The floors used in steel-skeleton construc tion may be of any of the ordinary fireproof types, but in designing the floors it is necessary to connect the columns by steel beams or gird ers, which act best for the stability of the build ing if arranged in continuous straight lines. The voids between the girders are spanned by beams, whose spacing is dependent upon the style of floor to be used, varying from 5 to 12 feet, the spaces between being filled by brick arches or porous terra-cotta tiles, or by concrete slabs. The amount of material in the beams must be exactly sufficient for the work— no more or less. This is essential, not only for economy, but also to reduce the dead loads on the joints, columns and foundations. There are many varieties of each of these systems, nearly all requiring the use of steel ties, plates or rods. For spans over 12 feet the monolithic concrete floor reinforced by steel bars or metal lath has been used, but there is a great tendency to de flection. The long-span systems are still in their infancy.
The girders of the exterior walls, commonly known as spandrel girders, are used at or near the level of each floor, and should be connected to the columns by knee or angle braces.
While the exterior walls of the building are carried in part on the spandrel girders, it is customary to rivet additional angles or chan nels on the outer face of the columns for the support of the outer four or more inches of the wall.
All projecting parts of the exterior, such as belt courses, cornices and balconies, must be supported by special framing. The ornamental finish of cornices having any great projection is often secured to the frame by iron hangers.
All parts of steel framework, except those buried in concrete, such as grillage beams, should be painted with the greatest care, as their preservation depends almost entirely on the quality of paint used and the way in which it is applied. All surfaces should be first thoroughly cleaned of scale and rust. It has been found that concrete adheres to a clean steel surface, and is a sufficient protection. All remaining parts should be given a coat of oil at the shops; they should then be painted with a coat of red lead or graphite paint upon arrival at the build ing, followed by a second coat after they are assembled. Sometimes a third coat is given, but it is scarcely necessary if the two previous coats have been properly applied. See CoN cazva Fireproofing.— While the steel-skeleton building is economical from the constructive standpoint, its usefulness and safety are greatly impaired if it is left unprotected against the ravages of fire. Many systems of fireproofing have been devised, all of which, however, con sist in enclosing the parts with a non-combus tible substance,— usually a clay product, or con crete or plaster,— applied in blocks or molded forms, set in mortar. For the outside of exte rior columns and girders it is considered suffi cient to lay the outer facing of the wall, if of brick or terra-cotta, directly against the metal. Granite, by reason of its friability under the combined action of heat and water, should be kept sufficiently far away from the structural parts to allow of the insertion of a layer of concrete. For all other parts of the skeleton
the usual protections consist of two inches of porous terra-cotta block, plaster block or cin der concrete. Columns and beams are some times enveloped with a sheet of wire cloth or expanded metal, and plastered.
None of these systems may be considered absolutely perfect, since they have all shown serious signs of deterioration under the contin ued action of a fierce fire, but it is a conceded fact that concrete, as a fire-resisting material, is unequaled.
In connection with fireproofing it is essential that interior partitions be built of non-combus tible materials: Those most frequently employed are of the same nature as the fireproofing just described. Porous terra-cotta blocks and plas ter blocks, three or four inches in thickness, dependent upon the height of the story in which they occur, have certain advantages by reason of the rapidity with which they can be set up, and the ease with which they can be removed where alterations are desired. Par titions are often made of small I or angle irons, over which is spread expanded metal or wire cloth in one or two thicknesses, to which the plastering is directly applied. Double thick ness partitions of this sort are more sound proof than those first mentioned.
In many so-called fireproof buildings wood finishes are desired, which with the contents are a menace; but experience has shown that fire can usually be confined to the room in which it originates, and can be checked in a few min utes. Methods of fireproofing wood have been devised, and consist of injecting a fireproofing solution into the pores, either under pressure or by capillarity. Its use is not frequent, however, being largely limited to war-ships.
The most fireproof structure yet built is probably the Printing Crafts Building, adjoin ing the Pennsylvania Terminal, on Eighth ave nue, New York city. No wood or other read ily burnable material is used in the construction. The elevators are all in the centre and isolated by fireproof partitions and steel doors from the rooms. The floors and ceilings are of heavily reinforced concrete, set into steel framework. All window sashes are steel, and all glass is wired. Each tenant's quarters are separated from every other like so many ovens, so that a fire may burn out the contents of one without the least chance of communicating to another. The floors are scuppered, like those of a steamship, so that water thrown on to extinguish a blaze is led by the scuppers to the outside of the instead of running down the hallways and stairs. There is ample water in the standpipes to drown out any blaze that might occur, and the system ensures any fire being confined to the place of its origin. This building is also remarkable for its heavy bracing. Priming presses produce great vibration, and to overcome this heavy L braces are placed at all cross joints.