Cast iron posts are superior to wooden posts, in that they do not decay, can be made smaller than wooden posts, and are not damaged by wear and tear. They also give the appearance of greater strength and durability, although this appearance may often be deceptive. When not protected by fireproof materials they are quickly injured by fire and water, and in a fire will stand no longer, if as long, as wooden posts.
The shell of cast iron columns that carry any weight should not be less than ¾ -inch thick, and should be cast straight from end to end; under no circumstances should a supporting column be cast as in Fig. 469. If the column is to be loaded with 60 per cent, of its calculated safe load, the thickness of the metal should be tested by boring one or more small holes through the shell and measuring the thickness of the opposite side by means of a stiff wire. A difference in thickness of more than 1/8 of an inch should not be permitted.
The ends of all posts should be turned in a lathe to a true plane at right angles to the axis of the columns. The lower post should rest on a cast iron plate with a raised cross cast on it to fit into the column. The portion of the plate which receives the column should also be turned. The upper posts should be bolted to those below. Fig. 470 shows the manner of casting the top of the columns where wooden girders are used. The top of the column should come about 4 inches above the top of the girder, unless the joists are framed flush, to facilitate bolting the columns together.
The ends of the girders should be tied together by straps on each side, as shown in Fig. 463.
If for any reason a very large cap plate or hollow box is required to support the floor construction, it may be cast separately and bolted to the top of a plain column, the bearing surfaces being turned to fit closely.
If an ornamental cap and heavy projecting base are desired below the girder, they should be cast in separate pieces and screwed to the columns, as in Fig. 471.
Steam Pipe Columns. - In many places a light, inexpensive column is required where only a slight load is to be supported. A particular demand for such columns is at the angles of show windows in one and two-story store buildings. As a rule, in such buildings the floor joists are supported by the side walls and the columns only have to support a light wall above. For such places steam pipe may be advantageously used for the columns, and also for the columns supporting the roof if one-story buildings or light floor loads. It should be borne in mind, however, that steam pipes have a very small sectional area, and they should be used for columns with extreme caution. Their length, in inches, should never exceed thirty times their diameter, if loaded with more than 50 per cent. of their safe load.
The ends of the columns should be turned perfectly true, and should be fitted with top and bottom plates like that shown in Fig. 472. These plates should also be turned where they bear against the column. Ornamental caps should be cast separate and screwed on. Where the columns carry less than 50 per cent, of their safe load, flange unions screwed to the pipe may be used for cap and base plates. The safe loads for ordinary steam pipe columns may be determined by means of table V, Appendix B. 261. Connection of Floor Joists and Girder. - In buildings of ordinary construction, that is with floor joists
2 or 3 inches thick, it is customary to drop the girder so that the floor joists may rest on top of it, as shown for the first floor, Fig. 463.
Very often the joists and girder are framed as shown for the second floor, a cross section of the girder being as in Fig. 52. In either case the joists should be tied together across the building by iron dogs (see Fig. 52) once in every
4 feet or by spiking the ends of the joists together.
If strength and economy alone are to be considered, placing the joists on top of the girder is the most economical method, and as strong as any.
From a slow-burning standpoint, however, this is the worst kind of construction as it leaves a space above the girder around which flames can lap, and also affords a chance for the accumulation of dust and dirt which in the course of time adds inflammable matter for a fire to feed upon. With the joists framed flush with the girder on top, no spaces are left and the girder will be much longer in taking fire. The flush girder also gives a much better appearance to a a room of moderate height.
When the beams are framed flush (in all buildings other than dwellings and small private stables) they should be supported either by steel or malleable iron joist hangers, or stirrups (see Sections 57 and 58), or by strips or angle irons bolted to the girder, as in Figs. 52, 53 and 55. Steel stirrups or hangers undoubtedly give the greatest strength, and if the ceiling is not plastered are to be preferred.
Fig. 472a shows the ideal method of framing of buildings of ordinary construction, although if the ceiling is to be plastered it would perhaps be best to use the Duplex or Goetz joist hangers, as they are not as much affected by shrinkage in the girder (see Section 57).