All wooden posts carrying girders and posts above, should have an iron cap with side plates to receive the girders, Fig. 171, allowing the post above to be supported directly by the post below, and not to stand ou the girder. This must be done for two reasons. One reason is, if the girder ran over the top of the post and the post above were set upon it, the natural shrinkage of the girders would be multiplied by eadh succeeding floor, and in a building of four or five stories; this might amount to two or three inches, so that the tipper floor beams at their inner end would be that much lower than the outer
ends which are supported by the rigid masonry. Another reason is,
that the crushing strength of the girder
in its longitudinal position is not so great
as the post standing on end, and it might
be unable to support the accumulated
weight of several stories. This support
was formerly obtained entirely by the
use of cast-iron pintles, Fig. 172, which
are cast with top and bottom plates to fit
the posts, the weight being transmitted
by the cross-shaped metal. This is an
effective method, but has been
seded by the more modern steel caps.
The bearing of the girders should be
at least five inches on either end, and a box hanger of some kind should be used to support the wall end of the girder, as shown in Fig. 173.
For large spans and heavy weights it will be often necessary to use iron or steel columns with wooden girders. If cast-iron columns are used, they should not be made with a shell less than three-quarters of an inch thick. This is necessary on account of the danger of an unequal thickness in the shell of the column. To obtain the hollow column, the casting must be made about a "core," and although this core is accurately centered, there is danger of its being displaced by the pouring in of the molten iron, as, being of a lighter composition, it will have a tendency to "float." This may result in an added thickness to one side of the shell of the column and -a corresponding lack of metal on the opposite side. To guard against this defect, cast-iron columns should always be tested by boring a small hole on opposite sides, and if more than one-fourth, of the thickness of shell is wanting in any column it should be rejected. The outer surface of cast iron should be smooth and clean, with sharp angles; and all projections, such as lugs, caps, or bases, should he closely examined to detect the pres ence of cracks which may occur at these points.
Cap and Base. The top and bottom of all cast-iron
should be turned off in a lathe to insure a bearing at right angles to the axis, and plates should be used to increase the bearing. These plates are cast with a ring or with projections to hold the column in place against movement, and they should be planed to a perfectly even bearing. The cap must never be spread out as a casting,
Fig. 174, but should carry up the line of the
itself, all orna mentation requiring a great projection being cast separately and fastened on, as in Fig. 175. This allows the shaft of the column to run straight up to bear the weight of the column above, while the girder is borne by the projections cast upon the column. The dis tance from the cap to the top of the column should be about four inches greater than the depth of the girder to allow roots for bolting the columns together.
With a wooden girder, the ends of the timber must be cut out to fit the diameter of the column (Fig. 176), and the girders must be tied across the cap by stout straps on each side. If steel beams are used, they can be tapped to the column plate or strapped together around the column.
Pipe Columns. Heavy wrought-iron pipe is often used for
a cap and base being sometimes cast and screwed on by a thread; and a. patented column formed by filling the pipe with cement, may be obtained ready fitted with cap and base.
Where the spacing of columns is required to be increased beyond the safe bearing of wooden girders, steel girders may be used. These usually consist of one or more steel I-beams, and they may be used with cast-iron columns or with steel columns. With cast-iron columns, the same methods will suffice as in the use of wooden girders, but with the use of steel columns a new method of construction arises, which it will be better to consider later in connection with steel framing. It may be properly said here, however, that the use of steel girders and steel columns, even if the floor timbers are of wood, is to be recommended for high buildings or for buildings which are subjected to the jar of machinery; the reason for this being in the greater rigidity which is possible in the girder and column connections, and the greater ease with which this construction can be braced.
The use of wood or steel for girders will require different methods of construction. If wooden girders are used they may be set wholly or partially below the floor timbers, or flush with them. If there are no objections to dropping the girder below, this is the simplest and strongest construction. In this ease, the floor beams should be sized down on the girders to maintain a per fectly level floor line, and a full bearing should be obtained for each timber. Where the girders are large the timbers may be brought to butt against each other upon the girder; and they should be secured end to end by iron dogs which turn down into each timber. (Fig. 177.) For small girders and light timbers, it will be better to let the timbers lap close and be spiked to each other, as in Fig. 178.