The timber, &c., used in the construction of wooden bridges is, according to the position it is made to occupy, obliged to undergo four kinds of' Curet-Ls, which tend to over come the cohesion exerted between the particles of which it is 'brined. These are, first, and principally, transverse pies sure, or a lbree acting transversely upon a beain, tending thereby to destroy the cohesion and continuity of its fibre, such is the force that the platform of a bridge must be made to resist; such, also, the roof, the joists, the flooring, and many of the levers, of which it is composed. This, however, is a compound force made up of the two succeeding forces. 2nd. A compressive or crushing force, which acts longitudi nally; this is called generally into play in the framing of the bridge, posts, struts, piles, &e. :;rd. A tensile force, in which there is a tendency to pull the particles of the wood from one another, and thus cause a total destruction of its parts by stretching and drawing it to pieces; this force is exerted on king-posts, tie beams, the under parts of the roadway, and of the joists, walings, &e.; and, lastly, there is a twisting force, by which the particles of bolts, rivets, trenails, gudgeons, &c., arc strained or wrenched fromn each other, or are constantly subjected to a force tending so to disunite and destroy them.
In order to prevent these forces front having the eff.•et of injuring time bridge, great care must be taken in the selection of the proper materials; in the manner of disposing them in the bridge, by adopting such an arrangement as will make one Voice act as a set-off to another by neutializing its inju rious etffets, and, above all, by distributing the pressures judiciously, so as net er to let any portion of the structure he loaded with more than one-third of the weight which it is calculated to bear without breaking.
In the case of a beam supported at its extremities, it has been found that if' it he cut on the upper side, near the middle, to about one-third of its depth, and a piece of hard wood he inserted, in the form of a wedge, so as to give the entire beam it slight camber, that its strength will be in creased about a sixth of mm hat it was previously. This plan may be practised with great advantage when the distance between the supports is small, because the rigidity of the material is so much greater in proportion to the weigth to he supported, that the deflexion is not injurious; but when the spans are great, and the props or piers for removed front each other, it beeomes necessary to truss the beacons to prevent their filling below the horizontal line. This deflexion is as the product of the weight and cube of the length directly, and inversely as the product of the breadth and the cube of the depth. A deflexion of about time fiftieth part of an inch is not very injurious, but, where possible, it should be provided against. It may be well to state here, that the deflexion
from a weight uniliammly distributed over it bridge is to the deflexion caused by the same weight placed in the centre, as 5 is to 6.
In the event of a beinn being loaded up to the point of breaking, it will bend before way, and the transverse strain which would produce this result is in the ratio of the breadth multiplied into the square of the depth, and inversely as the length; and the square of the secant of the angle of deflexion, immediately before giving way altogether, must also form an element of the calculation. When this is ascer tained, one-third or this breaking weight must be taken for the maximum load with which it must be weighted.
The inherent qualities of timber and other materials which enable them to resist the force of compression acting in the direction of their length, is directly as the tlairth power of their sides in cubic blocks, and as the square of their height inversely. The cohesion of wood, &c., by which it is able to resist the force of tension, or a drawing force, exerted in the direction of its length, varies in the proportion of its cross section. The pow er of resisting a tensile strain, in different kinds of wood, varies in the ratio of their different specific gravities.
All materials withstand the twisting force of tension when in the form of cylinders, in the ratio of the angle of tension, and the fourth power of the diameter directly, and in the in verse proportion of their length.
The resistance of different woods to pressure, as derived from actual experiment made with blocks of an inch cube, is found to be—for elm, 1,2S4 lbs.; white deal, 1,928 lbs.; oak, (English) 3,860 lbs.; and pine, (American) 1,606 lbs.
The resistance to tension for bars of an inch square of transverse section, is ascertained to be—for ash, about 11,970 lbs.; beech, 17,371 lbs.; elm, 1'2.231 lbs.; fir (pitch pine), about 12,917 lbs.; fir (red), 10,829 lbs.; fir (Memel), 10,662 lbs.; fir (Russian), about 9,992 lbs.; fir (American), 870 lbs.; fir (yellow deal), 8.316 lbs.; fir (white deal), 4,204 lbs.; fir (Scotch), 6.569 lbs.; larch, 10,875 lbs.; oak (English), 13,943 lbs.; oak (French), about 14,405 lbs.; oak (Baltic), 11,189 lbs.; oak (American), 10,154 lbs.; oak (Dantzie), 7,558 lbs.; and for willow, about 12,538 lbs.
Oak and fir are principally used in bridge-building, on account of their great durability and hardness, which is a point of very great importance; for the principal objection to wooden bridges in general is, that they are extremely liable to decay; still, their great cheapness, in comparison with bridges constructed of other materials, makes them very suitable in many instances.