The members of the steel skeleton are completed in their construction at the mill and are assembled and riveted to gether on the site of the proposed build ing. Angles, plates, channels and other rolled sections are built up into girders and columns. As much as is possible of the riveting is done at the mill, the rest being attended to at the site of the build ing. In order to distribute the weight of load columns, bases or bearing plates are placed between the column and its foun dation. The columns may be of steel or cast iron, hollow circles or hollow squares or H shapes being in use where the strength is sufficient as being the most economical. Slabs or arches of terra cotta or concrete laid between steel I beams make up the floor construction. The floor area is determined by its di vision into units of from 15 to 20 or more feet, according to the size of the rooms required. Single or double gird ers, built up or riveted, placed between the columns, hold up the ends of the floor beams. In high and narrow skele ton buildings cantilever girders are some times called for to distribute the weight evenly over the footings. The great
height of steel buildings in relation to the area of their site has likewise made methods of wind-bracing necessary, the systems employed being sway-bracing, knee-bracing, and portal-arch bracing, the aim in each case being to hold the steel frame in its several parts more securely to its foundation. While the ad vantages derived from steel skeleton con struction are many and obvious, there are some disadvantages. There is always the liability to corrosion in the metal frame, while the danger to life and limb from their great height and the concen tration of population made possible has been often illustrated. While they repre sent a tremendous development in the art of building, they are erected for con venience rather than for endurance or beauty, though in this latter respect numerous improvements have been made in late years. This disadvantage may be lessened by new rules and devices, but steel skeleton construction has be come a necessary adjunct to our material civilization, and greater development may be looked for in the near future.