In buildings requiring special provisions against the spread of fire, and where the artistic effect is not of prime importance, a frequent mode of construc tion is that known as gslow-burnine or (mill° construction. This has been brought about in a great measure through the efforts of the mutual fire-insurance companies in New Eng land. The system consists usually in building outside walls (generally of brick) of concen traced piers or buttresses, connected by a thin curtain wall; the girders, beams and interior columns are made of large timbers and the floors of plank of a suitable thickness. It is essential to avoid concealed hollow spaces, such as furring, where dirt would accumulate. The underlying theory of slow-burning construction is that whereas small timbers, such as the three inch joists and studs and the one-inch flooring of ordinary construction, readily burn through and are destroyed, large timbers, under the influence of severe heat, char but do not burn through readily, as the charred surface forms a non-conductor and protects the interior. If, however, for any reason they should burn to destruction, all connections are so made that the timbers can fall out of their places without disintegrating the masonry or columns on which they rest.
Beams are spaced every 8 or 10 feet be tween centres, and should not be painted for several years after completion of the building, in order to avoid dry-rot. The ends of timbers in masonry bear on iron plates with anchors, or rest in cast-iron boxes, with air spaces in the sides, which permit a circulation of air, and reduce the risk of dry-rot. Floor planks are not less than three inches in thickness, and for spans of 12 feet usually four inches. The larger spans are less desirable than the smaller. These planks should not be over nine inches wide. They should be planed on both sides, and grooved on the edges, the grooves being filled with hard-wood splines.
Top floors are made of PA-inch boards of southern pine, maple or other hard wood. It is desirable to lay top floors over a three-quar ter-inch bed of mortar, or two thicknesses of heavy sheathing paper.
For rooms where there is unusual risk of fire, such as hot-air drying, it is well to protect the ceilings with plastering on metal lath, filling in solid so as to avoid any cavities. Wooden posts should be covered with asbestos paper and tin.
Roofs are best when flat, and are constructed in the same way as the floors. They should be covered with tin, gravel or duck. Where the roof is pitched, it should be covered with shin gles or slate, laid over a three-quarter-inch bed of mortar.
Superposed columns are connected by iron caps, bases and pintles, arranged to give a proper bearing for the girders.
Partitions, if used, should be two-inch, tongued-and-grooved plank set on end, and plastered both sides, on metal lath.
Doors and shutters are built of two or more thicknesses of inch boards, covered on all sides with asbestos paper and tin, lock jointed.
The underwriters associations have formu lated in detail the best practice in mill construc tion, and are willing to advise on all question able points.
The closing years of the 19th century witnessed a develop ment in the structural use of steel for buildings which is wholly without precedent. While col umns and floor beams of iron or steel bad been in use for many years as interior supporting members, it was not until conditions demanded buildings of extraordinary heights that the metal framework was extended to the exterior as well as the interior structure. Exterior walls constructed entirely of masonry must be made too thick for economy of space and materials if the building which they enclose is more than six or seven stories high.
The first step was made by introducing iron columns in the masonry of the outside walls, with the sole purpose of supporting the adja cent floors, the masonry of the walls carrying itself on its own independent foundations. This system was found also to lack economy after the possible height of buildings had been increased a few stories. The culmination of the system was reached when the exterior frame was de signed to carry not only the floors and their various loads, but also the exterior walls. Each story now has its enclosing wall independent of the story above and below it, so that, as is fre quently the case, the outer facing or curtain wall of the high building is started at several levels at the same time at intervals of 5 or 10 stories.
The limit of height construction of such buildings is not yet known. Every year or two a taller one is constructed in New York city. They have an advertising value.
Vibration and deflection under the pressure of the wind must be provided against by stiffen ing braces or ties in the floors or partitions, more particularly where the height of the structure is relatively great.
Where streets are narrow, the crowding to gether of a number of such buildings darkens the streets and often produces disagreeable and even dangerous currents of air. The trend of legislation in large cities is toward restriction of height, Boston having already fixed a limit of 125 feet from the sidewalk level to the top of the cornice line.