used for other buildings than boiler houses or depots, except a ceiling be made below to prevent the contact of the air inside with the iron.

Fig. 5 is an elevation of nearly two of the three panels of one of the cast iron girders for connecting the columns, and carrying the transverse main gutters, which supported the roof of the English Crystal Palace. Figs. 6, 7, 8, 9, 10, 11, sections of various parts on an enlarged scale.

The depth of the girder was 3 feet, and its length was 23 ft. 3 inches. The sectional area of the bottom rail and flange in the centre (fig. 7), was 6 square inches; the width of both bottom and top rail (fig. 6), was reduced to 3 inches at their extremities. It will be observed that the section of the braces and ties are such as to give great stiffness, and the section of the braces at bb (fig. 8), is greater than at cc (fig. 9); the section of the tie (fig. 10), is the same as the brace at cc; they are all formed with a draft, that is, with a taper from the centre to the outside of from 1⁄2 to of an inch on a side, according to the depth of the feather.

The weight of these girders was about 1,000 lbs., and they were proved by a pressure of 9 tons, distributed on the centre panel.

A second series of girders were made of similar form to fig. 5, but of increased dimensions in the section of their parts. Their weight averaged about 1,350 lbs., and they were proved, as above, to 15 tons.

A third series, of increased section of parts, weighed about 2,000 lbs., and were proved to 22 tons.

Fig. 12 represents an elevation of two of the nine panels of one of the wrought iron trusses which carry the lead flat and arched roof across the nave of the Crystal Palace. These trusses are 72 feet long and 6 feet deep. The top rail G, shown in section fig. 13, consists of two angle irons 4 inches deep, 8 inches wide, and of an inch thick, with a plate 9 inches wide and thick, riveted on top. A space of 2 inches is left between the angle irons. The angle irons are in five lengths, and are connected by eight rivets passing through them, and through a plate or plates introduced between them. The top plate is in seven lengths, connected by inch rivets, through the angle irons, the plate, and a joint plate. The top plate is riveted to the angle irons by inch countersunk rivets, 5 inches apart. The bottom rail consists of two flat wrought iron bars, 6 inches deep, with a 2 inch space between them. It is in four lengths, jointed by six 1 inch rivets passing through joint plates 6 x 15 x inches on the outside, and three plates 17 x inches. The bars forming the central lengths of the bottom rail are ¡ of an inch thick, and those forming the side lengths are of an inch thick.

The end standards are of cast iron, 8; inches wide, 4 inches deep, and

1 inch thick, of a T form of section, secured to the column by six 11 inch bolts. The standard is 2 inches thick at top and bottom where it receives the rails. Two sockets are formed in the middle, to receive the diagonals I and J. I, being exposed to compression, is made of four angle irons, 2× 2 × inch, riveted together in pairs with inch rivets. The diagonal J is formed of two bars, 4 × inch, and is secured at each end by a 13 inch rivet. The ends are thickened by short plates riveted to them to make up in a measure the loss of strength from the large rivet hole. The diagonal K is formed of two bars 4 inches deep by 1 inch thick, and is fixed at each end by a 2 inch bolt and nut. The other diagonals, being exposed to much less strain, are formed of single bars 4 × inch, and are secured at each end by a 1 inch rivet.

The standards B and C consist each of four angle irons, 2 × 2 × inch, riveted together in pairs, and the two pairs riveted together with six small cast iron distance pieces between them, The next standard, that is, the third from each end, but not shown in the drawing, is of cast iron. It is of section, being at the centre 6 x 6 inches, thickness of metal to of an inch. The base, which rests upon the base of the bottom rail, is 18 × 4 inches, and the top is 18 x 3 inches. Triangular projections enter the top and bottom rail, where they are secured by 1 inch rivets. In the centre is a socket or slot through which pass the two light diagonals. The main strength of the truss consists in the top and bottom rails, the diagonals I, J, K, the first wrought iron standard, B, and the cast iron standard, D.

On the General Principles of Bracing.-Let fig. 42 be the elevation of a common roof truss, and let

resting against some fixed point, then the point B would support the whole downward pressure of the weight; but in consequence of the connection of the parts of the frame, the pressure must be resolved into components in the direction C' A and C' B, C'b will represent the pressure in the direction C' B, C'w the portion of the weight supported at B, C'a the pressure in the direction C' A, and w W the portion of the weight supported on A. The same resolution obtains to determine the direction and amount of force exerted on a bridge truss of any number of panels, by a weight placed at any point of its length (fig. 43.) In either case, the

effect of the oblique form C' A, upon the angle C, is evidently to force

Fig. 43.

it upwards; that is, a weight placed at one side of the frame has, as in case of the arch,

a tendency to raise the other side. The effect of this upward force is a tension on a portion of the braces, according to the position of the weight; but as braces, from the manner in which they are usually connected with the frame, are not capable of opposing any force of extension, it follows that the only resistance is that which is due to the weight of a part of the structure. Figs. 44 and 45 illustrate the results of overloading at single points

such forms of construction.

