zurick, Randsbcrg, "Wit-tingen, &c., has the tie-beam to constitute the abutments for the compressed timbers ; con-
Fig. 2175.
SECOND PRINCIPLE
sequently there is no thrust exerted against the walls, as the entire timber-work may be re-garded as a trussed frame.
The third principle was introduced at the Kandel Bridge by Joseph Ritters, but here itis requisite that theabutments should be made sufficiently strong to resist the entire thrust :sometimes the tie cannot be made use of; then we are obliged to adopt some such methodas here shown.
The fourth principle exhibits a different arrangement for beams that are compressed ;