' TRUSSING. The principle of trussing, as applied to the timber framework of roofs, is explained under ROOF. It remains to notice some of the methods in which that principle is applied to the support or strengthening of beams or girders, which may, by their judicious application, be made available for much Larger spans, and for the support of much greater weights, than simple beams of any attainable dimensions.
The rods or bars which are added to a girder for the purpose of trussing or supporting it may be applied in two sets, one on each Bide of the girder, and connected together by short cross-pieces at the necessary points ; or the beam or girder itself may be divided longi tudinally into two halves, or flitches, separated just so far as to admit a single truss between them, and held in the right position by the insertion of small blocks. In trussed girders formed in the latter way it is well to reverse the position of one of the flitches, so that the weaker end of one may lie alongside the stronger end of the other. One of the simplest methods of trussing girders is that represented in Aq. 1, in which a b is the beam, resting upon walls or other fixed points of support at its extremities, and c d and d e are two inclined struts, resembling the rafters of a roof. These abut, at their lower extremi ties, c and e, upon iron plates inserted in the timber ; and they sustain the centre of the beam by means of a king-bolt, d f, suspended from their apex, and passing through an iron plate which bears against the under aide of the This bolt corresponds with the king-post in the truss of a roof, and the lower part of the beam, between c and e, acts the part of a tie-beam.
By screwing up the nut f, on the lower end of the king-bolt, the beam is cambered, or slightly curved upwards, as shown in the cut, and so long as it retains this curvature the weight laid upon it must eventually press upon the trussing-bars, being transmitted to them through the king-bolt. The defects of this mode of trussing consist in the circumstance that the beam will not, so long as it retains the cambered form, sustain any part of the load, but will of itself throw considerable strain on the truss ; and that the lower edge of the beam, although required to be in a atato of tension, to act as a tic con necting the abutments c and e, is really in a state of compression, because it forma the inner part of the curve, which is necessarily shorter than the outer line formed by the convex top of the beam. "Notwithstanding these obvious defects," observes Mr. Ainger in a communication to the Society of Arts (` Transactions,' vol. xlviii. p. 101), " this mode of trussing continued to be much employed till about the year 1816, when Mr. Barlow, among other valuable experiments, compared girders trussed on the principle above described with a plain piece of timber of the same size, and found the latter to be on the average not considerably These defects are remedied by connecting the lower ends of the inclined bars, which are, though not very properly, called braces, by an iron rod stretching in a perfectly straight line from c to c, and capable of being brought to any required degree of tension by means of screws or keys. This addition makes the truss perfect in principle, its strength being limited only by that of the materials employed, which may be either iron alone, or iron and wood. In some cases the inclined bars are not continued upwards
until they meet in an apex, and a third bar, in a horizontal position, is placed between their upper ends. This horizontal piece resembles the straining-sill of a truncated roof, and the vertical bolts, of which two are used, take the place of queen-posts.
In the paper by Mr. Ainger above referred to, it is observed that trusses on the above principle are difficult and somewhat expensive to make in an effectual manner; and a more economical plan is described, which, though not equally efficient, adds very greatly to the strength of the timber. The description is illustrated by a representation, of which fig. 2 is a copy, of a girder thirty-four feet long, used to support a leaden flat, and which had been found to stand without alteration for two years. The beam a b is cambered in a similar degree to that shown in fig. 1, and the trussing consists of a series of iron rods, a c, c d, and d b, pulling against iron plates or abutments notched into the timber at a and b, and connected together at the joints c and d, by bolts similar to those used in the chains of a suspension-bridge, the rods cd being double, and embracing the ends of a c and d b between them. The truss forms, in fact, a suspension-bridge, supporting the middle of the beam at c and d by means of small blocks inserted between it and the connecting-bolts. The ends of the truss, a and b, are prevented from approaching each other by the upper part of the beam, which should therefore be in a state of compression ; and in order that it may be so, notwithstanding the extension of the fibres by the cambering of the beam, notches may be cut about one-third through the substance of the wood, as at e, e, e, e, e, which, after the curving of the beam, are filled in with wedges of hard wood or irou. The upper edge of the beam is thus enabled effectually to resist tho tension of the rods, the strength of which forms the only limit to that of the girder. Several varieties of this plan of suspension-trussing are given in Hebert's Engineer's and Mechanic's Encyclopaedia,' vol. i., pp. 158-161. It may be applied, like the former system, either to single girders or to those consisting of two flitches. Ainger gives a formula for calculating the size of the iron trussing-rods, which, for a beam thirty-four feet long, should have a cross-section of rather more than a square inch for every ton weight to be sustained in the centre of the beam. It was found by experiment that a fir beam eight inches square and twelve feet long between the supports, strengthened by iron rods one inch square, applied as in fig. 2, would support between 4000 and 5000 pounds, which is more than double the weight it would sustain without trussing. A girder of the same dimensions, trussed with iron braces on the principle of fig. 1, but with the addition of a horizontal tie-bar one ,,inch square, appeared to possess no greater strength to resist fracture, althoub* its deflection under similar loads was leas, owing to the iron braces being less compressible than the fir-weed, which, on the suspension principle, has to resist the tension of the rods.