CHAPTER XXIII.

FIRE-PROOF FLOORS.

THE term "fire-proof floor" is here understood to mean a floor constructed of fire-proof material, supported on or betwe. n iron or steel beams or girders, or fire-proof walls, and entirely protecting the ironwork from the action of fire. The various materials at present used in the construction of absolutely fire-proof floors are brick, hollow porous tile, hollow dense tile, thin plates of dense tile

-all products of clay; and concrete of Portland cement and either cinders, broken tile, stone, or brick; and also compositions made with plaster of Paris as a cementing material. The first three materials are generally used in the form of arches set between the beams. The thin plates of dense tile are used for forming vaults between girders. Concrete is used either in the form of an arch, or in flat slabs, forming floor and ceiling, with hollow interior; in the abs, iron bars, expanded metal, or wire ties are imbedded. Iron steel beams are generally laid in floors as shown in Fig. 1, the -ts either resting on top of the girders, as in Fig. 2, or bolted to sides of the girders.

Fig. 3 shows the detail of connection when the under sides are made flush; Fig. 4, the joint to bring the upper sides flush; and Fig. 5 shows the form usually adopted when the beams are of the same size, or the centre lines are brought together. Arrangements of this kind are also used to connect the trimmer-beams of hatchways, jambs, and stairways.1

The wail ends of the joists and girders should be provided with shoes or bearing plates of iron or stone, as the brickwork is apt to crush under the ends of beams, unless the load is distributed by this means over a sufficient surface. Anchor-straps should be bolted to the end of each girder and to the wall end of every alternate joist, binding the walls firmly from falling outwards in the event of fire or other accident.

Several simple modes of anchorage are shown in Figs. 3, 4, and 5.

When one beam does not give sufficient strength for a girder, it is customary to bolt together two or more with cast separators between them, as shown in Fig. 6.

1 The details of the connections and framing of iron beams are more clearly shown on pp. 365, 366.

Brick Arches.

The common way of making a fire-proof floor of brick is to fill the space between the joists with brick arches, resting on the lower flanges against terra-cotta or brick skewbacks. When this method is pursued, care should be taken that the bricks of which the arches are composed are of good shape, and very hard. They should be laid in contact with each other, without mortar; and all the joints should be filled with the best cement grout, and be keyed with slate.

Fig. 7.

The arches need not be over four inches thick for spans between six and eight feet, except for about a foot at each springing, where they should be eight inches thick for spans between six and eight feet, care being taken to form the skewbacks quite solid, and square to the line of pressure. The rise of the arch should be about oneeighth of the span, or an inch and a half to the foot; and the most desirable span is between four and six feet.

Above the arch the space is filled with cement concrete, in which wooden strips, three inches by four inches, are embedded for nailing the flooring to. The thrust of the arches is taken up by a series of tie-rods, usually from three-quarters to one inch in diameter, placed in lines from six to eight feet apart, and running from beam to beam from one end of the building to the other, being anchored into each end wall with stout washers; an

angle bar or channel serving as a wall plate for distributing the strain produced by the thrust of the first arch (Fig. 7).

The weight of a brick arch with cement filling is about seventy pounds per superficial foot of floor. Experience has shown that such a floor cannot be considered as fire-proof unless the lower flanges of the beam are protected by porous terra-cotta, fire-clay tile, or wire lathing, kept an inch away from the beam.

Brick floor arches are largely going out of use, owing to the fact that a fire-proof floor may be more cheaply constructed of other material.

Hollow Porous Terra-cotta and Hollow Dense Terra-cotta Floors.-For convenience, these materials will be referred to as Porous Tiling and Dense Tiling. A description of the materials, their nature and manufacture, will be found in Chapter XXV. They consist principally of clay, which is manufactured into hollow blocks, generally with angles on side or ends, according to whether the arches of the floors are to be of endmethod design or side method design. In some instances, to a limited extent, rectangular blocks have been successfully used, but this shape is not approved. The general practice in flat construction is to make bevel joints-radius joints are seldom used; the best workmanship and best results are found to be obtained with a bevel joint of about one inch to the foot. There are two general schemes of flat construction: one in which the tile blocks abut end to end continuously between the beams, and one in which they lie side by side, with broken joints, between the beams. In the end systems, it is not the practice to have the blocks in one row break joints with those in another, as it entails extra expense in setting. When this is done, however, the substantialness of the floors is increased.

In some forms of flat construction a side-method skewback (or abutment) is used, with end-to-end interiors and keys, or end-toend interiors and side-method keys. Experience has shown that in the side method of flat construction the skewback, or abutment, was the weakest-in case of failure, sometimes collapsing, but generally shearing off at the beam flange; consequently, the sidemethod skewback is not approved in the end-method construction unless provided with partitions running at right angles to the beam. Keys should be end to end, or solid. The latter, when made very small, are preferable.

A practice which has become somewhat general, especially in the East, is for the owner or general contractor to buy tiles, and the mason contractor on the job to build them in place in the building,

While with contractors of large experience this practice has worked very well, generally, the best workmanship and most satisfactory results have been obtained where the work was executed by some one of the regular fire-proof construction companies, who constantly have a corps of men experienced in that kind of work, and who make a specialty of building in fire-proof tiling.

When tie rods are placed near bottom flanges of beams the flat construction of tiling between the beams is found to be much stiffer and to have less liability to cracks in ceilings than when tie rods are placed in centre of beams. The reason is obvious. The greatest thrust is at the bottom flanges, and the nearer the tie rods to this point, the better the results.

The best form of centring for flat construction is that in which T-bolts are used, and double 2 x 6 sound lumber centrepieces below, midway between the beams and extending parallel with the