The posts may be either round or square. If round, it is better to leave the upper end square, as it gives a better' bearing for the iron cap. The bottom of the post should be round if the post is turned.
The New England Mutual Insurance Companies, which insure nearly all the mills and factories of the New England States, advise that a 1 ½-inch hole be bored through the centre of the wooden posts, with a ½-inch cross hole near the top and bottom to give a circula-lation of air through the post. It is claimed that this precaution prevents decay and dry rot, but it is very seldom that posts are bored in ordinary buildings.
If bored, the boring should be done entirely from one end, and if the auger comes out more than \ of an inch from the centre at the other end the post should be rejected. Boring from both ends is often done, but is not recommended, as it is difficult to make the holes meet at the centre.
All posts should rest on a cast iron plate, and never on a girder. The basement posts should rest on an iron plate bedded in Portland cement on top of a brick or stone pier, the top of the iron plate being kept a little above the concrete floor. The top of the posts should be fitted with an iron cap, which should support the girders and the post above.
Fig. 463 shows a tier of posts and girders for a two-story building.
The same construction is applicable for a five-story building by properly proportioning the size of the columns. The ends of the girders should be cut to fit closely around the bottom of the post, as shown at X, and be tied together longitudinally, either by bolting through the cap, as at D, or by iron straps, as at C and X. The bottom plate, A, should have a dowel in the centre to keep the post in position until the load is on.
The style of cap plate shown at B is the one most generally employed, and is about as good as any. When the girders and joist are in place, and especially when the building is occupied, there is no danger of the girders or posts slipping on the plate - in fact it would require a great force to move them - and the author doubts if any particular advantage is gained in fastening wooden columns together in a vertical line, for, if the building should take fire, the posts would certainly be destroyed as soon as the girders, and bolting the posts together top and bottom would in no way keep them from falling.
Many architects, however, prefer a cap with side plates like that shown at D. The side plates add somewhat to the strength of the cap and keep the upper post and girder in place while the building is being erected. Holes may be left in the side plates for securing the ends of the girders and the post above.
Figs. 464 and 465 show the Goetz and Duvinage caps, which are similar to the cap shown at D. These caps have lugs or dowels cast on the bearing plate to hold the ends of the beams, and the Goetz cap is made so as to be bolted to the posts, the patentee claiming that this will keep the columns upright, even if the beams and girders fall. Both of these caps are well adapted for their purpose and are extensively used. They are patented, however, and cannot be used without paying a royalty to the patentees. Caps made like D, without the lugs for securing the girders, can be made at any foundry without infringing on the patents. Fig. 466 shows a double post cap made entirely of forged steel.
Fig. 464. - Goetz Cap.
Fig. 465 - Duvioage Cap.
The side flanges, F, F, extend the full length of the cap, and the bracket bearing at N is riveted to the side flange and to the bottom ring. The post caps for girder in one direction only are made without the bearing N, as shown in Fig. 472.
The upper post may be capped by a wooden bolster (E) if preferred. The top of the post should be doweled into the under side of the bolster or secured by two square drift bolts, and the ends of the bolster should be spiked or bolted to the girder. The bolster should also be made of the hardest wood obtainable (preferably oak) so that the fibres will not be crushed where they bear on the top of the post. If the building has a flat roof, with a ceiling beneath, the ceiling joists should rest on a girder, and the rafters may be supported by a short partition of 2x6 studding, also resting on the girder.
Posts that are eccentrically loaded, i. e. from one side only, should have a greater cross section than if concentrically loaded.
The iron cap plates should be of such size that the girders will have a bearing of at least 5 lineal inches at each end, and the edge of the plate should not project more than 6 inches beyond the post, unless absolutely necessary to get a sufficient bearing area.
Fig. 466. - Van Dorn's Steel Post Cap.
The bearing area required for the end of the girder should be found by dividing one-half the load on the girder by 600 for oak, 500 for Georgia or Texas pine, 400 for Oregon pine and 250 for spruce. If the area thus found, divided by the width of the girder, is less than 5 inches, however, the latter distance should be taken as the minimum lineal support, except for very light girders.
Example of Bearing Surface. - A 10x12 Georgia pine girder is calculated to support 20,000 pounds. What bearing should it have at the ends ?
Answer. - One half the load would be10,000 pounds, and this divided by 500 gives 20 square inches for the bearing area. Dividing this by 10, the width of the girder, we have 2 inches as the distance that the girder should rest on the plate; for practical reasons this should be increased to 5 inches.
Round posts are generally preferred to square posts, as they are less in the way, but there is necessarily a loss of strength in turning, as the sectional area of the post is considerably diminished. For posts turned the whole length an iron cap like that shown at A, Fig. 467, may be used. The cap shown at B, is often used when the top of the post is left square and there is no post above.
Occasionally it is necessary to make the girders continuous over a post, or the post may support the end of a truss, in which case the post above cannot come down through the girder. For such construction a hollow cap plate should be used, something after the style of cap shown in Fig. 468. The cap should come down at least 3½ inches on the lower post, and should be bolted to the upper post.
Proportions for Cap Plates. - Rules for calculating the size of brackets, etc., for cast iron bearing plates are given in the later editions of the Architects' and Builders' Pocket Book, As a general rule the depth of the bracket should not be less than three-fourths of its projection.
The bearing plate should be 1¼ or 1 ½ inches thick and the socket plates ¾ or 1 inch thick.
The socket which encloses the top of the post should be made ½ inch smaller than the nominal size of the post, so that it will be sure to fit tightly over the post.
In designing iron castings the thickness of all the parts should be nearly uniform, that they may cool evenly and thus avoid initial strains.
Steel plates and caps being much stronger and not subject to internal strains need not be more than about one-half as thick, or even less.