Dovetail Joint

This is found occasionally in smiths' work, and an instance of its application occurs at the junctions of the ends of window bars in some varieties of wrought iron frames.

Dowel Led Joint

Wrought iron girders carrying floors are, amongst other contrivances, jointed to walls by means of a wrought iron dowel about 1 in. square and 3 in. or 4 in. long, leaded to the seating, a slot being cut in the girder flange to receive the same and allow a little longitudinal play. Rafter-chairs are likewise similarly secured.

Drilled Joint

A riveted joint in which the rivet-holes are drilled and not punched. Before drilling, however, all the parts of the girder should be temporarily put and held together with a sufficiency of bolts, and then the holes should be drilled to gauge through all the thicknesses, at the same time care being taken that all burrs left by the drill on each plate or piece are removed by slight countersinking before riveting. The holes thus made may be closer than when punched. A drilled joint costs more, and is supposed in some quarters to be weaker than one that is punched, though the drill being a cutting tool does not treat the metal so roughly as the punching machine. It leaves, however, the edges of the rivet holes sharper and firmer than when punched, which perhaps enables them to shear the rivets with greater ease. There is much to be said both for and against either practice. As regards their relative merits, it is sufficient to say that the conflict of opinion points to no surplusage of advantage on either side, and a safe and prudent course therefore is to drill the holes at the joints and where there are several thicknesses of plate and to punch elsewhere. In cast iron work, as already observed, holes if not cast must be drilled.

Elbow Joint

This is formed by means of an elbow piece, one form of which is shown in Fig. 81.

Expansion Joint

As stated in the Plumbers' Section, this form of joint is necessary to prevent accident and injury arising from the natural elongation and shortening of certain parts of structures exposed to changes of temperature. A difference of 60° Fahr. in its temperature alters the length of 200 ft. of iron by the space of 1 in., and in exposed situations a minimum provision should in consequence be made at the rate of about 2 in. for every such length to meet its fluctuations. Some engineers, however, consider an allowance of in. per 100 feet sufficient. The range of temperature in this country is about 83° Fahr., and a variation of 1 is theoretically equivalent to a change of load amounting to 1 cwt. per square inch of section, but the temperature of ironwork does not vary by a similar gradation to that of the atmosphere and therefore its effect upon a girder, etc, nduces less additional stress than would at first sight appear probable. As regards roof principals, in order to allow a little room for sliding crosswise on the wall, the bolt holes are elongated in the cast iron rafter-chairs on one side of a roof, or else the chairs are bedded on a plate of 9 lb. lead. An iron dowel, about 1 in. square, is, under these circumstances, sometimes leaded to the stone template, and entering a slot in the chair, duly limits its movements. If a long girder is bolted to a pier in its centre both ends are left free, but generally one end is fixed, the other being supported so as to allow slight motion. This is managed by merely planing the cast iron bed-plates respectively bolted to the girder and abutment, and lubricating the surfaces, or else by inserting between the plates from six to eight cast iron or steel bearing or expansion rollers fitted together in a wrought iron roller-frame superimposed upon a cast roller-bed, in the manner indicated in Fig. 70. Arched ribs and wide roofs, either curved or span, likewise require a similar bearing on one side for sliding motion, and usually it is necessary in works of magnitudo that over the rollers there should bo an adjusting joint to insure equal bearing upon them, together with due freedom to the structure as regards springing motion. In the Clifton Suspension Bridge there are jointed ends or flaps 8 ft long at both extremities of the roadway to give vertica and longitudinal freedom of motion to meet the effects of contraction and expansion as well as those arising from high winds and heavy loads. Fig. 87 represents a more or less satisfactory contrivance for meeting the requirements of variable length in a bridge rib. Cast iron becomes insecure at 32° Fahr. in the face of jar, vibration, or tensile strain.

Expansion Joint 82

Fig. 81.