Joist Courses.—By a joist course, or joists, is meant a system of hori zontal beams lying at fixed distances from one another and arranged be tween every two stories, or between the upper story and the roof; they are intended to carry both the ceiling of the lower story and the floor of the upper one. Those courses of beams which lie directly under the roof are called " attic courses," in contradistinction to the story courses. In the constrnction of these courses, it is of the first importance that the injurious sagging of the beams and disagreeable shaking of the floor should be pre vented by choosing beains of the proper cross-section. This is regulated by the load to be sustained, by the span, and by the distance of one beam from another.
Supporting-power of Joists.—The load is partly constant (dead load, comprising the weight of the beams themselves, of the ceiling, the floor, and the filling in) and partly variable (live load—furniture, goods, persons, etc.). The former may be calculated exactly; experience is the only guide in estimating the latter. For instance, in dwelling-houses, from sixty to eighty pounds are assmned for each square foot of floor-space; in schools and assembly-rooms, from eighty to one linndred pounds; in storehouses, from one hundred to one hundred and twenty-five pounds.
Span of Joists.—As regards the span of the joists, or the distance be tween the nearest supporting-points, it is necessary to keep within fixed limits, these being determined by the fact that the supporting-power of the beams does not increase pro rata with their own weight. As an average limit for the span, 25 feet may be assumed, the depth of the room not ex ceeding- this save in cases of necessity. The distance of the beams float one another lutist also not pass a fixed limit, chiefly in order to prevent the floor-boards from sagging; this distance may be from 2Y, to 3Y? feet, meas ured from centre to centre of the beams. The joists are given an oblong cross-section, as shoWn Figure 17 (pl. 3).
If the width of the rooms over which beams are to be laid exceeds 25 feet, a mere increasing of the cross-section will not suffice, but either arti ficial supports for the beams at various points in their length, consisting of wooden or iron girders or of posts, must be applied, or, if it be the uppermost conrse of joists, some method of suspension from above may be made inside the attic. This will be treated more fully under Roof-con struction. Some help may be had by laying two joists close together and connecting them with " dowels."
The following additional remarks respecting joist courses will be found pertinent: (1) The bearing at the ends of the joists must be at least equal to their height; (2) Wherever possible, the beams are to be laid in the direction of the depth of the room or building; (3) If a chimney or other obstacle occurs in the course of the joist, the latter must be trinnned for it —that is, cut off and connected with the adjacent joists by a beam-trimmer; (4) If a solid wall runs parallel with the direction of the beam, a beam lighter than the rest (wall-beam) is to be set at each side of the wall, pro vided the latter is to be continued higher up; but if the partition is of framework, the effort should be made to set a beam upon it, so that the beam may act as a cap for the lower partition and as a sill for the upper one.
The Ceilings may be constructed with visible joists, as in Figure IS (fi/. 3), or the joists may be concealed with boards or with lath and plaster. The space between the joists should not be left entirely vacant, but to render the ceiling proof against the penetration of moisture, sounds, and cold air, a false floor carrying some kind of solid waste material should be interposed. Figure 17 shows the arrangement of such a false floor, which is slid into the grooves or rabbets in the beams. It may also be rested on strips which have been nailed on.
Ceding Supports in supports for ceiling joists are often required, on account of the great width of the rooms. That these supports can be employed in a rich and tasteful manner is shown in Figure IS; it represents the interior of the great hall in the so-called " Giirzenich " at Cologne. Twenty-two columns of oak are connected by light wooden arches whose spandrels are filled in with Gothic fretwork. The effect of the whole is much increased by skilfully-disposed polychroming and gild ing. The length of the hall is 132 feet; the average height 46 feet, the span of the centre aisle 43Y, feet, and that of the side aisles and 21,1‘ feet respectively.
Paris, during a protracted period, much attention lias been given to methods for deadening the communication of sound and of fire between different stories of houses or buildings devoted to multifarious purposes and occupied by numerous tenants. A demand for the accom plishment of similar objects has materially increased in American cities since a considerable number of tenement-houses, French flats, and gigantic business edifices have been erected.