When splices are made at the building, the superintendent should see that the riveted surfaces are painted before being put together, and that a sufficient number of bolts are used to hold the pieces together rigidly while being riveted.
The use of splice-plates for connecting columns is to be preferred to a connection of plate and angles; and if any part of the column section projects beyond the column section below, it should not be left unsupported, but should be given a bearing on a filling plate riveted to the lower column. The rivet holes in the column and splice-plate should fit perfectly so that "drifting," i.e., straining the pieces together by means of a drift-pin, need not be resorted to; neither should heavy mauls be used to bring the members into posi tion, but wooden mallets only. The finished rivet head should be clean and smooth without cracks or flaws, and the two heads should be concentric with the axis of the rivet. All rivets should be examined to see that they are tight and well formed.
Loose rivets are often tightened by simply calking around the head with a calking tool. This should be watched for and never be allowed to pass, as a rivet so treated possesses no working strength, being tight ened simply by the contact of a thin edge against the steel members.
Grillage. The great loads which are concentrated at the bottom of the steel columns of a high building, will, in general, require a different treatment, to secure a firm foundation, from the stepped up foundations of stone or con crete which we have already considered, as in Fig. 203. Of course it would be possible to ex tend the area of the succeeding steps of an ordinary pier founda tion until it covered a sufficient footing; but this would in many cases be at a great expense and loss of room—two important fac tors in city buildings. To over come this, and to provide ade quate foundation within reasonable limits of depth and material, the use of steel beams for foundations has become general. This use of beams is known as grillage, and may be used for isolated bear ings or for the hearing of continuous walls.
Foundations of this sort are usually prepared by laying down a bed of concrete from four inches to twelve inches in depth, and laying upon this a series of steel beams. If a Ivan foundation, these beams
are continued at a spacing V:n•ving from six to twenty-four inches along the wall, as in Fig. 204. If a column or pier foundation, the concrete and beams will be similarly put down in a square, of an area sufficient to sustain the esti mated load, and these may be crossed by another or several tiers of beams laid at right angles, each series covering less and less of width until the footing is brought to the desired width to receive the base of the columns. (Fig. 205.) The distance apart of the beams will vary according to their length and the load to be borne, but should be at least enough to allow- of filling and properly tamping the con crete between the beams. The whole is then finished flush with a bed of concrete. Sometimes the nature of the soil and the closeness of the supports will snake it advisa ble to cover the whole area of the building with a thick bed of concrete, upon which the footings of beams or stone mav be laid. Before laying the steel beams for grillage, they should be cleaned and coated with asphalt; and they should be examined before cov ering with concrete, and all defects found in the coating thoroughly covered.
Timber is sometimes used as a grillage, in connection with piling, by bolting a series of timbers to the tops of the piles, this timber crossed by succeeding series, and the top covered with a floor of thick planks to receive the masonry.
Caissons. Where the soil is too yielding or the necessary area is not available, foundations are sometimes carried to the bedrock. This is done by the use of caissons which are sunk through the soft material to the rock.
These caissons consist of a steel chamber, having the bottom edge extending below an air-tight floor, far enough to form a working chamber into which compressed air is forced, which keeps out the water and soft material, and enables the work men to excavate and allow the caisson to sink to the bedrock. The exca vated material is hoisted up through air-locked shafts, and the masonry of the foundation is built within the caisson, and helps to sink it down. When bedrock is reached, the working chamber is filled solid with concrete and a solid foundation is secured.