401 



TRUNCATED. 



TRUSSING. 



402 



the horn, doubled up in a parabolic form, sounded by means of 

 mouth-piece, and subject to the same acoustical laws which govern a] 

 instruments of this class. [HORN.] The natural scale of the trumpet 

 as given by Karl Bargans, ia as follows : 



" By the assistance of a small brass tube, called the tuning-pipe, o: 

 shank, by which the tube of the trumpet is lengthened, the abovi 

 number of notes may be increased." [HARMONICS.] 



Music for the trumpet, as in the instance of the horn, is always 

 written in the natural key of c, and the key to which the iustrumen 

 is to be adapted is pointed out by the Composer. 



TRUNCATED, cut off, or abridged. Thus the part of a cone 01 

 pyramid which remains when the vertex and parts adjacent are sepa 

 rated from the rest by a plane section, is called a truncated cone or 

 pyramid. 



TRUNNIONS. When a heavy body has to be supported so as to 

 be freely moveable in a vertical plane, it is furnished with a couple o: 

 strong cylindrical pins, or trunnions, projecting from its sides, am 

 resting in semi-cylindrical grooves placed at equal heights in the sup- 

 porting pillars, so that a line connecting the trunnions is horizontal 

 lu this way the body may be placed in any position in a vertical plane 

 at right angles to such line. 



TRUSS. [HERNIA.] 



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 supporl 

 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 oi 

 trussing or supporting it may be applied in two sets, one on each side 

 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 go 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 

 fi<l. 1 , in which a 6 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 root 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, djf, suspended from 

 their apex, and passing through an iron plate which bears against the 

 under side of the beam. 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. 



Fig.l. 

 d 



By screwing up the nut /, 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 state of tension, to act as a tie con- 

 necting the abutments c and e, is really in a state of compression, 

 because it forms 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 weaker." 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 e, and capable of being brought to any required 

 degree of tension by mean* 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, 



AUTS ASD SCI. DIV. VOL. VIII. 



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 



Fig. 2. 



a leaden flat, and which had been found to stand without [sensible 

 alteration for two years. The beam a b is cambered in a similar degree 

 to that shown in Jig. 1, and the trussing consists of a series of iron 

 rods, ac,cd, and d b, pulling against iron plates or abutments notched 

 into the timber at a and 4, and connected together at the joints c and d, 

 by bolts similar to those used in the chains of a suspension-bridge, the 

 rods c d 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 6, 

 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 iron. 

 The upper edge of the beam is thus enabled effectually to resist the 

 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 fy. 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 jvj. 1, but with the addition of a 

 horizontal tie-bar one t inch square, appeared to possess no greater 

 strength to resist fracture, although ita deflection under similar loads 

 was less, owing to the iron braces being less compressible than the 

 fir-wood, which, on the suspension principle, has to resist the tension 

 of the rods. 



The late Mr. George Smart, inventor of the ingenious truss called 

 the " bow-and-string-rafter," in experiments tried to ascertain how far 

 the strength of a beam is increased by confining its ends, so as to 

 prevent them from approaching each other when the centre is heavily 

 loaded, found that a lath which, when simply laid on two points of 

 support, broke with a load of 11 Ibs. placed in the middle, would 

 sustain 270 Ibs. when the ends were firmly secured by wedges. These 

 experiments led him to the construction of trussed beams of unusual 

 lightness, in wrought-iron as well as in wood. In 1826 he submitted 

 to the Society of Arts a wrought-iron beam adapted for use in bridges, 

 roofs, floors, and other constructions in which stiflness, strength, and 

 lightness are required, for the invention of which he was rewarded 

 with their silver Vulcan medal. This beam or girder is made by 

 welding the ends of an arched bar of wrought-iron to a longer straight 

 bar, and then turning the ends of the straight bar either up or down, 

 as may be most convenient for fixing, according to the particular use 

 x> which it is to be applied. Blocks of well-seasoned wood are then 

 nserted at intervals between the arch and the straight bar, to prevent 

 ruckling, and the whole ia held together by iron straps inclosing the 

 ilocks and the iron bars. Mr. Smart conceived that wrought-iron 

 >eams made in this way would support so much more weight than 

 ,hose of cast-iron of similar dimensions, that they might be made of 

 any given strength for one-half the cost of cast-iron girders ; and he 

 refers to the application of such a beam to sustain a very heavy mass 

 of brickwork over a gateway leading from the Poultry, under circum- 

 stances which would have precluded the use of timber. Very light 

 timber beams were made by Mr. Smart upon the same principle ; and 

 so strong was big confidence in the application of trussing upon an 

 extensive scale, that he published a design for a foot-bridge of trussed, 

 timber to cross the river Thames at Hungerford by a single span. 



Cast-iron beams are frequently trussed with wrought-iron rods, in a 

 imilar manner to those of wood, and are applied to purposes for 

 which great strength is required. The brest-summers used over large 

 hop-windows, to sustain the front wall of the house, are often made 

 n thu way ; and similar girders are much used in the construction of 



D D 



