This is called the plenum or compressed air process. Caissons are also sunk by the vacuum process, which consists in exhausting the air from the interior of the caisson, and allowing it to sink by the aid of the pressure of the atmosphere. By this process, the water is caused to flow under the lower edge of the caisson, loosening the soil and assisting the sinking of the caisson.
Pile foundations are also used as already described, sometimes in connection with a timber grillage, but generally with a foundation of concrete or stone. (Fig. 200.) Cantilever Foundations. In buildings where, for any rea son, it is not desirable or permissible to build foundations under an adjoining building, cantilever girders are used to support the columns at the party line. The principle of this construction is shown in Fig. 207, where a simple cantilever is shown bearing upon a beam foundation and secured to an interior column. By a proper adjust tnent of the loads, this construction may he made as rigid as if the wall columns had each its own footing, and complications often avoided.
Girders. Next in importance to the columns will come the steel girders which run between the columns, and support the floor beams. Where the load is too great for a single beam, two or more beams may be bolted together, as in Fig 20S, or a girder may be built up of plates and angles, as in Fig. 209. 'These girders are connected to the columns by an gles, varying in size and the num ber of rivets, according to the load on the girder; and if they bear upon the mason work, steel or iron bearing plates must be provided to distribute the load over a surface large to sustain it safer-.
Girders which consist of two or more I-beams should be eounecte(l by means of bolts and cast iron separators, Fig. 210. The office of these separators is to hold the beams in position, and cause them to act as one beam, and also to prevent lateral deflection under heavy loading.
The separator shown in Fig. 210 is the type in general use, and consists of a series of bolts running through a plate-shaped casting made to fit accurately to the outlines of the beams, and having a width equal to the desired space between the webs of the beams. Another form of separator consists of spool-shaped castL gs of the required length to fit between the webs of t''e beams through which the bolts are run. These ,ao not form so rigid a girder as the plate separator;,, e=nd are only used for light loads. (Fig. 211.) Girder Connections. The connections of the girders with the columns are made by means of short pieces of angle iron riveted to the columns, above and below the girder, as in Fig. 212. The lower angle is made
heavy and set so as to form a seat for the girder, and is reinforced by upright pieces of angle iron, also riveted to the column, if the load is very heavy. The upper angle serves to hold the top of the girder in place.
Inspection. The workmanship of all riveting and splicing should lie carefully watched, with especial attention paid to the fitting of the pieces. It is usual to punch the holes by machine and when several pieces are to be fitted it is almost impossible for the holes to fit over each other exactly. If the variation is not too great, it will generally be overcome by using a rivet smaller than the standard size, depending upon the hammering to make the metal fill the irregularities (which it will rarely do), so that the pieces are im perfectly joined. This should be guarded against, and the holes should be drilled or reamed out to receive rivets of the stand ard size.
Bolting is often substituted for riveting if no objection is made, but this is not good work, as there is danger of the bolts working loose, while riveting is absolutely rigid if properly clone. Of course a certain amount of bolting will be necessary, and this should be watched to see that the bolts fit, and are long enough to receive the nuts properly.
When bolts are used on the sloping flanges of beams, bevelled washers should be used to give an even bearing.
Painting. Too much care cannot be taken in protecting columns and other steel work from corrosion. Where the members are painted, it is of great importance to see that no rust has formed under the paint, as the process will continue if once begun. If, however, the metal is perfectly clean, it will be protected as long as the paint remains whole. "hill scale," a coating which is produced by the process of roiling the steel, must be removed before painting, or it will peel off and bring away the paint. _ Cement Coating. Where the steel is completely encased in a cement or plaster fireproofing, there will be danger that the paint will decay in course of time and leave a minute crevice between the steel and the masonry which will allow rust to form; and for this reason it is a good plan to coat the carefully cleaned steel with a wash of Portland cement, which unites with the masonry and maintains a perfect contact with the steel_ Under these conditions, no further rust or corrosion will take place.