BUILDING permanent protection of all parts of the cage from corrosion is a most serious consideration. The safety of the structure depends upon the preservation of the absolute integrity of the cage. It must not only be strong enough to sustain all possible vertical loads, but it must be sufficiently rigid to resist without deformation or weakening all lateral disturbing forces, the principal of which are the pressure of wind, the possible sway of moving crowds or moving machinery, and the vibration of the earth from the passage of loaded vans and trolleys and slight earthquakes which at times visit almost all localities. In buildings wide in proportion to their height it is the ordinary practice to make the floors sufficiently rigid to transfer the lateral strains to the walls, and to brace the wall framings to resist them. In buildings of small width in proportion to their height this method of securing rigidity is generally found to be inadequate, and the frame is also braced at right angles to the outer walls to take up the strains directly. In each case all strains are carefully computed. The bracing is accomplished by the introduction at the angles of the columns and girders or beams of gusset plates or knee braces, or by diagonal straps or rods properly attached by rivet or pin connexions. All portions of the frame are united by hot rivets of mild steel or wrought iron, care being taken that the sum of the sectional areas of rivets affords in each case a sufficient amount of metal for the safe transfer of the stresses. The greatest care should be taken to see that all rivet holes are accurately punched, and if necessary that they are reamed so that each rivet will have its full value. It is evident that with this system of construction all serious inequality of settlement must be avoided, and unusual care, therefore, must be employed to da secure er ect tions ' P f foundations. As a preliminary to the design of the foundations a knowledge of the character of the site for a considerable distance below the footings should be gained by borings, and in cases where a certain settlement must be expected the footings must be so proportioned that the pressure at their bottom on each square foot of surface of the soil shall be uniform, so that inequality of settlement may be prevented. Experience has shown that a very close approximation to this result may be secured by assuming in the case of residences, hotels, offices, and other buildings in which the principal variable load is that of men and women, that the load on each lower column base is the sum of the dead load of the weight of materials borne by it, and 20 per cent, of the variable loads which may be imposed upon the floor and roof surface it sustains. In the case of shops, factories, &c., increased percentages of the variable loads must be assumed to be borne by the lower column bases in proportion to the relative amounts of the variable loads which will be imposed upon the floors for any considerable length of time. The foundations must be spread below the column bases until such areas are reached that the pressure on the soil is not greater than its safe sustaining power. This is accomplished by rackings of stone or brickwork, or steppings of concrete, or systems of inverted arches, or systems of steel girders and cantilevers bearing on grillages of parallel steel “ I ” beams kept a uniform distance apart by separators, and bolted together by steel rods. The grillage beams rest upon and are embedded in concrete, and in many cases the girders and cantilevers are also embedded in concrete. The tables of grillage must be so arranged and proportioned that there is no serious eccentricity of loading from the columns. Shear and bending moment, as well as the absolute strength of all girders, cantilevers, and “I” beams, must be carefully computed. Before building on an elastic or yielding bottom the sustaining strength of the soil must be ascer-
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tained. On the bottom is placed a table with a single leg of a recorded dimension arranged so that it can only move vertically. This is loaded upon the top with pigiron or other material of known weight, and the pressure is noted under which initial compression of the soil takes place, and the load under which the settlement becomes marked. At this point the increase of load is stopped and the soil is allowed to remain under pressure for several days, careful observations being taken at regular intervals of the rate of settlement until no further settlement occurs. A foundation which will not settle may be secured :— (a) By spreading the footings until the pressure on the bottom is less than the load at which the initial settlement occurred, which will be found to vary from 1| short tons per foot for ordinary wet clay or earth, to 3 or 5 tons per foot for retained sand or hard pan; (b) by building on piles driven to refusal (that is, until they will not move under the repeated blows of a heavy pile-driving hammer), in which case they must not be loaded beyond their safe computed strength as columns with free ends ; pile foundations should never be used unless the circumstances are such as to ensure that they with their cappings will be constantly covered with water; (c) by excavating to bedrock and laying foundations upon it. Where the rock is covered by dry earth or gravel this can be accomplished without particular difficulty by careful sheath-piling to retain the sides of the excavation, but where the covering is quicksand or swamp-mud pneumatic caissons must be sunk. A pneumatic caisson is a round or polygonal shell of wood or metal open at the bottom, which is called the “cutting edge,” about 6 feet above which is built a removable roof, which forms an air-tight chamber. This chamber is entered through the roof by a metal cylinder about 3 feet in diameter, which extends to the top of the caisson, and is called the “air-shaft.” On the top of this air-shaft is bolted a cylinder of greater diameter, provided with doors at its top and bottom, which is called the “air-lock.” Air is supplied through pipes to the airchamber, and constantly maintained at a sufficient pressure to keep water from entering the air-chamber, and to expel it from the ground immediately below it. The exterior of the caisson is made very smooth, and is thoroughly lubricated. The caisson is sufficiently loaded to cause it to sink as the material within it is excavated and removed. Access to or from the caisson for both men and materials is gained without altering the air-pressure in the caisson, by opening and closing the doors of the air-lock successively. When the sound bottom is reached it is faced oft to secure a good bearing, and concrete is introduced through the air-locks and bedded in the bottom of the caisson to a sufficient depth to form a water seal. The air-locks, airshaft, and roof are then removed from the top of the caisson, and an open well remains in which the building may be extended with appropriate materials to the concrete foundations at the bottom. No reliance is placed upon the strength of the caissons as a direct support to the building. A well-constructed and carefully-handled caisson, in which a proper air-pressure is maintained without interruption, may be sunk through flowing quicksand or mud in close proximity to the foundations of an adjoining building standing on the crust above the soft bottom, without injury to it. Until very lately it has been assumed that a depth of 110 feet below the waterlevel was the practical limit to which caissons could be sunk, for men cannot safely work in an air-pressure of more than about 50 pounds to the square inch, but it is now found that they can be safely sunk to a considerably greater depth, for recent experience has shown that the exterior water-pressure at the base of a caisson is in
