so placed as to receive blocks, or keys of hard wood. The keys Fig. 61-Represents a solid bailt beam with keys b, b of hard wood between the courses.. are sometimes made of two wedge-shaped pieces, (Fig. 62,) for a C Fig. 62-Represents the keys in the form of double, or folding wedges a, b let into a shallow notch in the beam c. the purpose of causing them to fit the notches more closely, and to admit of being driven tight upon any shrinkage of the woody fibre. The joints between the courses may be left slightly open without impairing in an appreciable degree the strength of the combination. This is a good method in beams exposed to moisture, as it allows of evaporation from the free circulation of the air through the joints. Felt, or stout paper saturated with mineral tar, has been recommended to secure the joints from the action of moisture. The prepared material is so placed as to occupy the entire surface of the joint, and the whole is well screwed together. 503. Open built Beams. In framing open built beams, the principal point to be kept in view is to form such a connection between the upper and lower solid beams, that they shall be strained uniformly by the action of a strain at any point between the bearings. This may be effected in various ways, (Fig. 63.) A B Fig. 63-Represents an open built beam; A and B are the top and bottom rails or strings; a, a, cross pieces, either single or in pairs; b, diagonal braces in pairs; c, single diagonal braces. The upper and lower beams may consist either of single beams, or of solid built beams; these are connected at regular intervals by pieces at right angles to them, between which diagonal pieces are placed. By this arrangement the relative position of all the parts of the frame will be preserved, and the strain at any point will be brought to bear upon the intermediate points. Two of the best known applications of this combination, when timber alone is used, are those of Colonel Long, of the U. S. Topographical Engineers, and of the late Mr. Town. 504. That of Colonel Long (Fig. 64) consists in forming both the upper and lower beams, termed by the inventor the strings, of three parallel beams, sufficient space being left between the one in the centre and the other two to insert the cross pieces, termed the posts; the posts consist of beams in pairs placed at suitable intervals along the strings, with which they are connected by wedge blocks, termed jibs and keys, which are inserted into rectangular holes made through the strings, and fitting a corresponding shallow notch cut into each post. A diagonal piece, termed a brace, connects the top of one post with the foot of the one adjacent by a suitable joint. Another diagonal piece, termed the counter-brace, is placed crosswise between the two braces and their posts, with its ends abutting against the centre beam of the upper and lower strings. The counter-braces are connected with the posts and braces by wooden pins, termed tree-nails. In wide bearings, the strings will require to be made of several beams abutting end to end; in this case the beams must break joints, and short beams must be inserted between the centre and exterior beams wherever the joints occur, to strengthen them. The beams in this combination are all of uniform cross section, the joints and fastenings are of the simplest kind, and the parts are well distributed to call into play the strength of the strings, and to produce uniform stiffness and strain. B 505. The combination of Mr. Town (Fig. 65) consists in twe A a a a Fig. 65-Represents an elevation A, and end view B, of a portion of Town's truss. a, a, top strings. b, b, bottom strings. c, c, diagonal braces. main strings, each formed of two or three parallel beams of two thicknesses breaking joints. Between the parallel beams are inserted a series of diagonal beams crossing each other. These diagonals are connected with the strings and with each other by tree-nails. When the strings are formed of three parallel beams, diagonal pieces are placed between the centre and exterior beams, and two intermediate strings are placed between the two courses of diagonals. This combination, commonly known as the lattice truss, is of very easy mechanical execution, the beams being of a uniform cross section and length. The strains upon it are borne by the tree-nails, and when used for structures subjected to variable strains and jars, it loses its stiffness and sags between the points of support. It is more recommendable for its simplicity than scientific combination. 506. A third method, called after the patentee, How's truss, has within a few years come into general notice. It consists of (Fig. 66) an upper and lower string, each formed of several thicknesses of beams placed side by side and breaking joints. On the upper side of the lower string and the lower side of the upper, blocks of hard wood are inserted into shallow notches; the blocks are bevelled off on each side to form a suitable point of support, or step for the diagonal pieces. One series of the diagonal pieces are arranged in pairs, the others are single and placed between those in pairs. Two strong bolts of iron, which pass through the blocks, connect the upper and lower strings, and are arranged with a screw cut on one end and a nut to draw the parts closely together. This combination presents a judicious arrangement of the parts The blocks give abutting surfaces for the braces superior to those obtained by the ordinary forms of joint for this purpose. The bolts replace advantageously the timber posts, and in case of the Fig. 66-Represents a, top string. d, single braces. e, e, steps of hard frame working loose and sagging, their arrangement for tightening up the parts is simple and efficacious. The timber of each string is not combined to give as great strength as its cross section is susceptible of, and the lower string, upon which a strain of tension is brought, against which timber offers the greatest resistance, has received a greater cross section than that of the upper. The preceding combinations have been applied generally in our country to bridges. In this application, the timber supporting the roadway of the bridge is usually placed on the lower strings; two, three, or four built beams being used, as the case may require, for supporting the transverse beams under the roadway, the centre beams leaving an equal width of roadway between them and the exterior beams. 507. Framing for intermediate Supports. Beams of ordinary dimensions may be used for wide bearings when intermediate supports can be procured between the extreme points. The simplest and most obvious method of effecting this is to place upright beams, termed props, or shores, at suitable intervals under the supported beam. be When the props would interfere with some other arrangement, and points of support can be procured at the extremities below those on which the beam rests, inclined struts (Fig. 67) may used. The struts must have a suitably formed step at the foot, and be connected at top with the beam by a suitable joint. In some cases the bearing may be diminished by placing on Fig. 67-Represents a horizontal beam C sup ported near the middle by inclined struts A, A the points of support short pieces, termed corbels, (Fig. 68,) and supporting these near their ends by struts. In other cases a portion of the beam, at the middle, may be strengthened by placing under it a short beam, called a straining beam, (Fig. 69,) against the ends of which the struts abut. Fig. 69-Represents a horizontal beam strengthened by a straining beam ƒ and inclined struts e, e. Whenever the bearing may require it the two preceding arrangements (Fig. 70) may be used in connection. F.g. 70-Represents a combination of Figs. 68 and 69. In all combinations with struts, a lateral thrust will be thrown on the point of support where the foot of the strut rests. This strain must be provided for in arranging the strength of the supports. 508. When intermediate supports can be procured only above the beam, an arrangement must be made which shall answer the purpose of sustaining the beam at its intermediate points by suspension. The combination will depend upon the number of intermediate points required. |