Bed Or Bedding Joint

Iron columns and stanchions may be bedded on stone or concrete by making the joint with asphalted felt, milled lead, iron cement, molten lead, sulphur, or Portland cement. With either of the two first a piece is merely inserted between the iron and base as a seating, but with the others the column, pilaster, or stanchion is first set up as nearly plumb as possible on its prepared base or bedstone, and correctly adjusted by means of iron wedges to the exact height and alignment, and then the bed joint is run with the cementing material, being further secured, if thought necessary, by bolts leaded to the base, as described under Bolted Joint, and by caulking where molten lead or rust cement is used. There appears to be no better plan than to set the column perfectly plumb with iron packings, and then to run the joint with neat Portland cement grout from the inside of the column, placing in the first instance a temporary curb round the base, so as to obtain a sufficient head of the semi-fluid to insure absolute solidity when set. Columns are sometimes cast with bed plates and bearing plates on the base and cap respectively, strengthened by vertical brackets termed feathers, stiffeners, or stiffening pieces, with bolt holes or caulkings, as the case may be, on the base plate for fixing to bed stones and holes in the bearing plate for bolting to iron overhead girders. The area of these expanded bearing surfaces should not much exceed three times that of the shaft, for if allowed to project too far danger will arise from cross strain in case of a slight yielding of the foundation, or the overhead girder deflecting. As a rule it is expedient to have the spreading base plate detached, as in Fig. 74, to obtain a perfect bearing and uniform distribution of load, rather than combined in the same casting with the column when large, and where this is the case the area of the base plate may be considerably greater than when it does not constitute a separate piece. Stanchions sometimes require specially cast spreading footings with lathe turned and truly faced surfaces of contact. Too much importance cannot be attached to the necessity of making the faced bearings and bed joints of pillars truly perpendicular to their axes, and fixing them sufficiently rigidly to prevent slipping in case of vibration or subsidence. Turned-up flanges or boxes, sometimes gracefully concealed with ornamental foliage, are cast on the cap plate when the columns support overhead girders of timber, which drop down upon snugs cast on the plate for a lateral tie. If there is a superimposed column, its base is cast with a flanged spigot for dropping into a socket, also flanged, formed by the prolongation of the cap of the lower column, the space between the bearing plate for the girder and the flanged and bolted bed joint of the columns being occupied by the floor. In many cases columns are bedded by direct bolting to massive cast iron base or foundation plates concealed below the ground-floor line. Iron bedded on iron requires true and parallel fitting and planing, or turning in a lathe, so that both head and foot, or top and bottom surfaces may be truly horizontal when fixed; and where a column fits into another, or only into its own base or capital, all the parts intended to be in contact should be turned and faced in a lathe, so that the proper touch may be preserved throughout. It is quite possible to chip and file, and finish off and bring the surfaces of ironwork to as smooth a face as that of glass. When iron rests on stone, the latter must be sufficiently thick, dressed to a true surface, properly-sunk for the plates of columns, or cut or bored for bolts, mortised for dowels, tenons, or caulkings, or sunk for the rivets of girders where the rivets are not countersunk. Cast iron girders carried by columns are sometimes forked out at the end to partially embrace the columns' head prolonged above the cap, a flanged necking or girder bed serving for their support, and a coupling link or shrunk-on collar forming the connection between two contiguous girders through the medium of burrs or snugs or stubs cast on them with that object. At other times the ends of the girders are flanged and bolted together over the cap. If a roof is to be upheld, the shoe or chair for the principal rafter is bolted to the girder near the column, which often acts as a downpipe. In bridge work a piece of thick sheet lead is sometimes inserted between the cast iron bed plates that are respectively bolted down to the granite bedding blocks capping the pier and the bottom booms of the girders. In other cases lead intervenes between the granite and iron, and in some instances the bed plates are planed to permit the girders to expand and contract, or they may be hinged, or else made to ride upon rollers according to the discretion of the constructor. Fig. 75 represents one method of bedding a superimposed column and overhead girder.

Bed Or Bedding Joint 75

Fig. 74.