To remedy this effect, if counter braces be introduced, as shown in dotted lines (fig. 46), the tendency of a weight moving across the structure is to compress the counters and extend the braces. But since, as we have said, braces are not usually framed, especially in wooden structures, to resist a tensile strain, it is necessary to overcome this force in another way; that is, by introducing wedges between the end of the counter braces and the joints against which they abut, or by means of the counters and the suspension rods, in any way straining the structure so that there may be an additional compression upon the brace more than the upward or tensile force exerted by any passing weight. In this case, therefore, the passage of a load would produce no additional strain upon any of the timbers, but would tend to relieve the counters. The counter braces do not, of course, assist in sustaining the weight of the structure; on the contrary, the greater the weight of the structure itself, the more will the counter braces be relieved.

If instead of the counter braces, the braces themselves are made to act both as the and as a strut, as has been done sometimes in iron bridges and trusses, then the upward force will be counteracted by the tension of the

brace, but in general counter braces are preferable, as it is better that the force exerted against any portion of the structure should always be in one direction.

It follows, from what has been shown of the effect of a variable load, that no bridge, either straight or arched, intended for the passage of heavy vehicles or trains, should ever be without counter braces or diagonal ties, and that only in the case of roofs or aqueducts of similar construction, when the load is uniform, or very small in comparison with the weight of the structure itself.

On the Truss by Tension Rod (fig. 47).—Since the limit of the elas

a

Fig. 47.

ticity of iron is very small in comparison with wood, when iron is thus used to truss timber, the rods must break before the beam reaches the deflection that the weight should produce. It is evident, therefore, that in construction the beam should not be cambered by the tension of the rod, but that the top of the beam should be arched, and be permitted to settle with the weight before it strains the rod at all. In general, the rods should be depended on to resist the whole of the tension, and act as the lower chords of an ordinary bridge; in this way the calculation becomes very simple and furnishes safe practical results. Thus (fig. 47.), to estimate the strain upon the suspension rod, multiply the weight supported at the point c or c' by the length of the rod a d or d' b, and divide the product by the length of the strut cd. The length of the rod and of the strut, may be measured from any horizontal line which completes the triangle.

Example.—What must be the tension on the rod of a truss 40 ft. span, supporting a load uniformly distributed of 80,000 lbs. ; the struts ed, e' d', being 3 feet, and the middle interval 12 feet, and the end ones 14 feet each? Then each point c, c', supports +40 or 3 of the weight,

=

15.5 sq. inches necessary to resist this tension.

=124,106 lbs., tension of rod.

Suppose a system to be composed of a series of suspension trusses, as

=

Fig. 48.

in fig. 48, in which the load is uniformly distributed. If we represent the load at each of the points, 4, 3, 2, 1, 2', &c., by 1, the load at 4 will be supported upon a and upon 3; hence the strut 3 will have to support a load of 1.5 1.5; of this, will be supported by 2 and by a; } of 1.5=1, 1+1=2, load on strut 2; of this load, or 1.5, will be supported at 1, and since from the opposite side there is an equal force exerted at 1, therefore the strut 1 supports 1 + 1.5 + 1.5= 4; the tension on the rod e2 is 4; on 2 3, 4 + 1 = 5; on 34, 5 + 1 = 6; on 4 a, 6 + 1 = 7; and the rod should therefore be increased in strength in these proportions from the central point c, to the point of suspension, a. The tension on the rods 34, 23, 12, may be easily resolved from their direction and the load upon the several struts.

If this construction be reversed, the parts which now act as ties must be made as braces, and braces, ties; then we have a roof truss, and the force exerted on the several parts may be estimated in a similar way as for the suspension truss.

It is evident that neither of these constructions would serve for a bridge trus, subject to the passage of heavy loads, but is only fit to support uniform and equally distributed loads.

To frame a construction so that it may be completely braced, that is, under the action of any arrangement of forces; the angles must not admit of alteration, and consequently the shape cannot. The form should be resolvable into either of the following elements:-Figs. 49, 50, 51.

Fig. 49.

In these figures, lines

Compression; lines

Fig. 50.

Fig. 51.

represent parts required to resist parts to resist tension only; lines

parts to resist both tension and compression.

It is evident that in a triangle (fig. 49.), an angle cannot increase or